# DIY PAR Meter



## Hoppy (Dec 24, 2005)

I just found this. I really like the idea of using a 3D printed housing, which would make the whole assembly job much easier and repeatable. One suggestion: don't use a lens at the top, use a diffuser - like a piece of frosted acrylic plastic, or make it from white acrylic, or even from clear acrylic that is sanded with about 200 grit sandpaper on both sides. I found that I could get repeatable results by just sanding clear acrylic. Or, you can use the clear lens and add a Rosco diffuser filter, like #116. The best Rosco filter combination I have found yet is #3313 and #373, plus an infrared blocking filter #1995, but the last one is not essential.

How expensive is it to make the 3D printed housing? As long as that is less than $10 this is a very good way to go.

What kind of cement are you going to use to seal the "lens" to the housing, and the electric cable to the housing?


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## Jalopy (Aug 11, 2013)

Hey Hoppy, sorry I didn't actually mean a lens literally. Yes, it's supposed to be a piece of frosted acrylic plastic. I was going to keep it as a surprise in case if it didn't work but the idea of the undercut is to make that sheet swappable for testing purposes. See the first post for an updated cutaway.

If I end up building the updated version, I will definitely integrate the new filters you've suggested. I've already bought the Rosco filters so I think I'm stuck with it for now. By the way, this version is optimized solely for the swapable acrylic sheet and ease of adjusting the thickness of the calibration spacer. The idea is to figure out if the acrylic top can be 3D printed as a part of the housing (no sealing on the top!) and if a very precise and repeatable calibration spacer can eliminate the the need for calibration in the future (I bought 2 sensors for this purpose). I have other ideas in mind for future improvements (reduced assembly time, improved sealing, etc.).

I was going to use what you used, Weldon #16, to cement the housing. Supposedly it can bond acrylic to a host of other plastics. As for the electric cable to the housing, I haven't decided yet. My fall back is to mix Weldon #16 with acrylic dust and form a bead around the seam between the cable and housing. In my next version (if it's built), I plan to have a nipple extend from the wiring exit with a small lip and use adhesive lined heat shrink.

By the way, supposedly the 3D printed black nylon is not waterproof so I might even bother sealing it at all unless I can find a suitable potting material. The 3D printed frosted acrylic IS waterproof though but more expensive so it's more appropriate for the next version.


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## Hoppy (Dec 24, 2005)

How would you assemble the sensor if the top lens/diffuser is an integral part of the 3D printed housing? One problem I find with Weldon #16 cement is that it flows all over the place, and if it contacts the Rosco filters it distorts them and changes the amount of light transmitted. You could probably coat the outside of the finished housing with black nail polish, but that can be a pain to work with too. Weldon #16 adheres very well to the insulation on the wire I use, so it seals the wire pass-thru very well.

Does the 3D printing process always generate a porous solid? Can it print opaque black acrylic?


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## Jalopy (Aug 11, 2013)

There is a black acrylic available and it is water tight. I'll try the black nail polish this time around. I'll upload a schematic for an integrated housing tonight to show you what I mean. There should be no issues with Weldon getting on the filters with this design.


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## Jalopy (Aug 11, 2013)

Hey Hoppy, here's what I have envisioned the integrated housing to look like. This is by no means my best attempt. One thing I want to point out is that I'd like to get rid of the preload spacer and wave springs altogether. I think that is doable and will measure the thicknesses of my two sensors to see what the tolerances are.


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## Hoppy (Dec 24, 2005)

Jalopy said:


> Hey Hoppy, here's what I have envisioned the integrated housing to look like. This is by no means my best attempt. One thing I want to point out is that I'd like to get rid of the preload spacer and wave springs altogether. I think that is doable and will measure the thicknesses of my two sensors to see what the tolerances are.


You can eliminate the spring if you make the bottom plug without a flange, so it can accommodate the variations in dimensions of the other parts. One thing that might make this configuration hard to work with is that if the calibration spacer isn't quite right it will be a chore to replace it to bring the sensor into calibration. That is a problem I have with my current design.

