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Date: Sun, 17 Nov 2002 10:44:24 -0500
From: "Raymond E. Rogers"
X-Mailer: Mozilla 4.76 [en] (Win95; U)
Subject: Re: Temperature Measurement Stability
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> Ray... just a few afterthoughts that may be useful. The NIST article
> is quite interesting and prompted me to dig out my notes from when I
> was selecting equipment for our system. I take back what I said about
> thermistors. From the notes (unfortunately, I no longer have the
> e-mails) someone told me NASA achieve something like 0.02ºC
> uncertainty calibrating every 5 years using YSI thermisters. Someone
> else made a similar statement about drift/stability tests on
> thermisters by Oak Ridge National Laboratories showing less than
> 0.01ºC in some types of thermisters over ten years. The NASA and ORNL
> papers may be available somewhere.
> It's a good time for me to review all this because I am currently
> designing a field analyser where I need to achieve ±0.1ºC at 60ºC at
> minimum cost. I had dismissed thermistors but am having another look
> at them after reading the NIST paper..
Posting any information would be appreciated. In particular ways to get
the NASA research.
We bought a cheap $18,000 IR camera to verify our solutions; the
alternatives are really expensive. Since the camera has a noise level
well above what I want to measure I have several "J" programs to aid me
in peeking into the noise (along with the thermometer and black body).
I am into tertiary inferences and that requires several different paths
> In the FAQ section of YSI's web site they state "with all YSI parts
> exhibiting less than 0.01°C drift at room temperature exposure for
> many years."
All parts? I will review thier site again.
> Sensor Scientific make interchangeable chip thermisters to ±0.2ºC for
> about $3 each. Is it possible to use something like that with a bias
> resistor in the probe assembly to achieve more accurate
> interchangability over a small specific temperature range?
It is really tempting. I could use this if there are no other sources
of error; assembly positioning, room temperature influences, etc.....
I have worked to eliminate these other error sources, but would have to
review them very carefully. $3.00 is tempting, the cost of calibrating
each assembly would be very high in comparison. Perhaps it could
simplify the calibration process, if they are stable. The real reaction
temperature control is nested inside of two other thermal loops to
stabalize it; thermal systems have an infinite number possible error
entry points and we have finite control systems. The other loops
basically are cruder, to reject room temperature variations, and to
prevent heat leakage paths around the border from creating temperature
gradients. Every wandering calorie is a temperature change. Our
reaction area is 3"X10". There are a lot of ways to control this but
control loops are fairly cheap, until one really gets down to accuracy;
besides the accountants and mechanical engineers have taken to wearing
garlic and making the sign of the cross when I start discussing this.
Sorry to carry on; I used to count electrons, now I count calories:)
Have I been promoted or demoted?
> "Raymond E. Rogers" wrote:
> > >The NIST paper is at:
> > http://www.cstl.nist.g
> > v/div836/836.05/papers/Wise92glass-disc_thermistors.pdf
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