Industrial thermometers come in a variety of forms and types, calibrating them can be quite complicated.
To calibrate industrial sensors the laboratory must compare them to standard thermometers whose characteristics have been defined and are traceable to National Standards. Such thermometers that are traceable to national standard are known as ‘Master Sensors’.
Master sensor is used as a reference sensor in comparison type calibration process.
Master sensor is put in the highly stable temperature zone along with sensor under calibration, their reading is compared and error is calculated.
Master should have high accuracy, and repeatability and should be recalibrated after certain period of time (generally time period suggested by calibration Laboratories).