# Cheapest PAR Meter DIY



## Corrie

You're my hero! LOL

...how about just throw the whole thing in a couple of zip-locks??


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## Hoppy

Waterproofing something like this requires more than a plastic bag. I made one attempt to water proof a sensor by using my old reliable WeldOn #16 acrylic cement. That worked perfectly to seal the white diffuser dome to the dark grey sensor housing. I just put a bead of the cement on the housing groove and pressed the diffuser dome in it, and held it for a minute to set up. But, when I tried the same thing for the two halves of the readout grey housing, it leaked badly when I tested it in a glass of water. That was because there is a notch at the dome end, where it is hard to get enough cement in to close up the notch. I think my next try will be with silicone for that joint. Using silicone can result in a very sloppy looking job, so I haven't decided yet how to avoid that. The hole where the electric cable enters the housing, which is really a notch in each of the housing halves, should probably also be sealed with silicone. The only remaining "holes" are the places where the two halves are held together with two screws. It looks like WeldOn #16 would work fine there.


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## Corrie

Hoppy, I meant the "whole thing"....sensor, cord, and meter
....can you operate the buttons through the plastic bag??


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## Hoppy

No buttons to operate except the on-off-hold switch, which you only use to turn it on before sticking it in the water. The problem would be the trapped air in the bag would make it hard to push it down to the substrate, and your hands and arms would interfere with the light. Sealing the sensor would only be hard once, not every time you try to use it.


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## Corrie

try something like these two....

Amazon.com: Star brite Liquid Electrical Tape: Sports & Outdoors

Amazon.com: 3M 06504 Marine Adhesive Sealant 5200, Black, 10-oz.: Sports & Outdoors


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## Hoppy

The Starbrite stuff looks like a winner! I can get it at the local hardware store in one ounce size - a little costly at about $7, but every visit to a hardware store is now costly. I still like the acrylic cement for sealing the diffuser dome to the housing, since it is clear, but this should work well for the rest of the sealing. Maybe tomorrow I will get some to try.


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## Corrie

Here's something else in case you don't know about it....

Amazon.com: J-B Weld 8277 Water Weld: Automotive

be sure and rough up the plastic with fine sandpaper and clean with pure ammonia first on any of them

personally, I would use the 5200 as my first choice


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## Corrie

you can get the 5200 in smaller tubes too....

http://www.boatersland.com/mmm05205.html?gclid=CMT85vniy7sCFTEV7Aodj0MAcQ

Boaters World, West Marine, etc


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## lochaber

How about filling it with epoxy? 
(yeah, I tend to think most problems can be solved with epoxy...)


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## KribsDirect

I'm looking forward to buying one of these if you decide to sell them on here. I'm also following your led build. You've got some serious diy skills Hoppy, very cool! I'm envious. You should have seen me trying to make a simple fake rock out of pond foam.. it was on the walls, in my hair.... I wouldn't even think about tackling something like this!


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## Mark13

Hoppy,

I worry about the chemicals in those products being fish safe. Here is a link to some black silicone aquarium adhesive. This one is only 3oz, but it can be had in a larger squeeze tube, and in a caulk tube. Source is Big Al's Online. Very few aquarium sources carry the black, or if they do, you must buy a box of 12 or 24 tubes.

http://www.bigalspets.com/silicone-aquarium-sealant-black-3-oz.html

http://www.bigalspets.com/black-silicone-rubber-3-oz.html


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## Hoppy

Mark13 said:


> Hoppy,
> 
> I worry about the chemicals in those products being fish safe. Here is a link to some black silicone aquarium adhesive. This one is only 3oz, but it can be had in a larger squeeze tube, and in a caulk tube. Source is Big Al's Online. Very few aquarium sources carry the black, or if they do, you must buy a box of 12 or 24 tubes.
> 
> http://www.bigalspets.com/silicone-aquarium-sealant-black-3-oz.html
> 
> http://www.bigalspets.com/black-silicone-rubber-3-oz.html


As far as I know, the only *cured* plastic substances that are harmful to the occupants of an aquarium are those containing mildew inhibitors. Those are designed to allow a chemical to "leach" out after they are cured. The others are just solid plastic of some form once they cure. But, of course, one would have to check the ingredients or msds for whatever substance was in question to be sure. The other significant fact is that a PAR meter will be in the tank water for only a few seconds, not for hours on end, and leaching takes time.


