Temperature control is ubiquitous in manufacturing processes, allowing you to select the appropriate operating mode or track changes in material condition. The temperature regime is equally important both when turning on the oven in the kitchen and in blast furnaces when melting steel, and deviation from normal operation can lead to accidents and injury to people. To avoid unpleasant consequences and to ensure the possibility of regulating the degree of heating, a temperature sensor is used.
Thermoelectric
The thermoelectric sensor is based on the thermocouple principle (see. Figure 1) - all metals have a certain valence (the number of free electrons in outer atomic orbits that are not involved in rigid bonds). When exposed to external factors imparting additional energy to free electrons, they can leave the atom, creating the movement of charged particles. In the case of combining two metals with different potential for the release of electrons and subsequent heating of the junction, a potential difference will arise, which is called the Seebeck effect.
Semiconductor
They are made on the basis of crystals with a given current-voltage characteristic. Such temperature sensors operate in a semiconductor switch mode, similar to a classic bipolar transistor, where the degree of heating is comparable to the supply of potential to the base. As the temperature rises, the semiconductor sensor will begin to deliver a higher current value. As a rule, the semiconductor itself is not used to measure heating, but is connected through an amplifier circuit (see. Figure 2).
They feature a wide range of measurements and the ability to adjust the sensor in accordance with the operating parameters of the equipment. They are a high-precision type, little dependent on the duration of operation. They have small dimensions, due to which they are easily installed in circuits, radio elements, etc.
Pyrometric
They work at the expense of special sensors - pyrometers, which allow capturing the slightest temperature fluctuations of the working surface of any object. Directly the sensing element itself is a matrix that responds to a certain frequency of the temperature range. This principle is the basis for measurements with a non-contact thermometer, which became widespread during the fight against coronavirus. In addition, their use is actively used for thermal imaging control of structural elements, equipment, buildings and structures.
Thermoresistive
Such temperature sensors are made on the basis of thermistors - devices with a certain dependence of resistance on the degree of heating of the base material. As the temperature rises, the conductivity of the resistor also changes, so you can monitor the state of the desired object.
The main disadvantage of a thermoresistive sensor is the small range of the measured temperature, but it able to provide a good measurement step and high accuracy in tenths and hundredths of degrees Celsius. Because of this, they are often included in the circuit using an amplifier that expands the operating limits.
Acoustic
Acoustic temperature sensors operate on the principle of determining the speed of sound transmission depending on the temperature of the material or surface. The sensor itself compares the speed of sound generated by the source, which will differ depending on the degree of heating (see. Figure 4). This type is non-contact and allows you to take measurements in hard-to-reach places or at high-risk objects.
Piezoelectric
The operation of the sensor is based on the effect of propagation of vibrations of a quartz crystal when an electric current passes. But, depending on the ambient temperature, the crystal oscillation frequency will also change. The principle of fixing temperature changes consists in measuring the vibration frequency and then comparing it with the established calibration of the ratings for different temperatures.