The thermocouple is a temperature sensor with many applications used for contact temperature measurement. It is one of the constituent elements of the thermocouple temperature probe, a measuring device whose main elements are the sensor and a data reading device. Usually having a liquid crystal screen. Often referred to as a thermostat, The thermocouple is one of the most used sensors. Alongside the infrared sensor and the thermistor, whose resistance depends on the temperature. The advantages of the thermocouple are its robustness, its affordable price, its reaction speed. But also its ability to measure over wide temperature ranges.
How does a thermocouple sensor work?
How the thermocouple works is based on a phenomenon called the Seebeck thermoelectric effect. This phenomenon is manifested by the appearance of an electrical voltage within an open circuit. It is often composed of two conductive materials of different natures during a temperature variation. The conversion of voltage into temperature is done using the Seebeck coefficient depending on the type of thermocouple. The precautions to take for optimal operation of thermocouples are the choice of the appropriate type according to the application and the type of junction. Taking electromagnetic disturbances into account is also important.
Temperature measurement using the thermocouple
The thermocouple sensor contains two wires of different conductive metals connected together by a junction called a hot spot. The cold point, connected to the device is the other end. When the hot spot is exposed to heat or cold, the electron density of each metal wire is modified. The electric current which passes through the sensor appears thanks to the temperature difference that exists between the hot point and the cold point. For measure temperature with a thermocouple precisely, you must either maintain the cold point at 0°C. Or set up what is called cold junction compensation.
The thermocouple comes in different types
There are several types of thermocouples that offer different temperature measurement ranges and adapt to several types of applications. The selection of thermocouple probe type appropriate depends on the environment and the budget allocated to the purchase. THE 8 types of thermocouple sensors mainly used are covered by the European standard IEC 60584.1. Some types contain base metals and are therefore inexpensive. Types B, R and S are made from precious metals such as platinum, which makes their price higher.
What affects thermocouple response time?
The thermocouple response time varies according to their design rather than their type. The characteristics that influence the reaction speed of these sensors are multiple. There is the protection of the probe, the junction method or even the connection cable. The more direct the contact with the environment whose temperature we wish to measure, the faster the response provided by the measuring probes. A thermocouple without a metal case or protective sheath therefore gives a rapid response. Bare wires can be subject to corrosion depending on their material and the environment, so each characteristic must be chosen carefully.
Conversion tables
A table or thermocouple conversion table is a tool for convert an electromotive force (emf) to a temperature in degrees Celsius or Fahrenheit. It also allows the calibration of the thermal probe by comparing the voltage obtained with the expected temperature. To read thermocouple conversion tables, you must know the type of thermocouple used, each type having its own Seebeck coefficient. To read the table, we match the potential difference noted with the temperature in degrees Celsius or Fahrenheit.
Thermocouple curves
An alternative to presenting the voltage/temperature ratio in table form is the graph. The temperature is on the abscissa and the voltage on the ordinate. The link between the two is not linear, it forms a curve. Each type of thermocouple has a distinct curve which allows the variations between the two data to be visually appreciated. Thermocouple curves, like tables, are based on the voltage measured on the sensor when the cold point is at 0°C. It is therefore necessary to maintain this weld in a bath of agitated ice water or to carry out compensation. This technique involves leaving the cold spot at room temperature. Then, we measure this temperature and convert it into voltage in order to add a corrective coefficient to the final calculation.
Measurement ranges
If the thermocouple is the most common type of sensor in the industry, it is largely because it allows a wide range of temperatures to be measured. Depending on the type, it can measure temperatures from -200°C to 1800°C. Thermocouples capable of measuring very low temperatures are types K, J, T and N. For high temperatures, types N, S, R and B are used. To choose a thermocouple suitable for a specific application, the temperature range to be measured and the measuring range of the thermocouple type must coincide.
Thermocouple calibration
What we call thermocouple calibration or calibration is a procedure consisting of check that the voltage indicated on the thermal probe voltmeter corresponds to the correct temperature in the conversion table. For this, the measuring point must be at a temperature that is known with certainty. The tests carried out make it possible to verify the accuracy of the probe at different points of its measuring range. If the results indicate a mismatch between the voltage and the corresponding temperature in the conversion table, a corrective coefficient must be added when using the sensor. Incorrect values can mean damage to an element of the probe, such as the sensor itself, the connector or the recorder.
How to carry out tests on a thermocouple?
Testing is one of the essential steps of calibration. It is recommended test a thermocouple when the probe experiences a change of assignment, regularly depending on the intensity of its use or in the event of suspected error. The procedures generally applied to test a thermocouple follow ASTM (American Society for Testing and Materials) recommendations for calibrating thermocouples. Two options are possible, calibration by comparison or fixed point. In comparison, we carry out a temperature measurement with the probe to be tested as well as with a reference sensor, which we know to be reliable. At fixed points, the sensor is exposed to the triple point temperature of the water.
Exchange with one of our advisors to choose the equipment best suited to your project. We offer a wide choice of temperature probes as well as a custom design service.
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
- Calibrating a thermocouple
- 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?