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## jeffdenney

Is this a new and improved diy par meter?? I own one of your older ones with the different filter housing. Does that read the same as this model?? 

Like filter wise is it the same and will it read the same spectrum? I never saw any graphs with the older model.


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## Hoppy

jeffdenney said:


> Is this a new and improved diy par meter?? I own one of your older ones with the different filter housing. Does that read the same as this model??
> 
> Like filter wise is it the same and will it read the same spectrum? I never saw any graphs with the older model.


As I said, my goal is to find a less expensive way to make a usable PAR meter. That starts with using one of the LX1010B lux meters, but with the fewest added parts, and least labor, to keep the costs down. One approach is to use the photodiode that is standard with the lux meter - that is the cheapest approach. But, that gives you a meter that basically measures the intensity of the green light in the spectrum, which can give you an accurate PAR reading as long as you don't try to use it to compare different types of lighting - it requires that each light you use it with have about the same percent of its spectrum in the green area, so measuring just the green light gives a reading proportional to the PAR for that light.

The other approach is to replace the luxmeter photodiode with the one I used on my other PAR meters, which measures a much, much larger part of the spectrum. That approach makes the meter work accurately for a lot more lighting types, and should be essentially as accurate as the other PAR meters I made, but it also costs more to make.

I'm still thinking about whether I want to make these to sell, and if I do, which version I would want to make.

And, I still haven't gone back to the problem of making the sensor waterproof, without spending much money.


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## jeffdenney

So the style that I have pictured below has the alternative photodiode correct?? 

I just wonder because if its the photodiode that comes with the lux meter ill have to guesstimate my par a little more.. add the blue and red in and give my best judgement...

But if I have the new diode it should be pretty bang on correct??

Basically what im asking is what photodiode and filters does this particular meter have??

Ive been using it for 6 months ish now and have been setting my photoperiod by this par meter.. its seems to b working for me..


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## Hoppy

jeffdenney said:


> So the style that I have pictured below has the alternative photodiode correct??
> 
> I just wonder because if its the photodiode that comes with the lux meter ill have to guesstimate my par a little more.. add the blue and red in and give my best judgement...
> 
> But if I have the new diode it should be pretty bang on correct??
> 
> Basically what im asking is what photodiode and filters does this particular meter have??
> 
> Ive been using it for 6 months ish now and have been setting my photoperiod by this par meter.. its seems to b working for me..


That uses the new photodiode, Excelitas VTB8441BH. It should continue to be accurate for a long time, possibly needing a new battery at times. This is the only photodiode I have used in any of the PAR meters I made.


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## Hoppy

Starbrite liquid electrical tape is *not* a good sealant to use. It is a noxious mix of unsafe solvents, in a tube where it can't be mixed as it is supposed to be, and so runny it gets on your hands and drips as you apply it. Brushing it would work better, but the noxious fumes are enough to avoid it. 

I'm going to try my reliable WeldOn #16 again. With a good technique it should work fine.


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## Corrie

..in a tube??
I've never seen liquid elec tape in a tube, usually it's in a can like PVC glue..and pretty thick



Corrie said:


> personally, I would use the 5200 as my first choice


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## jeffdenney

Hoppy said:


> That uses the new photodiode, Excelitas VTB8441BH. It should continue to be accurate for a long time, possibly needing a new battery at times. This is the only photodiode I have used in any of the PAR meters I made.


Awesome. Ive been using it for a while and it seems to be working great.. havent even changed a battery yet.. 

Thanks


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## Hoppy

Corrie said:


> ..in a tube??
> I've never seen liquid elec tape in a tube, usually it's in a can like PVC glue..and pretty thick


This is what I tried:









I also looked up the msds for this, and found the warnings were the most severe and varied of anything I have seen a msds for. It is just not good to use unless nothing else will work, and then only in a very well ventilated room, with hand and eye protection.

Today I finally was able to seal a sensor with WeldOn #16 acrylic cement, after several tries. The secret was to apply the cement inside, when assembling the sensor, then again over the outside of each joint. Without that last step the sensors just fill with water in a few minutes.

