Temperature measuring instruments are industrial automation instruments used to measure the temperature of objects. The earliest temperature measuring instrument was created by Italian Galileo in 1592. It is a large glass bubble with a slender neck, inverted in a container containing wine, and a portion of the air is extracted from it, causing the wine surface to rise into the slender neck. When the external temperature changes, the surface of the wine inside the thin neck rises and falls due to the thermal expansion and contraction of the air inside the glass bubble. Therefore, the height of the wine surface can indicate the temperature, which is actually an indicator without a scale.

The classification, characteristics, and applications of temperature measuring instruments are as follows:

1. The PN junction temperature sensor utilizes the characteristic that the forward voltage drop of the PN junction varies with temperature under certain current conditions. The characteristics of PN junction temperature sensors are small size, fast response, and good linearity. The interchangeability and stability of discrete component PN junction temperature sensors are not ideal. The integrated temperature sensor, which integrates the temperature sensing transistor with peripheral circuits such as amplification and compensation, has achieved miniaturization of the temperature sensor and overcome the disadvantages of poor interchangeability and stability of discrete PN junction temperature sensors. It is easy to use and has been widely used in temperature measurement, control, compensation, and other fields. Due to the limitation of PN junction's heat resistance, only temperatures within the range of -50~150 ℃ can be measured.

When using PN junction temperature sensors, the following issues should be noted:

(1) Control the working current to reduce the impact of PN junction self heating temperature rise on measurement.

(2) Operate under constant current to ensure the linearity of the sensor.

2. Infrared temperature sensors use the radiation energy emitted by measuring objects for measurement. The higher the temperature of an object, the higher its radiation energy. Infrared temperature measurement method belongs to non-contact temperature measurement, which has the advantages of high sensitivity, fast reaction speed, and wide temperature measurement range (-50~3500 ℃). It is suitable for measuring the temperature of small objects, motion, high temperature and other situations.

3. The natural oscillation frequency of quartz crystal changes with temperature, and this property of quartz can be used for temperature measurement. Quartz temperature sensors have the characteristics of small size, high sensitivity, high resolution, high measurement accuracy, good stability, and fast response speed. At the same time, due to the output signal being frequency, it is easy to achieve digital display, mainly used for high-precision and high-resolution temperature measurement and as a standard thermometer for value transfer. Attention should be paid to preventing mechanical vibration and impact when using quartz temperature sensors. The measurement range of quartz temperature sensors is generally within the range of -50~200 ℃.

4. Fiber optic temperature sensor is a temperature sensor that uses optical fibers as temperature sensitive components or transmits signals through optical fibers. The former type uses the characteristic of a certain parameter changing with temperature to measure temperature, which is called functional type; The latter type of optical fiber is only used to transmit information and is called conductive type. According to the function of optical fibers, optical fiber sensors can be divided into optical fiber type (functional type), transmission type (non functional type), and pickup type; According to the usage method, it can be divided into contact and non-contact types; According to the modulation principle, optical fiber temperature sensors can be divided into coherent and incoherent types; Coherent types include polarization interference type, phase interference type, and distributed temperature sensors; Non coherent types include radiation thermometers, semiconductor absorption thermometers, and fluorescence thermometers.

Due to the characteristics of anti electromagnetic interference and corrosion resistance of optical fibers, fiber optic thermometers have good electromagnetic insulation, large information transmission capacity, low loss, high strength, bendability, high sensitivity, high temperature measurement upper limit (3000 ℃), small size, light weight, simple structure, easy installation and use, and safe and explosion-proof. At present, the application of fiber optic thermometers in industry is not particularly widespread, mainly used for the following situations that cannot be measured by conventional thermometers: (1) temperature measurement in harsh environments such as high voltage and high current, strong electromagnetic interference, strong radiation, and flammability and explosiveness; (2) High temperature measurement; (3) Unable to observe or measure temperature in confined spaces. With the continuous development of fiber optic technology, fiber optic thermometers will have great potential.

In addition to the several new temperature measurement instruments discussed above, there are also new temperature sensors such as surface wave SAW temperature sensors (which convert temperature into frequency) and NQR thermometers that utilize the phenomenon of nuclear quadrupole resonance (whose resonance absorption frequency decreases with increasing temperature).