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From: Steve Turner
Subject: Re: Humidity sensor - peculiar behavior
References: <firstname.lastname@example.org> <3E170A5E.email@example.com> <firstname.lastname@example.org>
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Date: Sun, 05 Jan 2003 01:37:02 GMT
NNTP-Posting-Date: Sun, 05 Jan 2003 01:37:02 GMT
Organization: AT&T Worldnet
James Meyer wrote:
>>are you using AC or DC excitation?
> It looks like DC to me. At least DC applied to the device. It appears
>to be some sort of integrated sensor with drive, primary sensor, and output
>conditioning built into one "chip".
Excitation is indeed DC, at 5.0 v.
> The basic mechanism of the humidity sensor looks to me to be something
>that could suffer from "polarization" if driven from a DC source. I looked at
>the Honeywell web site but couldn't see any datasheets for that particular
The Honeywell site is poorly organized, IMO. I had the devil of a time
finding the datasheet that I have, and it's not very good. It's at:
I've heard of polarization problems with humidity sensors. Usually
these come with a firm admonition to use AC excitation only. There is
no such caution in the Honeywell datasheet, and in fact the supply
voltage is specified at 4.0-5.8 VDC.
> Given the measured results, I would be tempted to use the device in a
>sampled mode with no power applied for most of the time and only powering up and
>reading the output at intervals. Turn it on just long enough for the output to
>stabilize at a sample rate as long as I could tolerate and store the reading
>between samples. I can't imagine a humidity sensing application that would need
>more than a sample per minute.
This occurred to me, and it's presently the only approach that would
give acceptable results. It's doable, although it violates the KISS
principle. What bothers me more is not knowing WHY this is happening.
Thanks for your input.
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