A PAR Meter with Cast Plastic Parts
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Old 04-25-2014, 07:24 PM   #1
Hoppy
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A PAR Meter with Cast Plastic Parts


One problem with making a new sensor for a cheap lux meter, to convert it into a PAR meter, is the difficulty of reproducing the parts so they are so identical as to get the same result every time. There are so many variables that affect the sensitivity of the sensor to light that it would take machine shop precision to be able to make each sensor work exactly alike. Most of us, me included, don't have a machine shop. I have a hacksaw, a hand drill and sandpaper instead.

In this thread, http://www.plantedtank.net/forums/sh...ight=par+meter Jalopy proposed an alternative to having a machine shop - use an online 3D printing service to make the mechanical parts. That has inspired me to revisit my DIY PAR meter design.

I modified Jalopy's sensor housing design until I got it down to the smallest and least complicated shape that would fix the critical dimensions so each part would essentially be identical. http://www.plantedtank.net/forums/sh...9&postcount=30 I wanted it to be the smallest it could be, because 3D printing is priced by the volume of material in the part. Even with my "smallest design possible" version the housings cost $18, about $9 for the part and $9 for shipping. That is a significant cost for such a tiny part.

A bigger problem is the limited set of materials that 3D printing can be done with. This housing has to act as the cosine diffuser as well as containing the other parts. A good material for a diffuser will be white, it will not absorb or reflect differently for different parts of the light spectrum. It will also be the same for each part that is made, and it will have a transmissivity that allows the diffuser section to be about .05 inches thick - thinner would be too prone to mechanical failure and to damage during the cleaning process after the part is printed. Thicker would prevent limiting the light striking the photodiode to just the diffuser area since the whole housing would be made of the same material. It also has to be waterproof, not so porous as to allow water to quickly leak through it. I can find no such material among those that Shapeways, the 3D printing-for-DIYers company, has available.

Jalopy mentioned to me, in a PM, that he was looking at the possibility of molding the housings out of plastic. That started me looking at the possibility of casting the parts.

Amateurs can easily cast parts from polyester resin or epoxy resin, but not acrylic resin, which is ordinarily used for optical quality parts. Polyester casting requires mixing catalyst into the resin a few drops per ounce of resin, with the number of drops being determined by the thickness of the casting and other characteristics of the part to be cast. This makes making identical casting very difficult, since each part uses less than 1 ounce of resin.

Epoxy casting requires equal quantities of two resin components, a much easier to control mixture, and not a function of the thickness of the parts. Clear epoxy casting is well within the capability of an amateur, like me. So, I started teaching myself to cast epoxy parts!!
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Old 04-25-2014, 07:38 PM   #2
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I am very fortunate to have a TAP Plastics store in my area. http://www.tapplastics.com/product/m...sting_products Their website contains good instruction videos for the various casting materials they sell. And, of course, Google offers many other instruction videos with a simple search.

To cast an epoxy part requires first making a mold, a reverse 3D "image" of the part to be cast. A good mold will be easy to make, it will be capable of being used a hundred or so times before "wearing out", and will make cast parts that are almost exactly the same as the model the mold is based on. You can make a mold from urethane or silicone rubber. And, if you use silicone rubber, you can use either tin cured silicone or platinum cured silicone. Platinum cured silicone is by far the superior mold material, because the uncured solution has a very low viscosity, so it easily flows into every detail of the model, and doesn't hold air bubbles in solution, but lets the easily float to the surface. In addition, the finished mold is very strong and isn't affected by the resin you cast the part from. But, it is very expensive for big molds - fortunately this PAR sensor housing is very small.

The details for making a mold are well shown in the TAP Plastics videos, and in other videos on YouTube.

I had a sample housing made by 3D printing, and used that as my model for a mold. After experimenting with one cast part I modified that part slightly, and used that as the model for a final mold.

Since this part is the cosine diffuser, and the epoxy resin is clear, I had to add a white pigment to the epoxy to make white epoxy. TAP Plastics sells that pigment in a tiny little jar. The pigment is similar to toothpaste in consistency. I am using a "smidgeon" spoon to measure the pigment, adding one smidgeon of pigment to 25 ml of resin, to get the proper transmissivity. I determined this by trying both a half smidgeon and one smidgeon. By simple good luck the one smidgeon gives very close to what I need.

This cast housing costs me about $2 apiece, once I have the mold made.
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Old 04-25-2014, 08:39 PM   #3
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Hoppy you are truly an amazing individual. the level of detail you put into things just astounds me sometimes.

Of course we could just get them made in China at a fraction of the cost but where's the fun in that?
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Old 04-25-2014, 09:13 PM   #4
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This is a hobby - fun is the name of the game! And, learning new things is the best part of the game.

This epoxy casting method is good for almost any small plastic part. For example, to learn more about the limitations of epoxy casting I have also been trying this technique to make a drop checker, based on this design: http://www.plantedtank.net/forums/sh...078&highlight= So far it hasn't worked out very well. Casting epoxy gets much more difficult the thinner the parts are, because even though the liquid resin has a low viscosity, it is still a problem to make it flow into narrow passages and allow air bubbles to escape. I made one of those drop checkers with glued together acrylic, but had to use about a .04 inch wall thickness acrylic dome. That was too thin for the acrylic to clean itself of bubbles. And, a complex shape like that takes a lot of silicone for the mold - $$$ - plus is difficult to remove the casting from the mold. I'm still working on the idea, with the next approach probably being having Shapeways make a thicker wall version with 3D printing. I have the 3D CAD design done, but need to make some changes in it.

