piclist 1999\10\25\211758a >
Thread: Humidity sensors
www.piclist.com/techref/io/sensors.htm?key=sensor
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face BY : Chris Fanning email (remove spam text)



> > Hi, all - need a cheap, cheap, cheap "fog detector" sensor, to tell when
> > a piece of glass is getting fogged up.  I've thought of using a
> > LED/photodiode pair, which has possibly problems;  Could use an
> > enclosed/open thermistor pair, I suppose.  Anyone have ideas?
>
> Use the optical sensor, but set it to pick up the diffuse reflection.
> Most are designed to maximize the specular reflection, which will be exactly
> what you don't want.
>
> I designed a sensor like this to pick up paper behind a glass window. The
> omron sensors had a contrast ratio (target / no target) of 3/1 at best. The
> one I designed was 7600+ /1

That sounds good, but what about dirt?  Is that an issue here?  If it
is, the only reliable way I can think of doing this is to have two
temperature sensors and a hygrometer:

  1 temperature sensor monitoring ambient temperature
  1 hygrometer measuring humidity
  1 temperature sensor monitoring the glass surface temperature

If the temperature of the glass gets close to the dew point, then
kick the heater/fan on.

Not $1 but you could set it so the glass would never fog...

Chris

<199910260116.VAA55825@jingoro.prevmed.sunysb.edu> 7bit

In reply to: <00af01bf1f32$c0c62460$0200a8c0@xemu> from Dave VanHorn at "Oct 25, 1999 4:49: 2 pm"
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