Another concern: The "aperture" size is also a big factor in the sensitivity of the sensor. I'm using a .25" diameter "aperture" - larger was a pain because it provided too much light to the sensor, making me use heavier filtration, which, since gel filters don't have much affect, if any, on the IR part of the spectrum, left the IR contributing too much of the total light energy hitting the diode.

With 3D printing, can you switch materials during the process - make the top clear acrylic and the bottom an opaque plastic? Could you stack the internal parts and print the housing around them? (Seems unlikely.)


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## Jalopy (Aug 11, 2013)

I like your idea of making the bottom plug without a flange. I was also thinking of making it out of brass or some other heavier material since I think the sensor will float in its current design. Yes, I'm definitely worried about the calibration sensor. The spec sheet said I think 1-2% linearity but did not mention about repeatability across sensors. We'll see when I stick the 2nd sensor on there. 

I've stuck with the 0.25 diameter aperture. See the picture below. I was really faithful to your design with respect to the working features. 









As it is right now, you cannot switch materials. I'm sure someone makes a machine out there that lets you but it's not an option with Shapeway.com. You could stack the internal parts and print around with some limitations but again, not with the company I ordered these parts from.


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## Hoppy (Dec 24, 2005)

Any progress with this? Or any new design changes?


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## Jalopy (Aug 11, 2013)

Nothing yet. This week I plan on wrapping everything up related to the current design and get a reading out of it. Then it's onto the redesign. Sorry, for the lack of progress. I'm trying to find time between homework, kids, and work. Really wish I started this before I started classes.


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## Hoppy (Dec 24, 2005)

Jalopy said:


> Nothing yet. This week I plan on wrapping everything up related to the current design and get a reading out of it. Then it's onto the redesign. Sorry, for the lack of progress. I'm trying to find time between homework, kids, and work. Really wish I started this before I started classes.


This is very important - give up sleep! :icon_cool:red_mouth:red_mouth


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## Hoppy (Dec 24, 2005)

Have you made any progress with this?


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## Jalopy (Aug 11, 2013)

Hey Hoppy, I have all the parts... in pieces. I need to stop procrastinating and put it together. Thanks for reminding me though. I'll post a real update by next Friday.


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## Hoppy (Dec 24, 2005)

This is a way to make a sensor, using 3D printed parts, so its sensitivity is adjustable, filters are easy to install (but not replace), and sealing it from water is easy.

I bought a great CAD program for my Mac, the "Punch! ViaCAD" program, only $100, (vs about 5X that for any comparable program) and it does everything a CAD program can do, without having to install Windows on the computer first. The learning curve for this is steep, seeming impossible when you first start, but it only took me a few days to be able to make a computer model of the two parts for this. It helped that I first learned the basics of CAD back in the 1980's, using the Dassault CAD program that Boeing started using for commercial aircraft, and this Punch! program is based on that same program. I still have a long way to go to be proficient with the program, but it is certainly fun.


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## Jalopy (Aug 11, 2013)

Looks awesome, are you going to print it?


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## Hoppy (Dec 24, 2005)

Jalopy said:


> Looks awesome, are you going to print it?


I haven't decided, yet. First, this design needs the dome part to be somewhat translucent, so it acts as a diffuser, and I'm not sure what material will do that. Second, I don't know who to have do it, and if I can afford it. I'm really tempted though. I think I still have one of the photodiodes to use in it, but I'm pretty sure I'm out of the filters needed to make it work right. Last, I'm not sure that this is an optimum design yet.


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## Jalopy (Aug 11, 2013)

I think I have some Roscoe filter from one of your earlier designs. For the dome, I suggest the frosted or translucent acrylic material from Shapeways. You might have to print a dome in each of the materials to test it out.

The design looks good overall though.


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## Hoppy (Dec 24, 2005)

Jalopy said:


> I think I have some Roscoe filter from one of your earlier designs. For the dome, I suggest the frosted or translucent acrylic material from Shapeways. You might have to print a dome in each of the materials to test it out.
> 
> The design looks good overall though.


