The thermocouple temperature sensor owes its success to its great versatility. It allows you to create thermal probes suitable for many areas of application. The operation of the thermocouple is based on a phenomenon called the Seebeck thermoelectric effect, theorized by the physicist of the same name. The temperature is deduced 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 suits your needs.
The operation of thermocouples is based on thermoelectric voltage
A thermocouple is a sensor for various fields (industry, chemistry, food processing, etc.) and in various environments to measure temperature. It contains two wires of conductive metals or metal alloys of different natures.
The principle of operation of thermocouples
These wires are connected by two types of welds, the hot spot and the cold spot. 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 being degraded by the environment in which it is located. The cold point must remain at a known temperature. During temperature measurement, the cold junction can be maintained at a temperature precise via a refrigeration mechanism. It is also possible to measure its temperature and then carry out a differential calculation.
When the hot spot of the thermocouple is exposed to heat or cold, the electron density of each metal wire is modified. Temperature variations induce a dynamization of the electrons, they will then move towards the colder side of the conductive wires. Measuring equipment is used to evaluate this electromotive force. It measures the current entering the recorder box at the ends of each of the two wires. Some devices display the voltage, others display the temperature calculated according to the specifics of the thermocouple.
Precautions to take for optimal operation of thermocouples
If the two metal wires are welded, and not intertwined for example, it is to guarantee that contact is maintained despite external conditions. such as vibrations. There are several joining techniques: tin soldering, silver soldering, electric soldering, etc. Soldering should not be carried out at too high a temperature so as not to alter the alloy wires, which would modify the functioning of the thermocouple.
Being very weak, it may happen that electrical interference is disturbing the thermocouple signal. A noisy motor located near the temperature probe may also interfere with 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 right type of thermocouple and suitable sheathing in the middle to measure. A phenomenon of decalibration can occur when external elements such as too high a temperature cause the diffusion of metal particles into the metals of the thermocouple. The miscalibration can also come from wear of the insulating sheath, causing contact between the two wires.
How to choose your thermocouple?
The choice of thermocouples depends on the temperature measurement range in degrees Celsius of the medium to be measured and the expected response time.
The target temperature range and response time
In theory, all kinds of metal alloys can be combined 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 which compose them. Types J, K, T and E are the most common thanks to their moderate price and multiple applications. They allow measurement of high temperatures. Type R, S and B thermocouples are used to measure very high temperatures. They contain noble metals, hence a higher purchase price.
To avoid false results, it is essential to use the correct type of thermocouple for your application. Indeed, each of these types has its own characteristics, as the measuring range temperature offering optimal thermocouple operation. To obtain the most precise data possible, we therefore match the temperature range to be measured with the optimum range of the type thermocouple. Certain metals offering particular resistance are to be used for specific environments (acidic, basic, high pressure, etc.).
Response time varies depending on junction type 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 rapid.
Compatibility with the application domain
When choosing a sensor for a measuring probe, you must everything first define the variables you want to measure. For example, you have the option of opting for a humidity and temperature sensor. You can choose a programmable transmitter to configure the desired parameters.
To select an electronic probe, it is best to choose a measuring device easy to handle by users and adapted to the environment. For example, turning off the LCD screens of the sensors above an outside temperature of 70°C is a recommendation. To monitor temperature with contact and remotely, you can choose a device that combines infrared thermometer and thermocouple.
Among the most used sensors, there are also thermistors. These sensors operate on a principle close to that of the thermocouple since they react to temperature variations by modifying their resistance. Thermistors are either NTC (negative temperature coefficient) or PTC (positive temperature coefficient). These two types of thermistors depend on the materials they are made from.
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
- Thermocouples
