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'Calibration of NTC thermistor PIC based data'
2000\02\15@182706 by Dean Biddle

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Hi!

Could those who work with temperature logging equipment please help.

I have just completed a PIC based temperature data logger using a NTC
thermistor as a sensor.  A PC interface program contains a regression
routine to correct for the non-linear response of the NTC thermistor. I am
aiming to calibrate the device for better than +/- 0.02 C.  The zero point
should be no problem.  Has anyone experience with variations from 0 C due to
differences in purified water quality  (e.g. standard deionised, distilled
or ultrapure)?

The device is only required to measure temperatures between 0 and 50 C.  At
100 C, there are too few TMR0 derived counts to use this temperature for
calibration and I think it risky to depend on the accuracy of a
mercury-in-glass thermometer.  Does anyone know of a high scale temperature
reference around 50 +/- 0.02 C?

2000\02\15@184833 by Andy Kunz

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>The device is only required to measure temperatures between 0 and 50 C.  At

Do you have a picture of your circuit that you are using to measure this?

Andy

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2000\02\15@191811 by Gennette Bruce

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If a temperature nearby is Ok then you could use the melting point of
napthalene (C10H8) which is 80.6¡C at STP.  This is a safe chemical used in
colleges and universitys world wide as an exercise in melting point
determination.  Note - fresh napthalene must be used for the accurracy you
seek.

 Look here - http://www.chemfinder.com/  for other similar, safe compounds
to use.

Bye.

> {Original Message removed}

2000\02\15@201253 by Robert A. LaBudde

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<x-flowed>At 09:21 AM 2/16/00 +1000, Dean wrote:
>I have just completed a PIC based temperature data logger using a NTC
>thermistor as a sensor.  A PC interface program contains a regression
>routine to correct for the non-linear response of the NTC thermistor. I am
>aiming to calibrate the device for better than +/- 0.02 C.  The zero point
>should be no problem.  Has anyone experience with variations from 0 C due to
>differences in purified water quality  (e.g. standard deionised, distilled
>or ultrapure)?

The freezing point of water varies little with reasonable quality. Even tap
water should be within 0.01 or 0.02 of 0.00 C at freezing.

Your problem is going to be getting a proper ice bath to the exact
temperature. Use shaved ice and just enough water to fill the ice crevices.

>The device is only required to measure temperatures between 0 and 50 C.  At
>100 C, there are too few TMR0 derived counts to use this temperature for
>calibration and I think it risky to depend on the accuracy of a
>mercury-in-glass thermometer.  Does anyone know of a high scale temperature
>reference around 50 +/- 0.02 C?

The biggest error you are going to have is self-heating effects in the
thermistor, which will be about 0.1 C. You can calibrate them out at a
single temperature, but you can't fix them over the whole range to this
accuracy.

Measuring temperature to +/- 0.02 C over a reasonable range of temperatures
is a research-grade metrology problem.

What possible reason could you have for requiring absolute accuracy to this
level?

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</x-flowed>

2000\02\15@224311 by Russell McMahon

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Just a thought -

PT100 platinum resistors may be a better starting point for high accurate
temperature measurements due to their linearity.



     Russell McMahon
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