On Wed, 27 Apr 2005, vasile surducan wrote: > On 4/25/05, Peter wrote: >> >> 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). > > I think you've got the answer already by a previous poster. The > thermometer is not a perfect measuring tool. It "eat" the heat. Hmm, hungry thermometer not indicating its own temperature while violating several thermodynamic laws ;-) > However I suggest you a trick: use cheap termistors. If you need one > then buy 20. Aging them by keep all at 100C for one week or more into > an oven. Then choose one and calibrate it. Will be perfect. When > calibrate, use a thermostat and do not expect miliseconds in reaction > time if a huge thermometer is glued on the thermistor or if the > thermistor is huge. That's exactly what I am doing, except I did not have time to age this one. Its problem is the same even without the thermometer attached. It takes 5-10 minutes to get from 24 degrees to 22 degrees ambient. It is mounted far away from the board (~10 cm). I think that it's one of those hysteresis parts. I will try to bake it. thanks, Peter -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist