Measure the temperature by thermocouple?

Thermocouples are very widespread temperature sensors in the industry. They are robust, offer a quick response and can measure high temperatures. Thermocouple is a sensor that comes into contact with the environment to be measured, unlike the infrared thermometer for example. The thermocouple temperature sensor can be immersed in a medium that can be liquid or gaseous and connected to a recorder. To understand the thermal measurement method used by thermocouple, you must first take an interest in its composition. We will then see how the temperature is deducted from the measurement of an electric current. Certain practices are imperative to ensure that the temperature measurement by thermocouple is precise.

The Thermocouple probe uses electro-motor strength

THE Thermocouple temperature sensor contains Two wires of metal and conductive metal alloys. These wires are connected to one end, it is at the level of this junction that the temperature is measured. It is generally designated under the term of hot welding. Their other end is connected to the temperature measurement housing via what is called the welding or cold junction.

When hot welding has a temperature change, Low electric current is created. This electro-motor force (or fem) is proportional to the temperature in Celsius degrees of the measurement point. We use Thermocouples conversion tables to establish a correspondence between the measured current and the temperature.

How to choose the materials that constitute a thermocouple probe?

The temperature measurement range Thermocouples sensors varies according to the metals used for their manufacture. This difference depends on the conductivity of the alloys that constitute the couple's sons. These associations of two types of alloys are in a classification according to what are called the types of thermocouples.

Connection cables are generally between the threads of the thermocouple and the measuring devices, without distorting the results. This is possible thanks to The law of intermediate metals. According to this principle, even if a third homogeneous driver incorporates a circuit, the existing electro-motor force will have no modification.

How to ensure the temperature measurement accuracy of the thermocouple?

To properly use a thermocouple, pay attention to two essential elements. If hot welding is the part of the probe that seems the most important for the temperature statement, you should know that the principle works above all thanks to the difference between measurable tensions at the two ends of the wires. It is therefore necessary to proceed to what is called the compensation for cold welding. It is also necessary ensure the reliability of the measurement system By making a calibration at different stages of the life of the temperature probe.

Cold welding compensation

The measured electric current actually results from the temperature difference between hot welding and cold welding. There are several methods of compensation for cold welding. The most reliable is that which consists in Maintain cold welding at a temperature of 0 ° C By placing it in an ice water bath. If this method is easily applicable in a laboratory, it is not very suitable for an industrial environment.

A second method is to Place the cold welding in a calibration oven, which maintains the welding at a known temperature thanks to a thermostat. It is also possible to Leave the cold welding at room temperature And to measure the temperature in this junction. When you choose one of these last two methods, you have to Differential calculation To deduce the temperature of the hot welding. Indeed, the conversion tables start from a temperature of 0 ° C. The voltage measurement obtained is then converted into Celsius degrees as a function of Thermocouple type in question. There are measuring instruments capable of directly carrying out compensation calculation from customizable parameters.

Calibrate the thermocouple probe

The calibization of thermocouple is done right after its manufacture, before being put up for sale. There is another calibration at Whenever he changes assignment. It is also recommended to calibrate a thermocouple probe as soon as a dysfunction is suspected.

Two main calibration methods for a thermocouple sensor exist. When we do Comparison calibration, the temperature measurement of the thermocouple is compared to that carried out by another thermal measurement probe. The technique in fixed points consists in exposing the thermocouple probe to a specific temperature. These are 0.01 ° C, a temperature obtained when the water is equally in its three forms.

Thermal measurement in industry

Whatever the sector of activity, many industrial processes require the use of a detector to measure the temperature. Thermocouple is one of the most used devices. His response time is short, he is cheap and can measure high temperatures. Thermisting sensors are also very present in the industry. This type of probe measures the temperature according to the modification of the resistance of the materials. Finally, gas or liquid dilation thermometers (oil or glycerin) offer great precision. Their measurement range is however less than that of a thermocouple or resistance probe.