Selecting a temperature sensor requires more consideration than selecting other types of sensors. First of all, the structure of the sensor must be chosen so that the temperature of the measured fluid or the surface to be measured is reached within the specified measuring time of the sensitive element. The output of the temperature sensor is simply the temperature of the sensor. 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 a temperature sensor requires consideration of the following issues:
(1) Whether the temperature of the measured object needs to be recorded, alarmed, and automatically controlled, whether it needs long distance measurement and transmission.
(2) Temperature measurement range and accuracy requirements.
(3) Whether the temperature measuring element is appropriate or not.
(4) When the temperature of the measured object changes with time, the lag of the temperature measuring element can meet the temperature measurement requirements.
(5) Whether the environmental condition of the measured object has any damage to the temperature measuring element.
(6) How is the price and use convenient?
The temperature of the fluid in the vessel is usually measured with a thermocouple or a thermistor probe, but when the entire system has a lifetime that is much longer than the expected lifetime of the probe, or it is expected that the probe will be disassembled quite often to calibrate or repair it but not on the vessel. When opening, a permanent thermowell can be mounted on the vessel wall. Thermowells significantly increase the time constant of the measurement. When the temperature changes slowly and the thermal conductivity error is small, the thermowell does not affect the accuracy of the measurement, but if the temperature changes very quickly, the sensor does not track the rapid change in temperature, and the thermal conductivity error may increase, the measurement Accuracy will be affected. Therefore, the balance between maintainability and measurement accuracy must be considered.
All materials for thermocouple or RTD probes should be compatible with the fluid that may be in contact with them. When using bare element probes, the adaptability of each component material (sensitivity element, connection lead, support, partial protective cover, etc.) that comes in contact with the fluid to be measured must be considered. When using thermowells, only the material of the sleeve needs to be considered. .
Resistive thermal elements are usually sealed when immersed in liquids and most gases. They must have at least a coating. The exposed resistors must not be immersed in conductive or contaminated fluids. When they are required to respond quickly, they can be used for drying. The air and the limited number of gases and some liquids. Resistive components, such as those used in stagnant or slow-moving fluids, often require some sort of housing to provide mechanical protection.
When the tube, catheter, or container cannot be opened or closed, and the probe or thermowell cannot be used, it can be accessed and measured 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 isolated from the thermal radiation source, and the wall must be optimally thermally conductive to the sensitive element through proper design and installation of the sensor.
The measured solid material can be either metallic or non-metallic, and any type of surface temperature sensor will, to some extent, change the material properties of the surface of the material under test or subsurface. Therefore, the sensor and its mounting method must be properly selected so as to minimize such 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 characteristics of the measuring point or its surroundings are not changed in any way. There are a variety of available sensors of this type, including resistors (thin film thermal resistance, temperature sensors), as well as thin-film and fine-wire type thermocouples. The temperature of the surface jade shall be measured with a small embedded sensor or a threaded insert so that the outer edge of the embedded submarine or insert is flush with the outer surface of the material being measured. The material of the insert should be the same as the material being measured, at least very similar. When using washer type sensors, care must be taken to ensure that the gasket can reach as close to the desired temperature as possible.
The choice of temperature sensor 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 used. A narrower range usually requires that the sensor must have a relatively high basic resistance in order to obtain a sufficiently large resistance change. The sufficiently large resistance variation provided by the temperature sensor makes these sensitive elements very suitable for narrow measurement ranges. Thermocouples are more suitable if the measuring range is quite large. The freezing point is also preferably included in this range because the index table for thermocouples is based on this temperature. Sensor linearity in a known range can also be used as an additional condition for selecting a sensor.
The response time is usually represented 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 deciding factor for selecting the sensor. The time constants of bead-type temperature sensors and armored out-type thermocouples are quite small, while immersed probes, especially thermocouples with protective sleeves, have large time constants.