Next step is to put all of this together in one usable sensor. I'm inclined to concentrate on the least expensive build - using the original photodiode. Then I can try it with various kinds of light and see how much the calibration changes with the different kinds.


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## Hoppy

I finally have a cheapest PAR meter assembled, and it calibrates against the Apogee Quantum meter very well, within 5% of the reading. I had to abandon my attempt to make it read on the middle range scale because I couldn't get it to read close enough. So, I started all over, trying to get a broader spectral response and make it read on the lowest scale. Lots of spreadsheet calculations later I have a 5 colored filter, plus one diffuser filter combination that works. And, it has a broad spectral response, so it has a chance of working with a variety of light types. I'm waterproofing it now, to see if it continues to work the same afterwards. Then I will try it against a few more lighting types. And, finally, buy a few luxmeters and see it the results are the same with all of them.

The work required to modify one lux meter is pretty easy, goes fast, and doesn't require a lot of mechanical aptitude. But, arriving at that configuration was a royal pain.

Yesterday I tore down a readout module from a couple of my failed luxmeters, and to my surprise, the printed circuit boards in them are not the same, or even close to the same, for the same LX1010B meters. One circuit has 4 adjustable resistors, obviously used to adjust the readout to make it read correctly. (Unfortunately that readout isn't working well enough to test out how well it can be adjusted.) The other has no adjustable resistors that I could identify - either they were able to just use precision resistors, with no adjustment needed, or the circuit is considerably different. But the different Chinese companies making these appear to work out the design details independently.


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## Hoppy

There is always something to learn, and often it is learned with egg on your face!* My attempt to flatten out the spectral response of the lux meter photodiode by using colored filters didn't work out at all like I expected. The "PAR" meter response matched the Apogee meter response very well with a 50-50 bulb (half actinic), but was off considerably with a 10,000K bulb. Since the spectral response wasn't uniform, there are apparently big enough differences in peak responses for the two bulbs that one has peaks where the photodiode has peaks and the other doesn't. My attempt at forcing the lux meter diode to act as a PAR meter diode was a failure.

So, I changed the filters to neutral grey filters, leaving the spectral response a peak in the green band, but making the meter read the same numbers as the Apogee meter reads. Then I checked it with the two bulbs, and it works the same for both of them. That means it is better to just measure the light in a small part of the spectrum than it is to measure it at a combination of different parts of the spectrum. That isn't what I expected.

Next, I need to try this with a couple more different "white" bulbs, and sunlight.

*Egg on my face because I sealed the sensor before discovering this! I had to use a hammer to get it back apart. No problem, since I have about 50 left over sensors to replace it with.


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## Hoppy

After almost 2 years of working on this, trying everything I could think of, I finally shifted my brain into "park", and guess what? This lux meter has a closely fitting cap over the sensor, to block out the light to make the photodiode last longer when it isn't in use. What do you suppose would happen if you left that cap in place, but drilled a tiny hole in the center of the part over the diffuser dome? Duh! It acts as a very effective filter, reducing the lux meter reading as much as you might want to. So, I drilled a 1/16" diameter hole in it, and it reads about 25% low compared to the "PAR" that it should read. No filters needed, just a small increase in the hole size, probably to a #49 drill size (about .073 inches diameter). I will buy a drill bit that size and see if it works. If it does that means no parts, no filters would be required to make the lux meter read "PAR", and you could leave the drilled cover on to read PAR and take it off to read lux. At worst, you would need to add a tiny piece of diffuser filter from the Rosco sample book, over the back of the hole to adjust the reading down a bit.

I started thinking about this when I noticed that even the Rosco neutral gray filters badly distort the spectral response, and will probably make it mis-indicate some types of lighting, where a simple green light measurement would be more likely to be accurate. I should put my brain in "park" more often!:redface:

Here is what the modification looks like as a sketch:









Today I found that the best diameter for the tiny hole in the protective cover is .067 inch, which is a #51 drill size. I did this by trying 4 different drill sizes, plotting the resulting meter readings and using that to interpolate to the best diameter, giving the same reading on the lux meter as on a PAR meter. I still need to buy a #51 drill and try it to be absolutely sure of the size needed.

