28 Oct 2004 A
better cloud sensor may be a common IR Thermometer. The ZyTemp TN105 from Metric Instruments
($20) has the following readings:
overcast & rain
clear blue sky
clouds & clear sky
on where pointed
This sensor works around 10 nm wavelength and so detects
heat. If you stand up from a chair and point the sensor at
the chair it reads 80 deg F but a table top reads 72 deg F.
The box says the range is -33 to +220 C (-27 to +428 F) so the
above sky temperatures are within it's range. I believe that
these units are individually trimmed at the factory and that the
on board micro controller compensate for manufacturing variations
allowing this non cooled sensor to perform much better than raw
The TN9 IR thermometer could be combined with a small PIC microcontroller and packaged in a weatherproof way and could send the sky temperature back to your PC using RS-232. A plot of sky temp vs. time would give a good picture of the cloud cover.
The view angle of the TN105 & TN9 IR sensors is what they
call 1 to 1, meaning that at 1 foot from the sensor a 1 foot
diameter circle is being sensed. This is much too wide to
see clouds moving. By aiming the sensor down onto a
parabolic mirror that angle could be reduced and as clouds drifted
by the variation in temperature would give a feel for what was
going on. The trick is finding a parabolic mirror good ar
near IR (a slightly lesser requirement than for optical use, but
not by much) and has a useable f/D ratio.
Edmund Optics does
Mirrors that I think would work but they cost hundreds of
Mount versions of the TN105 that are already interfaced for
RS-232 (ZF-5283) are in development.
SparkFun IR Thermometer Evaluation Board - MLX90614
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