The measurement of dynamic temperature is more complex. Only through repeated testing, the conditions that often occur in the use of the sensor are simulated as closely as possible to obtain a reasonable approximation of the dynamic performance of the sensor.
In most cases, the selection of a temperature sensor requires consideration of the following issues:
(1) Whether the temperature of the measured object needs to be recorded, alarmed, and automatically controlled, whether it needs long distance measurement and transmission.
(2) Temperature measurement range and accuracy requirements.
(3) Whether the temperature measuring element is appropriate or not.
(4) When the temperature of the measured object changes with time, the lag of the temperature measuring element can meet the temperature measurement requirements.
(5) Whether the environmental condition of the measured object has any damage to the temperature measuring element.
(6) How is the price and use convenient?
The temperature of the fluid in the vessel is usually measured with a thermocouple or a thermistor probe, but when the entire system has a lifetime that is much longer than the expected lifetime of the probe, or it is expected that the probe will be disassembled quite often to calibrate or repair it but not on the vessel. When opening, a permanent thermowell can be mounted on the vessel wall. Thermowells significantly increase the time constant of the measurement. When the temperature changes slowly and the thermal conductivity error is small, the thermowell does not affect the accuracy of the measurement, but if the temperature changes very quickly, the sensor does not track the rapid change in temperature, and the thermal conductivity error may increase, the measurement Accuracy will be affected. Therefore, the balance between maintainability and measurement accuracy must be considered.
All materials for thermocouple or RTD probes should be compatible with the fluid that may be in contact with them. When using bare element probes, the adaptability of each component material (sensitivity element, connection lead, support, partial protective cover, etc.) that comes in contact with the fluid to be measured must be considered. When using thermowells, only the material of the sleeve needs to be considered. .
Resistive thermal elements are usually sealed when immersed in liquids and most gases. They must have at least a coating. The exposed resistors must not be immersed in conductive or contaminated fluids. When they are required to respond quickly, they can be used for drying. The air and the limited number of gases and some liquids. Resistive components, such as those used in stagnant or slow-moving fluids, often require some sort of housing to provide mechanical protection.
When the tube, catheter, or container cannot be opened or closed, and the probe or thermowell cannot be used, it can be accessed and measured 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 isolated from the thermal radiation source, and the wall must be optimally thermally conductive to the sensitive element through proper design and installation of the sensor.
The measured solid material can be either metallic or non-metallic, and any type of surface temperature sensor will, to some extent, change the material properties of the surface of the material under test or subsurface. Therefore, the sensor and its mounting method must be properly selected so as to minimize such 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 characteristics of the measuring point or its surroundings are not changed in any way. There are a variety of available sensors of this type, including resistors (thin film thermal resistance, temperature sensors), as well as thin-film and fine-wire type thermocouples. The temperature of the surface jade shall be measured with a small embedded sensor or a threaded insert so that the outer edge of the embedded submarine or insert is flush with the outer surface of the material being measured. The material of the insert should be the same as the material being measured, at least very similar. When using washer type sensors, care must be taken to ensure that the gasket can reach as close to the desired temperature as possible.
The choice of temperature sensor 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 used. A narrower range usually requires that the sensor must have a relatively high basic resistance in order to obtain a sufficiently large resistance change. The sufficiently large resistance variation provided by the temperature sensor makes these sensitive elements very suitable for narrow measurement ranges. Thermocouples are more suitable if the measuring range is quite large. The freezing point is also preferably included in this range because the index table for thermocouples is based on this temperature. Sensor linearity in a known range can also be used as an additional condition for selecting a sensor.
The response time is usually represented 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 deciding factor for selecting the sensor. The time constants of bead-type temperature sensors and armored out-type thermocouples are quite small, while immersed probes, especially thermocouples with protective sleeves, have large time constants.
The measurement of dynamic temperature is more complex. Only through repeated testing, the conditions that often occur in the use of the sensor are simulated as closely as possible to obtain a reasonable approximation of the dynamic performance of the sensor.
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