Frequently RTD’s are specified with close tolerances when it would be more appropriate,
and cost effective, to use a Class B tolerance calibrated.
This approach is only relevant where the input from the RTD is going into a PLC, or similar
system where the characteristics of each thermometer can be entered individually, and so
characterized perfectly with each sensor.
Close tolerance thermometers ensure that each is closely interchangeable, but does not
ensure the absolute “accuracy” away from 0°C. this is because of the importance of a tolerance
on the detectors performance at higher temperatures.
Equally, a situation that should be avoided if possible, is to specify close tolerance RTD’s
that are also going to be calibrated. The difficulty is that the international standards such as DIN
only define the tolerances of the bare detector, and not the assembled RTD. No account is made
for stem conduction errors, variations in RTD construction etc. In addition, close tolerance at
elevated temperatures can only be achieved with virtually perfect a detectors, which would be