On Mon, 13 Mar 1995, James L. Johnson wrote:

> Hello,

>   I am going to try to implement the solution to the Steinhart-Hart
> equation using a PIC.  This equation gives the relationship between
> the resistance of a negative-temperature-coefficient thermistor and
> its temperature.  It is an empirically determined equation, so is
> only an approximation to the relationship between resistance and
> temperature.  But, it is quite accurate over a large range as long
> as the coefficients are accurate.  The thermistor manufacturer gives
> you the coefficients so all you have to do is plug in the numbers.

>   This is a no-brainer in any high level language and we have it running
> in BASIC here in our lab.  However, it's probably rocket science to get
> it to work in a PIC.  At least it seems like it to someone who has no
> experience in programming microcontrollers (me).

For my hottub (http://hamjudo.com/cgi-bin/hottub) I use a 6811 (sorry, wrong
list) to read the thermistors.  I actually do the conversion to temperature
on a workstation, but I could do it on the 6811 if I wanted to.

I use linear interpolation into a lookup table.  I plotted the "correct"
values, the "approximate" values and the difference.  When I got the
difference small enough in the temperature range that I cared about, I said
enough, and went on to other things.

I'm using an 8 bit A to D convertor fed by a simple voltage divider.
If you're using a 16 bit A to D, you'll probably still want to use
interpolation, perhaps not linear interpolation.

---
Paul Haas   paulh@hamjudo.com     Web site: http://hamjudo.com/index.html
Home: (313) 487-8739      Office: (313) 487-4357      Fax: (313) 487-4371
      Finger or email my hottub at hottub@hamjudo.com, seen on TV