Calibration of a thermocouple

Whatever their field of application, temperature measurement devices need to be calibrated in order to offer good precision. There Thermocouple probe is useful for raising the temperature on large measurement ranges in industry, food or transport. The calibration of the thermocouple measurement devices is carried out by following different stages in order to ensure the concordance between the emitted signal and the measured temperature. Do you want to know when and how to calibrate thermocouples devices you use? Here are the principles of measurement as well as the techniques allowing to test the calibers of any type of thermocouple probe.

What is the calibration of a thermocouple?

To tackle the question of calibration, it is necessary to recall THE Thermocouple operation. This type of temperature sensor contains of two conductive metals wires of dissimilar natures. One of their ends is connected, generally thanks to a weld. The name of this junction is hot. When the hot spot is exposed to a temperature variation, A tension is created within the wires. The other end of the wires, the cold point, is linked to a measurement instrument that will allow Calculate the temperature from this voltage (or electro-motor force). Thermocouples are very useful in industry thanks to their reasonable price and the wide range of temperature they cover.

The calibration of a measurement device consists in ensuring that it gives precise temperature indications and possibly correct the errors noted. Many factors related to the environment in which the sensor is caused wear. Oxidizing or pressurized environments, for example, can decalibrate the thermocouple. The verification of the probe also involves a Measuring device test. This is a voltmeter which transcribes the current generated by the exposure of the probe to a temperature variation either in volts or directly in degrees Celsius. Finally, it is sometimes possible that the connection cables alter the measurement values. TEster all the constituent elements of the probe Thermocouple as well as the quality of their connection is a recommendation.

When to calibrate a thermocouple?

Thermocouples are calibrated The first time by the manufacturer. The calibration of the instruments is then necessary:

When the temperature probe is assigned to a new environment to be measured;
When change of assignment measurement probe;
In case of suspicion of inaccurate results;
Regularly, at a frequency which depends on the wear induced by its environment.

How to calibrate a thermocouple sensor?

The calibration procedure for a thermocouple is done By exposing the hot junction of the sensor to a known temperature in order to carry out control measurements. If the temperatures noted are not correct, it will be necessary to adjust the temperature calculation by adding a corrective coefficient.

To calibrate the thermocouple, you must first Make sure the probe is not damaged or soiled. Degradations can have effects on the precision of the probe. It is also necessary to have the necessary equipment to carry out the calibration. For this reason, calibration can be carried out internally as being entrusted to a provider specialized. Insofar as it is necessary to calibrate the probes regularly, it may be financially advantageous to equip yourself with calibration equipment.

Two main techniques Allow a test temperature to calibrate a thermocouple probe. Comparison calibration is a widespread technique in industry and calibration in fixed points makes it possible to measure with accuracy.

Comparison calibration

This technique is widely used in the industrial environment. Comparison calibration consists in placing the calibration probe and another type of probe which serves as a stallion in an environment where the temperature is stable and homogeneous. The type of probe chosen as a standard must offer greater precision than thermocouples. The heated medium in question is generally a dry well, a bath, or a tube oven.

Fixed points calibration

The technique in fixed points consists in exposing the hot welding of thermocouple to an environment whose temperature is known with certainty. The principle used is that of the triple point according to the international temperature scale of 1990. The triple point of water occurs when the water is equal in three forms: liquid, sparkling and solid. The temperature is then 0.01 ° C very precisely. The process allowing to reach this specific state is complex and therefore generally carried out by a specialized laboratory. Once the triple point of water has been reached, The water temperature remains stable for a long time. This stability makes it a technique to achieve a very precise measurement.

Interpretation of calibration data

The ASTM (American Society for Testing and Materials) has issued recommendations concerning the calibration of thermocouples in order to standardize the control measure. During calibration tests, it is necessary Calculate the errors from the output voltage of thermocouples thanks to standard tables. Calibration is a long process since by following these guidelines, the calibration process must be repeated at different temperatures until Test the probe on its entire measurement range.