Process Engineering Division – Instrumentation and Automation Section – Daily Question – Question from 2020-10-26

Thermometers are classified according to their measurement principles as follows: 1) Expansion thermometers: which are further divided into liquid expansion types (glass thermometers) and solid expansion types (bimetallic thermometers). It measures based on the principle of thermal expansion of liquids or solids. Such thermometers are simple and reliable, with flexible selection of temperature measurement points, but they are generally only suitable for on-site indication. 2) Gauge-type thermometers: There are three types, namely those filled with liquid, those filled with gas, and those filled with steam. Such thermometers, also known as bulb thermometers, measure temperature based on the principle that the expansion of a liquid, gas, or vapor enclosed in a fixed container due to heating causes a change in pressure. Such instruments are simple, inexpensive, and explosion-proof. However, its precision is low, and there is significant lag when transmitting over long distances. 3) Thermal resistance thermometer: It measures temperature by utilizing the principle that the resistance value of a conductor or semiconductor changes when heated. Such instruments offer high measurement accuracy, and are suitable for centralized measurement at multiple points as well as for remote transmission and control. However, its accuracy is poor at high temperatures, so it is only suitable for measurements at low and medium temperatures. At the same time, due to its large size, it is not possible to measure the temperature of a “point”. 4) Thermocouple thermometer: It measures temperature by utilizing the thermoelectric properties of a material. Such instruments are essentially the same as thermistor-based temperature meters; they can measure higher temperatures than resistive types, but their accuracy is lower at low temperatures, and cold-junction compensation is required during measurement. 5) Radiant high-temperature thermometer: The first four types of temperature measuring instruments are contact-type devices, while the radiant high-temperature thermometer is a non-contact type instrument; it measures the temperature of an object by utilizing its thermal radiation properties. Such instruments are easy to use for measurement, and they can detect high temperatures that other instruments cannot measure; however, they are only capable of measuring high temperatures, and their accuracy is affected by environmental conditions.

1. Temperature measurement using the principle of thermal expansion: By taking advantage of the principle that liquids or solids expand when heated, expansion thermometers can be constructed. 2. Temperature measurement using the principle of pressure variation with temperature: By taking advantage of the property that the pressure of a gas, liquid, or the saturated vapor of a certain liquid enclosed in a fixed volume changes as the temperature changes when heated, pressure gauge thermometers can be constructed. 3. Temperature measurement using the thermoresistance effect: By taking advantage of the property that the resistance of conductors or semiconductors changes with temperature, thermoresistive thermometers can be manufactured. Depending on the type of thermistor material used, there are platinum thermistors, copper thermistors, and semiconductor thermistors for temperature measurement. 4. Temperature measurement using the thermoelectric effect: Thermocouples can be fabricated by taking advantage of the thermoelectric properties of metals. 5. Non-contact temperature measurement (using thermal radiation to measure temperature) can only determine the surface temperature of the object being measured (luminance temperature, radiant temperature, color temperature, etc.).

1. Temperature measurement using the principle of thermal expansion: By taking advantage of the principle that liquids or solids expand when heated, expansion thermometers can be constructed. 2. Temperature measurement using the principle of pressure variation with temperature: By taking advantage of the property that the pressure of a gas, liquid, or the saturated vapor of a certain liquid enclosed in a fixed volume changes as the temperature changes when heated, pressure gauge thermometers can be constructed. 3. Temperature measurement using the thermoresistance effect: By taking advantage of the property that the resistance of conductors or semiconductors changes with temperature, thermoresistive thermometers can be manufactured. Depending on the type of thermistor material used, there are platinum thermistors, copper thermistors, and semiconductor thermistors for temperature measurement. 4. Temperature measurement using the thermoelectric effect: Thermocouples can be fabricated by taking advantage of the thermoelectric properties of metals. 5. Non-contact temperature measurement (using thermal radiation to measure temperature) can only determine the surface temperature of the object being measured (luminance temperature, radiant temperature, color temperature, etc.).

Direct measurement methods: bimetallic thermometers, thermal resistors, thermocouples. Indirect measurement methods: pressure-type thermometers, radiation thermometers

1. Contactless: radiation temperature measurement, brightness temperature measurement, colorimetric temperature measurement； 2. Contact-type: temperature measurement by volume change, temperature measurement by thermal resistance change, and temperature measurement by the thermoelectric effect.

