# Convert Lux meter to PAR meter?



## Hoppy

There are a lot of cheap Lux meters available now that look like they could become a hobby grade PAR meter with some modifications. Just for the fun of it I bought one. It is a Mastech LX1010BS, from Cybertech, in California, for only $14.99, $21.53 after shipping and tax. I ordered it last Saturday and it arrived yesterday, Tuesday. You can't beat that!

Here it is:









It takes a 9 volt battery, which I don't have right now, so I haven't tested it at all. I did partly disassemble the sensor though - 2 screws.









I have a LED light and a CFL-in-a-dome reflector light to calibrate it against a PAR meter. I'm guessing there will be a different conversion factor for the different types of light, but that's fine for what I want.

My requirement is to be able to use this under water, so that is the challenge. Ebay has a nice 4 inch diameter acrylic box that I could just seal the sensor in, but that has problems: temperature and atmospheric pressure changes would flex the box and probably break the seal, It would float so would be hard to push down to where I want it, it is too big for convenience. Another option is to discard the sensor housing and use a small 2 inch diameter screw top acrylic box as the housing. For the moment that is my plan.

What am I overlooking? And, is there a better way?

*EDIT: * To help those who don't want to read through 10 pages of frustrating failures, the final successful project starts at post #158


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## mistergreen

Can you find the name of that photodiode? Maybe we can look it up to find the spectrals for it.
I notice they use a ultraviolet filter on top of the photodiode.


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## plantbrain

I think the lux is divided by 60.6 for umol.

This assumes the PAr is 400-700nm though, lux meters measure the bell shape at 555 nm.

If the source of light is standard, this should work okay.
Better than nothing.

The other thing is to tie the probe next to a PAr meter and go a measuring various light sources and distances and see how close things are to the 60.6 conversion factor(this is what I'd do).


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## Imi Statue

I'm very interested in this thread! I hate the idea of paying the "Brightest" price for PAR performance.

You rock for checking this out Hoppy!

Imi


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## Hoppy

mistergreen said:


> Can you find the name of that photodiode? Maybe we can look it up to find the spectrals for it.
> I notice they use a ultraviolet filter on top of the photodiode.


I can't see the actual diode for the filter and a case below it. Here is another photo:









It looks like it would be pretty easy to remove the whole sensor assembly to put it into a different, and waterproof case. I think the white plastic dome, the cosine filter? - has to be the only optical thing between the light and the sensor, so I couldn't have that part inside the watertight case. Is that correct? The only effect I can figure out from having a layer of acrylic covering the dome is reflection off the flat layer, but there is reflection off the dome too, so I can't understand why that does much harm.

The 60.6 conversion factor could change if there is acrylic over the dome, I think, but that's not a problem either. And, if the conversion factor is accurate to +/- 5% that would be outstanding.


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## Hoppy

I found an acrylic round, screw-top box, 2.75 inches in diameter, which will work great as a housing for the sensor. It is available online, http://www.consumercrafts.com/store...e-jewelry-organizers-and-bead-storage/1125-53 Unfortunately, only one lid comes for the 4 boxes, but it also only cost me $2.25 at Michaels. Here is what it looks like:









I will have to cut the sensor housing down in length but it is empty in that area anyway. Now, how to make the sensor watertight? I'm thinking I could essentially pot it with silicone, so the hole for the electric cable will also be sealed. The lid will have to be cemented on, either with silicone or with acrylic cement.

The acrylic lid attenuates the meter reading from 99 to 89 lux with it sitting on my desk and the overhead CFL light lighting the room. That would be a 10% loss due to the lid, which would just change the conversion factor from 60.6 to 67. Not a problem, that I can see.

There is a tiny "instruction manual" with the luxmeter, which has this chart on it:









Clearly this is not consistent with a true PAR meter, but calibration against a PAR meter for specific light sources should take care of this easily enough.


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## wastedtime

Subscribed. I am interested in seeing how this works out for you. This is some great information. Thanks Hoppy!!


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## Hoppy

I always believe that when you decide how to do something, it is a good idea to go ahead and do it, so:









I sawed off each half of the housing separately, after first cutting the electric cable to it. Then, I found that it didn't quite fit in the little box, so I had to carefully file off the corners. Now it fits tightly.









I think I will do some more filing to make it fit less tightly, then take it back apart - one screw now - clean it with my handy air blower (my mouth), and go back to thinking about the sealing problem.

Does anyone have an idea about how critical it is to keep the sensor dark when not in use? These always come with a cap for the sensor, which I won't be able to use, but this one also came with a nice black cloth case which should do the same thing.


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## plantbrain

Hoppy said:


> The acrylic lid attenuates the meter reading from 99 to 89 lux with it sitting on my desk and the overhead CFL light lighting the room. That would be a 10% loss due to the lid, which would just change the conversion factor from 60.6 to 67. Not a problem, that I can see.
> 
> There is a tiny "instruction manual" with the luxmeter, which has this chart on it:
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Clearly this is not consistent with a true PAR meter, but calibration against a PAR meter for specific light sources should take care of this easily enough.



The scale is no good.

What you need to do is compare the PAr meter to this meter once it's set up and done.

Then see how the readings compare and then use that difference for the factor.

I suspect most of the typical white FL bulbs will be somewhat close as far as a PAR to Lux comparison.

Put another way: some standard measure of light is better than NONE.


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## Hoppy

I bought some 22-2 flexible shielded cable, 4 feet for $1.50, to reconnect the sensor to the meter. This, because the coiled cable that it comes with is too short for dunking the sensor 2 feet down in the tank. The cable didn't need to be shielded, but that was the closest I could come to the 24-2 cable on the unit. I soldered and taped the connections, with the new cable going through a tight fitting hole in the acrylic housing. To act as a strain relief I just knotted the cable right inside the housing. Without that the cable could be jerked loose from the diode. And, good news, it still works!









Now to decide on how to waterproof this. Suggestions???


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## mistergreen

I use a heyco grip cord to waterproof where the cable meets the housing but I'm going from beneath because it needs a flat surface, not a curved one from the side.

I use silicon to seal the cap.


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## Hoppy

mistergreen said:


> I use a heyco grip cord to waterproof where the cable meets the housing but I'm going from beneath because it needs a flat surface, not a curved one from the side.
> 
> I use silicon to seal the cap.


So, the curing silicone fumes don't do any harm? And, I know it doesn't adhere strongly to acrylic, but there shouldn't be any stress on it to keep it from sealing. I can't see any way to ever get back into the sensor housing once I seal it, whether with silicone or anything else, so is there a good reason not to use high viscosity acrylic cement to seal the lid on permanently? The solvent fumes? Remember it is threaded on too.


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## mistergreen

Fumes from super glue will corrode the photodiode. Silicone seems to be fine. I'm assuming the acrylic cement has the same stuff as super glue.


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## Hoppy

From the MSDS for "acrylic cement":









Do those look like what would corrode the diode?

It may not matter if I screw the lid on, then apply the high viscosity cement in a line where the lid meets the main body. Little, if any, fumes would then get into the box. I think???


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## d3snoopy

mistergreen said:


> Fumes from super glue will corrode the photodiode. Silicone seems to be fine. I'm assuming the acrylic cement has the same stuff as super glue.


I don't think so. Super glue is cyano acryliate. It like something like weld-on is dicholromethane. They work on two different principles - super glue adds a plastic-ish compound between the surfaces that grips both surfaces. Acrylic cement melts the acrylic, causing the two surfaces to re-crystallize as a single structure.


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## DarkCobra

Curing cyanoacrylate emits fumes which can deposit a white substance on nearby objects, _especially_ if they have any oil on them. It can and has been used by law enforcement to reveal fingerprints. But it's not corrosive.

Most hardware-store silicone produces acetic acid fumes as it cures. Which _can_ be corrosive, like any acid; especially if trapped in an electronic enclosure. I know there's specialized silicones that don't release acetic acid, which can even be used in direct contact with electronics to totally encapsulate them; but I haven't used them and can't make a recommendation.

However, since you have an outer housing which screws on before any of these substances are applied, I'd think you can use anything that provides an adequate seal.

And as for the sensor cap, I think it's just a dust cap. I've never heard mention of any need to keep a photodiode in the dark most of the time.

Oh, and Mistergreen came up with an idea in the DIY PAR meter thread that adding a purple filter gel to reject some of the green, while passing a higher proportion of photosynthetic red/blue, might give results closer to that of a real PAR meter. The filter I see on your LUX meter appears green, doing the exact opposite of what you want. I doubt it's a UV filter, as photodiodes have poor response to that. They are however very sensitive to IR, and if it does remove any of the non-visible spectrum; that would be it.


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## Hoppy

I have two sealing needs. One is around the joint between the cap and the box, or jar, both of which are acrylic plastic. The other is around the guts of the lux meter to hold it in place, and also to seal the pass thru hole for the electric cable. I was considering silicone for the second , and acrylic cement for the first (not methacrylic super glue).

My present plan is to leave the lid off the box until the silicone cures around the luxmeter guts, which would also reduce the volume of air in the box, but not in contact with any of the electric elements. Also a fillet of silicone outside around the electric cable. After the silicone cures, probably 2 days later, screw on the lid and apply high viscosity acrylic cement around the joint to seal it permanently. Would that adequately protect the diode, etc. from any fumes? And, do you think the limited adhesion of silicone to acrylic would be enough to seal up the electric cable hole - the cable fits pretty tight in the hole, so it is just the tiny surrounding gap the would be sealed.

Before I consider playing around with optical filters or other methods for changing the response of the diode, I want to try this as is, relying on calibrations using a PAR meter, for each type of lighting I will use it for. If that doesn't work well enough, then I can buy another lux meter and play with it further.


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## DarkCobra

Sounds like a good plan to me. roud:

Out of curiousity, I fully disassembled the sensor on my Extech LUX meter. There are no identifying marks whatsoever on the actual photodiode. But here's the filter:










Close to what I'd expect from a LUX meter that's intended to mimic the human sensitivity to green. A bit more blue than expected, but that may be to compensate for photodiode response; which is probably biased towards red.

I also tested it for UV/IR pass. Only slight UV reduction. But it's a very good IR filter, I can't see the otherwise bright flashes from a remote control with my camera at all.

May come in handy later should you ever decide to play with alternate filters.


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## mistergreen

DarkCobra said:


> Curing cyanoacrylate emits fumes which can deposit a white substance on nearby objects, _especially_ if they have any oil on them. It can and has been used by law enforcement to reveal fingerprints. But it's not corrosive.


Yeah, it does leave a residue but I also saw bits of the photo layer degrading like a burnt out LCD pixel. Maybe it reacts but not corrode.



> Oh, and Mistergreen came up with an idea in the DIY PAR meter thread that adding a purple filter gel to reject some of the green, while passing a higher proportion of photosynthetic red/blue, might give results closer to that of a real PAR meter.


Shouldn't it be a green gel to deflect green out? Like chlorophyll although magenta makes sense.


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## Hoppy

mistergreen said:


> Yeah, it does leave a residue but I also saw bits of the photo layer degrading like a burnt out LCD pixel. Maybe it reacts but not corrode.
> 
> 
> 
> Shouldn't it be a green gel to deflect green out? Like chlorophyll.


The color of a filter is deceptive. The color can be the transmitted color reflected back from outside the filter, or it can be light reflected off the face of the filter. But, it is the color coming through the filter that is the important one. This filter definitely looks green, and I think it is light that passed through the filter twice that gives it that color. Another guess: it is to adjust the peak transmission into the green band, the peak at 550 nm shown on that little chart. It might even come closer to a PAR characteristic with no filter at all.


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## mistergreen

Yeah, my photodiode peakes at 550 too but with some software & hardware fine tuning, it's pretty darn close to the real thing.


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## DarkCobra

mistergreen said:


> Yeah, it does leave a residue but I also saw bits of the photo layer degrading like a burnt out LCD pixel. Maybe it reacts but not corrode.


Ooo, that's weird. The active elements of the photodiode are usually well encapsulated. Are you sure it was that? Is it possible it had a thin-film external IR filter layer which was reacting with the glue?



mistergreen said:


> Shouldn't it be a green gel to deflect green out? Like chlorophyll.


You specified a "green light gel". Filter gels work by absorbing certain colors rather than by deflection, so a gel that's green in appearance is absorbing red and blue; which would be counterproductive. So I assumed you really meant "a gel that filters out green"; which by absorbing green and transmitting red/blue would appear purple. I may have assumed too much.

Now if you'd specified a dichroic filter, which does work by deflecting certain colors instead of absorption, then the correct filter would appear green.


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## Hoppy

I "potted" the sensor into the case this morning. It was difficult to get it to flow into all of the voids, so I gave up on trying to do that. There is enough there to hold the sensor in place, to seal the hole for the electric cable, and to greatly reduce the air volume, reducing "oil canning" due to temperature and barometric pressure changes. Now, a couple of days drying and I can cement the lid on.

The silicone added $4 to the total cost, now at about $28.


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## Hoppy

I studied the "manual" that came with the lux meter. It says that these need periodic recalibrating, and that is because the photo diode sensitivity constantly drops, proportional to the product of the time the diode is illuminated and the intensity of illumination. So, that answers my question about why they always have caps over the sensor - to keep the sensor dark to slow down the deterioration.