Shapeways looks interesting, and their "frosted" material looks ideal for a diffuser. I will be studying this more today to see if I want to proceed to a prototype or prototypes. If I do this I also want to do more work selecting the filter(s) to shape the spectral response to be at least as good as the Apogee Quantum meter response. 

Since this is hobby activity for me, the more work I have to do, the more fun it is!


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## Hoppy (Dec 24, 2005)

I just ordered one of each of the two parts, black acrylic for the body and white frosted acrylic for the dome. First, I added the hole for the electric cable in the body, saving the work of drilling the hole, and the risk of breaking it while drilling it. I learned the hard way that you have to be sure to specify that the drawing is in inches, not mm, when you order the parts! (Or do the drawing in mm.)

The cost is about $31 with shipping included. If I had ordered 10 the shipping cost would have been a lot less, per unit. Even so, the sensor is going to cost around $30 for parts only.


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## Hoppy (Dec 24, 2005)

About 2 years ago I began looking for a combination of Roscolux filters that would change the spectral response of an Excelitas photodiode to equal that of the Apogee Quantum PAR meter. I found many combinations that were usable, but until today I never found a combination that almost exactly duplicated the Apogee response. Here is that Apogee spectral response that is the goal:








The colored area is the ideal PAR meter response, the definition of PAR being all of the radiation between 400 and 700 nm wave length.

An Excelitas VTB8441BH photodiode has built in filters to give this spectral response:









Today I found a 3 filter combination that works very well, using the spectrum plots shown on the Rosco website, http://www.rosco.com/filters/roscolux.cfm Here is how I calculated the response:









These are the three filters, along with their transmission at each of 20 wavelengths. I used a spreadsheet to figure out the sensitivity of the diode at each wave length by multiplying the transmission percentages at each wave length together and multiplying that by the relative sensitivity of the diode at those wave lengths. Then to be able to plot that on a graph I converted each of those sensitivities so the the highest one is 1.0 and the others are fractions of 1.0.









When this is plotted on a graph, along with the Apogee Quantum PAR meter spectral sensitivity it looks like this:









You can see that a PAR meter using that photodiode and those filters would cover the whole PAR range better than the Apogee meter, but at the cost of including a little of the radiation outside the PAR range. I think that is a good trade off.

If the 3D printed parts here work out right, and if it is possible to adjust the assembly to calibrate it to read PAR accurately with just geometry changes, this should make a very good PAR meter.


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## Jalopy (Aug 11, 2013)

Looking good. You need to post some data when you're done.


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## Hoppy (Dec 24, 2005)

I just received a package of two of the LX1010B lux meters, which I didn't order, and with no indication of where they came from. Does anyone know anything about this???? If it is an anonymous donation to this project I really appreciate it.


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## Jalopy (Aug 11, 2013)

That's pretty awesome but it wasn't me. Put them to good use!

Sent from my DROID RAZR using Tapatalk


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## JoeFL77 (Sep 18, 2013)

Gentlemen,
Have you looked into what is available in smart phone. I have a Galaxy S4 and an app, Galaxy S4 Sensors, that has a Lux meter in it. I think it uses the front camera as the sensor.

I have on of Hoppy's DIY Lux meter and will try it as soon as I can find it. 

Of course, I worry about putting a $600 smart phone under water.

Joe


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## Hoppy (Dec 24, 2005)

Jalopy said:


> That's pretty awesome but it wasn't me. Put them to good use!
> 
> Sent from my DROID RAZR using Tapatalk


As I recall I saw a falling star last week and made a couple of wishes before it vanished: world peace and a couple of lux meters. Win some, lose some! :icon_mrgr


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## Hoppy (Dec 24, 2005)

The prototypes of the two sensor parts I'm having made have been shipped today, so I should have them by the end of the week. This is a slow process compared to making things yourself.