Thermometers are classified according to their measurement principles as follows: 1) Expansion thermometers: which are further divided into liquid expansion types (glass thermometers) and solid expansion types (bimetallic thermometers). It measures based on the principle of thermal expansion of liquids or solids. Such thermometers are simple and reliable, with flexible selection of temperature measurement points, but they are generally only suitable for on-site indication. 2) Gauge-type thermometers: There are three types, namely those filled with liquid, those filled with gas, and those filled with steam. Such thermometers, also known as bulb thermometers, measure temperature based on the principle that the expansion of a liquid, gas, or vapor enclosed in a fixed container due to heating causes a change in pressure. Such instruments are simple, inexpensive, and explosion-proof. However, its precision is low, and there is significant lag when transmitting over long distances. 3) Thermal resistance thermometer: It measures temperature by utilizing the principle that the resistance value of a conductor or semiconductor changes when heated. Such instruments offer high measurement accuracy, and are suitable for centralized measurement at multiple points as well as for remote transmission and control. However, its accuracy is poor at high temperatures, so it is only suitable for measurements at low and medium temperatures. At the same time, due to its large size, it is not possible to measure the temperature of a “point”. 4) Thermocouple thermometer: It measures temperature by utilizing the thermoelectric properties of a material. Such instruments are essentially the same as thermistor-based temperature meters; they can measure higher temperatures than resistive types, but their accuracy is lower at low temperatures, and cold-junction compensation is required during measurement. 5) Radiant high-temperature thermometer: The first four types of temperature measuring instruments are contact-type devices, while the radiant high-temperature thermometer is a non-contact type instrument; it measures the temperature of an object by utilizing its thermal radiation properties. Such instruments are easy to use for measurement, and they can detect high temperatures that other instruments cannot measure; however, they are only capable of measuring high temperatures, and their accuracy is affected by environmental conditions.

1) Temperature measurement using the principle of expansion 2) Temperature measurement using the principle of pressure variation with temperature 3) Temperature measurement using the thermal resistance effect 4) Temperature measurement using the thermoelectric effect 5) Temperature measurement using the radiation principle

There are expansion type, pressure type, thermocouple, thermal resistor, radiation type, infrared type, and others.

Thermometers are classified according to their measurement principles as follows: 1) Expansion thermometers: which are further divided into liquid expansion types (glass thermometers) and solid expansion types (bimetallic thermometers). It measures based on the principle of thermal expansion of liquids or solids. Such thermometers are simple and reliable, with flexible selection of temperature measurement points, but they are generally only suitable for on-site indication. 2) Gauge-type thermometers: There are three types, namely those filled with liquid, those filled with gas, and those filled with steam. Such thermometers, also known as bulb thermometers, measure temperature based on the principle that the expansion of a liquid, gas, or vapor enclosed in a fixed container due to heating causes a change in pressure. Such instruments are simple, inexpensive, and explosion-proof. However, its precision is low, and there is significant lag when transmitting over long distances. 3) Thermal resistance thermometer: It measures temperature by utilizing the principle that the resistance value of a conductor or semiconductor changes when heated. Such instruments offer high measurement accuracy, and are suitable for centralized measurement at multiple points as well as for remote transmission and control. However, its accuracy is poor at high temperatures, so it is only suitable for measurements at low and medium temperatures. At the same time, due to its large size, it is not possible to measure the temperature of a “point”. 4) Thermocouple thermometer: It measures temperature by utilizing the thermoelectric properties of a material. Such instruments are essentially the same as thermistor-based temperature meters; they can measure higher temperatures than resistive types, but their accuracy is lower at low temperatures, and cold-junction compensation is required during measurement. 5) Radiant high-temperature thermometer: The first four types of temperature measuring instruments are contact-type devices, while the radiant high-temperature thermometer is a non-contact type instrument; it measures the temperature of an object by utilizing its thermal radiation properties. Such instruments are easy to use for measurement, and they can detect high temperatures that other instruments cannot measure; however, they are only capable of measuring high temperatures, and their accuracy is affected by environmental conditions.

Commonly used: bimetallic thermometers, thermal resistance thermometers, and thermocouple thermometers.