After 11 hours I can still smell the ammonia given off by the curing silicone. This is the first time I ever cared about how long it takes, so it is probably typical. I decided to do the final cementing of the cap when the smell is gone.


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## Hoppy

This morning the odor from the silicone cure is gone, so I screwed on the lid, after cleaning it, and put a heavy bead of Weldon #16 acrylic cement along the joint to seal it permanently. Of course, first I verified that it still works ok.

Tomorrow I hope to borrow our club PAR meter and calibrate it.


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## alexei

Hoppy I am way excited to see what you come up with on this. Fingers crossed


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## audioaficionado

Subscribed!

You can never have enough PAR meters


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## Hoppy

I think the only real unknown at this point is how well it is sealed from the water. That will be the last test, because if it fails the whole sensor is junk. Right now I have to wait to get the PAR meter so I can calibrate it.

If I ever do this again I think I will completely disassemble the sensor and fit the cosine correction dome and the sensor alone in a much smaller acrylic container. Sealing it remains a question.


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## Hoppy

While I am stalled, waiting for access to a PAR meter, I've been thinking about the sensor part of the meter. Isn't that sensor just the photodiode mounted in something to hold it with the cosine diffuser over it? Can I buy another photodiode, the one mistergreen uses for his PAR meter, and package it, and use it in place of the sensor that came with my luxmeter? Obviously I don't know much about how these are hooked up.

If I did this, would it work?


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## mistergreen

It won't necessarily be a match. The voltage output may vary from diode to diode so the readings will be off even though they read the same spectrum.


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## Hoppy

mistergreen said:


> It won't necessarily be a match. The voltage output may vary from diode to diode so the readings will be off even though they read the same spectrum.


Since I have to calibrate it anyway, that doesn't bother me. I was concerned that it simply wouldn't work at all. I notice that the "cathode" is identified on the diode case. Is that the one that the negative polarity wire is soldered to? And, by convention, is the white or red wire the negative one? One last question, if it is connected backwards does anything happen other than it just not working?

End of today's photo electronics lesson for me:icon_redf


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## DarkCobra

Power flows from anode to cathode, just like a LED. So to get a positive voltage out, you ground the anode, then measure the voltage on the cathode.

General convention for low-voltage wiring is that when a colored wire is paired with a colorless (white or black) wire, the colored one is positive.

I just checked my LUX meter with a voltmeter to be sure, and it does follow the convention.

Hook one up backwards, chances are nothing bad will happen; apart from it just not working.


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## Hoppy

Well, it is good to know that I have a 50% chance of hooking up the photo diode correctly the first time! And, if I can see the tiny identifier dot well enough I can improve the odds. Thank you DarkCobra. When I assemble an electronic device now days it feels like walking a tight rope with no net.


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## mistergreen

On the photodiodes, there's a dot on the plastic that indicates the positive side.


Nothing bad will happen unless you run current through it.


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## Hoppy

This shows the dot marked "cathode". Is that the positive side?


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## mistergreen

yup


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## DarkCobra

Even if you can't find the dot, you can still identify the cathode by putting a voltmeter on it. The bare photodiode (with no filters) from mine produced 0.235V in room lighting during last night's test.


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## Hoppy

I just visited Tap Plastics and bought a 6 foot long 3/4" OD by 1/16" wall acrylic tube and some 3/4" diameter acrylic half spheres. (less than $6) I plan to use fine sandpaper to frost a surface of the half sphere, probably the flat surface. Then glue that to the end of the tube, and cut off an inch long piece, to be the housing/cosine "filter" for a new PAR sensor. My photodiode will be here late tomorrow.


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## Hoppy

Gluing the tube to the half sphere has been difficult. My first effort leaked badly at the joint. This is the second effort, using a new tube of Weld On #16.


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## Hoppy

The photo diode, beside a penny. Just barely big enough for me to solder the lead wires to it. This is the same one Mistergreen used in his pioneering PAR meter build, http://www.plantedtank.net/forums/showthread.php?t=120109


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## audioaficionado

How do you prevent the solvent glue from melting and clearing up the frosted surface of the half sphere?


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## Hoppy

audioaficionado said:


> How do you prevent the solvent glue from melting and clearing up the frosted surface of the half sphere?


That was a problem. Weld On #16 is pretty viscous, so it doesn't flow like the almost pure solven glues do. I put a bead of the glue over the end of the tube, working as fast as I could to beat the time it takes for it to skin over. Then I grabbed the half sphere and quickly put it in place (about 1/64" off, but that isn't a problem). The glue didn't run, and grabbed the pieces quickly. After about 5 minutes I ran a bead of glue around the joint, and let that cure for 30 minutes, then ran another bead. After an hour I tested it by weighting it with a fingernail clipper and stuck it into a glass of water. I checked for water inside every half hour or so. No leaks!

I knew this part was going to be difficult, so I bought 3 of the half spheres (45 cents apiece). I was lucky to make it work in two tries. 

It is interesting to see what the optical effect of a half sphere is. It seems to act as a lens, as it has to, but it collects light over a large area and concentrates it into a circle about half the diameter of the sphere. With the flat side frosted it diffuses the light so there is just barely an image visible through it.


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## plantbrain

Hoppy said:


> This shows the dot marked "cathode". Is that the positive side?


Cations are + eg K+, Ca++ etc........metals.........good way to think about it for some.


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## mistergreen

You might need a semi-opaque acrylic material to make it a real cosine diffuser.
The parabolic curve (output) may be too steep with your frosted semi-sphere. Calibrating that won't be fun. I found out that the light box acrylic I used works pretty darn well. It doesn't have to be a curve/lense.


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## Hoppy

This is how the calibration of the DIY meter looks, for the first version, with the sensor head inside an acrylic jar. This should be repeatable if anyone else does one this way.

The next test is the one it will probably fail - submerging it in water to see if it leaks.

EDIT: First a 5 minute dunk, then a 15 minute dunk. No leakage that I can see! Next will be to compare the readings in water to the PAR meter, to see if the calibration changes.

EDIT: Tested in aquarium, 2 feet high (2 feet head of water pressure on sensor). The data point falls on the same line as when calibrated in the air. This definitely works. However, the sensor is still too buoyant, floating up if your fingers slip at all. And, it needs a "wand" to be able to use it effectively without your hand getting in the way. 

I think the sensor-from-scratch that I'm making will be a big improvement.


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## Hoppy

Calibration chart above updated with under water reading.


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## Hoppy

Back to work on the sensor-from-scratch: I drilled the hole for the electric cable, a 6 foot long piece of 22 guage, shielded, round, flexible cable. Then cut it to length. The hole is 1/4 inch from the back of the half sphere end.









With the cable threaded into the hole, carefully, so as not to crack the acrylic.









Soldering the cable to the photo diode was easier than I expected, although it is at the limit of my vision capabilities. Once soldered on I carefully pulled and poked the cable back and pushed the diode up to near contact with the flat side of the half sphere. Looking through the other side I could near center the diode.









The next problem was holding the diode in place before potting the whole thing with Weld On #16. I ended up ripping a sandwitch bag in pieces, wadding the pieces into little balls and poking them in to hold the diode in place. This is primarily to keep the acrylic cement from flowing down over the sensing side of the diode. To be sure the connections were still good, I used a millivolt meter to check for an output at the other end of the cable. I had a couple hundred mV or so.

With the cavity half filled with Weld On #16: (the masking tape is to help hold the cable steady.)


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## plantbrain

Hoppy said:


> This is how the calibration of the DIY meter looks, for the first version, with the sensor head inside an acrylic jar. This should be repeatable if anyone else does one this way.
> 
> The next test is the one it will probably fail - submerging it in water to see if it leaks.
> 
> EDIT: First a 5 minute dunk, then a 15 minute dunk. No leakage that I can see! Next will be to compare the readings in water to the PAR meter, to see if the calibration changes.
> 
> EDIT: Tested in aquarium, 2 feet high (2 feet head of water pressure on sensor). The data point falls on the same line as when calibrated in the air. This definitely works. However, the sensor is still too buoyant, floating up if your fingers slip at all. And, it needs a "wand" to be able to use it effectively without your hand getting in the way.
> 
> I think the sensor-from-scratch that I'm making will be a big improvement.


Looks good over a wide range of lighting types.

I think the lux conversion Ole and some others came up with was less, around 60 vs this one which is 76.


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## Hoppy

plantbrain said:


> Looks good over a wide range of lighting types.
> 
> I think the lux conversion Ole and some others came up with was less, around 60 vs this one which is 76.


I knew this would be higher because of the loss through the acrylic jar lid. I had checked that before I started.

I just checked the new sensor with a mV meter and got about 380 mV in room light. The acrylic cement potting is still curing, so I will have to wait until tomorrow to calibrate the lux/PAR meter with it.












mistergreen said:


> You might need a semi-opaque acrylic material to make it a real cosine diffuser.
> The parabolic curve (output) may be too steep with your frosted semi-sphere. Calibrating that won't be fun. I found out that the light box acrylic I used works pretty darn well. It doesn't have to be a curve/lense.


I think you may be right now that I have it together. I can see the filter on the diode too clearly now. I may coat the spherical surface with white acrylic fingernail polish to fix that. How does that sound?


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## mistergreen

Hoppy said:


> I may coat the spherical surface with white acrylic fingernail polish to fix that. How does that sound?


Give it a try but I don't think it works by blocking out photons but rather bounce photons around in a medium until it hits the photodiode.


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## plantbrain

mistergreen said:


> Give it a try but I don't think it works by blocking out photons but rather bounce photons around in a medium until it hits the photodiode.


Basically spreads the light out nice and evenly over the area of the sensor. 

Hoppy, you could wire several of these sensors together and make a rail or a large area meter to measure the light also. Making a grid/linear slice if you will.....


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## Hoppy

As I understand it the "cosine" correction is to get the light from a large solid angle, as much as a half sphere angle, and the "frosting" would be to diffuse that light so it would be uniform over the whole sensor surface. Fingernail polish doesn't go on very uniformly without a lot of care in applying it, but a single coat does diffuse the light very well. I think I will calibrate it as is first, then coat it with the white polish and calibrate again. If I can find that photo diode locally these sensors are extremely cheap, and not too hard to make.

Tom, I'm not sure how I would use a grid of sensors. If they were wired in parallel it should give the average PAR over that grid (I think). Or, reading the PAR with a computer that plots the data points would give an interesting plot, too. But, it is hard enough to find spots in a planted tank where the light isn't partly blocked by foliage as it is, a grid would only work in a bare tank.


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## Hoppy

Calibrating the sensor as it was didn't work very well. It was too sensitive to the angle and not as linear as I wanted, plus it was too sensitive - too high readings. So, I coated it with two coats of white fingernail polish. On recalibrating it, it worked fine, but with a different calibration for LED light and 6500K CFL light. That is ok with me, since I will only use it with one of those. I now dub this the "PAR Volksmeter".









You can see that this has sensor a little smaller than the Quantum PAR meter. It is also easier to use than the Quantum meter for low PAR readings because of the 3 position switch on the readout.

The calibration is:










I will let the nail polish dry a few hours, then test it underwater. I see no reason for it to either leak or change calibration underwater.

Success, an easy DIY of a cheap lux meter to get a very cheap "PAR" meter, usable for most of our uses. I trust the calibration with LEDs more than the one with the 6500K CFL, because the latter was much harder to do, due to the small area of highest light under the CFL bulb. The cost: about $15 plus shipping for the lux meter, about $5 plus shipping for the photo diode, about $2 for the acrylic parts, $2 for the electric cable, plus odds and ends.

EDIT:
I recalibrated the PAR Volksmeter today. Yesterday I modified it by attaching a strip of acrylic to the bottom of the sensor, with a post at one end that will fit snugly into a 3/4" diameter acrylic tube, as a handle, so it is much easier to position and hold accurately.








Then, after a lot of thought I realized how much accumulated error is in each calibration point. For example, the PAR meter is only accurate to +/- .5 micromols, which is a large error at low values of PAR. Then just tiny mistakes in distance from the light source makes a big error in PAR readings. Plus minor mistakes in location of the two sensors adds another error. So, I tightened my positioning control, and weighted the data points by how much possible error each data point had. That gave me a big change in calibration constant, so I corrected the chart above. It also made the calibration constant be the same for the LED and 6500K CFL lights. That constant is 80 +/- 5 Lux per PAR unit. The chart above doesn't include a couple of data points that were way outside the area covered by the chart, but those points were included in calculating the constant. (Very high PAR means the sensor is close to the light source, where errors in distance are very significant and very hard to eliminate.)


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## mistergreen

Interesting. I wonder what the divergence in the CFL and LED. light color?

When I tested the DIY PAR meter, I used T5HO, LED, and spiral CFL. They came out pretty much close to PAR.


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## Hoppy

I don't know why the CFL data was so different from the LED. I'm still reluctant to accept that the data is right for CFL. For that one I taped the two sensors together, and moved them around under the light to get the maximum reading on both. For the others the area lit up about equally was much larger, and I just had the two sensors side by side, for one LED light, and placed at the same spot for the other light. I took several readings at each distance and/or LED current, to be sure I didn't have a sensor in a "dead spot". 

