Why calibration is important in an instrument?
Calibration is done on instruments to improve their accuracy and precision we can also determine the accuracy of an instrument during this process. Instrument calibration would make the work environment safe and it would also be useful to create valid data for future reference. Instrument calibration is important whenever measurement accuracy is important.
How often should we calibrate a thermocouple?
The frequency of calibration of a thermocouple is depended on the condition in which the thermocouple is used. If the thermocouple is used under extreme conditions, then it must be calibrated. If the thermocouple is exposed to high temperature or if it is subjected to thermal or mechanical shocks then it would affect its accuracy. So during these conditions, the thermocouple must be calibrated frequently.
How do we know if it’s time to calibrate or change the thermocouple?
Thermocouple calibration must be done
- Calibration must be done if its calibration cycle is due
- In case there is any variation in their performance
- Calibration should be done if thermocouples are used in extreme operating conditions
We should change a thermocouple when
- It exhibits an open circuit
- If there is any damage to the thermocouple
- If an ungrounded thermocouple exhibits low electrical isolation from the sheath or housing
- If the sheath is affected by corrosion or oxidation
How to calibrate a thermocouple?
There are different types of calibration for thermocouples and we can select any of these according to our required accuracy.
- Fixed point calibration
This is the most accurate type of thermocouple calibration and it will be done by using the thermodynamic fixed points. This calibration would be a slow process and it must be carried out by an experienced person.
At first, a controlled temperature must be established which should be stable and provides a constant temperature this could be an Ice bath. The ice bath should be uniform and must cover a large area where the thermocouple can be inserted into it. The sources of controlled temperature are called fixed points. The fixed points would be composed of a metal sample within a graphite crucible, with a graphite thermometer submerged in the metal sample. So while this metal sample reaches a freezing point then it would maintain a stable temperature. A reference junction temperature must also be established mostly zero degrees Celsius is used and a fluke 702 calibrator can be used to do the thermocouple measurements.
2. Liquid baths
This is the most common method of thermocouple calibration and it would use a stirred liquid bath or furnace according to the temperature requirements. In this method, the thermocouple will be placed together with a previously calibrated sensor in a liquid bath. Both of the sensors would be stabilized at a specified temperature and the thermocouple reading would be compared with the calibrated sensor. So when the liquid flows through the sensor probes then it would improve the heat transfer.
3. Dry block calibration
This type of calibration is not usually done in laboratories like the other two methods. In this type of calibration, the thermocouple would be inserted in a calibrator which consists of a metal block with holes that are a snug fit with the sensor. The metal block would be heated or cooled to the required temperature and after that, the thermocouple readings would be compared to the block temperature. Mostly the block temperature reading would be determined by an internal sensor. This type of calibration can be done if you require moderate accuracy for your device.
Calibration procedure of thermocouple ( J and K type)
- Connect the terminals of the thermocouple to the multimeter
- Make sure that the positive and negative terminal of the multimeter is properly connected to the thermocouple
- Set the multimeter to the 200mv range
- Take cold water into a kettle and boil it, and make sure it is boiling before you turn it off
- Place the thermocouple and the thermometer in the water
- After that take readings from the thermometer and the multimeter voltage, take as many readings as possible in the range of 80 to 20 degrees Celsius
- Pour some cold water into the kettle to speed up the change in the temperature after that wait for a few seconds for the temperature to settle and take the reading
- Repeat the last steps a few times so that the temperature of the water comes down to the room temperature or until the thermocouple voltage is zero
- Then create a table of the readings
Calibration of thermocouple
- The calibration device must be selected in a way that it must provide temperature variation over the full temperature sensor process range. We can use a fluke calibrator to do this
- Do the connections according to the above image
- During the variation of the heat source, temperature time shall be allowed for the temperature to stabilize before the reference temperature is taken for the calibration purpose
- Thermocouple calibration should be done by equipment that would be capable to produce the regulated temperature source
- The output of the thermocouple must be compared to the reference temperature probe indication, which would be the thermometer
- The analog input signal must be simulated over the full range of thermocouples under the test
- The generated analog input value will be displayed by the standard test equipment
- The input signal should be applied by checking the calibration sheet
- So while applying, the input signal comparison must be made between the signal input, which is displayed in the standard test equipment monitor, and the signal input which is displayed by the indicator which is under the test
- The error between the measured and the converted value should not be beyond the manufacturer’s stated lists.