PAR Meter Diffuser Material
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Old 02-05-2015, 06:54 PM   #1
Hoppy
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PAR Meter Diffuser Material


I'm stumped!

I'm trying to refine my DIY PAR meter, which uses a cheap lux meter as the starting point. One of my goals is to have a waterproof sensor, so it can be used in water filled aquariums. My design has evolved to the point where I am casting the parts with epoxy resin, containing either black or white pigment - black for the housing, white for the diffuser. At this point it works great, better than any I have made before, except when I check it in water it reads about half what it should read, based on the reading from an Apogee PAR meter. This happens with even 1/16 inch of water over the top of the sensor!

I tried coating the epoxy diffuser with clear nail polish (acrylic), but that had no effect at all. I tried putting a piece of plastic wrap (saran) over the diffuser, held in place with a rubber band. It read correctly that time. I repeated this 3 times after trying other things, and it worked correctly every time. I tried a shiny finish on the diffuser and a matte finish on the acrylic nail polish, and there was no difference.

So, I'm stumped.

Suggestions as to why this is happening, or how to get around it are very, very welcome.
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Old 02-05-2015, 07:01 PM   #2
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You heard my theory on the layer of air helps correct the weird light refraction issue.

Have you tried using another material for the diffuser?
The 2447 white plexiglass worked pretty well for me. It diffused light without drastically diminishing the intensity. I can snail mail you some samples for free.
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Old 02-05-2015, 08:01 PM   #3
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It is very likely that the saran wrap worked because it trapped a very thin layer of air between it and the diffuser, but it is possible that wasn't why it worked. The index of refraction of saran is slightly higher than that of epoxy and a bit more higher than acrylic. I can't yet get a handle on why that would give the effect it had.

If I can't cast the diffuser I will probably not continue with this design at all, so finding a different diffuser material isn't going to help a lot. But, thank you very much for the offer. Another possibly good diffuser material is the frosted acrylic rod that Tap Plastics sells - it worked well for me with other designs. Cutting discs of uniform thickness from that rod isn't at all easy for me with my lack of a workshop, so I already dropped that as a material in favor of casting a diffuser.

The cast diffuser I'm using is made from a mold I made using a 1.5 inch diameter acrylic sphere, from Tap Plastics, which took 3 iterations to get right.
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Old 02-05-2015, 08:36 PM   #4
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Have you considered 3d printing?
My public library recently got a 3d printer.


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Old 02-06-2015, 02:13 AM   #5
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Quote:
Originally Posted by Hoppy View Post
It is very likely that the saran wrap worked because it trapped a very thin layer of air between it and the diffuser, but it is possible that wasn't why it worked. The index of refraction of saran is slightly higher than that of epoxy and a bit more higher than acrylic. I can't yet get a handle on why that would give the effect it had.

If I can't cast the diffuser I will probably not continue with this design at all, so finding a different diffuser material isn't going to help a lot. But, thank you very much for the offer. Another possibly good diffuser material is the frosted acrylic rod that Tap Plastics sells - it worked well for me with other designs. Cutting discs of uniform thickness from that rod isn't at all easy for me with my lack of a workshop, so I already dropped that as a material in favor of casting a diffuser.

The cast diffuser I'm using is made from a mold I made using a 1.5 inch diameter acrylic sphere, from Tap Plastics, which took 3 iterations to get right.
for fun, try coating the resin w/ vegetable oil.. See what happens..
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Old 02-06-2015, 02:56 AM   #6
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Quote:
Originally Posted by mistergreen View Post
Have you considered 3d printing?
My public library recently got a 3d printer.


Sent from my iPad using Tapatalk HD
The mold for the body of the sensor is made from a modified 3D printed body that is about the 3rd or 4th one I had printed. 3D printing is really too expensive to make "production" parts from, compared to epoxy casting.

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Originally Posted by jeffkrol View Post
for fun, try coating the resin w/ vegetable oil.. See what happens..
That will be an interesting experiment, so I will try it tomorrow. My tank light is off now and I am too lazy to manually turn it back on.
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Old 02-06-2015, 04:39 AM   #7
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Maybe try another type of epoxy? They all can't be the same chemical makeup.


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Old 02-06-2015, 04:40 PM   #8
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Quote:
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Maybe try another type of epoxy? They all can't be the same chemical makeup.


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I had a hard time finding the one I'm using now. I want a relatively fast cure, taking only 24 hours, instead of 72 hours, for obvious reasons. And, it has to be reasonably priced and available in small quantities. The one I'm using now I found on Ebay, and later, on their own website. All of the others I have found come in very large containers. And, I think they are all almost identical, based on the descriptions.

I'm now looking for a good, but inexpensive adhesive that will bond acrylic to epoxy. If I find it, I can use the Tap Plastics frosted rod and work out a way to cut uniform discs from it. So far I have found that neither epoxy cement, nor acrylic cement will work well enough.
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Old 02-06-2015, 06:58 PM   #9
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Any way to physically hold the diffuser in place? Epoxy is really good at that and no need for glues. Maybe mold your casing with the diffuser in the mold?
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Old 02-06-2015, 08:48 PM   #10
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Quote:
Originally Posted by Hoppy View Post
I had a hard time finding the one I'm using now. I want a relatively fast cure, taking only 24 hours, instead of 72 hours, for obvious reasons. And, it has to be reasonably priced and available in small quantities. The one I'm using now I found on Ebay, and later, on their own website. All of the others I have found come in very large containers. And, I think they are all almost identical, based on the descriptions.

I'm now looking for a good, but inexpensive adhesive that will bond acrylic to epoxy. If I find it, I can use the Tap Plastics frosted rod and work out a way to cut uniform discs from it. So far I have found that neither epoxy cement, nor acrylic cement will work well enough.

http://www.resinobsession.com/ItemPa...que-White.html
http://www.aeromarineproducts.com/casting-resin.htm
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Old 02-07-2015, 02:21 AM   #11
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I'm going to try "Loctite Plaastics Bonding System", a two part methacrylate cement made to bond different plastics together. It's cheap, less than $4, and easy to use - no mixing. I bought one, but now I need to test it bonding a frosted acrylic rod to a piece of epoxy casting. Tomorrow.
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Old 02-07-2015, 03:08 AM   #12
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How big is the part you cast, and the area that needs to be "clear"?
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Old 02-07-2015, 02:36 PM   #13
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Default Diffrent meter

I have a Milwaukee MW700 lux meter. Can I buy anything to convert it to read PAR. Thank You
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Old 02-07-2015, 03:16 PM   #14
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Quote:
Originally Posted by Dead2fall View Post
How big is the part you cast, and the area that needs to be "clear"?
It is about 1/2 inch in diameter, and the "clear" area is about 1/4 inch in diameter to 1/2 inch in diameter.

Quote:
Originally Posted by mistergreen View Post
Any way to physically hold the diffuser in place? Epoxy is really good at that and no need for glues. Maybe mold your casing with the diffuser in the mold?
The epoxy resin flows into just about any size opening, so it would likely leak around the diffuser if I tried to mold it in place. Also, I adjust the sensitivity by altering the dimensions of the diffuser and it's distance from the diode. (I make it so it isn't sensitive enough, then trim the distance and the diffuser thickness to get more sensitivity.) I haven't found a way to hold dimensional tolerances close enough to make it so it is always right on without adjustments. And, it takes quite a bit of sanding of the diffuser to make a small adjustment in sensitivity, so I never overshoot.
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Old 02-08-2015, 01:05 AM   #15
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Quote:
Originally Posted by jeffkrol View Post
for fun, try coating the resin w/ vegetable oil.. See what happens..
I coated the sensor diffuser with olive oil, no improvement. The reading was even a bit lower. This is the sensor with the nail polish already on it.
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