I expect to play with calibration some more, just to build up my confidence. Right now I am just tickled to see that the idea works well enough to make a usable PAR meter. We aren't scientists, so "hobby grade" testing is all we really need.


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## Hoppy

Now that I no longer need the sensor head from the lux meter I completely disassembled it. This is the active parts, which are held in by one tiny screw, and a snap-in holder for the photo diode, and 4 snap-in tiny knobs for the cosine diffuser. From this it would be easy to make a little larger sensor like the one above, using an acrylic tube about 1.25 inch inside diameter. Tap Plastics has 1 1/2 inch O.D. clear tube with 1/8" wall, which would work well. It costs about $17 for a six foot piece. Or, there are lots of small acrylic boxes and other small containers on Ebay which would work. I probably won't spend any more on this, unless I get a great idea I can't resist.


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## mistergreen

One big difference I noticed looking at the color filter, the LUX sensor is green while the VTB88... is blue-ish. So I guess it's somewhat more suitable for PAR readings


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## Hoppy

Calibration was redone, and comment #53 revised, above.

I checked my light level in my 65 gallon tank. With the Quantum meter I get 25 +/-5, depending on where I put it, and how much shading I get from the leaves, etc. With the Volksmeter, I get 20, and I couldn't get it in the exact same place I was able to get the Quantum meter, because the handles are too different. I'm pleased with this, and will probably use it for my routine measurements now.


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## Hoppy

If anyone is interested in making one of these, but doesn't have easy access to the plastic parts and electric cable, I can put together a kit of those parts:









With that kit plus the lux meter and photodiode, which anyone can buy on the internet:








There are other vendors for those parts, with different and possibly lower shipping costs.

You can make a usable PAR meter for about $31 plus three shipping costs. PM me if you are interested. You would have to make your own handle to hold it at the bottom of the tank so your hand and arm don't block part of the light. And, of course, for best accuracy you need to have access to a good PAR meter to calibrate yours.


----------



## Chlorophile

I would have taken the acrylic round box you had and sprayed it black. 

Then I would have just taken the cosine filter and cut a hole in the top of this.








Cut the hole equal to roughly the diameter of the cosine filter and siliconed it in place so that the dome of the filter protrudes through the hole. 
Once its dry just seal it up with the diode fixed beneath the cosine filter.


----------



## Hoppy

Chlorophile said:


> I would have taken the acrylic round box you had and sprayed it black.
> 
> Then I would have just taken the cosine filter and cut a hole in the top of this.
> Cut the hole equal to roughly the diameter of the cosine filter and siliconed it in place so that the dome of the filter protrudes through the hole.
> Once its dry just seal it up with the diode fixed beneath the cosine filter.


That would work, but gain very little. The big problem with that one was the size and buoyancy of the sensor assembly. It was very difficult to use it because of that. If you want to do that, just remember that the layer of acrylic over the dome reduced the lux reading by 10%.

Using just those parts, I would use a smaller acrylic container, and just the sensor and cosine filter in it. That helps a lot with both the buoyancy, if most of the remaining volume is filled with silicone, and the size of the sensor assembly. Incidentally, when I tore that original assembly down, I noted that there was very poor adhesion between the acrylic and the silicone. It came apart extremely easy.


----------



## Hoppy

I stopped by Tap Plastics today and found a tiny acrylic box, about 2" x 1" x .75", which should be perfect for mounting the guts of the original Lux Meter in a waterproof box.









I will have to do some minor trimming of the photo diode package and the cosine diffuser before they will fit. But, at 90 cents, the little box is a perfect size and price.









The only trimming I had to do, to this point, is file a small, shallow notch on the top edge of the box so the round diode filter holder will fit flush with the top. It is glued in place with Weld On #16 now.

It needs some "ballast" to help it lose its buoyancy. So, I added some pool filter sand and some more WeldOn #16.


----------



## plantbrain

Hoppy, you might also look into R/B ratios for lighting types, sometimes the makers give this data, this will skew the PAR/Lux data some but is interesting none the less. 

Ivo Busko used to discuss this, he's a pretty smart guy and did a lot with lighting on the APD a decade or and 1/2 ago.


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## plantbrain

Hoppy, just make a rod like we have for the PAR meter, no need for weighted ballast.


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## Hoppy

plantbrain said:


> Hoppy, just make a rod like we have for the PAR meter, no need for weighted ballast.


I have to eliminate most of the air trapped in the little box, or I found that it gets difficult to hold the sensor steady due to its constant attempts to float up. Rather than just pour more acrylic cement in, which eventually softens and distorts the whole box, I decided to use mostly sand as the filler. So it helps in two ways.


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## Hoppy

All of the ballast sand in place. After it sets (dries) some more the top gets glued on. The sand grain on top of the filter, I blew off.










This is before I glued on the cosine diffuser, which is acrylic too.










Complete, waiting for the cement to set/dry. I like the size of this, and the lack of buoyancy, but I suggest using something more coarse than filter sand as the ballast/filler. This is certainly better than my initial try.










The two sensors I now have. I still like the last one, but this one is a nice size too.


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## audioaficionado

I got one of the first six Good Enough PAR meters. The only issue is that the sensor wants to float like a cork. I don't want to open it and break the water tight integrity of it. I really like how you got the one on a flat base. You can get it right down at the substrate level while mine will be about an inch above. I'm happy I got that PAR meter even if the sensor isn't as compact as a commercial unit. $60 vs $400 makes it worth dealing with the float issues.


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## Hoppy

audioaficionado said:


> I got one of the first six Good Enough PAR meters. The only issue is that the sensor wants to float like a cork. I don't want to open it and break the water tight integrity of it. I really like how you got the one on a flat base. You can get it right down at the substrate level while mine will be about an inch above. I'm happy I got that PAR meter even if the sensor isn't as compact as a commercial unit. $60 vs $400 makes it worth dealing with the float issues.


I agree, and we now have the third option of just buying a cheap lux meter and modifying it. The Good Enough PAR meter has the advantage of reading out PAR directly, instead of having to use a conversion factor to convert a reading to a PAR number. I suspect it may also measure a spectral range a little closer to the PAR spectral range. But, anyone who doesn't want to pay $300 for a real PAR meter can pay $60 or so for the Good Enough PAR meter, unless even that is too much for their budget. Then they can make the PAR Volksmeter for around $30, using that last sensor design. I think anyone who wants to measure PAR should now be able to do so.


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## Hoppy

The calibration constant for this one is 67. The lux reading divided by 67 equals the PAR. I took great care keeping the two sensors the same distance from the light. Did two readins at 26 and 45 micromols of PAR, and got 1750 and 3030 lux. I haven't decided which sensor to use for my PAR meter - most likely this one. When I measured the PAR with this at the substrate level in my tank, I got 22 micromols of PAR, in between the 20-25 readings I got with the PAR meter and my other sensor.

Now if anyone wants to make their own PAR meter, this version is as easy as possible to make, and costs:
$15 for the lux meter
$8.50 for the Weld On #16 cement
$2 for the electric cable
$1 for the little acrylic box
$26.50 plus tax and any shipping costs

And, it took me about 2 hours to make the sensor, at most.


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## Hoppy

Is anyone interested in buying one of these?








It would be complete - the lux meter, the sensor, with a 4 or 5 piece holder and rod attached, to make it easy to hold it in position in the tank - for $35 shipped. Until Amazon runs out of the lux meters I can make these in batches of 5. And, they only have 19 left as of this morning.

PM me right away if you are interested. I will only do this if I can do 5 at a time. And, this will not be profitable for me - I just want to see more PAR meters in use.


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## Bad-Daddio

PM'd.

Hoppy - Just one question. How stable can I expect readings to be over time? Would any re-calibration be necessary?


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## Hoppy

Bad-Daddio said:


> PM'd.
> 
> Hoppy - Just one question. How stable can I expect readings to be over time? Would any re-calibration be necessary?


Of course I don't know from experience, but the lux meter maker says the photo diode slowly loses sensitivity, with the loss being proportional to the intensity of the light and the time it is exposed to that light. I doubt that using as we do, with a couple of readings maybe every 6 months, will cause any degradation at all. Plus, the lux meter is made to read up to 100,000 lux, which, for this modification, would be 1500 micromols of PAR, almost 100 times the intensity we will use it with.


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## mistergreen

you should cover the sensor when not in use.


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## Bad-Daddio

Got my Volksmeter in the mail today.

Wanna know how I know that it's really cool? As I explained what it is, my wife's eyes slowly rolled back in her skull and my daughter called me a nerd and walked away. My step son got all excited - He gets it!

I wasn't planning on setting my tank back up until after summer - Probably not going to be able to wait that long now....

Thanks again Hoppy!


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## Hoppy

I made 5 of these and sold them at cost, primarily to share this, but also in hopes of getting some more PAR data on various lights. One glitch has shown up - the tiny amount of solvent left inside from the acrylic cement causes some frosting of the acrylic box. This slightly reduces the sensitivity of the meter. I corrected for that when I calibrated them, so I don't think it will ever be a problem. After another week or so I will again recalibrate mine to see if any more frosting occurs.


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## sowNreap

Hoppy said:


> Is anyone interested in buying one of these?
> 
> 
> 
> 
> 
> 
> 
> 
> It would be complete - the lux meter, the sensor, with a 4 or 5 piece holder and rod attached, to make it easy to hold it in position in the tank - for $35 shipped. Until Amazon runs out of the lux meters I can make these in batches of 5. And, they only have 19 left as of this morning.
> 
> PM me right away if you are interested. I will only do this if I can do 5 at a time. And, this will not be profitable for me - I just want to see more PAR meters in use.


Wish I'd seen this thread before I bought the Lux meter from Amazon. I'd have bought one of these from you. 

Hopefully I can make use of the lux meter as is since I don't currently have the time to try to mod it and maybe not the skill either. If it took you 2 hrs probably would take me 8 or more. LOL

EDIT: I noticed the Lux meter in this thread is 100,000 lux, but the link you posted for me in another thread was to the 50,000 lux meter. Did I mess up getting that one???


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## Hoppy

sowNreap said:


> Wish I'd seen this thread before I bought the Lux meter from Amazon. I'd have bought one of these from you.
> 
> Hopefully I can make use of the lux meter as is since I don't currently have the time to try to mod it and maybe not the skill either. If it took you 2 hrs probably would take me 8 or more. LOL
> 
> EDIT: I noticed the Lux meter in this thread is 100,000 lux, but the link you posted for me in another thread was to the 50,000 lux meter. Did I mess up getting that one???


No mistake. I didn't think through what I needed well enough before I bought mine. 50,000 lux is almost 900 PAR, way beyond what we need, so I never use the high range setting on my meter - wasted money.

I did the mods on mine mostly for fun, and to prove it could be done. If I were doing this again I would just buy the 50,000 lux meter and limit my use of it to dry tanks, or out of tank measurements. There are some reasons to measure PAR in the tank, but if all you want to do is pick a light, verify that it will give you the PAR you want, and what distance from the substrate it should be, a simple lux meter works just fine. Measure the lux, divide by 61, and you have the PAR with the accuracy we need.


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## thelsegse

They have a lux meter app for the iPhone. Don't know how good it is though.


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## sowNreap

Hoppy said:


> No mistake. I didn't think through what I needed well enough before I bought mine. 50,000 lux is almost 900 PAR, way beyond what we need, so I never use the high range setting on my meter - wasted money.
> 
> I did the mods on mine mostly for fun, and to prove it could be done. If I were doing this again I would just buy the 50,000 lux meter and limit my use of it to dry tanks, or out of tank measurements. There are some reasons to measure PAR in the tank, but if all you want to do is pick a light, verify that it will give you the PAR you want, and what distance from the substrate it should be, a simple lux meter works just fine. Measure the lux, divide by 61, and you have the PAR with the accuracy we need.


Good deal. After I get a light fixture picked out, set up & post the readings here then I may try to mod it to be able to use in under water. Just for fun and self edification. 

Thanks for your help Hoppy.


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## HD Blazingwolf

So have u calibrated the meter to read in par. Or lux and divide by a set number to get par?


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## Hoppy

I just calibrated it to get a conversion factor. When you encase it in acrylic like that it no longer reads true lux. The acrylic reduces the amount of light reaching the photodiode, so it is reading less than true lux. And, some etching of the acrylic happens until all of the solvent fumes get chemically incorporated into the acrylic. That changes the calibration a little bit, but i allowed for that when I calibrated it.

Unless you have a need to check the PAR at various locations in the tank, under plants, near plants, etc. it works best to just use the lux meter as is, out in the air, and divide the reading by 61 to get PAR. Then you can, if you wish, add 10% to account for the focusing effect of the water. The PAR you get at the substrate depends on the distance of the light from the substrate and little else.


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## HD Blazingwolf

so would it be feasable then to have a lux meter and seal the photo diode with it and use that?
i can't imagine that the casing could not just be sealed with some silicone.