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## Hoppy (Dec 24, 2005)

The parts arrived today. The translucent dome seems to pass more light than I expected, and the fit isn't what I expected, so I will have to do a little sanding to get the dome to fit over the body part. Other than that they look very good, and should work fine.

Now I just need to assemble this, and experiment to see how I can alter the design to make it always work without laborious testing, adjusting, re-testing, etc. Ultimately, I hope it will be possible to make this work with a single 3D printed part, so the cost is much less.


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## Hoppy (Dec 24, 2005)

It works, but is far too sensitive to be usable as it is now. It reads about 3500 when the PAR is about 41, so on the 50,000 scale it reads a little low, and on the 20000 and 2000 scales it reads way too high. Adjusting the height of the translucent dome above the diode works for about a +/-5% adjustment, which is good, but the overall sensitivity is just too high. It sure looks good though!

The three fixes for this would be making the dome from white acrylic instead of translucent acrylic, or making it much thicker above the diode, from ,050 inch to perhaps .50 inches, which is impractical, or making the dome much higher so it is much farther from the diode. I think the best approach will be using white acrylic, which is still a bit translucent. This material just transmits too much of the light.

I think I will try a different design for the next iteration, and use white acrylic for it. The design I'm thinking of is much closer to Jalopy's original design, but a one piece design.


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## mistergreen (Dec 9, 2006)

Heh, It does look good. That's the material of the diffuser?
I'm assuming the diffuser (acrylic 2447) you used before this one works pretty well.


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## Hoppy (Dec 24, 2005)

mistergreen said:


> Heh, It does look good. That's the material of the diffuser?
> I'm assuming the diffuser (acrylic 2447) you used before this one works pretty well.


The diffuser dome is made of a translucent acrylic that Shapeways uses. It will never reduce the intensity of the light striking the diode enough - just too transparent. The one I just ordered will be made of white acrylic, which may end up being too opaque, but I like this design better, and it "only costs" me $18 for one, with over half of that the shipping cost. This will be a one piece body, with the walls, other than right over the diode, thick enough to block most of the light. But, only trial and error will work to find a final design.

The acrylic I was using before, for diffusers, is the "frosted acrylic" rod from Tap Plastics. It works very well, but has to be cut with more precision than I can apply, in order to make the sensors reproducible without lots of adjusting and testing - very labor intensive.


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## mistergreen (Dec 9, 2006)

Oh, btw. Thanks to your hard work with finding the 3 roscolux filters, I'm going to add them to my sensor. This will make it more accurate. I just bought the roscolux swatch, hopefully, they'll have all 3 filters in there. It's cheaper than buying a big sheet plus shipping.


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## Hoppy (Dec 24, 2005)

mistergreen said:


> Oh, btw. Thanks to your hard work with finding the 3 roscolux filters, I'm going to add them to my sensor. This will make it more accurate. I just bought the roscolux swatch, hopefully, they'll have all 3 filters in there. It's cheaper than buying a big sheet plus shipping.


Those little sample books are supposed to be free! I just picked up another one yesterday, and they had a whole carton of them to give away. My original book of samples is pretty well shredded from all of the pieces I have cut out to try over the past 2 years.

Jalopy mentioned casting housings for PAR meters, as a cheaper alternative to 3D printed ones. I've been looking at that too - TAP plastics has all of the resins, mold making materials, dyes, microballs, etc. needed, and they have a store near me. It is an interesting DIY method, but with a part having detail on both inside and outside it isn't the simplest thing to make a mold for. And, getting the right mix of resin, dyes and/or microballs to make a diffuser work right, plus being able to duplicate the mix easily will be a challenge.

EDIT: With a little more googling, it looks like making the mold would be pretty easy, so this could drop the price of the housings to a few cents each! (But, only after the trial and error process to get the proper mix for a translucent diffuser.)