This is an absurdly simple way to make a usable PAR meter. Drill a tiny hole in the protective cover. Use silicone sealant to make the sensor body waterproof. (Silicone worked best for this, by testing various sealants with partially assembled sensors.) The lux meter is then unharmed and, by removing the protective cover, reads lux as it was designed to. This enables you to read from zero to about 50,000 PAR! Take it on a trip to the sun and measure the PAR there!


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## Wannaberooted

Well done! I don't know how you could make it any easier. When I have all my other projects done, I'll try one.


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## Hoppy

I was able to make the lux meter read almost exactly what a PAR meter reads, but not just by adjusting the hole diameter. A very small change in the hole diameter makes a big change in the lux reading. I wasn't able to find the #51 drill (.067") I wanted, so I got a #50 (.070) instead. That made the lux meter read 37% too high! But, I added a Rosco #111 and #101 diffuser filter under the hole, and the reading was then right on the button. This may be an advantage for other reasons. By sealing around that pair of filters inside the protective cover it prevents water from filling the cover, making it less "sloppy" to use in the tank. Still, if I could find a #51 drill, it should also be very accurate.

Now, I need to combine sealing the sensor to make it water tight, with the drilled protective cover, and calibrate this with various lighting types. Since this sensor measures only green light I expect that there will have to be a correction factor for some types of lighting.


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## Hoppy

I finally got my hands on our club Apogee Quantum PAR meter, and calibrated this meter with two colors of PC bulbs, an incandescent bulb, a 6500K CFL bulb, and open shade outdoors. The results are:








The meter reads a bit low, from 3 - 14% low, so if you wanted better accuracy you could add 8% of the reading to the reading and be much more accurate. One thing I discovered is that the meter works well with a light typical of an aquarium light overhead, but out in the sun or in shade outdoors it doesn't do well. A really good PAR meter will have a cosine diffuser on it, so it is measuring light from a large conical area above the meter. With the tiny hole as the primary filter on this meter it doesn't have a very effective cosine filter, so it misses a lot of the light out doors. For our typical planted tank use it works fine though.

Two more tasks to be done:
Demonstrate that I can waterproof the sensor without interfering with the geometry of the tiny hole to the diffuser to the photodiode.

Demonstrate that this works the same when I make more of these. (Sometimes something will work ok once, but due to small geometry variations, not work well when repeated.) So, I guess I need to buy a few more of the lux meters.


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## Hoppy

Obviously the easiest and cheapest way to use one of these lux meters as a PAR meter is to just use it as is, measure the light intensity in lux instead of PAR units, then divide the reading by *78* to get the PAR reading. That coversion factor I got by reading the intensity with the small hole filtering the light, then removing the cover withe hole in it, and reading the same light again, in lux units. Dividing the lux reading by the PAR reading, then multiplying it by .92 to adjust for the approximately 8% low readings from the modified lux meter, gave me 78 as the conversion factor. Perhaps if I were to do this again another time with another lux meter I would get a slightly different factor, but 78 will always be true for this specific lux meter.

I ordered a couple more lux meters, which I will test the same way to see if the 78 factor stays the same.


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## saiko

Hoppy, so these results basically are from two #111 and #101 filters, and the 0.067" hole?


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## Hoppy

saiko said:


> Hoppy, so these results basically are from two #111 and #101 filters, and the 0.067" hole?


Yes, that's the configuration I used. The question is whether this will work for a random lux meter. This particular meter is one I replaced the photodiode with a Excelitas diode, then reinstalled the original diode. Doing that may have slightly altered the optical path, which would mean this wouldn't be the right combination for a new lux meter. The two new ones I ordered should be adequate for answering that question. I should have them by the end of this coming week.


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## Hoppy

I have been wondering why the cosine diffuser works so poorly with the .067" dia hole. One possibility is that the hole is too small to expose any curvature of the diffuser. So, I found a filter combination that lets me enlarge the hole to 15/64" drill size. I used 2 each layers of Roscolux #15 Deep Straw, and one layer of Roscolux #386 Leaf Green. This narrows the spectral band a bit, and shifts it a bit towards yellow. It should work as well as the existing lux filter alone. (The green line is the modified filter response, and the fuzzy black line is that of the lux meter alone.)









Tomorrow I will check the readings in open shade again to see if I still have a cosine diffuser problem.

