Thermocouples are very common types of temperature sensors in industry. They are robust, offer rapid response and can measure high temperatures. The thermocouple is a sensor which 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 which can be liquid or gaseous and connected to a recorder. To understand the thermal measurement method used by the thermocouple, you must first look at its composition. We will then see how the temperature is deduced from the measurement of an electric current. Certain practices are imperative to ensure that thermocouple temperature measurement is accurate.
The thermocouple probe uses electromotive force
The thermocouple temperature sensor contains two wires of conductive metals or metal alloys. These wires are connected at one end, it is at this junction that the temperature is measured. It is generally referred to as hot welding. Their other end is connected to the temperature measuring box via what is called the solder or cold junction.
When the hot weld has a temperature change, a weak electric current is created. This electromotive force (or emf) is proportional to the temperature in degrees Celsius of the measuring point. We use thermocouple conversion tables to establish a correspondence between the measured current and the temperature.
How to choose the materials that make up a thermocouple probe?
The temperature measurement range thermocouple sensors vary depending on the metals used for their manufacture. This difference depends on the conductivity of the alloys which constitute the wires of the couple. These combinations of two types of alloys are classified according to what we call the types of thermocouples.
Connection cables are generally between the thermocouple wires 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 conductor integrates a circuit, the existing electromotive force will not have any modification.
How to ensure the accuracy of thermocouple temperature measurement?
To use a thermocouple correctly, you must pay attention to two essential elements. If the hot junction is the part of the probe which seems the most important for the temperature reading, you should know that the principle works above all thanks to the difference between the measurable voltages at the two ends of the wires. It is therefore necessary to carry out what is called cold junction compensation. It is also necessary ensure the reliability of the measurement system by performing calibration at different stages of the temperature probe’s life.
Cold junction compensation
The measured electric current actually results from the temperature difference between the hot junction and the cold junction. There are several methods of compensating the cold junction. The most reliable is that which consists of maintain the cold junction at a temperature of 0°C by placing it in an ice water bath. Although 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 junction in a calibration oven, which maintains the solder at a known temperature using a thermostat. It is also possible to leave the cold junction at room temperature and measure the temperature at this junction. When choosing one of these last two methods, you must perform a differential calculation to deduce the temperature of the hot junction. In fact, the conversion tables start from a temperature of 0°C. The resulting voltage measurement is then converted to degrees Celsius based on the thermocouple type which it is about. There are measuring instruments capable of directly performing the compensation calculation based on customizable parameters.
Calibrate the thermocouple probe
The thermocouple is calibrated just after its manufacture, before being put on sale. There is another calibration to every time he changes assignment. It is also recommended to calibrate a thermocouple probe as soon as a malfunction is suspected.
Two main calibration methods for a thermocouple sensor exist. When we carry out calibration by comparison, the temperature measurement of the thermocouple is compared to that carried out by another thermal measuring probe. The fixed point technique consists of exposing the thermocouple probe to a precise temperature. This is 0.01°C, the temperature obtained when water is in equal parts in its three forms.
Thermal measurement in industry
Regardless of the sector of activity, many industrial processes require the use of a detector to measure temperature. The thermocouple is one of the most used devices. Its response time is short, it is inexpensive and it can measure high temperatures. Thermistor sensors are also very present in the industry. This type of probe measures temperature based on changes in the resistance of materials. Finally, gas or liquid expansion thermometers (oil or glycerin) offer great precision. However, their measurement range is less than that of a thermocouple or resistance probe.