So i am buying the mastech meter. with the cosine filter it comes with. what is the conversion factor you came up with? i saw 80 +/- but i think that was with ur self made acrylic lid. and i've see 67 thrown around. i believe u came up with 67 on the second diy diode u made, and that used the factory cosine filter


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## Hoppy

If you use the lux meter as is, I believe 61 is a good conversion factor. Once you put anything between the light and the photodiode, other than what is in the lux meter, it no longer reads lux accurately, so you have to calibrate it. It might be possible to just seal the existing sensor housing with silicone, but if you don't succeed you might have destroyed the photodiode. That housing has a lot of places to seal if you try it. I would definitely remove the two screws holding the housing together and study it before starting to seal it.

Then you have the buoyancy problem. With air inside the housing it will float up and be a pain to hold down without also blocking some of the light. You could probably silicone some pennies inside the housing to add more weight and reduce the buoyancy. I think it is a great DIY starting point however you do it.


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## HD Blazingwolf

I plan on getting some small .15 mm or .30 mm needles and edging the cosine filter with silicone. trying to leave as small of footprint of silicone as possible to reduce light loss

it will be fun that's for sure.
i do have access here to lead weights that i can easily silicone in place


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## Hoppy

This hasn't worked out as well as I hoped. Two problems: One is that I would like to measure lux occasionally, but after modifying the sensor I can't. The second is that I began to doubt that the calibration of the PAR modified sensor was staying the same. It appeared to have lost some sensitivity over the past 2 weeks or so. Then, I noted that Amazon had a 50,000 lux meter for sale at only $10, so I decided to start over. I bought one of the meters - about $18 with tax and shipping:









My scheme this time is to put a bayonet connector in the cable from the sensor to the meter. So, I found these at my local hobby electronic supply place:








They were about $1 each, for one male and 2 female connectors.

Then, the hard part. I decided to use the sensor I made before, for measuring PAR inside the tank:









So I soldered one of the male connectors to the cable on that sensor, which I found was just beyond my visual capability, at first. To be able to do this I had to first solder two short lengths of single conductor wire to the connector, just barely in my visual capability. Then solder the sensor cable to those short lengths of wire, tape it up with electricians tape, and as luck had it, it worked.

Next was cutting the cable connecting the lux meter sensor to the meter and soldering the other two connectors on. The problem with this is that the coiled cable is very springy, making it hard to slide the connector plastic cover back over the cable end before doing the soldering. After a few appropriate cuss words that finally worked out, leaving me with:



























Now, I have a working lux meter and a PAR meter that is self calibrating. I can read the lux from a light at a distance, divide by 61 to get PAR. Replace the sensor with the waterproof "PAR sensor", get a reading and divide that by the PAR I just calculated and I get the lux per micromol of PAR calibration constant for the PAR sensor. If I next use the PAR sensor a month later I can just repeat the calibration and have an accurate measure of PAR even if the PAR sensor has degraded.

I like this version of the Mastech lux meters better than the more expensive one, even though it only measures to 50,000 lux. As far as I can tell it is made exactly the same as the other one, just a slightly different cosine diffuser, and lower maximum range.


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## sowNreap

Glad you got it all fixed up. I'm anxious to see how your spiral bulb dome fixture readings compare to mine. To see if I need to change my setup.


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## Hoppy

sowNreap said:


> Glad you got it all fixed up. I'm anxious to see how your spiral bulb dome fixture readings compare to mine. To see if I need to change my setup.


I"m starting that in a few minutes. Stand bye


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## Hoppy

One minor problem keeps a luxmeter from being as good as a PAR meter for our use. That is the fact that luxmeters have the sensor filtered to approximately match human eye sensitivity to colors - allowing much more yellow-green light to be measured than red or blue light. Since we almost always use lights with similar spectral outputs, this isn't a serious concern. But, we could add a tiny filter to a DIY sensor, as discussed in http://www.plantedtank.net/forums/showpost.php?p=1915574&postcount=428 For example a Roscolux 4930 filter would change the response of a typical luxmeter sensor from one with a peak at 550 nm to:









This is similar to the response of the Quantum meter, at least much more similar than the bare luxmeter sensor is. I'm thinking about making a sensor with a disc of this filter gel in it. Then, it would make a converted lux meter into a nearly genuine PAR meter. The filter material is extremely cheap per sensor, at about $8 per 20 x 24 inch sheet. With this addition a sensor could be made for about $12 in batches of 10. Then modifying a cheap lux meter as I just did would get you a self calibrating PAR meter for less than $40.


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## toffee

Hoppy said:


> One minor problem keeps a luxmeter from being as good as a PAR meter for our use. That is the fact that luxmeters have the sensor filtered to approximately match human eye sensitivity to colors - allowing much more yellow-green light to be measured than red or blue light. Since we almost always use lights with similar spectral outputs, this isn't a serious concern. But, we could add a tiny filter to a DIY sensor, as discussed in http://www.plantedtank.net/forums/showpost.php?p=1915574&postcount=428 For example a Roscolux 4930 filter would change the response of a typical luxmeter sensor from one with a peak at 550 nm to:
> 
> 
> 
> 
> 
> 
> 
> 
> 
> This is similar to the response of the Quantum meter, at least much more similar than the bare luxmeter sensor is. I'm thinking about making a sensor with a disc of this filter gel in it. Then, it would make a converted lux meter into a nearly genuine PAR meter. The filter material is extremely cheap per sensor, at about $8 per 20 x 24 inch sheet. With this addition a sensor could be made for about $12 in batches of 10. Then modifying a cheap lux meter as I just did would get you a self calibrating PAR meter for less than $40.


Want to make a few? I will buy one from you, just let me know.


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## Hoppy

No, I found that making more than one of these sensors is a very unpleasant task for me with my less than great vision and not too steady hands. But, it is basically an easy project for someone younger. I was hoping someone would jump on this as a chance to make some money selling these on the for sale forum.

If I were doing the sensor over I wouldn't use acrylic cement as a casting material inside the housing with the photodiode. It just releases too much plastic altering gas for that application. Instead I might use "plumbers" epoxy - an easy epoxy to work with. It is fun to work with this, so someone should step up now.....anyone?


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## toffee

Hoppy said:


> No, I found that making more than one of these sensors is a very unpleasant task for me with my less than great vision and not too steady hands. But, it is basically an easy project for someone younger. I was hoping someone would jump on this as a chance to make some money selling these on the for sale forum.
> 
> If I were doing the sensor over I wouldn't use acrylic cement as a casting material inside the housing with the photodiode. It just releases too much plastic altering gas for that application. Instead I might use "plumbers" epoxy - an easy epoxy to work with. It is fun to work with this, so someone should step up now.....anyone?


I am in the poor vision and shaky hand category. If anyone wants to step in and follow Hoppy's 'manufacturing' design, I could be the first customer :wink:.


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## Ductapemaster

I wouldn't have a problem making a few of these for interested parties. I just read this all the way through and it's a simple device.

Would someone mind putting together a single post with all of the necessary materials and maybe a few notes on construction? I'm pretty certain I understand how to put one of these things together, but I don't want to be making them incorrectly or be missing some feature...


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## Hoppy

If I did this again I would use this basic design:









But I would use http://www.homedepot.com/h_d1/N-5yc...d=-1&keyword=epoxy&storeId=10051#.UD2HGWjOzfI or a similar epoxy putty, to fill the cavity of the acrylic housing. I would still use the acrylic cement to seal the hole where the electric cable comes into the acrylic housing. I would try to minimize the use of acrylic cement inside that housing. See http://www.plantedtank.net/forums/showpost.php?p=1906452&postcount=29 and following posts for more details.

Anyone who makes these has to work out most of the details to match their abilities and materials available. Make a prototype or two to be sure you have it worked out.


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## Ductapemaster

I'm going to order some parts today so I can play around with them. Apparently there's a plastics supplier on the next street over from work, so I'm going to check that out at lunch. Hopefully they have what I need.

I have a bunch of epoxy so I was planning on using that anyways as well. I'm assuming you glued the half sphere to the pipe using cement as well?

Also, I'd like to get an idea of how many people would want one of these. Would it be kosher to post in the for sale forum to judge interest? I want to make sure its worth my time.


----------



## Hoppy

Ductapemaster said:


> I'm going to order some parts today so I can play around with them. Apparently there's a plastics supplier on the next street over from work, so I'm going to check that out at lunch. Hopefully they have what I need.
> 
> I have a bunch of epoxy so I was planning on using that anyways as well. I'm assuming you glued the half sphere to the pipe using cement as well?
> 
> Also, I'd like to get an idea of how many people would want one of these. Would it be kosher to post in the for sale forum to judge interest? I want to make sure its worth my time.


Yes, I glued the half sphere to the tube with acrylic cement. That part was pretty easy.

I think it is against the rules of the for sale forum to take orders for something, but I could be wrong. You could post something in the lighting forum to see how many would be interested. I have had about half a dozen people PM me asking for me to sell them one, so there should be a market for 10 or so, at least.

I sold 5 at $35 each, and barely broke even. I briefly considered making more to sell at $45-$50, so there would be some reward for the labor. I think that would still attract the same number of buyers.


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## Ductapemaster

I'll throw a post up in there and see what happens!

$45-50 is about what I estimated for materials + my time. Considering what something like this costs to make new, I'd say its worth it.


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## toffee

Ductapemaster said:


> I'll throw a post up in there and see what happens!
> 
> $45-50 is about what I estimated for materials + my time. Considering what something like this costs to make new, I'd say its worth it.


Count me in, I was going to buy a lux or rent a PAR meter. 

Make sure you do caliber the new unit against a PAR though. I am going to use it to decide how many solartubes etc., getting the data wrong could be really really bad as Solartube is almost $800/unit installed and HOA may not allow me to keep making changes to their roof.

Thanks


----------



## Hoppy

toffee said:


> Count me in, I was going to buy a lux or rent a PAR meter.
> 
> Make sure you do caliber the new unit against a PAR though. I am going to use it to decide how many solartubes etc., getting the data wrong could be really really bad as Solartube is almost $800/unit installed and HOA may not allow me to keep making changes to their roof.
> 
> Thanks


Why do you want to use a PAR meter for that, instead of a standard lux meter? Is that for lighting a big aquarium?


----------



## toffee

Hoppy said:


> Why do you want to use a PAR meter for that, instead of a standard lux meter? Is that for lighting a big aquarium?


8'x30"x20" room divider. Planning to use solartube as main source of light if possible, then LED as night time lighting source. Just want to measure PAR under a solartube.


----------



## Hoppy

toffee said:


> 8'x30"x20" room divider. Planning to use solartube as mine source of light if possible, then LED as night time lighting source. Just want to measure PAR under a solartube.


You can use a lux meter, divide the reading in lux by 61 and be very close to the accurate PAR reading. It is only when you want to measure the light in the water filled aquarium that a PAR meter, with waterproof sensor, is needed. I'm curious about how much light you get from one of those solar tubes too.


----------



## toffee

My idea probably won't work as the end of solartube will be 5ft from the top of the tank. Tank will be 20" deep, so 6.5ft2 to the bottom. The manufacturer claim that each tube is good for 6ft diameter of area, so using 2 tubes with 2.5ft apart means a lot more light in towards the center of tank. A rough drawing of the space where the tank will go.








Hobbyists, mostly reef guys extend the tubes to about 1ft on top of the tank, that has proven to work, but mine will be 5 ft away... or 4ft too far.

Like to measure to make sure though.


----------



## Hoppy

Toffee, be sure to start a thread on this when you start building it. It will be very interesting, and I predict that you will have very high light from just one solartube.


----------



## toffee

Hoppy said:


> Toffee, be sure to start a thread on this when you start building it. It will be very interesting, and I predict that you will have very high light from just one solartube.


Hoppy, I have multiple of solartubes in my houses so I am familiar with them, I have been using them since the early 2000s. I must be one of their pioneer customers. I have no doubt that it will work if the tubes can be extended closer to the tank, like 2ft or so. At 5-7ft, I am simply not sure. 

But I really don't want the tubes to be extended as it really won't look right to have something blocking the view. The end of the house, extreme right on my drawing, faces south and has very nice view. Besides not sure how extra weight will affect the roof structure considering it's almost 30 ft un-supported.

If I would to start a thread about this on going project, where should I put it? I really would benefit from inputs during the planning stage.


----------



## toffee

Hoppy said:


> Toffee, be sure to start a thread on this when you start building it. It will be very interesting, and I predict that you will have very high light from just one solartube.


Gotten about 6000 lumens 1ft from a north facing solartube. Future tubes on top of aquarium will be south facing, so may be a bit more lumens. As it is, I may get 3 - 4 hours of appr 30 micromols at the substrate level from solartube. That plus the evening LED for another 3 hours or so. So together 6 hours of 30PAR and another 3-5 hours of morning and evening light from solartube.


----------



## Hoppy

Start a thread in this forum - DIY forum. We have discussed solartubes here before, but not in any detail. As I recall at least one member in Mexico uses them to light his tank.

If you can get 30-40 micromols of PAR you will have great light!


