How does a thermocouple work?

The thermocouple temperature sensor owes its success to its great versatility. It creates thermal probes adapted to many areas of application. The functioning of the thermocouple is based on a phenomenon called the thermoelectric effect Seebeck, theorized by the physicist of the same name. We deduce the temperature from the electrical voltage level generated by the temperature change within the thermocouple. But what exactly is the operating mechanism Thermocouples ? Here are some keys to better understand the secrets of the temperature probe and help you choose the one that meets your needs.

The operation of thermocouples is based on thermoelectric voltage

A thermocouple is a sensor for various fields (industry, chemistry, food, etc.) and in various environments to measure the temperature. It contains Two wires of metal and conductive metal alloys of different natures.

The operating principle of thermocouples

These sons are connected by two types of welds, the hot spot and the cold point. The hot spot is in the direction of the environment whose temperature we want to measure. Hot welding often has Protection by a metal sheath. In order to prevent it from degrading by the environment in which it is located. The cold point must remain at a known temperature. During the temperature taking, Cold welding can be maintained at a temperature specifies via a refrigeration mechanism. It is also possible to measure your temperature and then carry out a differential calculation.

When the hot spot of the thermocouple is exposed to heat or cold, The electronic density of each metal wire is modified. The temperature variations induce an energization of the electrons, they will then move towards the coldest side of the conductive wires. We use measurement equipment for Evaluate this electro-motor force. It measures the current entering the recorder box at the ends of each of the two wires. Some devices display voltage, others display the temperature calculated according to the specifics of the thermocouple.

The precautions to be taken for optimal thermocouples operation

If the two metal wires are welded, and not intertwined for example, it is to guarantee the maintenance of contact despite the external conditions such as vibrations. There are several junction techniques: tin welding, money to silver, electric welding, etc. Welding should not be carried out at a too high temperature In order not to alter the alloy threads, which would modify the operation of the thermocouple.

Being very low, it can happen that Electric interference enabled The thermocouple signal. A noisy engine located near the temperature probe can also hinder the operation of the thermocouple and thus distort the results. It may then be necessary to calibrate it again.

It is essential to use the good type of thermocouple and suitable sheathing In the middle to measure. A decalibrage phenomenon Can occur when external elements as a too high temperature cause the diffusion of metallic particles in the thermocouple metals. The decaling can also come from wear of the insulating sheath, causing contact between the two wires.

How to choose thermocouple?

The choice of thermocouples depends on the temperature measurement range In degrees Celsius of the environment to be measured and the expected response time.

The temperature range and the response time targeted

In theory, All kinds of metal alloys can be associated To form a thermocouple. However, 8 Types of thermocouples are mainly used. They are the subject of a European standard and appear in a classification according to Metal combinations who compose them. The J, K, T and E types are the most common thanks to their moderate prices and their multiple applications. They allow a high temperature measurement. Thermocouples of types R, S and B are used for Measure very high temperatures. They contain noble metals, hence a higher purchase price.

To avoid false results, it is essential to use the adequate thermocouple type according to its application. Indeed, Each of these types has clean characteristics, like the measurement range temperature offering the operation of the optimal thermocouple. To obtain the most precise data possible, therefore made Concue the temperature range to be measured with the optimal range of the type thermocouple. Certain metals offering a particular resistance are to be used for specific environments (acids, basic, high pressure, etc.).

The response time varies depending on the type of junction at the end of the thermocouple. In the case of the exposed junction, the junction is not in the protective sheath. Contact with the environment being direct, the response time is therefore fast.

Compatibility with the field of application

When you choose a sensor for a measurement probe, you need everything first define the variables you want to measure. For example, you can opt for a humidity and temperature sensor. You can choose a programmable transmitter to configure the desired settings.

To select an electronic probe, it is best to choose a measurement device easy to handle by users and adapted to the environment. For example, extinguishing the LCD screens from the probes beyond an outside temperature of 70 ° C is a recommendation. To monitor the temperature with contact and remotely, you can choose a device combining infrared and thermocouple thermometer.

Among the most used sensors, there are also Thermists. These sensors work on a principle close to that of thermocouple since they react to temperature variations by modifying their resistance. The thermists are either CTN (with negative temperature coefficient) or CTP (positive temperature coefficient). These two types of thermistors depend on the materials from which they are designed.