In order to maintain consistent quality of manufactured products, it is necessary to perform periodic calibrations on process sensors and instruments. There are several philosophies for calibration of measurement and control circuits. The basis of the chosen method should always be to include the temperature sensor.
It does not make sense to only calibrate and adjust the electronic part of the loop. A rule of thumb says that only 10% of the total error is in the electronics, the other 90% is due to the sensing element. So it is essential that the temperature sensor is tested, meaning physically exposed to the desired temperature. A dry-block calibrator is an easy method to create the 'process' temperature.
output from the sensor can be taken from anywhere in the loop. And
the rest of the loop may be tested electronically.