My thermistors cost US$0.15 and are (I think) 20% tol. I need to get quotes on 10% thermistors, that may be money well spent! Supply voltage is pretty constant. Thermistor is checked by using an AN512 style r-c time constant circuit in a PIC. It is only measured every ten second s or so to prevent self-heating. The rest of the time there is zero (10e-8 amps?) current through it. The tolerance on the capacitor in the RC , and power supply votlage s compensated by measuring a standard carbon film resistor of known value, then dividing by this value and multiplying by a constant. The variations between PICS are probably a major contributor to tolerances, especially variations in TMR0 trigger voltage. -----Original Message----- From: DAZLOGAN@aol.com To: lilel@toastmaster.com Date: Tuesday, May 04, 1999 9:34 AM Subject: Re: In System Calibration >Tight tolerance thermistors (to 0.3 degC accuracy) aren't much more expensive >than the poorer tolerance ones. See FARNELL catalogue under R-T matched >thermistors. > >Im not totally sure about your application but other contributing errors are >of course variations in supply voltage (use a cheap fixed voltage reference >or resistance bridge type arrangement), self heating of the thermistors (keep >current low through thermistors as recommended by manufacturer). > >How is the thermistor being read by the controller ? - ADC, frequency or what >? >The associated components of a thermistor measurement might contribute major >errors to the temperature reading. >For example, sticking a thermistor in as a timing component of a 555 is a >reasonable solution, but 555 chractersitics change from one device to another >and their tempco. is not good, yet alone the other timing components used >around it. > >Give me more information. > >Darren