Le fonctionnement du thermocouple se base sur un phénomène appelé effet thermoélectrique Seebeck. Ce phénomène se manifeste par l’apparition d’une tension électrique au sein d’un circuit ouvert composé de deux matériaux conducteurs de natures différentes lors d’une variation de température. La tension obtenue est convertie en température grâce au coefficient Seebeck en fonction du type de thermocouple. Les précautions à prendre pour un fonctionnement optimal des thermocouples sont le choix du type adéquat selon l'application, le type de jonction et la prise en compte des perturbations électromagnétiques.

The thermocouple sensor is made up of two different conductive metals wires Connected together by a junction called hot spot. Their other end, the cold point, is connected to the measuring device. When the hot spot is exposed to heat or cold, thea electronic density of each metal wire is modified. The electric current that travels the sensor appears thanks to the temperature difference that exists between the hot spot and the cold point. To measure the temperature with a thermocouple in a precise way, either keep the cold point at zero degrees Celsius, or put in place what is called the compensation for cold welding.

There are several kinds of thermocouples that offer Different temperature measurement beaches and adapt to several types of applications. The selection of the appropriate thermocouple probe type depends on the environment in which it will be used and the budget allocated to the purchase. THE 8 types of thermocouple sensors mainly used are covered by the European standard CEI 60584.1. Some types are made up of common metals and are therefore inexpensive. Types B, R and S are designed from precious metals such as platinum, which makes their price higher.

The response time of thermocouples varies according to their design rather than their type. The characteristics that influence the reaction speed of these sensors are multiple: Protection of the probe, the junction method or the connection cable. The more contact with the environment whose temperature is wished to measure, the faster the response provided by measurement probes. A thermocouple without metal box or protective sheath therefore gives a quick response. Naked wires can be subject to corrosion according to their material and the environment, so you have to choose each characteristic carefully.

A Table or Thermocouple conversion table is a tool used for Convert an electro-motor force (FEM) 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 have to 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.

An alternative to the presentation of the voltage/temperature ratio in the form of a table is the graph. The temperature is in abscissa and the tension in order, the link between the two not being linear, it forms a curve. Each type of thermocouple has a separate curve which makes it possible to visually assess the variations between the two data. The thermocouples curves, such as tables, are based on the tension noted on the sensor when the cold point is 0 ° C. This welding must therefore be maintained in an agitated ice water bath or compensation. This technique consists in leaving the cold point at room temperature, measuring this temperature and converting it to voltage in order to add a corrective coefficient to the final calculation.

If thermocouple is the most common type of sensor in the industry, it is largely because it allows you to measure a large temperature amplitude. Depending on the types, it can measure temperatures from -200 ° C to 1,800 ° C. Thermocouples capable of measuring very low temperatures are the types K, J, T and N. For high temperatures, we use types N, S, R and B. To choose a thermocouple adapted to a precise application, The temperature beach must be measured and the measurement range of the type of thermocouple coincident.

What is called calibration or calibration of thermocouple is a procedure of Check that the voltage indicated on the voltmeter of the thermal probe corresponds to the right temperature in the conversion table. For this, the measurement point must be at a known temperature with certainty. The tests to check the accuracy of the probe are carried out on different points of its measurement range. If the results indicate a gap between the voltage and the corresponding temperature in the conversion table, a corrective coefficient must be added during uses of the sensor. Erroneous values ​​may mean that an element of the probe is damaged, Like the sensor itself, the connector or the recorder.

The test is one of the essential stages of calibration. It is recommended to test a thermocouple when the probe experiences a change in assignment, regularly depending on the intensity of its use or in the event of suspicion of error. Procedures generally applied to test a thermocouple follow The recommendations of the ASTM (American Society for Testing and Materials) for the calibration of thermocouples. Two options are possible, calibration by comparison or in a fixed point. In comparison, a temperature measurement is carried out with the probe to be tested as well as with a reference sensor, which we know reliable. In fixed points, the sensor is exposed to the temperature of the triple point of water.

 The response time is the time taken by the sensor to reach 66.6 % of the final reading and is the standard means of the industry to measure the response times of the probes. Five times the response time cited is the figure normally required to obtain 100 % of reading. The response times depend on the measured substance and, in the case of a liquid or a gas, the degree of agitation. It is therefore difficult to quote a specific response time without knowledge of the application.

The results presented in this catalog have been obtained in an agitated oil bath and can differ from those obtained in other conditions but can be used as a general guide when choosing probes.

If necessary, each probe is supplied with a hexagonal handle, small round, ribbed for heavy loads or in T -shaped. To reduce bacterial growth, the probe handles contain an antibacterial additive "Biomamaster".

Hexagonal handle Made of nylon and available in black. The maximum temperature is 105 ° C.

Small round handle Made of nylon and available in black. The maximum temperature is 105 ° C.

T -shaped handle Made of polypropylene and available in black or white. The maximum temperature is 85 ° C.

The robust ribbed handle (with color code caps) is made of polypropylene and available in black or white. The maximum temperature is 85 ° C.

The right cord in PVC is a cord for general use and available in lengths up to 100 meters. As standard and if necessary, each probe is delivered with a right PVC cord of a meter and a connector. As an alternative, a one meter spiral cable is available for standard, portable, type K or T thermocouple probes, replace the first figure (1) of the control code with the figure 3. The maximum temperature for PVC and PU is 80 ° C.

Certain industrial and high temperature probes are available with an overwhelmed cable isolated in stainless steel and fiberglass. As a standard and if necessary, each probe is supplied with a cable and a two -meter stainless steel connector. Maximum temperature of 350 ° C.