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PICList Thread
2001\11\01@061241 by moshe

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.

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2001\11\01@064026 by Vasile Surducan

If you need a capacitor meter there are several projects with 16F84
one is at
Is not the best analogic design for the converter but is very well
If you like to know various conversions tehniques using operational
amplifiers then search for the book: Operational amplifiers ( tehnique
? ) by Gerald Graeme, senior engineer Burr Brown. It's an old but valuable
book. U/f, i/f, u/period etc
Another capacitor meter is at picmicro webring, do a search.


On Thu, 1 Nov 2001, moshe wrote:

{Quote hidden}

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2001\11\01@092138 by Thomas McGahee

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:
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

Fr. Thomas McGahee

{Original Message removed}

2001\11\02@035920 by Vasile Surducan

Dry/wet temperature measurement methode have the disadvantage of
great errors.
The name of the tools using this principle is psychometer, or absorption
psychometer. And the error cames from response time of the wet temperature
sensor and the problem of keeping clean the lithyum-clorure sock on this
sensor. Also if you don't have a fan which continuously absorbe the
unknown air on the sensors your humidity meter will show a false


On Thu, 1 Nov 2001, Thomas McGahee wrote:

{Quote hidden}

> {Original Message removed}

2001\11\02@101032 by Herbert Graf

IMHO the worst con about this method is that it's useless below the freezing
point of the liquid being used (usually water). TTYL

> {Original Message removed}

2001\11\02@103109 by Lawrence Lile

I am using a dry bulb/wet bulb humidity controller on an environmental
chamber now.  I find it to be quite reliable, due to it's simplicity.  But I
have highly accurate thermocouple temperature measureing equipment
available, I have deionized water available to keep the little wet bulb sock
clean, and a permanent fan blowing on the wet bulb.  If these things are not
handy, then dry/wet bulb becomes a messy hassle.  Go with a commercial
sensor if you have any money available at all.

-- Lawrence

{Original Message removed}

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