EVDRIVER wrote:I don't mean the actual thermistor but a pot that allows the instrumentation to be adjust to match the correct temp. I just calibrated 30 thermostats with remote sensors that varied 5 degrees up and down.
But my point is that, if the circuitry is designed correctly, an adjustment pot is not only unnecessary but IT usually becomes the largest source of error. IMHO, calibration pots in this type of temperature measurement are a crutch used in poor circuit designs. My latest thermistor circuit design uses a PIC16F88, which only has a 10-bit ADC section, but I anxed over a 0.3 degree error and found the problem was that I didn't correctly understand some undocumented features of the ADC reference voltage input. After I figured out the problem, the circuit measured properly with high accuracy thermistors to better than 0.2F.
One hint I have is to put the reference resistor on the top (connected to Vref) of the voltage divider and the thermistor at the bottom (connected to ground). This puts the non-linear R-T curve for the thermistor in opposition with the non-linear curve for the voltage divider, which makes the temp vs. voltage relationship more linear, rather than less linear as it would be if the thermistor was at the top and the reference resistor was at the bottom.
Most of our users have problems understanding my design philosophy at first, but soon come to appreciate it. "There is no calibration adjustment or procedure. It either measures within specifications or its broken and must be repaired or replaced." We do implement calibration with other types of sensors, but not thermistors...