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Subject: Re: Humidity sensor - peculiar behavior
Date: Sat, 4 Jan 2003 23:29:04 -0700
Organization: CompuServe Interactive Services
NNTP-Posting-Date: Sun, 5 Jan 2003 06:29:05 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
"Steve Turner" wrote in message
> James Meyer wrote:
> >On Sun, 05 Jan 2003 01:37:01 GMT, Steve Turner
> >>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
> > 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
> >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 .
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