Author Topic: Homebrew Pyranometer  (Read 2933 times)

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Offline rossw

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Homebrew Pyranometer
« on: January 08, 2016, 03:11:35 am »
It's been a terrible long time since I posted anything, and I'm sorry! Too busy and lots of stuff not actually RE-related.

So, background - several years ago I added a Pyranometer to my weather info page. It was a reasonably expensive professional device made by Apogee Instruments, and it gave great results. However the first one failed after around a year and was quickly replaced (kudos to Apogee, who admitted a manufacturing problem at the time). The new one failed before christmas after 2 years in service! So I've decided that I'll build my own and use the Apogee to calibrate and check it. I've got enough raw data to be sure it's working right, and a calculated model that I've compared to the observed data each day for a couple of years, so pretty high confidence there.

So I thought about different ways to make it, and researched what others have done, and although nobody has used exactly this construction technique, it seems pretty logical to me. A 20mm plastic conduit adaptor was about the right size, height, had a threaded end etc. Reasonably UV stable, readily available, inexpensive.


In order for the lens/diffuser to mate later, I faced it to make a nice, flat, square surface to work with


To make the lens, PTFE (teflon) is universally accepted as an ideal material because of its bandpass characteristics, optical properties, UV stability etc. So I got a length of 25mm rod. Turned the required angles, faced the top, machined a shoulder to mate with the conduit adapter above.


This was just before I cut the lens off the stock


Lens is a firm press-fit into the conduit end


I used the CNC to make a tapered plug from 6mm corion, completed with holes for the BPW21 photodiode to fit through, to hold the diode in exactly the right place forever. The conduit adapter is tapered, so the plug press-fits just this far in.


When completed, it looks quite spiffy. (It needs a clean here from my grubby fingers before being installed)


While I was at it, it seemed reasonable to do one specifically for UV - the sensor I chose is sensitive from 240nm to 370nm, which is pretty useful. Also very cheap! Another 6mm plug was made on the CNC.


I included holes to fit cables so the board could still sit flat.


It too fits inside a 20mm conduit adapter, and sits down just far enough for the lens to drop in.


When completed, its lens gets sealed on too with neutral-cure silicon. Looks identical to the pyranometer!


Next is to make the transimpedance amplifier, which I've got figured out but haven't actually made yet.
It is important because the photodiode needs to operate into a short circuit to produce ideal linear output proportional to the power received. The transimpedance amplifier ensures 0V is across the cell, so is a near perfect current-to-voltage device.

Offline bj

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Re: Homebrew Pyranometer
« Reply #1 on: January 08, 2016, 05:39:40 am »
   Nice work Ross.
   And yes, I had to look Pyranometer  up.  :)
"Even a blind squirrel will find an acorn once in a while"
bj

Offline oztules

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Re: Homebrew Pyranometer
« Reply #2 on: January 08, 2016, 06:11:54 am »
Have not the slightest idea what I would do with one.... but if I wanted one I would copy yours..

Very nicely built.


................oztules

Flinders Island...... Australia

Offline MadScientist267

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Re: Homebrew Pyranometer
« Reply #3 on: January 08, 2016, 04:38:32 pm »
Came out pretty good Ross. Be cool to see it pit up against the big boys...

Steve
Wanted: Schrödinger's cat, dead and alive.