Detecting Cloudy or Clear night

[zl3gp]

I have a simple detector. See:-

http://www.weather-watch.com/smf/index.php/topic,46978.0.html

[TokKiwi]

Dendrite - (I thought I posted a reply again on here yesterday but it is not here...) anyway, what i said was i think you are trying to be too clever with the thermos flask as far as expense goes and functionality - little gain and more awkward to get wires out etc. more difficult to fix outside to something.  I would, for my next offering, use a UV stabilised, plastic o-ring sealed junction box with a pass through gland for the wiring and a cut out on the top for the lens and fix it through the bottom with screws and perhaps add som silica crystals for moisture abatement.  Mine is develping 6ºC difference at nighttime and that is more than enough for cloud cover definition inside WD.  jm2c

TK

[niko]

I agree with TK, if it's radiation you are blocking then styrofoam, esp the foil covered stuff, will be a lot simpler to work with and equally effective. Also that big steel container will have a lot of thermal mass and could even make things worse if it cools below air temp. IMHO the IR filter is overkill too for this simple application. Also jm2c

[dendrite]

I agree with TK, if it's radiation you are blocking then styrofoam, esp the foil covered stuff, will be a lot simpler to work with and equally effective. Also that big steel container will have a lot of thermal mass and could even make things worse if it cools below air temp. IMHO the IR filter is overkill too for this simple application. Also jm2c

Well we'll see. Vacuum insulation has a much higher R-value than styrofoam. The biggest influence on the temp probe should mainly be diabatic processes from the IR absorption/emittance.

Definitely agree about the "too clever" post a bit up, but I'd like to see just how much of a temp differential I can get with this method. I like the desiccant idea to remove as much moisture as possible from the jar. In a perfect scenario I'd think we'd want as little water vapor as possible in the jar so that the black body could rapidly radiationally cool without reaching saturation (I think that was mentioned a bajillion pages ago but I can't remember).

I saw some people used heatsinks for the black body. I have some Krylon ultra flat black paint that has a very high emissivity. I'll probably just spray the probe and a big metal fender washer black with it. The old 3M super black velvet paint I believe had an emissivity near 1.00, but I'm not sure it is possible to find it anywhere anymore.

I got the 1000nm IR filter for the cover so I will lose some of the near infrared wavelengths re-emitted by clouds, but most of the wavelength spectra are above 1000nm anyways. Plus this will really cut down on the radiative flux from the sun making it into the jar during the daytime hours. Overkill? Maybe. But I'm a meteorology grad student and love this stuff so it isn't just a hobby to me. I'm hoping to be able to use this jar enough after sunrise and before sunset to pull off smooth 24hr cloud cover obs combined with my Bird Clear Sky Model solar ones.

[niko]

Well we'll see. Vacuum insulation has a much higher R-value than styrofoam. The biggest influence on the temp probe should mainly be diabatic processes from the IR absorption/emittance.

But R value is mostly thermal resistance/conduction. If a vacuum was a good blocker of radiation we would all have frozen to death eons ago 

Overkill? Maybe. But I'm a meteorology grad student and love this stuff so it isn't just a hobby to me.

Good for you. You should have fun with this project and explore all your ideas, take our inputs for what they are worth and with a pinch of NaCl

[dendrite]

But R value is mostly thermal resistance/conduction. If a vacuum was a good blocker of radiation we would all have frozen to death eons ago 

Good for you. You should have fun with this project and explore all your ideas, take our inputs for what they are worth and with a pinch of NaCl

Well I took everyone's considerations and the more I thought about it the more I decided not to splurge too much $$ into this. I have a little rinky dink jar out there with the bottom a mix of powdered graphite and Krylon #1602 flat black paint. I get a decent 6F differential with clear skies, but I've decided to go another route.

I had an old IR temp gun lying around and the more I played around with aiming it at the sky the more I want to go in this direction. The plan was to keep everything Davis related, but I think now is the time to dive into some of the hobby sensors and experiment with them.

Thanks for all of the comments though everyone.

[niko]

I posted in another thread recently that I think IR is the best way to go for this. Peltier will work well too. Parallax.com has an IR module with a simple interface to their processors.