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From: John Popelish
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X-Mailer: Mozilla 4.7 [en] (Win98; U)
Subject: Re: Humidity sensor - peculiar behavior
Date: Mon, 06 Jan 2003 05:55:35 GMT
NNTP-Posting-Date: Mon, 06 Jan 2003 00:55:35 EST
> "Steve Turner" wrote in message
> > James Meyer wrote:
> > >On Sun, 05 Jan 2003 01:37:01 GMT, Steve Turner
> > >wroth:
> > >
> > >>I've looked at both the sensor output and the follower output with a
> > >>scope. The sensor output shows some noise, perhaps 10 mV p/p, plus
> > >>very sharp spikes of about 400 mV p/p at about 5.9 KHz. The follower
> > >>output shows a 5.9 KHz fundamental signal as well, attenuated (maybe
> > >>50 mV p/p), full of harmonics with a decaying envelope. I don't know
> > >>if any of this is relevant to anything. It is definitely not square
> > >>wave and looks nothing like oscillating op amps that I've seen in the
> > >>past.
> > >
> > > To me, that indicates that the sensor element is probably being driven
> > >with AC and its output is being converted to DC.
> > >
> > > Is there any information, datasheet?, for the whole sensor? I looked
> > >briefly at the Honeywell web site and didn't see one.
> > You're referring to an equivalent schematic for the sensor itself?
> > Not that I've been able to find. As I mentioned elsewhere, the
> > Honeywell web site and their device documentation leave something to
> > be desired. The actual sensing is capacitive, but the remainder of
> > the sensor operation is completely unknown to me.
> > > One other observation... Have you tested more than one of the sensors?
> > >A sample of one isn't very scientific especially when the sample doesn't
> do what
> > >you think it should.
> > I agree entirely. Problem is, the sensors are close to $40 apiece.
> > On a hobbyist budget that's a bit stiff. I've not had more than a
> > single specimen available for testing.
> > What seems suggestive to me is that the initial sensor reading is
> > right on the money at three different humidities. You just can't
> > leave the sensor powered up. :) But you're right, I have no way to
> > prove that this isn't coincidental.
> > Steve Turner
> > Real address contains worldnet instead of spamnet
> Steve do you have any kind of temperature compensation the specifications
> sheet shows up to .22%/degC for the variations caused by temprature. So
> possibly the self heating is such that the probe inclosed in your jars is
> miss reading .
Or the voltage regulator or opamp on the board is heating it.
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