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## Hoppy (Dec 24, 2005)

My latest attempt to make a housing for a PAR meter through Shapeways has arrived. It is the all white one shown above. The basic design looks promising, but finding the right combination of dimensions is going to be very difficult. As is, this one is too sensitive except in the 50,000 range, where it is a little too insensitive. So, it takes more than minor tweaks of the geometry to make it work as it is now. And, I'm not sure that having the whole housing somewhat translucent is going to work. It looks like much of the light getting to the diode is from areas that are thick, where I expected to get only trivial amounts of light.

I will be doing more experimenting to see what to try next.

EDIT: After using very fine sandpaper to "polish" the diffuser part of the housing, I made it read almost exactly 2X the correct readings, with the readout meter set at the 50,000 range. I can work with that! My next step is to drill the housing for the electric cable, do a little more "polishing", and try to cast one with clear epoxy http://www.tapplastics.com/product/...ing_products/easycast_clear_casting_epoxy/386 colored with white pigment http://www.tapplastics.com/uploads/pdf/TAP Premium Pigment.pdf I should be able to change the sensitivity of the diffuser/diode combination by adjusting how much pigment I use.


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## Hoppy (Dec 24, 2005)

I just successfully made a mold for casting the PAR meter housing/diffuser in epoxy resin. It looks like it will work ok, but until I actually try it.....?

The mold is made of platinum cured silicone, following the directions in http://www.tapplastics.com/product_info/mold_making_guide This looks like a very effective way to DIY something you have designed/made in big enough quantities to sell.


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## Hoppy (Dec 24, 2005)

Today I tried to cast one of the housings in epoxy resin. Unfortunately, it takes 2-3 days for it to fully cure, so I won't know how well I did until Tuesday. For sure, it was a difficult job. The housing uses about 2 ml of resin, but the smallest amount I could mix was 20 ml and even then I'm not sure I was accurate enough with using equal amounts of resin and hardener. I just used clear epoxy because, for now, I'm just trying to learn to cast one. If it works the next one will be with epoxy with white pigment in it. And, that means finding a way to measure 1/32nd of 20 ml! The pigment has the consistency of toothpaste, making that even harder to do. I suspect I will use a tooth pick and "scoop" what will fit on about 1/4" of the end of it. I need to be able to adjust the amount of pigment to get the translucence I need. Fun!


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## Hoppy (Dec 24, 2005)

The casting cured faster than I expected, and turned out pretty good!










The two problems I see are:
The mold isn't done quite right, so the casting is about 1/16 inch longer than the original, caused by the impossibility of shoving the original part into the half mold all the way in - trapped air in the recess stops it from going in all the way, resulting in a longer casting than the original.

And, there are a couple of air bubbles, not in critical places, but demonstrating that the mold isn't quite right for allowing the bubbles to float out as the resin is poured in.

There are a lot of things to learn about how to do this well. I need to decide whether to make another mold, or live with this imperfect one.

EDIT: I decided to continue with the current mold. All it does is thicken the diffuser area, which I was intending to do eventually anyway. So, I just finished casting a new body, this time with white pigment in the resin. That worked out very well procedurally.


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## Hoppy (Dec 24, 2005)

This is the first white housing I made with epoxy resin, clear resin with one smidgen of white pigment mixed in. This one test pretty close to what I want - it read 31 when it should have read 41. I need it to read either correctly or a little high (so I can add a diffuser filter to drop the reading down to the correct value. Today I will try this with 1/2 smidgen of pigment.


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## Kaduhn1 (Aug 3, 2013)

You need a machine shop to machine a mold for you, I just so happen to work in one. have any 3D drawings of the parts and mold that are finalized and good? I have Solidoworks 3d software and work in a machine shop. Hoppy those molds look dang close to final part and the look pretty easy to machine, nothing crazy


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## Kaduhn1 (Aug 3, 2013)

Reading a little more about the program you use Dassault is who makes solidworks so those parts you have drawn up i should be able to open up. are they .sldprt file?


