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From: firstname.lastname@example.org (Bill Sloman)
Subject: Re: Temperature Measurement Stability
Date: 17 Nov 2002 21:32:48 -0800
References: <3DD2EC20.2FB10158@maine.rr.com> <3DD65E25.100323B8@maine.rr.com>
NNTP-Posting-Date: 18 Nov 2002 05:32:49 GMT
"Raymond E. Rogers" wrote in message news:<3DD65E25.100323B8@maine.rr.com>...
> John Larkin wrote:
> > On Wed, 13 Nov 2002 19:19:44 -0500, "Raymond E. Rogers"
> > wrote:
> I understand the the common ones are only stable to .1-.05 degC for a
> year, and the better ones are more expensive. There is also the problem
> that I want (perhaps I will not be able to we have physical
> implementation problems) to have the system reference attached to a
> stage that moves around. The test strips are put on and taken off of a
> stage that has to move in and out of the instrument for accesability. I
> want this test stage to be nailed to the right temperature. Now the
> reaction of the Platinum RTD's to temperature is far less sensitive than
> the thermistors. Use of the thermistors would allow less concern for
> the intergrity of the various connections leading back to the measuring
What you want is a four-terminal measurement set-up. You do have to be
able to set upper limits to the impedances of the four connections,
but within those limits a properly designed four-terminal measuring
system is insensitive to the impedance of the leads.
Most instrumentation amplifier data sheets have circuit diagrams for
four-terminal set-ups, and several of the fancy 24-bit sigma-delta A/D
converters lend themselves to job by sampling the reference voltage
and the voltage to be converted on flying capacitors - we could have
done it with the Crystal Semiconductor CS5508 we used back in 1993.
If you are looking for good-quality connections for frequent
disconnection, ther e are quite lot of high specification conventional
connectors around if you have got the space. If you are short of space
in two dimensions, the "bed of nails" connectors used in ATE machines
might be interesting - Farnell stock a fair range, and they aren't all
Bill Sloman, Nijmegen (but in Melbourne, Australia, right now)
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