I also found that I can seal the diffuser to keep water out by using silicone. I had to apply it on the inner surface of the top part of the sensor housing, first to seal the white diffuser to housing interface (making sure not to have any silicone build up on the bottom lip of the diffuser which fits against 4 posts in the lower housing), then around the lip that mates with the lower housing, making sure just a little overlaps the lip to make good contact with the lower housing. I immediately reassembled the housing halves, reinstalling the two screws that hold it together. This was to make sure no silicone interfered with the fit - these parts have to fit just right to maintain the dimensions of the optical path inside. I wiped off the excess silicone that squeezed out, to keep it from interfering with the fit of the drilled cap on the upper housing. After several hours to cure I checked it for leaks by dunking in a glass of water for 30 minutes, held down under water with a rubber band. It didn't leak at all.


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## Hoppy

I checked the accuracy of the modified lux meter against the Quantum PAR meter in open shade outdoors this morning. The reading is now about 30% low, a big improvement over the approximately 70% low reading I had before. Of course we don't use open shade to light up an aquarium, but we do need to measure both the direct light from above and the reflected light off the glass, so a reasonably good cosine diffuser is still necessary. This should be adequate.

EDIT: Bad news! (This is like a soap opera.) I tried to recalibrate the modified lux meter, using all of my various types of light. It was immediately obvious that the calibration with a 50-50 PC bulb didn't even come close to matching that for a 10,000K bulb. Clearly, the sharper peak in the spectral response, from using the filters I selected, makes the meter very sensitive to the spectral output of the light source. Where it calibrated right on the money with a 50-50 bulb, with a lot of blue in the spectrum, it was way off for the 10000K bulb with less blue, and, as might be expected, the 10,000K bulb gave much higher readings than the 50-50 bulb.

Back to the drawing board.


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## Hoppy

In today's episode of "As the World Turns".....

I replaced the filters I was using with two diffuser filters, a Roscolux #101 and a #111 and recalibrated the meter against the Apogee Quantum PAR meter. 










This calibrates perfectly - no correction factor needed, and it works for 10,000K and 50-50 (half actinic) PC bulbs, plus a 26 watt 6500K CFL and a soft white 40 watt incandescent. In open shade outdoors it isn't very accurate, but at least it reads in the right ballpark there. With this configuration the range switch on the lux meter readout has to be in the 20,000 setting, which means the meter actually reads 10X the actual PAR. I also noticed that the lux meter readout isn't the same in the 2000 and 20,000 range settings - the 2000 setting doesn't give exactly 10X the 20,000 range reading. That is just the built in inaccuracy in the lux meter, which is claimed to be +/-5%.

Unless I come up with a new idea, this is the best configuration I can find for using these lux meters as PAR meters. And, again, this still needs to be shown to work for more than one of the lux meters.


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## ckmullin

And the latest build was with the new Excelitas VTB8441BH ?


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## Hoppy

ckmullin said:


> And the latest build was with the new Excelitas VTB8441BH ?


No, it is just the LX-1010B lux meter modified so it reads PAR numbers. This is supposed to be a minimum cost usable PAR meter.


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## KribsDirect

:bounce: Congrats! A major milestone in your build!

I hope the future tests prove well


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## ckmullin

Hoppy said:


> No, it is just the LX-1010B lux meter modified so it reads PAR numbers. This is supposed to be a minimum cost usable PAR meter.


Thanks. Thought that but wanted to check.


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## Hoppy

To make this the easiest possible modification, I switched to a 3/16 inch hole, a hole drilled with a 3/16" bit, with no filters or diffusers added. That gave me a reading the same as the Apogee Quantum PAR meter, within about 5%. Today I received a couple more lux meters, from Ebay, made by a different Chinese company, but still the LX-1010B model. I tried each of them with the same diameter hole in the cover, and both gave the same result, the same as the Apogee meter within 5%.

So, you can have a "PAR" meter for as little as about $15, if you are willing and able to drill a 3/16" hole in the center of the protective cover (after removing it from the sensor, of course.) If you want it to be usable under water you also need to use silicone to waterproof the sensor, which is a little bit tricky because you need to get a good seal everywhere, but without having any on the upper surface of the sensor, where it would interfere with the fit of the protective cover.

This should be well within the reach of all of us. The three that I have modified, but not waterproofed, I plan to sell on the For Sale forum sometime in the next few days.


