Common Techniques to Calibrate Thermocouples
Thermocouples are usually calibrated in one of three ways depending on the accuracy needed and the conditions where the calibration takes place.
The most accurate calibration is done in a laboratory using thermodynamic fixed points. This can be a slow process and requires a knowledgeable technician to accomplish.
The next most common method is a comparison calibration using a stirred liquid bath or furnace depending on the temperature required. This method is also usually done in a lab, and in this case the thermocouple is placed in a liquid bath or furnace along with a previously calibrated sensor. Both items are stabilized at a specific temperature and the thermocouple reading is compared to the calibrated sensor.
The third method, and the one that lends itself to in-situ calibrations in non-laboratory locations, is the dry-block calibrator. The thermocouple is inserted into a calibrator that includes a metal block with holes that are a snug fit with the sensor. The metal block is heated or cooled to the required temperature and then the thermocouple reading is compared to the block temperature. The block temperature may be determined by an internal sensor, or a second calibrated sensor could be used.
How Often Should I Calibrate a Thermocouple?
The frequency of calibration really is up to the user. Many thermocouples are calibrated annually, but more frequent calibrations may be required depending on the conditions under which the thermocouple is used. Conditions such as extended high temperature exposure, severe environmental conditions or thermal or mechanical shock conditions can affect thermocouple accuracy.
How Do I Know if it's Time to Calibrate or Change a Thermocouple?
Thermocouples should be calibrated when:
-- Their calibration cycle is due.
-- Their performance comes into question.
-- When subjected to unexpected or unusual operating conditions.
Thermocouples should be changed when:
-- The thermocouple exhibits an open circuit.
-- When an ungrounded thermocouple exhibits low electrical isolation from the sheath or housing.
-- If there is any damage to a thermocouple due to mechanical forces.
-- The sheath is compromised due to corrosion or oxidation.
-- When calibration identifies deviation of the accuracy outside of the specifications.