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
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