The thermometer achieves thermal balance through conduction or convection, so that the indication value of thermometer can directly represent the temperature of the measured object.
Generally, the measurement accuracy is high. In a certain temperature range, the thermometer can also measure the temperature distribution inside the object. However, for moving objects, small targets or objects with small heat capacity, large measurement errors will occur. The commonly used thermometers include bimetal thermometers, liquid glass thermometers, pressure thermometers, resistance thermometers, thermistor and thermoelectric couple. They are widely used in industry, agriculture, commerce and other sectors. These thermometers are often used in daily life. With the wide application of cryogenic technology in national defense engineering, space technology, metallurgy, electronics, food, medicine, petrochemical and other departments and the research of superconducting technology, cryogenic thermometers for measuring temperatures below 120K have been developed, such as cryogenic gas thermometers, vapor pressure thermometers, acoustic thermometers, paramagnetic salt thermometers, quantum thermometers, low-temperature thermal resistance and low-temperature thermoelectric couple Etc. Low temperature thermometer requires small volume, high accuracy, good reproducibility and stability. The carburized glass thermal resistance made of porous high silica glass is a kind of temperature sensing element of low temperature thermometer, which can be used to measure the temperature in the range of 1.6 ~ 300K.

working principle

Sensor based on metal expansion principl
The metal will produce a corresponding extension after the environmental temperature changes, so the sensor can carry on the signal conversion to this kind of reaction in different ways.

Bimetallic sensor

Bimetallic sheet consists of two pieces of metal with different expansion coefficient sticking together. With the change of temperature, the expansion degree of material a is higher than that of the other metal, which causes the metal sheet to bend. The curvature of the bend can be converted into an output signal.
Bimetal rod and tube sensor

With the increase of temperature, the length of the metal tube (material a) increases, but the length of the non expanding steel rod (metal b) does not increase, so that the linear expansion of the metal tube can be transferred due to the change of position. In turn, this linear expansion can be converted into an output signal.

Sensor designed for deformation curve of liquid and gas

When the temperature changes, the volume of liquid and gas will also change accordingly.

Various types of structures can convert this change of expansion into a change of position, which produces a position change output (potentiometer, inductive deviation, baffle, etc.).