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## Hoppy

I have been trying find a simple way to make the lux meter readout be the actual PAR number. It seems like it should just take a voltage divider to do that, but after asking mistergreen some questions I started having doubts. I tried to use a 10K trimmer pot as a voltage divider, but that didn't work at all. So, I bought a 1 megohm potentiometer to play with. I first found that the readout portion of the lux meter does not supply any voltage to the sensor, so it has to be reading just millivolts from the photodiode. Then I shorted the connected sensor with the potentiometer to see how much resistance I can use in a voltage divider without skewing the reading. One megohm did nothing to the reading, but at around 200-300Kohms it did drastically change the reading. So, a one megohm pot would work as a voltage divider, as far as the sensor part is concerned. In order to get enough sensitivity to adjustments I decided that a 1 megohm resistor in series with a 10Kohm resistor and 10Kohm pot would work best. But, having the 10K ohm resistance across the readout (milivoltmeter) might be a problem. I'm still thinking about that part. (The photodiode resistance is around 10 Kohms, if that matters, and I think it does, since it seems to show that 10 Kohms in the circuit with the milivolt generating photodiode doesn't affect the readout - maybe. And, the milivolts from the sensor, divided by 61, across a 10 Kohm resistor is essentially the same as having the photodiode connected to the readout) More to follow!


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## Hoppy

Into each life some rain must fall! I find that you can't change the readout of these cheap lux meters so they read about 1/60th of the values they now read. At least you can do it with a voltage divider. After spending about $30-$40 on resistors, pots, etc. I find that the readout is the same whatever I put in the circuit between the sensor and readout. As I was trying to sleep last night I think I figured out why that is: They advertise these meters as having automatic zero compensation, which has to mean that it readjusts its readout as needed to maintain the original accuracy. So, when I add a voltage divider, it just adjusts the readout to ignore the voltage divider. Just how that is done, I don't know, but I do know I am not going to succeed with this approach.

Fortunately, as I was still trying to sleep I figured out an easy way to accomplish the same thing with Rosco gel filters - it's obvious, isn't it? If I use multiple neutral density filters, plus a spectrum adjusting filter, with combined transmissivity of .0166, I am dropping the readout by a factor of 1 to 60, which is what it takes to make the reading a good approximation of PAR instead of lux.

After a few hours of work I find that I can do this with 4 layers of filters:










It won't be perfect, but it will greatly reduce the emphasis the lux meter puts on 550 nm, yellow green, light, and drop the sensitivity down to .0166 of what it is now. The sensor is so simply made it will be a cinch to put the stack of filters in place. So, now I just need to find those specific filters at a reasonable price.


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## Hoppy

Roscolux filters are sold at a theatrical lighting store in Sacramento. They have the free sample books, with what appears to be every color filter that Rosco sells in it. The samples are about 1.5" x 3", enough to make filters for at least 8 "PAR meters". When I get time to concentrate on this I will modify mine with a stack of filters. Even if a full sheet of these filters costs $10, it would add a trivial amount to the cost of modifying lux meters in quantity.


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## toffee

Keep up the good work hoppy!


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## Hoppy

The first chart is the frequency response of the basic Mastech luxmeter sensor, from the tiny instruction sheet that comes with it. The second chart is what it would be by placing a stack of Roscolux filters - 3 layers#3308, #398, #397, 3 layers #4315 - over the photodiode assembly in the sensor. This theoretically will reduce the readings by 1/57.4, to convert the reading to an approximation of PAR, plus change the frequency response to better match the needed response for a true PAR meter.

I calculated this using the frequency response numbers from http://www.rosco.com/filters/roscolux.cfm and a spreadsheet calculator. This was about the 6th attempt, and I think it is the best I can do, other than adjusting the attenuation, if desirable after calibrating this against a PAR meter. I would like to get less attenuation of the green part of the spectrum, but so far I can't find a combination of filters that will do that. It will probably be next week before I get around to actually trying this.

EDIT: I can't stop doing this!










With a couple of small changes in the filter stack, changing to 2 layers#3308, #97, #398, #397, 3 layers #4315 gives less attenuation of the green area and possibly a better response overall.


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## sowNreap

Oh no .. Hoppy's addicted to Lux2par ... no it's not a new designer drug but something much better! :icon_lol: 

I have faith that you'll get it almost exactly the way it needs to be and then a lot of people will benefit from all your hard work.roud:


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## Hoppy

Last night I disassembled the sensor to see how I could stack filters in it:









The electrical tape is due to my experiments with a voltage divider.









You can see how simple the assembly is, and how small the filters can be.










This thing is designed with someone like me in mind! I can cut squares of filters, about 3/8 inch square, and just drop them in the square recess. Maybe a tiny drop of silicone sealant to hold the stack in place. 

I think the next step will be to measure the actual attenuation of each filter, by before and after lux readings for each filter type. Possibly today.


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## Hoppy

Now I'm making progress! I tried measuring the % transmittance for each filter I want to use, by laying them on top of the little round case for the photodiode, then carefully putting the cosine diffuser half of the housing back on top, without installing the screws. I measured reductions in readings that were similar to the specified transmittance for each filter, but higher, in each case. I decided that was due to light leakage around the filters, so I stuffed all but one of the filters in the square box that now holds the factory installed filter. This time when I measured the % transmittance for the stack, I got 2.1% instead of the spec 2.0% - very close! So, now I have a PAR meter that probably reads a little high, but I can add one more filter on top of the little round lid for the diode, to finish adjusting this to read correctly, when I get our club's PAR meter to use. I expect to be off about 15%, which I can correct with one filter, if I select it right. And the spectral response for the meter will be: (If the final filter is the neutral density filter I think I can use.)










This will have a nice broad spectral response, not terribly different from the Quantum meter response:


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## drhemlock2

*thanks for the hard work*

i understand a little OCD is good for the soul i do this all the time else nothing would get done...i will follow with great interest your findings.
doc


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## Hoppy

I tested my PAR Volksmeter against a Quantum PAR meter and found I was reading about 40% high, so I added the last filter that I had calculated would be needed. Then rechecked it against the Quantum meter:









Now, I would call that a success! Since I have the Quantum meter for awhile I think I may set up a LED light (the above check is with a 36 watt 6700K /AH Supply PC light) and check at various PAR levels, to be sure.

The final filter stack is: (All Roscolux filter numbers, all of which are in the free sample booklet)
2 - Roscolux #4915: CalColor 15 Lavender 
3 - Roscolux #398: Neutral Grey 
1 - Roscolux #397: Pale Grey

I measured the PAR of full sunlight at noon PST with both meters:









That is incredibly close!


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## drhemlock2

*cool so cool*

so how much do you think it would take to put one of these together say for a project all together?. i would like and i know others would a list of all the hardware you used to get her this far i am satisfied it is close enough for horse shoes or my needs. i have read the whole thread but i know you made some changes up to this point.
doc
:fish:


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## Hoppy

The cost would be about $15 plus shipping for the lux meter. The filters are from the free sample book you can pick up at all dealers in Roscolux filters. All of the other modifications I made didn't work out at all well, other than the pair of connectors that allow me to make another sensor and use it in place of the original sensor. That was about $5 as I recall, if you want that feature. Adding the filters is very easy to do, so no real skill is needed, just reasonably steady hands. All of the filters will fit in the square "box" over the photodiode. And, there are just 4 screws to remove to get to that "box". At this point the PAR meter isn't water proof, but I'm planning another effort at making a sensor that will be.


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## sowNreap

Way to go Hoppy! I knew you'd get it. roud: I think even I do the filter mod. :smile:


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## Hoppy

I finally made a waterproof sensor that reads in PAR units, but reads about 10% low. I ended up using this design: 









It is made from a piece of 3/4 inch diameter acrylic tube, an acrylic half dome, a strip of 1/8 inch thick acrylic, for a handle. First I cut a piece of tube about .02-.03 longer than the photodiode with its leads - Excelitas VTB8441BH - from Newark Electronics, about 3/8 inches long. Then I drilled a hole in one side at about the height that I expected the electric cable to come in, just big enough to get the cable through it. I had a 1" by 5" piece of acrylic !/8 inch thick sheet, so I glued that tube to that with Weldon #16. When that dried I used black nail polish to paint the inside of the tube a solid black. 

To solder the cable to the photodiode I drilled a shallow 5/16 inch diameter hole in a scrap of plywood. Then I jammed the photodiode in that holes with the leads up. This acted as a third hand to hold it when I wrapped the two cable wires around the leads and soldered them on. 

To get the diode inside the tube I poked the other end from inside the tube, through the hole in the side, and pulled it until the diode could fit into the tube. I centered the diode, with the ends of the leads against the bottom of the tube enclosure, and used masking tape to hold the cable steady against the bottom piece. 

I used Loctite instant mix, 5 minute setting epoxy to form a pool of epoxy about 1/8" deep, deep enough to cover the cable hole in the tube, to anchor the photodiode in place.

Then the hard part began: selecting Roscolux filters to convert the reading to PAR. First I wanted to change the wave length sensitivity from a peak at about 600 nm to something more nearly flat over the 400-700 nm range. At the same time I needed to greatly reduce the sensitivity of the photo diode. This required a spreadsheet calculator, with all of the transmissivity data for lots of filters, by wave length. First I found a filter to knock off the 600 nm peak, without too much damage to the rest of the sensitivity. Then a filter to correct, as much as I could, the damage that was done to the rest of the sensitivity. (This means first reading the graph of sensitivity for the diode at every 20 nm and putting that in the spreadsheet, then multiplying all of the filter sensitiviies by the diode sensitivity, and dividing by the resulting maximum peak sensitivity to get a graph with the peak at 1.0.) To act as a diffuser to emulate the cosine diffuser, I used a Roscolux diffusing filter, and multiplied those sensitivities in with the others. When I had the best looking spectral distribution I could get, which took several tries, ending up with 5 filters, I cut half inch diameter discs of each and stacked them on top of the photodiode, and sat the hemishpere on top, and measured the output with my "standard" light. I got 1112 vs the 25 I wanted. So, I had to find a stack of neutral filters that would drop that 1112 down to 25. That took another 6 filters. 

After a final trim of the stack of 11 filter discs I restacked them on top of the photodiode and laid the hemisphere on top and repeated the reading - I got 22 this time, which was close enough, since no other combination was nearly that close.

The filters I used are: #163, #4915, #4230, 2 #R09, #97, 2#397, #98,#398, and #3318. Unfortunately there is no Rosco filter in the sample book that cuts out the infrared and near infrared.

By now the acrylic cement and the epoxy had set up very well, so I glued the hemisphere on top with Weldon #16, and put a filet of that cement around the electric cable on the outside to better waterproof it.

After it set up good, I put a bead of Weldon #16 around the joint at the hemisphere to tube interface to better seal that.

One more calibration, and it still read 22, where 25 would be the desired reading. A final calculation of the sensitivity vs wavelength gave me:










It looks pretty ratty, but it should still work ok. Eventually I will try it at noon in full sun, to see if the 10% error is the same at high light.

Much of the cost of this was in Epoxy and nail polish! About $9 for them, plus $4.26 for the photodiode and about $7 for shipping.


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## sowNreap

Your way over my capabilities now! LOL 

But I do wish I had a waterproof one. I moved and altered my lighting setup a bit and now need to know what it's reading in the tank itself. 

If you decide you want to make a couple let us know. I'd like to buy one. I think I could do the adapter part to be able to switch between this sensor and lux sensor. But like I said this mod is beyond my ability.


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## drhemlock2

seems like this is not going to leave you alone until you get it the way it should be this is good news. i am at the stage now where i am designing my lighting using your info as well as others. so many choices . i have found the lux meter on the ebay and hope to get after it later on once i figure out the type of photo diode you end up using as well as the filters along with it.
thanks for your work.


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## Hoppy

This morning I rechecked the reading vs a Quantum PAR meter. It read 21 with the PAR meter reading 24-25. Close enough for my purpose. I will recheck it again in a few days to verify that the reading is stable.

I also made another try at selecting filters the give a more even spectral response. It came out worse than what I used. The difficulties are finding a filter to knock off the near infrared and near UV, without distorting the rest of the spectrum too much. Then finding another combination to even out the response, without getting too many peaks, and still another combination to drop the sensitivity sufficiently without really exaggerating the peaks. I keep thinking it can be done better, but I'm out of ideas now. My "obsession" has about run its course now.


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## Hoppy

sowNreap said:


> Your way over my capabilities now! LOL
> 
> But I do wish I had a waterproof one. I moved and altered my lighting setup a bit and now need to know what it's reading in the tank itself.
> 
> If you decide you want to make a couple let us know. I'd like to buy one. I think I could do the adapter part to be able to switch between this sensor and lux sensor. But like I said this mod is beyond my ability.


It is really an easy mod, especially if you have good vision and steady hands, both of which I am a bit short of. The only tools I used were a hacksaw to cut the acrylic tubing, a utility knife to score the flat acrylic for breaking off a strip, a soldering iron to connect the wires to the diode, an electric drill (cordless) to drill for the electric cable in the piece of tubing, scissors to cut circles of filters, a pair of wire strippers for the electric cable wires a sheet of fine sand paper, to flatten and square the ends of the tubing, toothpicks to manipulate the filters and diode, and some masking tape to act as a third hand at times.

You can PM Ductapemaster to urge him to make some, too.


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## sowNreap

Appreciate all your hard work on this, Hoppy. 

I've got all the tools, just not the skill to use them very good. I had a hard enough time just trying to solder a headphone cable to a new jack today. But I might see what I can do when I get some spare time. 

Otherwise I'll check with Ductapemaster (I'm really good at using duct tape to mod things :icon_bigg ) to see what we can work out.


