Re: TASS Cameras and rain.

[petemanly@BIX.com]

>I've seen a description somewhere of a simple
>cloud sensor that uses the fact that during a clear night the sky is much
>colder than on a cloudy one.  Thermoelectric sensors are pointed towards
>the sky and the ground and the resulting differential signal gives an
>indication of the sky's temperature, and therefore its clearness.

     I have the design including mechanical drawings and interface circuits.
It's been written up several places and the one that Shawn saw may just have 
been one of my articles.

A rain sensor could be added to give additional warning - something like an
>array of exposed wires with alternating polarities that would conduct a
>current when rain drops completed the circuit between two wires.  Push a
>little current to keep it warmer than the dew point and I think it would be
>reasonably sensitive to even a light rain.

     The rain and dew sensors are a pair of identical printed circuit boards
with two patters looking like interleaved combs. You measure the resistance
between the two patterns. The rain sensor board has a heater circuit under it
(requires two tempersture monitoring thermistors) which keep the board slightly
warmer than air temperature so it doesn't get confused with dew (I assume we can
operate in light dew conditions by heating the window on the TV camera box).
The dew sensor is not heated. In fact, the back of it is insulated from IR
radiation from the ground. The only problem with these devices is maintenance.
They have to be cleaned regularly, especially after a heavy rain that can 
deposit gunk on the sensor. Since the traces on the printed circuit boards are
exposed, they must be made of gold and not some oxidizing metal like copper
or silver. -Pete-