Thermocouple

Thermocouple is a temperature sensor with many applications used for the contact temperature measurement. This is one of the constituent elements of the thermocouple temperature probe, a measurement device whose main elements are the sensor and a data reading device, generally with a liquid crystal screen. Thermocouple is one of the most used sensors, Alongside the RTD (PT100, PT100, PT1000 etc.) and the thermistor, the resistance of which depends on the temperature. The assets of thermocouple are Its robustness, its affordable price, its reaction speed and its ability to measure on large temperature ranges.

How does a thermocouple sensor work?

The functioning of the thermocouple is based on a phenomenon called Seebeck thermoelectric effect. This phenomenon is manifested by The appearance of an electrical voltage within an open circuit composed of two conductive materials of different natures during a temperature variation. The voltage obtained is converted into temperature thanks to the Seebeck coefficient as a function of the thermocouple type. The precautions to be taken for optimal thermocouples are the choice of the appropriate type according to the application, the type of junction and taking into account electromagnetic disturbances.

How does thermocouple measure temperature?

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.

What are the types of thermocouple?

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.

What influences the thermocouple's response time?

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.

What is a conversion table for thermocouple?

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.

What is the thermocouples curves?

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.

What is the measurement range of a thermocouple?

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.

How to calibrate a thermocouple?

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.

How to carry out tests on a thermocouple?

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.

What is the response time for thermocouples?

 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.

What is the difference between the different handles offered?

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".

What types of cables are available?

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.

Exchange with one of our advisers to choose the equipment that best suits your project. We offer a wide choice of temperature probes as well as a tailor -made design service.

If you can't find the probe you are looking for, contact Guilcor To get a tailor -made probe.