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## Hoppy (Dec 24, 2005)

Kaduhn1 said:


> You need a machine shop to machine a mold for you, I just so happen to work in one. have any 3D drawings of the parts and mold that are finalized and good? I have Solidoworks 3d software and work in a machine shop. Hoppy those molds look dang close to final part and the look pretty easy to machine, nothing crazy


I'm using the Shapeways 3D printed part as a model for casting a silicone mold. That mold is good for at least 100 castings, so I don't really need another mold. It would be easy to machine the model if I had a small machine lathe, but I don't, and Shapeways price is substantially less than the cost of a lathe, even when I add the $100 cost of the computer program for CAD.

Right now I'm quite happy with the shape of the model, so all I'm doing now is trying to find a pigment to resin ratio that will give me the translucence I need. Once I have two data points for two different ratios I can find out with considerable accuracy what ratio would work best.


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## Kaduhn1 (Aug 3, 2013)

yeah, buying a few machines to make one part wouldn't be very cost effective and I didn't know how long that mold would last, 100 parts isn't bad though. Is this something you're eventually going to produce and sell? If you ever need an actual metal mold made let me know. I'd exchange it for a meter if they don't cost too much, I'd be making it on my free time after work so not a big deal to do, wouldn't be charging you a shop rate to make it- more so just helping you out. Go all out and make an injection mold  make 2 of the holders and two lenses in one shot. How awesome would that be.


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## Hoppy (Dec 24, 2005)

Kaduhn1 said:


> yeah, buying a few machines to make one part wouldn't be very cost effective and I didn't know how long that mold would last, 100 parts isn't bad though. Is this something you're eventually going to produce and sell? If you ever need an actual metal mold made let me know. I'd exchange it for a meter if they don't cost too much, I'd be making it on my free time after work so not a big deal to do, wouldn't be charging you a shop rate to make it- more so just helping you out. Go all out and make an injection mold  make 2 of the holders and two lenses in one shot. How awesome would that be.


Thank you! I appreciate the offer. I do plan to make several more PAR meters, assuming this technique works out well enough. I doubt that I will make another 100 though. First step is finding out if this will work well enough. The big question is how sensitive the design is to variations in the mix of pigment and resin. If that has to be held constant within 10%, for example, it isn't really workable because the pigment is like sticky toothpaste and is very hard to measure accurately. But if it only needs to be held constant within 25%, I think I can make that work ok.


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## Kaduhn1 (Aug 3, 2013)

are you weighing the pigments to mix them? we do ink for pad printing and its measured out by weight not volume.


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## Hoppy (Dec 24, 2005)

Kaduhn1 said:


> are you weighing the pigments to mix them? we do ink for pad printing and its measured out by weight not volume.


I have considered weighing it, but the amount of pigment is so tiny it would be very difficult to do it well. I do have the gram scale to do it. The resin and hardener are designed to be measured by volume, so I would need to carefully measure out the desired volume and weigh it, then try to hit that weight each time. If I decide to make a lot of these that is probably the best approach, just for consistency.


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## Kaduhn1 (Aug 3, 2013)

Just throwing some ideas for you  maybe a syringe if the pigment can be sucked up.


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## Hoppy (Dec 24, 2005)

Kaduhn1 said:


> Just throwing some ideas for you  maybe a syringe if the pigment can be sucked up.


I asked the Tap Plastics clerk if I could do that, and she just laughed, telling me I can't even pour it out of the little jar. It has to be scooped out like toothpaste. It adheres to everything extremely well, so once in the scoop, getting it out into the mixing cup is a problem too. There are liquid pigments available, but not in white, at Tap Plastics.


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## Kaduhn1 (Aug 3, 2013)

http://www.smooth-on.com/Urethane-Plastic-a/c5_1119_1213/index.html
Here's another plastics company we buy stuff from. Looks like there is more of a liquid maybe


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## Hoppy (Dec 24, 2005)

If I decide to buy online there are many places to shop, but my Tap Plastics store is very close to me, so I prefer shopping there when possible. Once I pass the experimenting stage I may do more shopping around. (I just used all but an ounce or so of my silicone rubber, and it is very expensive, so more shopping will probably be necessary.)


