> -----Original Message-----
> From: Spehro Pefhany [SMTP:speff@INTERLOG.COM]
> Sent: Monday, November 18, 2002 1:26 PM
> To:   PICLIST@mitvma.mit.edu
> Subject:      Re: [EE]:Pressure sensor confusion
>
> At 08:20 AM 11/18/02 +0000, you wrote:
>
>
> >   You can then select what type of function to fit, linear,
> >polynominal, exponential, log etc.  For a thermistor, a 3rd order or
> better
> >polynominal does the job very well.  Under the "Options" tab of the trend
> >line menu, check the "Display equation on chart".  You can then use this
> >equation to obtain any arbitrary value.
>
> You must be using the thermistor over a very narrow range, n'est pas?
> (as opposed to the full usable range of the sensor)
>
I suggested this method it as Lile had a collection of values he wanted to
interpolate.  I personaly use the Beta values provided by the manufacturer,
and yes, I do tend to use a fairly limited range.  Typicaly the operational
range would be no more than 20C to 40C, but my linearisation tables stretch
from around -10 to 95C.  Obviously the accuracy tends to suffer at the
extreme ends of the scale but that's not an issue in this instance.

> I've found even high order polynomials a very poor way of doing R->T
> conversion for thermistors. Breaking it into segments and using
> a number of lower-order polynomials works, however.
>
Certainly to use the full range of the component a single polynomial (even a
high order one) may have considerable errors.  However, if absolute accuracy
over a wide range is required, a thermistor is possibly not the best choice
to start with.

Regards

Mike

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