Have you considered that the reference thermometer is storing heat ? The
greater the mass of an item the longer it takes to cool down. 

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
Peter
Sent: 25 April 2005 21:36
To: pic microcontroller discussion list
Subject: [EE]: thermistor drift


Hi all, I have a small problem. I have a new thermistor and a new thermostat
I am working on. The thermistor is paired with a standard thermometer. As I
cycle the thermistor (between about 20 and 50 deg C) I can see it does not
go back to the original value. Just how much is a new thermistor supposed to
drift (in percent). It's a low cost unit but not that low. I have worked
with thermistors before and this one seems way out of line. I see a 4%
difference after a cycle from 20 to 50C, coming back to 20C, and it takes
ages (~10 minutes ~) to return to the original value for the last 5 degrees
(way more than the reference thermometer glued to it). That is enormous
imho. I think that I have a faulty part, but I would like to know if someone
has seen such problems. 
My circuit dissipates less than 1mW in the thermistor so that cannot be the
reason.

tia,
Peter
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