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## Hoppy (Dec 24, 2005)

I just tested the housing made with 1 smidgeon of pigment to 20 ml of resin. That increased the sensitivity too much, so that it looks like I would need about .8 smidgeon of pigment. I can't possibly do that repeatably.

All of this was done with the lux meter set on the 20000 range. If I switch the range to 2000, I would need to use about 3.5 smidgeons of pigment in 20 ml of resin. That is a crude estimate, so it might take 4 smidgeons to work. The maximum I can use without affecting the cure of the epoxy resin is 2.5%, or .5 ml, which is 3.1 smidgeons. That would make the meter read about 30% too high, and I can adjust that downward with a diffuser filter. I think my next step will be making a housing with 3 smidgeons of pigment in 20 ml resin. This is a slow process, because it takes 3 days for each casting to cure to the hard condition.

I can't accomplish much by moving the diode farther from the diffuser portion of the housing because the thicker, non-diffuser portions still contribute some light to the reading, and that compensates for most of the reduction in light from moving the diode farther away. It might be that this design can't work out.:frown:

EDIT: Another possibility: I made a mistake in casting the silicone mold, leaving the diffuser section about .13 inches thick instead of .05 thick. If it were correct there would be much less light contribution from the non-diffuser portions, and adjusting the sensitivity by moving the diffuser farther from the diode would be much more effective. So, my next move will be to cast another silicone mold. $$$$$

EDIT2: I sanded the top of the housing to reduce the 1 smidgeon pigment diffuser thickness from .11 in. to .09 in. and it now reads right on! Best of all, the thickness required isn't so critical that great precision is needed. A .005 in. variation made only a tolerable little difference in sensitivity. So, now I need to make a silicone mold of the modified housing ($$$) and see if I can duplicate this. It is $$$ because I am out of liquid silicone, and it costs about $40 for another 15 ounces. It takes only a few ounces for the mold, so I will have some left over for other projects, one of which I'm already working on.


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## Hoppy (Dec 24, 2005)

Progress report: I have been able to make castings from the successful housing. It also looks like I can make castings of the bottom part of the PAR meter, so I'm pretty sure this will work as a good way to make PAR meters that are more accurately reproducible, easier to assemble, and still not very expensive. I plan to start a new thread detailing how I'm doing this, as soon as I can get a prototype made, and some more data on how different diffuser filters affect the calibration of the sensor. That should be the middle of next week.


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## Hoppy (Dec 24, 2005)

I have finally completed a working PAR meter! See http://www.plantedtank.net/forums/showthread.php?p=5859178&posted=1#post5859178 for more details.


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## Kaduhn1 (Aug 3, 2013)

How much does it cost to make these? The iowa aquatics club is looking at buying a par meter to rent out to members.


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## Hoppy (Dec 24, 2005)

The cost of making these is not just the cost of the parts, but also a pro-rated part of the cost of developing the design, which was a few hundred dollars. See http://www.plantedtank.net/forums/showthread.php?t=615762 for more information about them. Right now I have stopped making them, so I can devote more effort to selling my house, finding another place to live, and moving. Once that is done I plan to continue making them.


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## jimn1680 (Jul 16, 2014)

Have you verified the spectral response with Excelitas? This is the reply that I received from them:

*From:* Desfonds, Eric [mailto:[email protected]] 
*Sent:* Thursday, July 10, 2014 12:12 PM
*To:* James Howard (JHN)
*Cc:* Larsen, Ella; Ruiz, Luz; Chepulis, Dennis; Charlebois, Alain
*Subject:* RE: Response Curve

I agree it is a bit off. I took the liberty to digitize the curve, see attached.

The first datapoint is around 330nm. The 720nm corresponds to about 10% relative to the peak. We more routinely would have use the 10% points in the low- and high-range, but given the steepness of the low wavelength side, we used 330nm. And we used the 10% point on the long-wavelength side.

