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Old 10-09-2013, 06:51 AM   #1
Jalopy
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DIY PAR Meter


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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Last edited by Jalopy; 10-12-2013 at 07:24 AM.. Reason: Updated diagram.
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Old 10-12-2013, 04:36 AM   #2
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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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Old 10-12-2013, 07:48 AM   #3
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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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Old 10-12-2013, 04:58 PM   #4
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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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Old 10-12-2013, 08:49 PM   #5
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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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Old 10-13-2013, 06:48 AM   #6
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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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Old 10-13-2013, 04:49 PM   #7
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Quote:
Originally Posted by Jalopy View Post
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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Old 10-14-2013, 04:33 AM   #8
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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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Old 10-19-2013, 10:07 PM   #9
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Any progress with this? Or any new design changes?
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Old 10-20-2013, 05:31 AM   #10
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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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Old 10-20-2013, 04:12 PM   #11
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Quote:
Originally Posted by Jalopy View Post
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!
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Old 02-26-2014, 06:23 PM   #12
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Have you made any progress with this?
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Old 02-27-2014, 06:51 AM   #13
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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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Old 03-04-2014, 07:38 PM   #14
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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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Old 03-05-2014, 03:56 AM   #15
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Looks awesome, are you going to print it?
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