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Thread: Transient response of a thermistor...
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face BY : David Cary email (remove spam text)



Dear "Jose S. Samonte Jr.",

Chris Cox may be on to something.

Chris Cox <STOPspamcvhcoxRemoveMEspam.....BELLSOUTH.NET> on 2001-04-12 11:01:25 AM wrote:
> armpit temp(a remarkably stable temperature
...
> The curve "shape"
...
> Does it mimic an RC time constant?

Yes, take a bunch of data look at the graph.

It always takes about 7 time constants to settle within 1 part in 1000 --
because ln(1000) is roughly 7. In particular, if, hypothetically, the human
armpit had a time constant of 1 minute, then when it is cooled by a 10 K
transient, it will take about 7 minutes to settle to within 0.01 K of the final
value.

Do you see the same response when you warm your probe up slightly above body
temperature before measuring ?

Since human body temperatures normally change throughout the day (Is that the
``circadian rhythm'' ?), I would much rather use a fixed reference. I know
several people who test their thermocouples by dropping them in icewater, which
is always very close to 0 C.
Maybe you could try sticking your probes to the outside of a metal can of
icewater. (Or would plastic more closely emulate a human body ?)

--
David Cary

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