What are the thermal sensors in temperature sensors?

Temperature sensor

Choosing a temperature sensor requires more consideration than choosing other types of sensors. Firstly, the structure of the sensor must be selected to achieve the temperature of the measured fluid or surface within the specified measurement time of the sensitive element. The output of the temperature sensor is only the temperature of the sensitive component. In fact, it is often difficult to ensure that the temperature indicated by the sensor is the temperature of the measured object.

In most cases, the selection of temperature sensors requires consideration of the following aspects:

(1) Does the temperature of the tested object need to be recorded, alarmed, and automatically controlled, and does it require remote measurement and transmission.

(2) The size and accuracy requirements of the temperature measurement range.

(3) Is the size of the temperature measuring element appropriate.

(4) Can the hysteresis of the temperature measuring element adapt to the temperature measurement requirements when the temperature of the measured object changes over time.

(5) Is there any damage to the temperature measuring element caused by the environmental conditions of the tested object.

(6) What is the price and is it convenient to use.

The temperature of the fluid in the container is generally measured using thermocouples or thermal resistance probes. However, when the service life of the entire system is much longer than the expected service life of the probe, or when the probe is expected to be disassembled quite frequently for calibration or repair but cannot be opened on the container, permanent thermowells can be installed on the container wall. Using a thermowell can significantly extend the measurement time constant. When the temperature changes slowly and the thermal conductivity error is small, the thermocouple sleeve will not affect the accuracy of the measurement. However, if the temperature changes rapidly, the sensitive component cannot track the rapid temperature change, and the thermal conductivity error may increase, the measurement accuracy will be affected. Therefore, it is necessary to balance and consider the two factors of maintainability and measurement accuracy.

All materials of thermocouples or thermal resistance probes should be adapted to the fluid that may come into contact with them. When using exposed component probes, consideration must be given to the adaptability of the materials of each component in contact with the measured fluid (sensitive components, connecting leads, supports, local protective covers, etc.). When using thermowells, only the material of the sleeve needs to be considered.

Resistive thermistor components are usually sealed when immersed in liquids and most gases, with at least a coating. Exposed resistive components cannot be immersed in conductive or contaminated fluids. When their rapid response is required, they can be used in dry air and a limited number of gases and certain liquids. Resistance components, such as those used in stagnant or slow flowing fluids, usually require some type of housing to provide mechanical protection.

When pipes, conduits, or containers cannot be opened or are prohibited from opening, and therefore cannot use probes or thermowells, measurements can be made by clamping or fixing a surface temperature sensor on the outer wall. In order to ensure reasonable measurement accuracy, the sensor must be thermally isolated from the ambient atmosphere and from the thermal radiation source, and the appropriate design and installation of the sensor must ensure optimal thermal conductivity of the wall to the sensitive components. The measured solid material can be metallic or non-metallic, and any type of surface temperature sensor will to some extent change the material characteristics of the surface or lower layer of the measured object. Therefore, it is necessary to make appropriate choices for sensors and their installation methods in order to minimize this interference. The ideal sensor should be made entirely of the same material as the measured solid and integrated with the material, so that the structural features of the measurement point or its surroundings will not change in any way. There are various types of sensors available, including resistance (thin film thermal resistance, temperature sensor) types, as well as thin film and fine conductor type thermocouples. When measuring the temperature of surface jade using small sensors or threaded inserts that can be embedded, the outer edge of the embedded salt transmitter or insert should be flush with the outer surface of the measured material. The material of the inlay should be the same as the measured material, at least very similar. When using a gasket type sensor, attention must be paid to ensuring that the temperature that the gasket can reach is as close as possible to the desired temperature.

The selection of temperature sensors is mainly based on the measurement range. When the measurement range is expected to be within the total range, a platinum resistance sensor can be selected. Narrow ranges typically require sensors to have a relatively high basic resistance in order to achieve sufficient resistance variation. The sufficiently large resistance variation provided by temperature sensors makes these sensitive components very suitable for narrow measurement ranges. If the measurement range is quite large, thermocouples are more suitable. It is best to also include the freezing point within this range, as the indexing table of the thermocouple is based on this temperature. The linearity of sensors within a known range can also serve as an additional condition for selecting sensors.

The response time is usually represented by a time constant, which is another fundamental basis for selecting sensors. When monitoring the temperature in the storage tank, the time constant is not as important. However, when the temperature in the vibrating tube must be measured during use, the time constant becomes the determining factor in selecting the sensor. The time constant of bead type temperature sensors and armored exposed type thermocouples is relatively small, while the time constant of immersion probes, especially thermocouples with protective sleeves, is relatively large.

The measurement of dynamic temperature is quite complex, and only by repeatedly testing and simulating the conditions that often occur during sensor use as closely as possible can a reasonable approximation of the sensor's dynamic performance be obtained.

temperature controlApplication design:

Electronic thermometer, electronic calendar, electronic clock temperature display, electronic gifts;

Heating and cooling equipment, heating constant temperature electrical appliances;

Automotive electronic temperature measurement and control circuit;

Temperature sensors and instruments;

Medical electronic equipment and electronic toilet equipment;

Mobile phone batteries and charging appliances.