Temperature sensor selection guide

July 29, 2021

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

In most cases, the following aspects need to be considered for the selection of temperature sensors:
(1) Whether the temperature of the measured object needs to be recorded, alarmed and automatically controlled, and whether it needs to be measured and transmitted remotely.
(2) The size and accuracy requirements of the temperature measurement range.
(3) Whether the size of the temperature measuring element is appropriate.
(4) When the temperature of the measured object changes with time, whether the hysteresis of the temperature measuring element can meet the temperature measurement requirements.
(5) Whether the environmental conditions of the measured object damage the temperature measuring element.
(6) What is the price and whether it is convenient to use.

The temperature of the fluid in the container is generally measured with a thermocouple or thermal resistance probe, but when the service life of the entire system is much longer than the expected service life of the probe, or the probe is expected to be removed quite frequently for calibration or repair, it cannot be placed on the container. When opening, a permanent thermowell can be installed on the vessel wall. The use of thermowells will significantly extend the time constant of the measurement. When the temperature changes very slowly and the thermal conductivity error is small, the thermowell will not affect the accuracy of the measurement, but if the temperature changes very quickly, the sensitive element cannot track the rapid temperature change, and the thermal conductivity error may increase again, measure The accuracy will be affected. Therefore, it is necessary to weigh the two factors of maintainability and measurement accuracy.

All materials of thermocouple or thermal resistance probe should be compatible with the fluid that may come into contact with them. When using exposed element probes, you must consider the adaptability of the materials (sensitive elements, connecting leads, supports, partial protective covers, etc.) that are in contact with the measured fluid. When using thermowells, you only need to consider the material of the sleeve. .

Resistive thermistors are usually hermetically sealed when immersed in liquids and most gases, and at least they must be coated. Bare resistive elements cannot be immersed in conductive or contaminated fluids. When fast response is required, they can be used for drying The air and a limited number of gases and some liquids. If the resistance element is used in stagnant or slow-flowing fluid, it usually needs to be covered by some kind of housing for mechanical protection.

When the pipe, conduit, or container cannot be opened or the opening is prohibited, so that the probe or thermowell cannot be used, the measurement can be carried out 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 heat radiation source, and the heat conduction of the wall to the sensitive element must be optimally designed and installed through the sensor.

The measured solid material can be metallic or non-metallic, and any type of surface temperature sensor will change the material properties of the surface or subsurface of the measured object to some extent. Therefore, the sensor and its installation method must be properly selected in order to minimize this interference. The ideal sensor should be made entirely of the same material as the solid being measured and integrated with the material, so that the structural features of the measuring point or its surroundings will not change in any way. There are a variety of available such sensors, including resistance (thin film thermal resistance, temperature sensor) type, and thin-film and thin-wire thermocouples. Use the embedded small sensor or threaded insert to measure the temperature of the surface jade. The outer edge of the embedded salt transfer device or insert should be flush with the outer surface of the material to be measured. The material of the insert should be the same as the material tested, at least very similar. When using a washer sensor, care must be taken to ensure that the temperature reached by the washer is as close as possible to the temperature to be measured.

The choice of temperature sensor is mainly based on the measurement range. When the measurement range is expected to be within the total range, platinum resistance sensors can be used. The narrower range usually requires the sensor to have a fairly high basic resistance in order to obtain a large enough resistance change. The large enough resistance change provided by the temperature sensor 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 include the freezing point in this range, because the thermocouple index table is based on this temperature. The linearity of the sensor within the known range can also be used as an additional condition for selecting the sensor.

The response time is usually expressed by a time constant, which is another basic basis for selecting a sensor. When monitoring the temperature in the tank, the time constant is less important. However, when the temperature in the vibrating tube must be measured during use, the time constant becomes the decisive factor in selecting the sensor. The time constants of bead temperature sensors and armored exposed thermocouples are quite small, while immersion probes, especially thermocouples with protective sleeves, have relatively large time constants.

The measurement of dynamic temperature is more complicated. Only by repeatedly testing and simulating the conditions that often occur in the use of the sensor as closely as possible, can a reasonable approximation of the dynamic performance of the sensor be obtained.


The working principle of NTC negative temperature coefficient temperature sensor

NTC is the abbreviation of Negative Temperature Coefficient, which means negative temperature coefficient. It generally refers to semiconductor materials or components with a large negative temperature coefficient. The so-called NTC temperature sensor is a negative temperature coefficient temperature sensor. It is made of metal oxides such as manganese, cobalt, nickel and copper as the main materials and made by ceramic technology. These metal oxide materials have semiconductor properties because they are completely similar to semiconductor materials such as germanium and silicon in terms of conduction. When the temperature is low, the number of carriers (electrons and holes) of these oxide materials is small, so its resistance value is higher; as the temperature rises, the number of carriers increases, so the resistance value decreases. The NTC temperature sensor has a range of 100 to 1,000,000 ohms at room temperature, with a temperature coefficient of -2% to -6.5%. NTC temperature sensor can be widely used in temperature measurement, temperature compensation, surge current suppression and other occasions.