The principle of operation and features of temperature sensors, classification and scope

Modern production is simply unthinkable without the automation of various technological processes. Starting from a nuclear power plant and ending with cars, elements of automatic control and regulation of the necessary parameters can be found everywhere. Pressure, angular and linear velocities, temperature and many other parameters must be controlled for more efficient operation of the entire production or machine.

Among the total variety of monitored parameters, about half is taken by temperature measurement and control. And one of the most important parts of the entire system is the sensor. Given that conditions and temperature ranges can vary greatly, sensors and primary converters are made with different properties and qualities, depending on the technological requirements.

The temperature measurement sensor itself is a device capable of receiving a measured value and converting it into a signal for subsequent processing and regulation by a controlling device. Simply put, it is a converter of one quantity (temperature) into another quantity (electric current, resistance), which is capable of process the device (for example, a temperature controller) and, based on the data received, perform the actions for which this device. For example, when the temperature reaches above the set point, the device can turn off the actuator to stop the heating source (medium).

Types of temperature sensors

Due to the fact that the conditions and measurement ranges for different tasks can be very different, and the requirements for measuring various temperature parameters may be different, respectively, and to perform certain tasks thermal converter must meet these conditions and certain requirements. Therefore, they can be different and use different properties of materials in work. Thus, sensors are:

• Semiconductor;
• Thermal resistive;
• Acoustic;
• Thermoelectric;
• Piezoelectric;
• Pyrometers.

We will briefly describe the features of each of them so that you can imagine in what cases it is necessary to use this or that device.

Semiconductor thermoelectric

Thermal converters of this type are in demand in industries, as they are inexpensive and fairly accurate instruments with a low error. Under the influence of temperature, such a sensor registers changes in the properties of the pn junction. Here can be used almost any diode or bipolar transistor. High accuracy of semiconductor temperature sensors is achieved due to the dependence of the voltage across the transistor on the absolute temperature.

Thermal resistive thermoelectric converters

The main advantages of such temperature sensors are their durability, stability and high sensitivity. They fit perfectly into almost any scheme.

The operation of such thermal converters is based on a change in resistance under the influence of temperature on a conductor or semiconductor. Simply put, they contain a thermistor in their design, which responds to changes in the measured medium.

Depending on the material used in thermoresistive temperature sensors, they are divided into:

1. Silicon resistive, which are characterized by long-term stability and high accuracy.
2. Resistive temperature detectors featuring high stability, robustness and accuracy. Their work is based on the ability of metals to change their resistance when exposed to temperature. Most often, such sensors use platinum or copper, and when controlling especially high temperatures, tungsten. Their only drawback is their relatively high cost.
3. Thermistors are based on the use of metal oxide compounds. They are used only for measuring absolute temperatures. The main disadvantages are the need for calibration and fragility.

Acoustic proximity devices

This type of temperature sensor is mainly used for measuring high temperatures. Their principle of operation is based on changing the characteristics of sound at different temperatures. Consists of this thermal sensor from receiver and emitter. The sound, passing through the investigated medium, enters the receiver, where its parameters are recorded, and on their basis the temperature is determined.

Acoustic thermal sensors are often used in medicine and where it is impossible to measure temperature by contact methods. One of their main disadvantages is the low accuracy of the measured temperatures and the high error due to additional features.

Thermoelectric sensors

Thermoelectric sensors, or, more simply, thermocouples are distinguished by a wide range of measured values ​​- from -200 to 2200 degrees Celsius. Moreover, their capabilities depend on the materials used. For example, base metal thermocouples can measure temperature up to 1100 ° C, with noble up to 1600 ° C, and to measure especially high thermal conditions, thermocouples with refractory metals such as tungsten are used.

The principle of operation of thermoelectric sensors is based on the Seebeck effect, i.e. junctions of dissimilar metals are used, forming a closed loop in which an electric current arises when the junctions have different temperatures. The thermocouple consists of two ends: working and free. The former dives directly into the work environment, while the latter does not. Thus, a temperature difference arises, which is displayed in the form of an output voltage, which is recorded by a multivoltmeter, often included in a set with a thermoelectric sensor.

Piezoelectric quartz devices

The principle of operation of a piezoelectric temperature sensor is based on the use of a quartz piezoresonator. The piezomaterial used in it plays the role of a resonator. When an electric current is applied to it, this the material starts to hesitate when exposed to different thermal modes, and the vibration frequency also changes, which is the basis of piezoelectric sensors.

Non-contact thermal converters pyrometers

Non-contact sensors capable of detecting thermal radiation from heated bodies are called pyrometers. The convenience of such devices is that there is no need to place it directly into the environment. However, without direct contact, the accuracy of their readings is relatively low, because there may be side effects that affect the readings.

There are three types of pyrometers:

1. Interferometric pyrometers emit two beams, which pass one through the medium, and the second is the control one. These two beams hit a silicon sensitive element, after which the refraction and length of the beams, which directly depend on the heating of the medium, are compared.
2. Fluorescent Thermal Sensors they work according to a more complex principle: phosphorus-based components are applied to the surface where it is necessary to measure the amount of heat. After that, the object is exposed to ultraviolet pulsed radiation, as a result of which certain reactions occur, and the radiation is analyzed.
3. Sensors that contain solutionsthat can change color under the influence of temperature. Cobalt chloride used in such pyrometers, upon contact with the measured medium, is capable of changing the color spectrum depending on the degree of heating. Thus, the amount of light passing through the solution makes it possible to measure the required thermal parameters.

Selection rules

All of the above sensors perform excellently within the specified limits. However, you need to understand that it is necessary to choose and use them based on the requirements in a particular case.

Therefore, when choosing one or another thermal converter, you should pay attention to the following points:

1. The value of the temperature range.
2. The ability to immerse the sensor in the measured environment. If this is not possible, then it is worth resorting to using pyrometers or acoustic sensors.
3. Measurement conditions are one of the most important considerations when choosing a sensor. Here it is worth considering not only the aggressiveness of the environment, but also parameters such as: pressure, humidity, etc. Therefore, it is worth choosing either contactless sensors or in corrosion-resistant housings.
4. The nature of the output signal must always be considered as well. Indeed, some thermal converters can immediately convert the signal into degrees, while others give it only in the amount of current.
5. Some sensors are quite unstable and short-lived, which should also be taken into account. Therefore, if a long work is required without replacement and calibration, then this nuance must also be taken into account.
6. When choosing a sensor for certain needs, it will be useful to pay attention to the response time, resolution and error, operating supply voltage, housing type.

Taking into account all of the above nuances, you can choose a sensor that is fully consistent in its characteristics in a particular situation and for specific tasks.