On Tue, 8 Aug 1995, Clyde Smith-Stubbs wrote:

> Has anyone had any experience with sensing relative humidity? Suggestions
> on techniques and/or commercial sensors would be appreciated.

An excelent question.  I just recieved a couple samples from
   HY CAL Engineering
   9650 Telstar Ave
   El Monte CA 91731
   (818) 444-4000

They produce a 0.8 to 3.8 volt signal that is proportional to relative
humidity.  More precisely, my sample is 0.803v at 0% and 3.111v at 75.3%
relative humidity (traceable to NIST standards).  These things can only drive
5 microamps.  So I want an 8 bit or better 0 to 5 volt A to D with a serial
interface that will work with a 5 microamp signal.  I only plan on making a
couple of these, so I'll probably use a Basic Stamp rather than a naked
PIC (trade off unit cost versus design time).

My question as posed, suffers from their being too many correct answers.
I suspect that there are several dozen A to D's that meet my specs.  What
I really want to know, is what databook should I ask for?  I'll be doing
many other projects on a similar scale.  I'd only want to learn a few
serial protocols.  I like parts that can be bought from DigiKey quantity
2 (I learned; never buy just 1).

Back to the humidity sensors.  They come with and without attached
temperature sensors, in a variety of enclosures.  The IH-3602-C is packaged
with a 1000 Ohm platinum RTD in a 6 pin metal TO-5 can with an integral
hydrophobic filter.  The IH-3605 is a 3 pin device without the temperature
sensor.  I have no idea what they cost.  (Hint, send me free samples and I
may eventually put them on my web page and write about them.)

I've tried the IH-3602-C with a sensitive volt meter and it works, but I
haven't hooked one up to a PIC yet.  It's 2.86 volts here, which means the
humidity is 67% (it is very humid outside).

--
Paul Haas paulh@Hamjudo.com