It's too bad that polyester resin casting is so difficult for such thin parts. That is a much better material, or it seems to be.

EDIT: I keep forgetting: the major disadvantage of epoxy resin as a material is the slow curing it needs. It takes about 4 days to reach a hard cure, and the full hardness takes even longer. If you are trying to make several castings, it takes either multiple molds - $$$ - or you can only do 2 per week or less. I haven't found a faster curing epoxy resin yet.
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Old 04-26-2014, 01:25 AM   #5
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This is the base of the sensor, which seals off the inside of the housing, making it waterproof. I will use 5 minute epoxy to glue the two parts together, and to seal where the electric cable goes into the housing. The mold was formed from a model made with acrylic parts.

And, this is another housing being cast. I bought 100 6 ml polypropylene test tubes on Ebay to use for more accurate measurements of the two epoxy components. This is the first housing made with that degree of accuracy, which I have found is essential. This one cured a little faster, as a result.
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Old 04-27-2014, 05:33 PM   #6
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This is the first PAR meter I have completed using the cast epoxy parts. It seems to work fine, but I still want to try it under water to be sure it doesn't leak. That has to wait until the epoxy glue holding it together cures overnight.
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Old 04-27-2014, 07:59 PM   #7
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Now this is my kind of project! I like what you've done with the casting and using epoxy as a material.

I'm not sure if you know about it but if you want to make more complicated parts in the future and tap into a great source of knowledge about everything casting related you can visit the forum at http://www.alloyavenue.com/vb/forum.php. The forum there deals primarily with metal casting, but has a great group of people who have tried basically everything and can give you help and advice on any sort of casting related projects. A good number of them also have 3D printers, and CNC machines.
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Old 04-27-2014, 11:06 PM   #8
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Quote:
Originally Posted by Zapins View Post
Now this is my kind of project! I like what you've done with the casting and using epoxy as a material.

I'm not sure if you know about it but if you want to make more complicated parts in the future and tap into a great source of knowledge about everything casting related you can visit the forum at http://www.alloyavenue.com/vb/forum.php. The forum there deals primarily with metal casting, but has a great group of people who have tried basically everything and can give you help and advice on any sort of casting related projects. A good number of them also have 3D printers, and CNC machines.
Thanks for the link. I may spend some time there to see what I can learn.
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Old 04-27-2014, 11:48 PM   #9
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Try Smooth on also Hoppy.

http://www.smooth-on.com/

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Old 04-28-2014, 03:08 AM   #10
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Smooth-On does have a faster cure clear epoxy, but it isn't retailed anywhere near me. I would need to drive 200 miles round trip, or pay high shipping costs for it. And, it seems to be a more expensive product too. But, it is encouraging to see that there are choices.
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Old 04-28-2014, 04:25 AM   #11
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some really great efforts. cool stuff!
Do you plan to add an acrlyic rod as holder?
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Old 04-28-2014, 05:08 AM   #12
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Quote:
Originally Posted by saiko View Post
some really great efforts. cool stuff!
Do you plan to add an acrlyic rod as holder?
At least for the first ones I make I don't plan to add an acrylic rod as a holder. It isn't hard for someone to add something like that if they want it. And, the super expensive Li-Cor PAR meter doesn't come with that - just with a platform with leveling screws.

I just found a faster cure, hobby grade epoxy casting resin on Ebay, and bought some to try out. It is called Resin Obsession Clear Epoxy Casting Resin. It is just a little more expensive than what I can get at my TAP Plastic store, but not by much. I think the white pigment I have will work ok with it, and it cures in 24 hours instead of 3-4 days.
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Old 04-29-2014, 09:20 PM   #13
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The PAR sensor passed its water dunk test. I kept it under water for a couple of minutes, and no sign of air bubbles, which would indicate a leak. After drying it off I tested it again and it reads exactly the same.

The only problem I'm still looking at is how to make sure the sensor is attached to the base reasonably near perpendicular to it. This first one looks like about 10 degrees off perpendicular. Fortunately, the PAR reading stays essentially constant with the sensor at angles over 30 degrees off of perpendicular to the light source. And, testing it out in the sun worked well too, with a reading reasonably close to the expected PAR - I didn't have an Apogee PAR meter on hand to check it for accuracy in the sun.

Another possible problem I checked for is stray light coming from the sides and bottom of the sensor. The most PAR I got that way is 1-2 PAR units, an insignificant amount, so the sensor is reading the light passing through the filters, and not stray light that isn't filtered.

I plan to assemble another one tomorrow to see if I can duplicate the results.
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Old 05-05-2014, 09:20 PM   #14
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I'm finding that it is very hard to get exactly the right amount of pigment in the epoxy, so the sensitivity of the sensor varies a lot from sensor to sensor. But, I also found an easy fix for that. I can easily sand off the top of the sensor, maintaining about the same curvature, and slowly bring the sensitivity up to where it should be. In other words the sensitivity depends on the thickness of the white epoxy diffuser area above the diode, but it varies by something like the square root of the thickness, so it takes quite a bit of material removal to nudge the sensitivity by about 10%. It isn't a hair trigger type of thing, making it an easy adjustment to make. I really like this design so far.
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Old 05-05-2014, 10:20 PM   #15
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How long till you start mass producing them and spreading them to the hobby?
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