Martin McCormick <martin@dc.cis.okstate.edu> 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.

Quite practical. In fact, a proposal for an article for Circuit Cellar
on that exact topic crossed my desk recently. It will probably appear
sometime this year.

The fogged mirror is the approach used. You can either try to servo
the cooler to maintain it at the dew point temperature, or cool it
down from ambient for each measurement. According to the author, the
former has a tendency to rapidly accumulate dust and grime on the
mirror (since it's constantly damp), requiring more frequent cleaning.

-- Dave Tweed
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist