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DIY PAR Meter

20K views 59 replies 9 participants last post by  Hoppy 
#1 · (Edited)
Hey, so I know this has been done before so not much new here. After reading the various DIY PAR meter posts though I found that everyone has had to calibrate their sensors. Hoppy's latest iteration uses a sliding tube to adjust distance between the sensor and the lens/filters before securing in place.

This is a DIY PAR meter build from a mechanical perspective and hopefully I can get good readings just after assembly. I have all the components and will try to finish up this build in 2 weeks.

Here's a cross section of my design. The number, order, and distance of the filters to the sensors are all taken from one of Hoppy's designs.
EDIT: I've replaced the original diagram with this. This shows my design intent better. This version of the housing is meant to be a test bed for an integrated 3D printed acrylic housing. The undercut is meant to accept a Tap Plastic 2447 frosted acrylic sheet or a 3D printed acrylic part for testing purposes. Also I've updated it to show the filters I'm using. No big surprise here, it's a copy of what Hoppy used to have.


Here's a picture of the 3D printed housing. It's porous black nylon so it'll have to be sealed before dunking in water. The surface quality turned out much better than I hoped.
 
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#34 ·


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.
 
#35 ·
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!
 
#36 · (Edited)
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.
 
#37 ·


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.
 
#38 ·
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
 
#40 ·
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.
 
#41 ·
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.
 
#42 ·
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.
 
#44 ·
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.
 
#46 ·
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.
 
#48 ·
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.)
 
#49 · (Edited)
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.
 
#50 ·
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.
 
#53 ·
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.
 
#58 ·
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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