Olin,

Thanks for the reply to my first post on this list.

It was easy for me to get the curve using a curve fitting program I have
and to write some quick code to generate the resistances for every one
degF step.  I would like the accuracy to be within +/- 1 degF.

> The Ranco controller is too simple and can't perform all that I need
it
> to do.  I want to build a couple of temp controllers - one for the
wort
> making, i.e., the hot end, and one for the fermenting, i.e., the cold
> end.

>Do these need to be varied in temperature, or is each one held at a
fixed
>temperature?

The temps will be fixed at a value for a set interval of time, and then
varied to another temperature, i.e. a step function over an hour or so
on the wort controller and over several days or weeks on the fermenter
controller.

> I need help in figuring out how to read the resistances from the
>Ranco sensor and convert the readings into temperatures.

>You have a large resistance range, from 1.5Kohms to 300Kohms.  It won't
be
>easy translating this to an analog voltage and still maitain the
>resolution you want everywhere in the range.  You could have the PIC
>switch in two or three different pullup resistors to switch which part
of
>the range gets the best response into the A/D.  Of course your
>measurements won't be any more accurate than the resistors unless you
do
>calibration.  Especially for a one off hobby project it makes sense to
>throw in any old resistors, then calibrate one to known temperatures
with
>values saved in EEPROM.

What would the circuit look like that you are suggesting?

Thanks,
John


-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Olin Lathrop
Sent: Tuesday, November 04, 2003 5:51 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE:] interfacing a temperature sensor that will fit inside
a thermowell immersed in a liquid

John Murray wrote:
> The temp vs resistance curve is very nonlinear.  I was able to fit it
> very well with a 13th order Chebeshev polynomial,

Yikes!  How accurate do you need this controller to be?  A simple 3 or 4
order polynomial won't do?  What about a plain old fashioned lookup
table
with linear interpolation between the points?

> The Ranco controller is too simple and can't perform all that I need
it
> to do.  I want to build a couple of temp controllers - one for the
wort
> making, i.e., the hot end, and one for the fermenting, i.e., the cold
> end.

Do these need to be varied in temperature, or is each one held at a
fixed
temperature?

> I need help in figuring out how to read the resistances from the Ranco
> sensor and convert the readings into temperatures.

You have a large resistance range, from 1.5Kohms to 300Kohms.  It won't
be
easy translating this to an analog voltage and still maitain the
resolution you want everywhere in the range.  You could have the PIC
switch in two or three different pullup resistors to switch which part
of
the range gets the best response into the A/D.  Of course your
measurements won't be any more accurate than the resistors unless you do
calibration.  Especially for a one off hobby project it makes sense to
throw in any old resistors, then calibrate one to known temperatures
with
values saved in EEPROM.


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