Common Techniques to Calibrate Thermocouples

A thermocouple is essentially a sensor that detects a temperature change. A thermocouple has two dissimilar wires welded at one end and free at the other. When these wires experience a temperature difference, a voltage is produced, creating a potential difference at the junction. This voltage at the junction is measured and correlated with the temperature. Thermocouples are made rugged and robust. They can withstand a wide range of temperatures. However, since the temperature measurement depends on the voltage, thermocouple calibration at regular intervals is necessary to ensure that the device can successfully recognize the voltage. The process of calibration involves comparing the thermocouple’s measurement accuracy against a known and standard reference.

Calibration of thermocouples requires specialized equipment. There are mainly three ways we can calibrate a thermocouple. Thermodynamic fixed-point calibration: The thermodynamic fixed point calibration is the most accurate way to calibrate a thermocouple. This method involves comparing the thermocouple’s temperature readings against the globally accepted, fixed temperature points of common elements and compounds where their physical state changes. For example, the freezing point of metal like Tin is 231.928 degrees Celsius as per the ITS-90 or the International Temperature Scale developed in 1990. Keeping the reference junction at 0-degrees-Celcius, the thermal EMF (Electromotive Force) from the thermocouple is measured during the fixed point transition where the metal materials transition from a solid to liquid. This EMF is then compared with standard measurement charts to determine the thermocouple’s measurement accuracy. Thermodynamic fixed-point calibration is the most accurate way to calibrate a thermometer. Stirred bath or furnace method: The option between stirred bath or furnace is used based on the temperature requirements. When the temperature is at the desired level, the thermocouple to be calibrated is used to measure temperature along with a known accurate thermocouple. If the thermocouple needs calibration, the two thermocouples will show different readings. This method is also conducted in a lab, but it is less accurate than the thermodynamic fixed-point calibration. Dry Block Calibrator: The Dry block calibrator method uses a dry-block machine. The thermocouple probes are inserted into the dry-block. The metal block is then cooled or heated to a specific temperature, and thermocouple readings are measured. If the thermocouple reads the same temperature set in the dry-block, it doesn’t require calibration. However, if you spot a variance, then calibration may be due.

Thermocouples are critical components of a system that closely measure a physical property. Such devices are expected to perform without compromise because an error or faulty reading can lead to something catastrophic. In industries, thermocouples may get subjected to varying temperatures all day long for months on end. Generally, annual calibration is advised for every thermocouple. However, for thermocouples that see constant use, calibration should be made at shorter intervals. With constant use, the efficacy of the thermocouple deteriorates over time. So it is important that calibration should be made to ensure that the thermocouple works smoothly. Also, some companies or applications require to have a calibration certificate due to regulations.

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.

There is no surefire way to determine the last service of a thermocouple from its physical appearance. However, industries keep a log of parts that have undergone calibration and the ones scheduled for future maintenance. 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.

With a temperature change, a standard thermocouple gives a reading that corresponding to the temperature change. However, how can we verify that what we see is reliable? To sort this issue, we adhere to international standards that are put in place to provide a reference for measurements. ISO and NIST are among the most widely renowned standards used to gauge the performance of thermocouples. At Omega Engineering, we provide calibration services that adhere to international standards. Omega uses various fluid baths, furnaces, and calibrators to perform temperature calibrations over a wide temperature range. Each heat source is outfitted with an independent NIST traceable standard. Contact us for your thermocouple calibration requirements and experience top of the line service and unparalleled work quality.