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## Sugarcat

Do you have any thoughts on using this with LED lighting, Hoppy?


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## Hoppy

I have checked this type of "PAR" meter with a LED light, having half cool white and half warm white. It worked fine. Where it might not work as accurately is if you have a lot more red or blue in the spectrum than is typical for fluorescent bulbs normally used to light an aquarium.


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## Sugarcat

Good to know...one of these nifty little guys would be more than adequate for our tanks then. I'd be glad to buy one of your samples whenever you're finished with them.


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## Hoppy

This is how I sealed the sensors, to waterproof them. Put a ring of silicone around the inside of the top half of the sensor housing, to seal the white cosine diffuser to the grey housing. Let that set-up for an hour, then put a ring of silicone around the inside of the lower half of the housing, so the top half will touch it when it is assembled, and being sure to have silicone all around the electric cable. Then put the two halves together carefully so you don't spread silicone all over the place, and install the two screws that hold it together. Then fill the screw head recesses with a drop of silicone on each. Scrape off any silicone that squeezes out, so it doesn't interfere with the fit of the cover. It takes about 12 hours to fully cure.


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## yashmack

im gonna have to dig out my lux meter and drill bits now...
Awesome!! thanks for all the experimentation and sharing what you found!!


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## ipkiss

Hoppy, 

thanks for persisting in your quest! so you said that the size of the hole affected your readings quite a bit but what about the position of the hole in the "center" of the protective cover? What if I'm a hair off center in drilling this hole? How exact does it need to be?


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## Hoppy

ipkiss said:


> Hoppy,
> 
> thanks for persisting in your quest! so you said that the size of the hole affected your readings quite a bit but what about the position of the hole in the "center" of the protective cover? What if I'm a hair off center in drilling this hole? How exact does it need to be?


I tried to hit the center of the spherical recess, but I doubt that I was more accurate than +/- 1/16 inch if that. But, the results were very close for the two that I just did. One read about 4-5% high and the other was about 4-5% low. Theoretically it shouldn't have much effect if you are close to the center, since all it is doing is limiting how much light strikes the diffuser and the diffuser does diffuse the light, spreading it out a lot before it gets to the photodiode.

I would like to make a jig for drilling them, but I haven't figured out an accurate way to make it yet. (I just use an electric hand drill, drilling a much smaller pilot hole first.)


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## ipkiss

hmm. that's comforting to know. thanks!


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## jrill

Hoppy said:


> To make this the easiest possible modification, I switched to a 3/16 inch hole, a hole drilled with a 3/16" bit, with no filters or diffusers added. That gave me a reading the same as the Apogee Quantum PAR meter, within about 5%. Today I received a couple more lux meters, from Ebay, made by a different Chinese company, but still the LX-1010B model. I tried each of them with the same diameter hole in the cover, and both gave the same result, the same as the Apogee meter within 5%.
> 
> So, you can have a "PAR" meter for as little as about $15, if you are willing and able to drill a 3/16" hole in the center of the protective cover (after removing it from the sensor, of course.) If you want it to be usable under water you also need to use silicone to waterproof the sensor, which is a little bit tricky because you need to get a good seal everywhere, but without having any on the upper surface of the sensor, where it would interfere with the fit of the protective cover.
> 
> This should be well within the reach of all of us. The three that I have modified, but not waterproofed, I plan to sell on the For Sale forum sometime in the next few days.


Hoppy,
OK, just so I have this straight. I got a little confused reading all the posts, but to use this as a par meter all we need to do is drill the hole?

And thanks for posting this and doing all the work testing. Cool beans.


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## Hoppy

jrill said:


> Hoppy,
> OK, just so I have this straight. I got a little confused reading all the posts, but to use this as a par meter all we need to do is drill the hole?
> 
> And thanks for posting this and doing all the work testing. Cool beans.


The hole serves no purpose except to make the meter read the right numbers for PAR, when on the 20000 scale range. You can also just use the lux meter as is, and divide the reading by 78.


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## jrill

Awesome, Thanks.


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## Hoppy

I made a few more of these, to further verify that the hole size works for all of them. It does, and I have listed these for sale in the For Sale forum. (For those who are all thumbs!)


