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From: "Al Hephy"
Subject: Re: Humidity sensor - peculiar behavior
Date: Mon, 6 Jan 2003 09:06:59 -0500
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Steve Turner wrote in message =
> Many thanks to those who have contributed ideas for solutions to this
> I believe I have eliminated the possibility of a faulty sensor. I
> have removed the sensor from the circuit board and suspended it naked
> in the 75% constant humidity chamber. The reading is spot on with
> only negligible drift.
> Oddly, though, I cannot reproduce the drift problem by plugging the
> end of the sensor cable into same circuit as exists on the printed
> circuit board. This configuration also gives a stable reading! The
> only difference here is that the circuit is on a solderless breadboard
> outside of the chamber instead of on a printed circuit board inside
> the chamber.
> I was able to recreate the original problem by soldering the sensor
> back into the circuit board. Inserting a 100 ohm series resistor
> between the sensor output and the op amp input, and a 0.1 uF cap
> between this input and ground, may have slowed down the output voltage
> decay, but the output still ends up far out of tolerance from where
> it's supposed to be.
> I'm stumped. I guess I'll power the sensor only when actively taking
> a reading.
> Again, thanks to all who replied. I appreciate the help.
> Steve Turner
> Real address contains worldnet instead of spamnet
The experiments you describe above would imply that your PCB is =
sensitive to humidity...
I'd look very carefully at the input to the op amp.
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