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## Hoppy

sowNreap said:


> Appreciate all your hard work on this, Hoppy.
> 
> I've got all the tools, just not the skill to use them very good. I had a hard enough time just trying to solder a headphone cable to a new jack today. But I might see what I can do when I get some spare time.
> 
> Otherwise I'll check with Ductapemaster (I'm really good at using duct tape to mod things :icon_bigg ) to see what we can work out.


Remember our difficulties measuring PAR for CFL bulbs? I think the problem may be related to the need for those lux meters to be turned on with the sensor covered. That lets the electronics set an accurate zero for the readout. I was shocked when I first turned on my meter with the new waterproof probe, and it read around 1300 instead of 25! I noticed that the reading kept rising and falling too. Then I remembered that these are supposed to be self adjusting on the zero point, so I turned it off, laid the original sensor black-out cap over the sensor. Then turned it on again. When I removed the cap, it read 21, and the reading was steady. I'm not sure this fouled up your lux meter readings before, but it might have.


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## sowNreap

It's possible that's what happened. I think I usually took the cover off first then turned it on. Later I uncovered and turned on and then made final height adjustments to give a bit of time for the sensor to stabilize. Finally on the last few tests using the aquarium hood, I just left in on and uncovered because it didn't take that long to get the height adjustment right. 

Good to know though. In the future I'll make sure to turn on, then uncover.

If I get time I may test those spiral CFL's again. Just to see if I can get more consistent readings.


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## Hoppy

OK, the way we learn is from our mistakes! Today I tried my new waterproof sensor in the noon sun. It doesn't work well. The acrylic half sphere on the end is much too sensitive to the angle to the light. I could rock the sensor back and forth towards the sun and a little off from towards the sun, and it made a big difference in the reading, ranging from about 450 to 1500 with only maybe 20-30 degrees of movement. That makes it unusable if any accuracy is needed.

If I had listened to mistergreen I wouldn't have used the half sphere, but used just a flat disc as the measuring end of the sensor. One other bad effect of the half sphere is the magnification it gives, which just adds to the difficulty in reducing the reading by a factor of 60 to get it to read PAR instead of lux. I now think that it would work best to get a white acrylic 1/8" thick end for the sensor. But, I don't really need a waterproof sensor, so I may just let this go. (But, I may also get the PAR fever again and try one more time later!)

EDIT: Well the fever came back in about 3 hours! I realized I might be able to hacksaw off the top of the sensor, and reuse the rest of it with a flat top. So, I successfully removed the top, found the guts all in good shape, and then searched again for a better arrangement of filters. I now have it back together with new filters, a flat acrylic top, painted white with nail polish on the inside and lightly sanded to eliminate most reflection from the top.

When I did the final assembly and glued the top on, I accidentally left one filter out, so it reads high now, 35 instead of 25. I think I can glue another flat top on it, with another filter sandwitched between them to get back to the right reading. That will be tomorrow's project.


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## Hoppy

I cut another disc of 1/8 acrylic, lightly sanded one face to stop reflections, sandwitched a couple of filters between that and the existing top of the sensor, glueing it in place around the outer edge with Weldon #16. Now it reads 21 instead of 25, and the non-waterproof sensor reads 27. Both are close enough. When the sun comes out I will try a full sun reading again.


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## drhemlock2

*way to go hoppy*

i knew it would not keep ya down keep plugging away and let us know what you find this seems like a weekend project to me....
doc
:icon_roll


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## Hoppy

I think it is well worth the effort to convert the original sensor to read in PAR, and it is about as easy as it could be to do it. Once we know what the optimum set of filter squares is, to use the minimum number and have the best response curve, it is about a one hour job, at most. (I may make a few more calculations to try to find the best set of filters.)

Making a waterproof sensor is probably not worth the effort, and, while the last design I used was fairly easy to assemble, any mistake is very hard to correct, since it is all glued together. But, it does work. If you decide to do a waterproof sensor you almost have to use a disconnect fitting so you can use the original sensor to calibrate the new sensor. So, I think that is worth the effort too.


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## Hoppy

This is my last try - I mean it this time!

Yesterday I tried a different technique for selecting filters to get the PAR correction on the Mastech sensor. I tried to visualize the appearance of the sensitivity plot needed to make the change to the sensor sensitivity I wanted. Then I searched until I found one that looked like it should, and calculated the sensitivity with that filter in place. Then I just repeated this process until I was near the amount of attenuation of the output that I needed, and assembled the sensor with those filters to see how close I was. That let me calculate the % transmission I needed in one last filter. After several tries I got the spectral sensitivity near what I wanted, and the readout very near what I wanted. I ended up using 6 filters - Roscolux #17, 36, 55, 363, 4430, and 3304.

Two more discoveries: Most important, the conversion from lux to PAR for this meter isn't dividing lux by 61, as I have been saying, but it is dividing lux by 76 - big mistake on my part. This is the reason the data we got for CFL bulbs was so far off from the older data. Then I find that the Mastech sensor electronics are easily confused. Every time I open the sensor case and replace it, the readout goes crazy for a few minutes, before settling down to read correctly. This is probably because of the cheap way it is made.

The sensitivity chart for this final sensor looks like:









When I check this against the Quantum PAR meter I get 32 PAR with the Mastech meter and 29 with the Quantum meter. In the sunlight, the Mastech meter reads appropriately. (I wasn't looking for exact readings for that much light)

I found the easiest way to add these filters to the Mastech sensor is to just lay the stack of .5 inch square filters on top of the circular housing for the diode, and use a couple of strips of Scotch tape across opposite corners to hold them in place. That part is extremely easy. If a correction is needed it is easy to remove the stack and substitute for one of the .5 inch squares.

When you stack filters it tends to result in exaggerated peaks and valleys in the response curve. First, the sensitivity is cut by a factor of 76, and the filters that are appropriate tend to have peaks and valleys in roughly the same places, which quickly amplifies the magnitude of those peaks and valleys. One would think that neutral gray filters wouldn't have those peaks and valleys, but they do, and the darker they are, the bigger the peaks and valleys. Filters that are the darkest also tend to totally block certain parts of the spectra, which isn't good at all, so multiple different filters are necessary.


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## Hoppy

I'm sure no one will be surprised that I am still working on this. I'm still not satisfied, and it is a very challenging problem.

This week I made a couple of "breakthroughs". First, the Mastech LX1010B luxmeter I have has a very easy to modify sensor, because the photodiode is enclosed in a cylinder of plastic almost exactly 1.25 inches in diameter, so it will fit well inside a 1.5 inch diameter acrylic tube. Also, the assembly is very easy to take apart and put back together. Plus, the cosine filter "dome" is just a bit bigger than 1.5 inches in diameter, and isn't permanently attached to the housing - it pops out. So, the photodiode and cosine filter are easy to fit into a short piece of 1.5 inch acrylic tube, which is easy to buy in one foot lengths on Ebay.

Next, after a week of trying to find a combination of Roscolux filters that will convert the lux readings to PAR readings it finally occurred to me that I don't need to do that. I can just convert it to read 10XPAR instead, which takes far fewer filters, making it much easier to find a set that do exactly what I want. And, when a sensor good for reading 20,000 in its indicator is going to be marginal for accurately reading 20 on that indicator. By shooting for 10XPAR, it will be reading 200, or 10% of the lowest scale, and by putting the scale selector on the 20,000 range, it reads PAR with no conversion needed.

Today I found a workable method for selecting the filters, and got it down to only 4 filters. Those make it read exactly the same as the Quantum PAR meter, and the spectral response is also good:









By requiring only 4 different filters, the Rosco sample book goes a lot farther for modifying these. I can get at least 4 sets of filters from one book, or buy just 4 sheets of filters, and have enough for 100+ PAR meters.

By the middle of next week I will have my 1.5 inch tubing, and be ready to work out the last details for converting it to a true, waterproof PAR meter. One absolute rule for this is to do nothing to interfere with the optical path in the sensor, besides adding the filters.

I have also finally figured out that my connectors that allow me to switch sensors aren't a workable idea. When working with such low voltages and currents, the contact resistance in the connectors is too big a problem and a variable one at that. Also, there is no reason to use more than one sensor, if I do this well.

My biggest problem will be finding another equally interesting project when I finish this.:redface:


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## drhemlock2

*hello my friend*

still at it i see let me know what i need to do with the sensor and what link did you use for the acrylic tube i need some also. are you going to use the cosine filter "dome" on the tube? let me know.
doc


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## Hoppy

This is a sketch of what I plan to do. Notice that the optical path will be the same as the original sensor.

For the acrylic tube, go to Ebay and search for acrylic tube. You will find numerous listings, all using the same illustration of a bunch of clear plastic tubes. Click that sellers "store" and you will find a listing for 1.5 inch tube. It would be cheaper at Tap Plastic, but they won't sell less than a 6 foot length, and that costs more than one foot plus shipping on Ebay. I get my sheet acrylic from Home Depot. It is cheaper at Tap Plastics, but I have to drive 20 miles, and that costs gas! HD is very close to me.


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## Hoppy

I have now checked, double checked, and triple checked my dimensions and I'm convinced this will work. I also visited Harbor Freight and bought a Dremel type rotary tool, not the toy one, but the $20 one. Ready to start!!


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## Hoppy

The rotary tool worked pretty well for roughly cutting away the unwanted parts. Then some filing and sandpaper finished it off. So far, so good!


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## drhemlock2

*new tool so cool*

i love it when a plan comes together and the ability to accquire a new tool i mean tools are cool! and if you can justify them that is even cooler. i like the idea of keeping most of the sensor body and just adding the tube w/acrylic bottom to make it water tight/ i have been thinking of a way to use it in a tank w/water by adding a aluminum arrow blank that you can get at the wally world and attaching it so you can hold it at the level you need for a reading. or even a telescoping antenna thing you can get at the shack. now i have to think of a way of a good attachment method. i have been experimenting like i told you with the hamburg filter thing and found that besides weldon gorilla CA glue will work also the other types would not. i have almost decided on the design as the one i used in he ten gallon QT tank will work but not very pretty. i have better plans for the 75 gallon one. "keep er up and get er done" hoppy!
doc


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## drhemlock2

*new tool so cool*

i love it when a plan comes together and the ability to accquire a new tool i mean tools are cool! and if you can justify them that is even cooler. i like the idea of keeping most of the sensor body and just adding the tube w/acrylic bottom to make it water tight/ i have been thinking of a way to use it in a tank w/water by adding a aluminum arrow blank that you can get at the wally world and attaching it so you can hold it at the level you need for a reading. or even a telescoping antenna thing you can get at the shack. now i have to think of a way of a good attachment method. i have been experimenting like i told you with the hamburg filter thing and found that besides weldon gorilla CA glue will work also the other types would not. i have almost decided on the design as the one i used in he ten gallon QT tank will work but not very pretty. i have better plans for the 75 gallon one. "keep er up and get er done" hoppy!
doc


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## Hoppy

Until I get the acrylic tube I have gone as far as I can. I just cut off the wires to the photodiode and soldered a new cable on. This was to avoid having to do any soldering near the plastic parts. A milivoltmeter indicates that the diode works fine now - about 250 mV in my room light. The tubing should arrive Tuesday or Wednesday.

The way I hold a PAR meter in the water is with an acrylic tube, which fits snugly into a next size larger piece of tube cemented to a strip of acrylic sheet which is the bottom of the sensor and arm to get the sensor away from the influence of the tube acting as the "probe". This is how I have our local aquarium club Quantum meter set up.


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## drhemlock2

*hello*

that answered the question very well thank you
doc


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## Hoppy

Today my one foot length of 1 1/2 inch dia acrylic tube arrived, and with some difficulty I cut off a piece for the housing. First I found that the vendor had not cut the tube square on both ends, so I marked off a 21/32" length and used a small plastic miter box from HD and a hack sawa to cut it off. My cut was 1/32 out of square. So, I cut about a sixteenth of an inch off taking care to keep it square, then hand sanded it so it was square. I then clamped a "stop" to the miter box 11/16 inch from the blade, and cut that while slowly rotating the tube with the end hard against the stop. That worked very well, so after sanding to clean it up, I have my housing.

Next problem: the tube is 1 1/4" ID and the cylinder from the lux meter is also 1 1/4 " OD, so it won't fit inside the tube. Tomorrow I will sand either the inside of the tube or the outside of the cylinder until they fit together. (Probably inside the tube so I don't have to handle that lux meter cylinder any more than necessary.) As with any DIY project, each step hands you another little problem.


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## Hoppy

After just a little sanding on both the OD of the lux meter cylinder and the ID of the acrylic cylinder:









Next, mark the acrylic cylinder for the wiring hole and drill it. Then put it together:









The hardest part was getting the tiny screw that holds the diode plate in place without hitting the electric cable behind it. I used needlenose pliers to put a kink in the cable to clear the mounting screw.

Finally, I got the lid back on top of the lux meter cylinder:









Somewhere along the way I put a hairline crack in the filter that fits in the lid, above the diode. I'm assuming that will have negligible effect on the operation.

To hold the lux meter cylinder in place, 4 small drops of Weldon #16.


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## Hoppy

Shown with the filter pack of 4 filters attached with small strips of Scotch tape. The tape is primarily to keep them in place while the cosine diffuser is being glued on.










