So I built a 24hr FARS for my Davis Pro2 Temp/Hum Sensor.
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Anyway, I already had the Radiation shield parts including the welded core, Davis fan and insulated tube that I bought A La Cart. I just never purchased the solar panel, NiCad batteries and PCB board that the whole package comes with. I always planned to make my own because the OEM version just didn't seem reliable. Well a few years later and I finally completed my project.
It's a Sched-40 1-1/2" x 10' Pipe set 3.5' into the ground with Quickcrete. This puts the sensor 6 ft over grass. I got a Universal Shelter housing a Sunsaver 6 Charge controller, with some straight thru terminal connectors a Fused terminal connector contacts and a Midnight Solar DC Breaker all DIN rail mounted with two 12v 7a/h AGM batteries and a 30W Monocrystaline PV panel mounted on top of the pole using a re-purposed sat dish mount, some super-strut and PV clamps on spring nuts. The black line is a braided steel anchor cable supporting the SHT31 w/25ft cable going to the ISS/rain bucket.
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The Fan is a 92mm 12vDC Computer Case Fan 'Panflo' 2.5w moving about 30CFM. I put some screen material over the fan to keep the lizards out. Duct seal compound in the 4 gaps left by a square fan in a round hole. Waterproof connectors (the kind like current automobile headlamps use).
I'm getting 5.5 hour direct sun on the panel. Batteries are going into float mode with 2.5 hrs of sun left after all night on battery power. So I should have adequate reserve for less optimal sun days. It could operate fine on one 7 ah battery but my calculations showed 2 of them would give me 2 days reserve power just in case.
ALTERNATE : The other option that I tried for a few days was adding a PAM1 relay with coil power connected to the PV terminals and the N.O. lead switching the + fan conductor. This made it daytime fan that switched on about 7am and turned off about 8:30 PM. I may go back to this as this will double the life of the fan. Night time aspiration is not that crucial after all.
As you can see, I did this instead of cutting my grass