Humidity is probably best measured using a technique that measures
what is called the "relative humidity".

In this method you compare the temperature of a "dry" temperature
sensor and a "wet" temperature sensor.

The "dry" temperature sensor is simply exposed to the air, which
will have a humidity content that we wish to measure. The "wet"
temperature sensor has a piece of gauze around it, and the bottom
end of the gauze resides in a container that supplies water. The
water will wick up the gauze and will provide the equivalent of
100% humidity. The two sensors will report the exact same temperature
only at 100% relative humidity. The lower the relative humidity, the
greater the difference between the two readings.

There are tables available that will tell you what the relative
humidity is across various temperature ranges. A look up table
or a mathematical algorithm can be used to convert the two
temperature readings into a relative humidity reading.

************
If you want to use a sensor that changes capacitance with humidity,
then my capacitance meter project may be useful:
http://www.redrival.com/mcgahee/
You would not need all the switching ranges in your application.

Another useful method is to make a little RF oscillator whose
frequency is a function of the capacitance of the humidity
sensor. You then convert frequency or period into humidity
by means of a lookup table or mathematical algorithm. You can
often help to stabilize such a setup by including a small
fixed capacitance in parallel with the capacitance to be
measured.

Fr. Thomas McGahee

----- Original Message -----
From: moshe <moshe@GAD-UNIV.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, November 01, 2001 5:40 AM
Subject: [PIC]:HUMIDITY SENSOR.


> Hi everyone.
> trying to implement humidity sensor type:H1-PHILIPS, to my pic 16F84.
> I build a capacitor to frequency converter ( arround the 555
multivibrator),
> but the results are poor.
> does anyone have any good/new  idea about humidity measuring, application
> note etc.??
>
> thank you for advanced.
> moses.
>
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