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## Plant_Haus

Awesome very nice, looks like something useful


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## gerbillo

Hoppy said:


> The hole serves no purpose except to make the meter read the right numbers for PAR, when on the 20000 scale range. You can also just use the lux meter as is, and divide the reading by 78.


Just to be sure, when set to 20000, we still have to multiply the value on the LUX meter by 10 (as indicated by the display), then divide by 78, right?

I got the LX-1010B, decided to not modify it and just do the quick math instead. The readings seem to be roughly in the range that I expected, thanks for all your hard work!


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## Hoppy

gerbillo said:


> Just to be sure, when set to 20000, we still have to multiply the value on the LUX meter by 10 (as indicated by the display), then divide by 78, right?


No!! With the range set to 20000 the value shown on the Lux meter is the PAR reading. No multiplying or dividing. This is with the sensor cover in place, with the hole in the cover. Remove the cover and it is just an ordinary Lux meter, except the sensor is waterproofed.


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## gerbillo

Hoppy said:


> No!! With the range set to 20000 the value shown on the Lux meter is the PAR reading. No multiplying or dividing. This is with the sensor cover in place, with the hole in the cover. Remove the cover and it is just an ordinary Lux meter, except the sensor is waterproofed.



Now this is confusing. Just to be sure that we're talking about the same thing, I decided use it without the cover, so no hole nor filter, just as a lux meter set at 20000. You said earlier to divide the lux meter value by 78 to get PAR:



Hoppy said:


> You can also just use the lux meter as is, and divide the reading by 78.


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## Hoppy

gerbillo said:


> Now this is confusing. Just to be sure that we're talking about the same thing, I decided use it without the cover, so no hole nor filter, just as a lux meter set at 20000. You said earlier to divide the lux meter value by 78 to get PAR:


If you use the lux meter as a lux meter, without the cover with the hole in it, you are measuring lux as the original meter was designed to do. In the 20,000 range you multiply the reading by 10, to get the lux reading. To convert that to a PAR reading you divide lux by 78.

I thought you were asking about using the meter as a PAR meter, with the cover with the hole in it on the sensor. If you do that, the meter reads PAR directly. 

In all cases, any conversion of lux to PAR is an approximation. The accuracy varies with the type of light you are measuring. When I calibrated these I noted that the PAR reading on a Quantum PAR meter was 1/78 of the lux reading, for the light I was using. If I had been using a different type of light, the conversion would have been at least a little bit different.


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## Jsack12

Hoppy I'm thinking about ordering one of these as is. I'm gonna take apart the sensor and silicone like you showed to waterproof. Then using the lux meter reading lux, I then divide whatever number it says by 78? Correct? This cheap diy stuff can get confusing


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## Hoppy

Jsack12 said:


> Hoppy I'm thinking about ordering one of these as is. I'm gonna take apart the sensor and silicone like you showed to waterproof. Then using the lux meter reading lux, I then divide whatever number it says by 78? Correct? This cheap diy stuff can get confusing


Yes, that's correct. You use the meter as it is designed to be used, and it measures in lux. Divide the lux value by 78, or for some lights the number to use is as low as 70, and you get about the right PAR reading.

Don't forget how hard it is to get the sensor completely waterproof. I was successful once, but other times it would eventually seep a little water in, and stop working until I dried it out, not easy to do. This was so frustrating to me that I finally decided that it would be best for me to just use it by propping my light across a couple of chairs, and measuring the lux at the same distance from the light as the substrate was, then divide by 78 (I finally switched to 70 as the divisor, but I forget why )


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## jeffkrol

Hoppy said:


> As I said, my goal is to find a less expensive way to make a usable PAR meter. That starts with using one of the LX1010B lux meters, but with the fewest added parts, and least labor, to keep the costs down. One approach is to use the photodiode that is standard with the lux meter - that is the cheapest approach. But, that gives you a meter that basically measures the intensity of the green light in the spectrum, which can give you an accurate PAR reading as long as you don't try to use it to compare different types of lighting - it requires that each light you use it with have about the same percent of its spectrum in the green area, so measuring just the green light gives a reading proportional to the PAR for that light.
> 
> The other approach is to replace the luxmeter photodiode with the one I used on my other PAR meters, which measures a much, much larger part of the spectrum. That approach makes the meter work accurately for a lot more lighting types, and should be essentially as accurate as the other PAR meters I made, but it also costs more to make.
> 
> I'm still thinking about whether I want to make these to sell, and if I do, which version I would want to make.
> 
> And, I still haven't gone back to the problem of making the sensor waterproof, without spending much money.


The lux sensors "should" just have a green filter over them.. any chance of removing it???

Actually just opened my INS sensor.. def. a green filter..


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## Hoppy

Yes, the filter is easily removable, but then you have a IR radiation sensor, not a visible light sensor. And, I found it essentially impossible to substitute a filter that would widen the wave length response evenly enough to make it a usable PAR sensor, while also blocking the IR radiation. You can certainly make a better sensor for PAR than it starts out, but the diode is just not sensitive enough in the near 400 nm area to ever work well as a PAR sensor. IR blocking filters are not that cheap either, unless you don't mind giving up the 600-700 nm portion of the spectral sensitivity.

We have no good reason to need accurate PAR readings anyway. If we can get within about 10 PAR accuracy at 50 PAR, that is still a useful device. And, you can't beat the cost.


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## jeffkrol

Hoppy said:


> . And, I found it essentially impossible to substitute a filter that would widen the wave length response evenly enough to make it a usable PAR sensor, while also blocking the IR radiation. You can certainly make a better sensor for PAR than it starts out, but the diode is just not sensitive enough in the near 400 nm area to ever work well as a PAR sensor. IR blocking filters are not that cheap either, unless you don't mind giving up the 600-700 nm portion of the spectral sensitivity.


"In water" IR is absorbed quite readily, and w/ LED's little 700nm or above light anyways..


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## Jsack12

Hoppy said:


> Jsack12 said:
> 
> 
> 
> Hoppy I'm thinking about ordering one of these as is. I'm gonna take apart the sensor and silicone like you showed to waterproof. Then using the lux meter reading lux, I then divide whatever number it says by 78? Correct? This cheap diy stuff can get confusing
> 
> 
> 
> Yes, that's correct. You use the meter as it is designed to be used, and it measures in lux. Divide the lux value by 78, or for some lights the number to use is as low as 70, and you get about the right PAR reading.
> 
> Don't forget how hard it is to get the sensor completely waterproof. I was successful once, but other times it would eventually seep a little water in, and stop working until I dried it out, not easy to do. This was so frustrating to me that I finally decided that it would be best for me to just use it by propping my light across a couple of chairs, and measuring the lux at the same distance from the light as the substrate was, then divide by 78 (I finally switched to 70 as the divisor, but I forget why
> 
> 
> 
> 
> 
> 
> 
> )
Click to expand...

Would propping it on some chairs still give you an accurate reading? I would think the dead air would give you a higher reading then it would be going through water.


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## jeffkrol

Jsack12 said:


> Would propping it on some chairs still give you an accurate reading? I would think the dead air would give you a higher reading then it would be going through water.


somewhat the opposite.. Water refracts the light and light hitting objects/glass has a tendency to bounce it back in..

not much of a difference but some..esp. deeper in the tank.


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## Jsack12

jeffkrol said:


> Jsack12 said:
> 
> 
> 
> Would propping it on some chairs still give you an accurate reading? I would think the dead air would give you a higher reading then it would be going through water.
> 
> 
> 
> somewhat the opposite.. Water refracts the light and light hitting objects/glass has a tendency to bounce it back in..
> 
> not much of a difference but some..esp. deeper in the tank.
Click to expand...

So even doing it in dead air no water would give you about the same reading as it would in water?


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## Hoppy

Jsack12 said:


> So even doing it in dead air no water would give you about the same reading as it would in water?


Yes. When I have checked this I almost always got about the same PAR reading in the water as out in the air. Considering all of the other inaccuracies in measuring PAR the lack of water has a minor effect.


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## Jsack12

Hoppy said:


> Jsack12 said:
> 
> 
> 
> So even doing it in dead air no water would give you about the same reading as it would in water?
> 
> 
> 
> Yes. When I have checked this I almost always got about the same PAR reading in the water as out in the air. Considering all of the other inaccuracies in measuring PAR the lack of water has a minor effect.
Click to expand...

Well hell im just gonna get one of those lux meters since they aren't even $20 and do it that way


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