With the cosine diffuser glued on and the glue drying. I used Weldon #16, of course. When it dries well, I will put a filet of glue around the diffuser to cylinder joint on the outside. Before gluing this I tested it - it reads the same as it did when the original sensor was together, with the same filters.


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## Hoppy

All that's left is to glue the sensor to the "handle", a 1 5/8" x 5" piece of acrylic, that seals the bottom of the sensor and lets me hold it in place under water without my arm or hand interfering with the measurement.

I think this is the ultimate PAR meter for the DIYer. Easy to make and cheap.


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## h4n

very nice Hoppy!
i'm going have to order some acrylic tube and give this a try haha.
I noticed you now use 4 filters. Using the 6 you sent me orginally does the same thing correct?


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## Hoppy

h4n said:


> very nice Hoppy!
> i'm going have to order some acrylic tube and give this a try haha.
> I noticed you now use 4 filters. Using the 6 you sent me orginally does the same thing correct?


No, with the 4 filters, the meter reads 10X PAR, so the 20,000 scale on the meter reads actual PAR. What I sent you made the meter read PAR, but not as accurately as with the 4 filters.

Look for a Rosco dealer near you and pick up a free sample stack from them, then you have enough of each of those 4 filters to make at least 4 meters. You could make a lot more, but then you have to worry about centering the filter stack over the window for the photodiode. With the 3/4 inch squares I used, the filters can't slide off far enough to be a problem.


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## HEINEKEN357

Great job on the new design, Have you tested in your tank how does it compare from the old test? Also what 4 filters are you using should i remove the other 2 filters you sent or just keep with the 6?


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## sowNreap

Looks really good Hoppy! Appreciate the detailed instructions and especially the photos.  When I get some time I'm going to try to make one. 

But which 4 Rosco filters are you using? I thought you mentioned it before but can't seem to find it now. 

I may ?? be going to Smoky Mountain Park next week so while down that way might try to find the Rosco dealer in Knoxville and pick up a sample stack.


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## h4n

I see! So should I remove two filters then? Or just leave the 6?


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## Hoppy

The four filters used are Rosco #398, #4830, #53, #302.









This is a different group from those I mailed out to a few people. Selecting the right filters is sheer drudgery, but drudgery like working a good jigsaw puzzle is. You have to get filters whose total transmissivity is 1/7.6 (.1316), and whose spectral transmissivities add up to give you a good final spectral sensitivity. This required a spreadsheet where I copied the spectral sensitivities of many of the Rosco filters and combined different groups until I found a good grouping. There is a lot of logic involved in doing that, but there are also several different ways to approach it, only one of which finally worked for me.

I think I have enough of the four I selected left for perhaps 3 more meters, if anyone wants them.

I know this has been a confusing thread, since this final effort is at least the 4th version, and all are different in significant ways. It was a learn as you go experience for me. If it helps in figuring this out, this final version starts at http://www.plantedtank.net/forums/showpost.php?p=2034019&postcount=132


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## sowNreap

Let me see if I have this straight. I think you sent me these 6 filters: Roscolux #17, 36, 55, 363, 4430, and 3304 none of which are the ones you used in this last mod. But if I was to use those filters it would still read in PAR just not as accurate as this new better way. ???

If so, and you have too many requests for the new filter stack, I can just use what you sent. But IF you have any extras after that, I'd appreciate if you could send me a new stack. Just in case I can't make it to the Rosco dealer in Knoxville .. which is about 70-100 miles away.


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## Hoppy

sowNreap said:


> Let me see if I have this straight. I think you sent me these 6 filters: Roscolux #17, 36, 55, 363, 4430, and 3304 none of which are the ones you used in this last mod. But if I was to use those filters it would still read in PAR just not as accurate as this new better way. ???
> 
> If so, and you have too many requests for the new filter stack, I can just use what you sent. But IF you have any extras after that, I'd appreciate if you could send me a new stack. Just in case I can't make it to the Rosco dealer in Knoxville .. which is about 70-100 miles away.


Yes, you have it right - but one big difference between the two filter stacks is that the stack of 6 you have makes the meter read PAR directly, while the new stack makes it read 10X PAR, so if you use the 20,000 lux scale, it reads PAR directly, and more accurately. I mailed you a set of the 4 new ones.


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## sowNreap

Thanks Hoppy for the clarification and for mailing the new filter stack. 

I just wanted to make sure everyone that might want to do a mod on their lux meter had at least 1 of the filter stacks before I asked you for the new one. The more people we have that are willing to take lux/par readings on their light fixtures the more everyone on TPT will benefit.


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## shd17

So if I use the 4 filters in your final versions, how would I tell that the lux meter is reading par correctly (or close to it)?


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## Hoppy

shd17 said:


> So if I use the 4 filters in your final versions, how would I tell that the lux meter is reading par correctly (or close to it)?


You would have to borrow a Quantum PAR meter and verify the accuracy. However, each of the Rosco filters has a percent transmission number, which is shown on the Rosco website. If you multiply those 4 numbers together, .88 x .64 x .40 x .61 = .1374 which is 1 divided by 7.28. If there were no other factors involved, it should be 1 divided by 7.6, or .1316. However, since the lux meter photodiode isn't equally sensitive to all wave lengths, it worked out that the .1374 was a near perfect total transmissivity.

The only things this modification does are:
Filter the light striking the photodiode to reduce it by a factor of .1374.
Adjust the spectral response curve to make is a bit more consistent with what a PAR meter response curve should look like.
Using the same optical path, make a waterproof sensor, so it can be used in the aquarium.

The resulting "PAR meter" will be as good as the Mastech lux meter is, but it will just read differently. And, as far as I can tell so far, the Mastech lux meter is all that it needs to be to be a good meter for our uses.


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## shd17

So i tried measuring a 14w cfl in 10in dome light and i got 77-80 par at 12", 40-42 par at 19". Does that sound reasonable? Or is it off?


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## Hoppy

Those PAR readings are very consistent with the reading in Diagram 7 of http://www.plantedtank.net/forums/showpost.php?p=837592&postcount=21


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## Hoppy

I have been working steadily, day after day, trying to make this DIY PAR meter work better, and be easy to duplicate. I think I now have it about as good as it is going to get. After a lot of wheel spinning, and wasting parts, I finally sat down and figured out what it should take to get where I want to be. First, the cosine diffuser, I figured out, is just a way to put a light source for the sensor's photodiode at a fixed distance from the diode, and with a brightness that is proportional to the brightness of the light being measured. The amount of light this provides to the diode for measuring depends on how far it is from the diode, with the brightness dropping with the square of that distance, making the distance a critical dimension.

Next, I figured out that filtering the light to reduce its intensity at the diode, and modify the spectral sensitivity of the diode, isn't a good idea. This is because the diode does have some small sensitivity to infrared and UV, even though there is a glass filter mounted on it to greatly reduce its sensitivity to the non-visible light. But, the Rosco gel filters are not able to block the infrared light at all, or at least they only block a small portion of the infrared. That means as I filter out visible light to flatten the spectral response, and reduce the intensity, I am also making the IR light sensitivity go up as a percentage of the total sensitivity - not good at all. So, I have to use a minimum of filtering, preferably only one relatively weak filter.

But, you can "filter" the light by reducing the aperture the light shines through. The amount of light going through is roughly proportional to the area of the aperture - the square of the aperture diameter. So, the aperture is another critical dimension.

The best way to be able to reproduce both the aperture and the distance between the photodiode and the cosine diffuser is to make each of those parameters be set by an individual part, with one easy to control dimension. Along with that, the design has to position the photodiode in a fixed location.

With all of that in mind I came up with this: (Using an Excelitas photodiode http://www.newark.com/jsp/displayPr...Vactec&mckv=siOiAZ7Nw|pcrid|15779268381|plid| instead of the lux meter diode, to get around the filter on the lux meter diode.)

















This is made by first making the 6 acrylic pieces. I used plastics from Tap Plastics, http://www.tapplastics.com/product/plastics/plastic_rods_tubes_shapes/cast_acrylic_discs/137 http://www.tapplastics.com/product/plastics/plastic_rods_tubes_shapes/clear_acrylic_tubes/141 http://www.tapplastics.com/product/plastics/plastic_rods_tubes_shapes/frosted_acrylic_rods/149 Also, I had a scrap piece of 1/16" thick acrylic I used for one piece.

Starting at the top: I drilled a centered .25" hole through one of the 3/4" diameter acrylic discs. Then I used black nail polish to paint the inside face of that disc black. I poked and tapped in a short piece of .25" dia. frosted acrylic rod in the hole with a bit of Weldon #16 cement on it. When it cured I hack sawed off the excess rod and used fine sandpaper to trim it flush on the top side of the disc, with the rod already flush with the bottom side.

For the aperture, I drilled a .125" diameter hole through a 3/8" square of 1/16" acrylic. and painted the inside face of it with black nail polish. When that cured I cemented it on the bottom of the disc with the frosted rod in it, so the hole was centered over the frosted rod end. This is the cosine diffuser plus aperture.

Next, I cut a piece of 3/4" OD, 1/16" wall acrylic tube about 3/8" long with square ends. (I used a small cheap miter box and hacksaw.) Before I cut it, I drilled a hole centered on one side, with the hole diameter the same as the electric cable diameter.

To mount the diode, I used another of the 3/4" diameter acrylic discs, with a carefully centered .312 dia. hole through it. (Drilling centered holes is easy if you first make a jig from a piece of plywood, drilling a shallow 3/4" dia recess to fit the disc, then using the center of that hole to locate a .25" or .312" hole the rest of the way through the plywood. Push the disc into the hole, clamp the plywood to a workbench with the disc captured between the plywood and the bench, then drill the hole through it, using the hole in the plywood as a centering guide.) The Excelitas diode is .312 in diameter, with two parallel cutoffs making it a rounded rectangle, so it fits snugly into the .312 hole in the disc. With the face of the diode at the top of the disc, I used Weldon #16 to glue the diode into the disc.

Poke the electric cable through the hole in the 3/4" tube piece far enough so you can strip the wire so you have two stripped wires about 1/2" long. Use the plywood drill guide to hold the disc that holds the diode, so you can locate the electric cable in between the two electric leads to the diode, and wrap one wire around one lead and the other around the other lead. The positive lead is identified with a "+" marked on top of the diode. The red wire goes around that lead. Solder the two connections and cut off the excess wire.

Carefully push and pull the wire back to get the diode inside the piece of 3/4 tube, with the disc sitting on top of the tube. Use Weldon #16 to cement it on, being careful not to get it on the diode sensing surface. Apply a filet of Weldon #16 around the cable inside the 3/4 tube, and outside as well. This seals the water out of the inside, and acts as a strain relief to avoid pulling the wire off the diode.

Cut a .25 inch long piece of the 3/4" acrylic tube to be the spacer that positions the diode .25" from the cosine diffuser. Keep the faces parallel and square, and use fine sandpaper to get the length accurate. Use Weldon #16 to glue that on top of the diode mounting disc.

Cut a rough circle of Lusco #3313 Tough Minus Green filter and put that on top of the diode - it can be loose since it can't go anywhere.

Use Weldon #16 to glue the cosine diffuser/aperture to the top of the assembly, with the aperture to the inside. After that cures, glue the assembly to a strip of acrylic about 1.25" x 5" to seal off the bottom and act as a handle for the sensor. Add another coat of Weldon #16 to the outside of all cemented joints to be sure they are waterproof.

Use black nail polish to paint the exterior surfaces, being very careful not to overlap the frosted rod diffuser disc.

Connect the end of the electric cable to the luxmeter readout. I cut the cable to the readout off as close to the luxmeter sensor as I can, to have a straight section of wire to make the soldered connections. Connect red to red. Use shrink tubing to insulate and reinforce the connection. Since the luxmeter cable wires are very thin, and weak, I use the ground wire on the electric cable, not otherwise used for anything, to overlap the insulation of the luxmeter cable by an inch or more, and use black electric tape to tightly hold it to the insulated cable to relieve the load on the soldered wires.

The performance of this PAR meter is:


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## mistergreen

Did you find a big different with and without the minus green filter?


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## Hoppy

mistergreen said:


> Did you find a big different with and without the minus green filter?


No, that filter is just there to theoretically broaden the spectral response. What I did find, was that using many filters stacked up to get the broad response I wanted, jacked up the IR respons excessively, so hot light sources gave 10X response by the meter compared to cool light sources. So, I went for the minimum of filtering. The 1/8" aperture alone didn't drop the meter reading enough, and 3/32" dropped it too much, so the aperture size to correct that would have required buying a set of drills in very small increments of size around 1/8th inch. So, I looked for a filter to fine tune it, one which would do a minimum of IR emphasis, and not drop the response in the blue area. This one fit that criteria.

Before I can do actual calibrating of the sensor I have to glue it all together and paint it black, making it a permanent assembly, so I was very reluctant to do that kind of testing. I just used black electrical tape to hold it together and block light leakage as I experimented, with just a 10000K PC bulb. Even with this I used up 3 of the photodiodes in unsuccessful assemblies. But, I plan to try to saw them apart to salvage the diodes. This has been a very interesting project, but also extremely frustrating.


