thermistor II

[Martin Czech]

The problem with repeatability (text below)  disapeared.
One has to wait a really long time >12min until the NTC
settles. So the values below have ~2% error, because I
waited only 3 min. Tomorrow I'll see how a resonable lower leg
NTC can be constructed with constant Rs and this 10K resistor.

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I'm  just coming again out of the lab. I put a "10kohm" NTC disk thermistor into
the thermostream: Here's the data:

Temp Ohm
273  34180
293  12700
303  8006
313  5173
323  3421
333  2318
343  1596
353  1122
373  586.2

Temperature in K, I waited 3 min. for each step.  The time constant of
the heating process was aprox.  0.0275/s, this means that the process
reached 99.3% of the end temperature after the 3 min.

A good aproximation curve for the NTC is

k1 = 1.525873e+10 Ohms
k2 = 4.768422e-02 1/K
r0 = 2.984176e+02 Ohms
r(T)=k1*exp(-k2*T)+r0  (T in K!)

An aproximation with r0=0 doen't seem to fit very good.

I tryed to reverse to process, i.e. going back from 373 to 273 K.
After waiting the 3 min. the NTC was still far away from the first
measure point. After 6 min. still a considerable error remained. I
don't believe there's a kind of hysteresis, but what else could make
the deviation from the exponential time/temp law?    <---- this is no longer valid!

m.c.

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