Martin McCormick wrote:
> 	How practical would it be to use a Peltier module to read
> relative humidity by cooling a surface until either dew or frost
> formed, then comparing that value with ambient air temperature?
>
> 	There could be a number of ways to determine the
> formation of dew such as either looking for sudden electrical
> conductivity between two electrodes over a normally nonconductive
> path or even by photometry as in when a shiny surface fogged up.
>
> 	I guess you could even skip the Peltier part and stick an
> ice cube on one side of the shiny surface and wait until it
> fogged, as long as one could accurately read the surface
> temperature.
>
> 	The electronic humidity gauges I have read about appear
> to use substances whose resistances vary with relative humidity
> and have calibration issues.  I thought of this when we recently
> bought a room humidifier which was quite technically flashy with
> an electronic sensor for humidity which, when powered up, would
> never go below a reading of about 55% even when the house was as
> dry as a bone and the local weather conditions as given via the
> local airport readings showed the outside air as having about 20%
> relative humidity.  In our house, it should have been even lower,
> what with it being chilly outside.
>
> 	We took it back to the store where we had bought it and
> got a heavy dose of what I call "store talk" which is what
> merchants spew out when they don't believe you and want to not do
> their job.
>
> 	The replacement unit we did buy from the same store seems
> to be just fine but it uses an electromechanical humidistat.  You
> can hear a micro switch click as you turn the knob past the set
> point.
>
> 	If you wanted to get really nifty, you could put your
> humidity sensor across the room and use RF or IR to tell a relay
> to open or close, stopping or starting the fan on the humidifier.
>
> 	With the exception of calculating the relative humidity,
> control of the rest of the system should be a straight-forward
> PIC project or maybe a pair of PIC projects.
>
> Martin McCormick WB5AGZ  Stillwater, OK 
> Systems Engineer
> OSU Information Technology Department Network Operations Group
>   
Try a google search using "chilled mirror hygrometer"

This link seemed interesting

http://www.yesinc.com/products/data/cmh/index.html

This type of humidity sensor is as accurate as the temperature sensor on
the mirror, measuring dew(frost) point. Main problem is keeping the
mirror clean. Any contamination leads to errors due to (usually) premature
condensation. Also you would want the mirror to have a very low thermal
mass.

Jim
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