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## Hoppy

I think I am finally satisfied with my design, after a lot more work. I finally thought to look up the spectral response for the Apogee Quantum PAR meter, to see what they use and how good it is. I was surprised to find that they also have a far from perfect match with the 400-700 nM PAR spectral range, and, as a result their meter is only good to about +/- 4% for typical aquarium lights, and less accurate for incandescent lights. So, I tried to match their spectral response as best I could with the photodiode and Roscolux filter gels that I have. That proved to be relatively easy, once I worked out the details of what the filter response should look like. I settled on using the Roscolux # 4815 filter gel, giving this spectral response vs the Qualtum meter spectral response:









That left the problem of how to adjust the calibration of the meter without changing the spectral response. So, I noticed that in the Roscolux sample book are 19 different diffuser filters, white diffusers which would not alter the color of the light passing through them. I set up a PAR meter and checked the reduction in the reading that each of those diffusers gave me when I put one on top of the PAR meter sensor. The transmissivities ranged from 45% to 90%, so by using different combinations of those diffuser filters I could adjust the sensitivity of the meter over a very wide range.

With that problem solved, I made some modifications to the last design of the sensor, all aimed at making it easier to make the parts and to assemble them with minimal variation from assembly to assembly. My final design, which I prototyped, was:









With this design I can make 5+ of each part in about 30 minutes, with the aid of a few very simple fixtures. Assembling the parts was very easy, even with the tiny parts.

Today I received a batch of 5 more lux meters, so I was able to assemble a complete PAR meter for testing and to determine what diffuser filters I needed. It took just 2 diffuser filters, Roscolux # 102 and #114, to get a near perfect match with a Quantum meter. This PAR meter is now well within my goal of +/- 10% accuracy. I have two more just about assembled now.

If you have reasonable mechanical skills you should be able to easily duplicate this and make them to sell, if you wish to do so. The Roscolux filter sample books are free at a dealer, or you can buy one at Amazon. The acrylic tubes are available on EBay. You should have little difficulty finding usable flexible 2 conductor cable at a local electronics supply store, although Radioshack doesn't seem to have any.

I have a list of 7 people who have asked that I make them one, so I will finish off that list, and maybe make a few more, then I hope someone else picks up this relatively easy, high demand "product" for others to buy.

Here is the assembly sequence I am following:
Assemble Photodiode to item 5.
Apply acrylic cement to holes in item 5 to attach diode.
Thread cable through hole in item 6, and solder cable wires to leads.
Assemble above assembly to item 6, with acrylic cement.
Paint outside cylindrical surface of above assembly with black nail polish.
Glue item 4 to item 1 with acrylic cement.
Paint outside cylindrical surface of that assembly with black nail polish.
Load filters into item 4-1 assembly and push item 3 in to retain.
Use masking tape to assemble above assemblies for testing.
Adjust diffuser filters to get correct PAR reading.
Apply 2 drops of acrylic cement to hold item 3 in place.
Glue the two sub-assemblies together with acrylic cement.
Glue above assembly to item 7, and paint bottom of item 7 under item 6 with black
nail polish. Glue on a “socket” for holding wand made of piece of acrylic tube to
above assembly.

This modified meter works on the maximum scale of the lux meter, the 50,000lux scale. I just use a couple of drops of acrylic cement to lock the range selector slide switch in the right position.


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## sowNreap

Great work Hoppy!! I figured that you wouldn't quit until you had it perfect in your eyes.


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## jerrytheplater

Is this the meter you are offering for sale here?: 
http://www.plantedtank.net/forums/showthread.php?t=308809

I just bought one from you and did not see this thread till now.


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## Hoppy

jerrytheplater said:


> Is this the meter you are offering for sale here?:
> http://www.plantedtank.net/forums/showthread.php?t=308809
> 
> I just bought one from you and did not see this thread till now.


Yes, this is the PAR meter you bought. As I make more of these the mechanics of doing it get a bit easier so they tend to be more nearly exact duplicates. It is still difficult to get the photodiode mounted at exactly the same distance from the diffuser, so I am finding that I have to tweak the diffuser filter selections a bit at times. When all goes well I use the same 2 diffuser filters in all of them. (The meter calibration is done by choosing diffuser filters which also change the total amount of light passing through them.)


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## Hoppy

I am still having problems with duplicating these exactly. Any minor error in the geometry make the sensitivity different, and I have to experiment to calibrate it against a Quantum meter, using different diffuser filters. So, I have spent some time trying to make it less variable. This is the design I will be using for any next batches:










Except for one part of the assembly, this is easier and faster to assemble, and the geometry is more consistent. It works exactly as well, once calibrated. The part that remains difficult to assemble is the photodiode to the tiny circuit board, cut from a much bigger one I got at Radio Shack, which will produce 28 tiny circuit boards. But, soldering the diode to the circuit board, the cable to the diode on the circuit board, and maneuvering this assembly into the 3/4" acrylic tube is still difficult. I think this will get easier as I make more of these.

I tried to photograph this as I assembled it, but my camera macro function isn't good enough to focus on the tiny parts.

I plan to make about 5-6 of these at a time, probably one batch every week or 10 days, and sell these on the For Sale forum. But, I was informed that there would also be a market for these with Reef tank keepers if they work equally well with typical Reef tank lighting. So, I ordered cheap 50-50 actinic/10000K PC bulb to verify that it is equally accurate with that mix. I haven't received the bulb yet, so I don't yet know how well it will do with that light.


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## Hoppy

I have been asked if these PAR meters will work ok for reef tanks. To find out, I bought a 50-50 36 watt PC bulb, and tested a couple of them with that. One read 32 vs the Quantum meter reading of 33, and the other was only 2 units off at about the same reading. So, now I know they do work with 50-50 white/actinic light.


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## Hoppy

While experimenting with putting 3 photodiodes in one sensor, in order to improve the spectral sensitivity of the PAR meter, I found that I could fit a photodiode into a 1/2" diameter acrylic tube. This led me to redesign the one diode PAR meter to be 2/3 the size of what I had been making - a 1/2" diameter vs. 3/4" diameter sensor. 









Assembling this is much easier if you make a simple fixture to hold it while you work on it, while also keeping the photodiode located where it should be. This is just a piece of 3/8" tube, with a longitudinal saw kerf in one side, to make it fit inside a 1/2" acrylic tube easier, with a length sticking above the base equal to the desired distance from the top of the 1/2" tube housing to the top of the photodiode (9/32 inch), cemented into a sheet of acrylic.








Fit the 1/2" tube housing on the assembly fixture, and poke the electric cable through the hole, and up out of the housing far enough to let you strip the insulation about 1/4" back. Don't do this before trying to stick the cable through the hole - it is much easier to do before stripping the wires.









After stripping the wires, gently move the cable back so the stripped wires are inside the housing. 









Now, look at the face of the photodiode and locate the mark, either a "+" or just a dot, which identifies the cathode of the diode. This is the lead that connects to the red wire in the cable. Turn the housing over and carefully drop the diode in, so the leads straddle the cable, with the cathode lead on the red wire side. Put the housing back on the fixture, so the diode rests on the top of the stub of tube in the fixture. Use a wood toothpick or something similar to bend up the wires to get them in contact with the diode leads, ready to solder the connections.









Continued on next post:


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## Hoppy

Continued from last post:
Solder the cable wires to the diode leads.









Trim off the wires and diode leads that extend past the end of the acrylic housing. Install a 1/8" long 1/2" tube retainer (the retainers have a saw cut on one side. Use diagonal cutter to trim off more of the acrylic so it will clear the cable). Push the retainer down until it holds the photodiode against the tube stub in the fixture. This locates the photodiode in the right position. Put a drop of acrylic cement on the cable where it comes through the wall of the housing, and one other place on the retainer, to hold the retainer in place. Be sure to pull the assembly partly back off the tube stub in the fixture to make sure you don't cement it to the fixture.









Cut out two 5/16" diameter discs of Rosco #4815 pink filter, and carefully put them on top of the diode, with the assembly turned over and not on the fixture.









Install a 1/8" long retainer on top of the filters, to hold them and the diode in place. This will fit tightly inside the housing, but can be held in place with a drop of acrylic cement, if you wish.









Continued on next post:


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## Hoppy

Continued from last post:
Cement the bottom of the housing onto the base by putting a puddle of acrylic cement where you want the housing to be attached and pushing the assembly into the puddle, moving it a bit to be sure you have a good contact. Hold the assembly in place for about 3 minutes to let the cement set. The electric leads may slightly interfere with this, so you need to hold it until their push won't separate the joint.









Cut the sensor off the lux meter - disassemble the sensor so you can cut with a long cable lead left ahead of the coiled part of the lux meter cable. Strip the wires on that cable, and the cable to the new sensor. Leave the ground wire on the new sensor extending about 3/4 inch beyond the wires so it can be used as a strain relief for the soldered connections.









Tin all 4 bare wires. Place shrink tube segments on the cables before soldering the two connections. I use shrink tube on both connections, with two layers over the whole cable connection, with the long ground wire wrapped around the lux meter cable so the shrink tubes will grab it to help relieve the strain on the connection.









Cut a 5/16" diameter disc of Rosco #114 diffuser filter and place it on top of the upper retainer in the sensor assembly. This filter is essentially invisible, so it takes great care to be sure it is in place before attaching the top "lens".









Continued on one more post:


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## Hoppy

Continued from last post:
Use black electricians plastic tape to hold the top lens in place, and black out the sides of the sensor, for calibration. 









With the sensor shimmed up to match the height of the Quantum meter sensor, measure the PAR from a fixed light with both meters. They should match.









They do match! So, all that's left is to cement the lens in place, put acrylic cement over the cable to housing joint to waterproof it, and cement the socket for a wand to hold the sensor at the other end of the bottom strip.









The last step is to paint the exterior of the sensor housing with black nail polish, two coats, to block all light except that that comes in from the top. You now have a usable PAR meter!


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## Jalopy

Hey Hoppy, how do you calibrate your PAR meter? Thanks.


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## Hoppy

Jalopy said:


> Hey Hoppy, how do you calibrate your PAR meter? Thanks.


I use a Quantum PAR meter, or one of my PAR meters that I have very carefully calibrated several times. I set the two sensors side by side under a 50-50, 36 watt PC AH Supply light that is rigidly mounted above a stand I made for a 10 gallon tank. If my PAR meter reading isn't within 5% of the other meter, I adjust the spacing between the photodiode and diffuser, for the version I am making now, or for the older versions I adjusted the number of white diffuser filters I used, until I get the one being calibrated to be within about 2 digits at about 40 PAR. Since there is no way to adjust the slope of the PAR vs Quantum PAR, I only calibrate at one PAR reading.


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## Jalopy

Hey Hoppy, does the thickness of the 1/2" acrylic disc in this version of the PAR sensor matter? Thanks.


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## Aquaticz

How do you convert Lux to par - please


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## Hoppy

Aquaticz said:


> How do you convert Lux to par - please


Lux and PAR are measurements of the same thing - light intensity at a surface. "Lux" is the units used when measuring the light, with the spectrum weighted to match human eye sensitivity. PAR is the units used when measuring light over the complete 400 to 700 nm wave length range, with all parts of the spectrum treated equally. Because lux involves suppressing wave lengths not easily seen by humans, the two sets of units can't be directly converted from one to the other, without using the spectral distribution of the light source to adjust the lux measurement to one that treats all wave lengths equally. So, the conversion factor for going from lux to PAR is different for different types of light sources - fluorescent, the sun, LEDS, CFLs, T8 bulbs vs T5 bulbs of different color temperatures, etc.

Fortunately, for our use, we don't need to know PAR to within 1% or better. If we know it to within 10% that works perfectly well. So, it is possible to use a lux meter with the reading divided by about 60 - 80 to convert the measurement to the equivalent of PAR. Or, you can use various filters on a lux meter to tweak the frequency sensitivity of it, and the overall sensitivity of it, and make it read in PAR units, accurate to +/-10%, and possibly better, for the commonly used lighting types on planted tanks. That is what I was doing in this thread.

I have stopped making these "PAR meters", out of boredom, mostly. But, I think I can just slightly modify a lux meter like the ones I used, so it reads in PAR units almost as accurately as those I made. I'm making good progress towards doing so. That would make it an easy DIY project for anyone, or it would make selling modified units for $25 and making some profit for little work, possible. If this works out, I will start a new DIY thread about how to do it.


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## jerrytheplater

I realize this is a very old thread. And I also just read how Hoppy calibrated his sensor when making it by comparing it to a better meter and adjusting the sensor construction. I tried sending him a PM and received an error saying he has chosen not to receive PM's. So, this is why I'm reviving this thread.

I have two of these Lux meter PAR meters made by Hoppy (Hoppy meters). One for our club, and one for me. I just tested them side by side with a one week old Apogee PAR meter, which I'm assuming is calibrated. The readings on the Hoppy meter were about half of the ones from the Apogee unit. They were tested in a planted tank with readings varying from 40-120 PAR on the Apogee. We also tested in full sun. Apogee about 2700 PAR, Hoppy 1140 PAR. Diffused shade under a tree: Apogee 300 PAR, Hoppy 145 or so. 

I can live with doubling the Hoppy readings. But is there any way to electrically calibrate the Hoppy meter?


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