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Date: Thu, 21 Nov 2002 20:22:08 -0500
From: Raymond Rogers
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Subject: Re: Temperature Measurement Stability
References: <3DD2EC20.2FB10158@maine.rr.com> <3DD65E25.100323B8@maine.rr.com> <email@example.com>
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Bill Sloman wrote:
> "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"
>> 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
> that expensive.
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