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'[EE]: Humidity Sensor'
2001\09\22@192841 by Herbert Graf

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Some of you may remember my interest in making a humidity sensor,
unfortunately none of the options seemed appropriate, so I have settled on
getting a retail one. I have searched the only sensor I could find (for a
reasonable price) was a capacitive one which is unfortunately out of stock.
So I decided to see what products use. I bought a household digital
thermometer with humidity reading and opened it. However the humidity sensor
it uses seems to be a resistive sensor, something I didn't even know
existed! :) I looked up this type of sensor and it mentions that you can't
just drive it with DC, it will "polarize" the sensor, why this is bad I
don't konw (perhaps for electrolysis reasons?). Anyways, does anyone have
experience with this type of sensor? Since it will be outside I need to know
how sensitive to temperature changes it is, and if it is how to compensate
for it? Thanks for any info. TTYL

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2001\09\23@075039 by Walter Banks

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Herbert Graf wrote:

> I bought a household digital . . . the humidity sensor
> it uses seems to be a resistive sensor. I looked up this type of sensor and it
> mentions that you can't
> just drive it with DC, it will "polarize" the sensor, why this is bad I
> don't konw (perhaps for electrolysis reasons?).

Your guess is right. Years ago I worked on a greenhous control project. It is
amazing how fast a copper from a PC board will move across a open resistive
humidity sensor with DC component applied to sense it.  The same is true for
Wet / Dry  thermistors. Great care needs to taken to assure that the leads on
the
wet thermistor are not in contact with the wet wick. The capacitive sensors used
in
home humitifiers work very well and are inexpensive Capacitive sensors are
reliable but not paricularly accurate (3-5% error)  I haven't looked recently
where they can be purchased.

Walter Banks

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2001\09\23@102746 by Steven Bakaletz

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> Some of you may remember my interest in making a humidity sensor,
> unfortunately none of the options seemed appropriate, so I have settled on
> getting a retail one. I have searched the only sensor I could find (for a


You might want to take anouther  look at Peter Anderson's suggestion
http://www.ibutton.com.
Data sheet
catalog.sensing.honeywell.com/datasheet.asp?FAM=humiditymoisture&PN=H
IH%2D3610%2D004#SPECIFICATIONS

I did, I bought a couple unsoldered them from the board and hooked them up
to the analog inputs
of a PIC16F873.  Works great.

Having worked on humidification systems for automotive spray booths, 20
bucks for a sensor
that give you a direct voltage output is a pretty good deal.

(You can buy the chip alone from Newark for $40.00
www.newark.com/psearch/searchResults.jsp?action=0&First=0&QText=19c66
78)
go figure!!

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2001\09\23@111442 by Herbert Graf

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$20 US is just not worth it since I can buy these retails ones for $25 CND.
TTYL

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2001\09\23@132739 by Tsvetan Usunov

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>Some of you may remember my interest in making a humidity sensor,
>unfortunately none of the options seemed appropriate, so I have settled on
>getting a retail one. I have searched the only sensor I could find (for a
>reasonable price) was a capacitive one which is unfortunately out of
stock.

Did you check SMARTEC?
http://www.smartec.nl they have nice low cost sensors.

Best regards
Tsvetan
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2001\09\23@143729 by Steven Bakaletz

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> $20 US is just not worth it since I can buy these retails ones for $25 CND.
> TTYL


I agree $16 US is a good price.  I bought from ibutton because the only source
I could find was Newark at 40 US. It was worth the 20 bucks saved for the
3 sec it took to unsolder it.  Where are you getting them from?

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2001\09\23@150105 by Herbert Graf

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Actually no, thanks for the link, unfortunately they don't have a
distributer in Canada and the US distributer doesn't seem to sell them
direct. Any other sources? :) Thanks, TTYL

> {Original Message removed}

2001\09\23@150314 by Herbert Graf

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Well the sensor I have right now is part of an indoor household thermometer,
and that is part of the problem, I don't think it is a temperature
compensated reading. The sensor itself is a simply humidity sensitive
resistor, which is perfect for my application, but if I could find the
manufactuerer of it I would be better informed as to whether it is a good
choice.
The iButton sensor looks much more advanced, the question is if it's worth
it to me to go through the hassle of ordering one online, paying for the
thing (which would end up costing around $40 US after shipping, taxes and
import fees) and waiting for it to arrive, or just working with this sensor.
It's amazing how easy it is to get a temperature sensor but how difficult it
is for any other type of sensor! :( (an atmospheric pressure sensor is
something I will also want eventually, haven't found a cheap source for that
either...).

Anybody knows who makes these sensors and who might sell them? The "big
ones", ie. Digikey, only seem to sell very expensive systems, not the
sensors alone.

Thanks, TTYL

> {Original Message removed}

2001\09\24@035531 by Alan B. Pearce

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>(an atmospheric pressure sensor is something I will also
>want eventually, haven't found a cheap source for that
>either...).

I am after something similar to this as well. What I actually want is
something to measure lb/sq in or metric equivalent, rather than pressure in
atmospheres or mm mercury though.

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