The peak in the specific curve is closer to 530nm than 580nm, which is around 85% of the peak response. Apologies of the confusion.

That being said, the datasheet with the graphs is a more general sheet, and some products like the VTB8441BH have varying Silicon thickness, so I would trust the datasheet values MIN/MAX/NOMINAL (330, 720, 580) as valid for the VTB8441BH part. Other VTB parts also have varying peak wavelengths, the graph was meant as a reference really.


*Éric Desfonds*


There is a big difference between a curve of 330,720, 580 and the general curve Absolute Spectral Response “B” Series Filtered curve published in their attachment which you quote.

Thanks,
Jim



Hoppy said:


> About 2 years ago I began looking for a combination of Roscolux filters that would change the spectral response of an Excelitas photodiode to equal that of the Apogee Quantum PAR meter. I found many combinations that were usable, but until today I never found a combination that almost exactly duplicated the Apogee response. Here is that Apogee spectral response that is the goal:
> 
> 
> 
> ...


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## Hoppy (Dec 24, 2005)

No, of course I have no method for verifying spectral response of a photodiode. But, I also don't understand the response you received - what are they saying the true response looks like?


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## jeffkrol (Jun 5, 2013)

O/T a bit.. above got me thinking about better photodiodes:
http://www.atmo.arizona.edu/student...ne/lecture_24/lect24_ground_optical_meas.html
http://www.atmo.arizona.edu/student...ne/lecture_24/lect24_ground_optical_meas.html










Needs a blue filter (as mentioned) and of course a uv/ir cut filter.. 

UDT 020A Photodiode: Electronic Components: Amazon.com: Industrial & Scientific

Since this is NOS tech.. thought I'd find a "horde"..
http://www.surplussalespa.com/catalog/product_info.php?cPath=35_60_371&products_id=14564


> PIN-3CDP


http://www.physics.utoronto.ca/~ast...X-ray PD TIAs/Docs/Photodiode_UDT_catalog.pdf

pg21 for specs...


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## kzeller (Dec 31, 2014)

Hoppy, have you sold any of these. If so for how much?


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## Hoppy (Dec 24, 2005)

Yes, I sold a few of them. But, then I started getting ready to move to an apartment, with much less room. So, I quit making them. Now I'm back working on this, but since I now have a Finnex Planted + light, with much more red in the spectrum, I want a PAR meter that includes more of the red light than this one did. It looks like it is very possible to do.


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## CallMeJay (Sep 8, 2015)

Hoppy said:


> About 2 years ago I began looking for a combination of Roscolux filters that would change the spectral response of an Excelitas photodiode to equal that of the Apogee Quantum PAR meter. I found many combinations that were usable, but until today I never found a combination that almost exactly duplicated the Apogee response. Here is that Apogee spectral response that is the goal:
> 
> 
> 
> ...



Hoppy, how do you measure the spectral sensitivity of VTB8441BH when using filters? So you can draw the line of spectral for before and afteradding filters?

Regards.


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## Hoppy (Dec 24, 2005)

CallMeJay said:


> Hoppy, how do you measure the spectral sensitivity of VTB8441BH when using filters? So you can draw the line of spectral for before and afteradding filters?
> 
> Regards.


Unless you possess some pretty expensive equipment you don't measure spectral sensitivity of anything. Fortunately, manufacturers of photodiodes publish the spectral sensitivity data for their products. Once you know the spectral sensitivity of a specific diode, and you have a source of optical filters with spectral transmissivity data for them, you can combine the filter data with the diode data and get what the modified sensitivity of the diode will be with a specific filter or filters.

For a given small band of wave lengths you can multiply the spectral sensitivity (0 - 100%) at that wave length by the spectral transmission of the filter at that same wave length (0 - 100%) and get the sensitivity of the combination. This will give a very low number, like .055. To make that into a usable number you "normalize" the results by dividing each of them by the highest number. That makes the maximum result be 100%, with other wave lengths having lesser results. It sounds confusing, but a simple spreadsheet makes it an easy calculation.


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