What is Calibration
Calibration is the process of determining or adjusting the deviation of a instrument (which is under calibration) with reference to a said standard. This is done by comparing the device or the output of an instrument to a standard having known characteristics.
For Thermocouple calibration at Comparison method we have required
- Stable temperature source
- Calibrated master sensor
- Calibrated meter
The device with the known or assigned correctness is called the standard Sensor and the second device is the unit under Calibration, test instrument, or any of several other names for the device being calibrated
- The Thermocouple under calibration is physically checked for tts hot and cold junction to be intact.
- Each Test Thermocouple is attached with an identification sticker using a tag.
- The Standard Thermocouple and the test thermocouple is inserted in the holes of equalizing block inside the HT furnace in such a way that the hot junctions of all TC's on the same place in the block.
- Set the HT Furnace to desired temperature and wait until the temperature is not stable.
- Observation on thermo-emf are recorded at each set temperature for both standard as well as Test Thermocouple when the temperature in the furnace is stable for both ( Standard and Test Thermocouple)
- Minimum 5 measurements are recorded for each calibration points.
- Similar procedure is followed for all other calibration temperature points say, as 400, 700 & 1000 Deg C.
- When the calibration is completed the thermocouples are not removed immediately, but left to cool down to ambient slowly to avoid the sudden change is temperature.
- The readings are recorded systematically for all thermocouples with readings of reference junctions if placed at ambient temperature.
- The environment data for room temperature and relative humidity (RH) are also measured and recorded.
- At each mean value of Thermo-emf measured for standard thermocouple in the furnace, the actual
temperature is calculated from its calibration certificate.
- The mean valued of corresponding emf of TC under calibration also calculated from the reference table if the reference temperature is maintained at 0 Deg C
- In case the reference junction is maintained at ambient, the emf at each temperature is calculated by adding the emf corresponding to measured reference junction to the emf of measured temperature.
- The Standard and Test Thermocouple are handled carefully by a well trained Engineer
- The Hot Junction of all thermocouples must be same in HT furnace.
- Readings are always taken at stable condition of the furnace temperature.
- The HT furnace while operating at high temperature above 400 Deg C, should not be suddenly switched off. This may reduce the life of furnace.
Why we need for thermocouple calibration
Thermocouples are calibrated by essentially comparing the device for calibration to another device with a proven accuracy. During this process of comparison, we can then identify our sensor device's error of margin.With this information, we can then properly measure temperature using our thermocouple. Calibration can be done in a laboratory under strict and controlled conditions or we can also carry out field calibration which is much quicker. To maintain the accuracy and quality of our temperature readings, make sure that our thermocouples are calibrated regularly. Annual calibration is recommended.