Whatever their field of application, temperature measuring devices need to be calibrated in order to provide good accuracy. There thermocouple probe is useful for measuring temperature over wide measurement ranges in industry, the food industry and even transport. The calibration of thermocouple measuring devices is carried out by following different steps to ensure agreement between the emitted signal and the measured temperature. Do you want to know when and how to calibrate the thermocouple devices you use? Here are the measurement principles as well as the techniques for testing the calibers of any type of thermocouple probe.
What is the purpose of calibrating a thermocouple?
To address the question of calibration, it is necessary to recall the thermocouple operation. This type of temperature sensor contains two conductive metal wires of dissimilar natures. One of their ends is connected, generally by welding. The name of this junction is hot spot. When the hot spot is exposed to a temperature variation, tension is created within the wires. The other end of the wires, the cold point, is connected to a measuring instrument which will make it possible to calculate the temperature from this voltage (or electromotive force). Thermocouples are very useful in industry thanks to their reasonable price and the wide temperature range they cover.
Calibrating a measuring device consists of ensuring that it gives precise temperature indications and possibly correcting any errors noted. Many environmental factors in which the sensor is located cause wear. Oxidizing or pressurized environments, for example, can decalibrate the thermocouple. Verification of the probe also involves a testing the measuring device. This is a voltmeter which transcribes the current generated by 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 first time by the manufacturer. Calibration of the instruments is then necessary:
- When the temperature probe is assigned to a new environment to measure;
- During the change of assignment of the measuring probe;
- If inaccurate results are suspected;
- Regularly, at a frequency which depends on the wear induced by its environment.
How do you calibrate a thermocouple sensor?
The procedure for calibrating a thermocouple is done by exposing the hot junction of the sensor to a known temperature in order to carry out control measures. If the temperatures recorded 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 may have effects on the accuracy 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 or entrusted to a service provider specialized. As it is necessary to calibrate the probes on a regular basis, it can be financially advantageous to equip yourself with calibration equipment.
Two main techniques allow you to generate a test temperature in order to calibrate a thermocouple probe. Comparison calibration is a widespread technique in the industry and fixed point calibration allows for accurate measurement.
Calibration by comparison
This technique is widely used in the industrial environment. Calibration by comparison consists of placing the probe to be calibrated and another type of probe which serves as a standard in an environment where the temperature is stable and homogeneous. The type of probe chosen as standard must offer greater precision than thermocouples. The heated medium in question is generally a dry well, a bath, or a tube furnace.
Fixed point calibration
The fixed point technique consists of exposing the hot junction of the thermocouple to an environment whose temperature we know with certainty. The principle used is that of the triple point according to the 1990 International Temperature Scale. The triple point of water occurs when water is found in equal parts in three forms: liquid, gas and solid. The temperature is then very precisely 0.01°C. The process of achieving this specific state is complex and therefore generally carried out by a specialized laboratory. Once the triple point of water is reached, the water temperature remains stable for a long time. This stability makes it a technique allowing 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 measurement. During calibration tests, it is necessary to calculate errors from the thermocouple output voltage using standard tables. Calibration is a lengthy process since following these guidelines requires repeating the calibration process at different temperatures until test the probe over its entire measuring range.
Go further on thermocouples
To go further on the subject of thermocouples, we also recommend these articles:
- Thermocouple probe category
- What are the different types of thermocouples
- How is temperature measured by thermocouple?
- Thermocouple Response Times by Type
- Thermocouple curves by type
- How to test a thermocouple?
- Thermocouple measuring range
- Thermocouple conversion table
- How does a thermocouple work?
- Thermocouples
