Pressure thermometer although belongs to the expansion thermometer, but it is not by the thermal expansion of the material volume change or size change reflects the temperature, but by the liquid or gas in the closed container after the heat pressure rise reflects the measured temperature, so this thermometer indicator is actually an ordinary pressure gauge. The main features of the pressure thermometer are simple structure, high strength and good vibration resistance.
Pressure thermometer is mainly composed of a temperature package, capillary tube and pressure-sensitive components (such as spring tube, membrane box, bellows, etc.), shown in Figure 2.2.3. Temperature package, capillary tube and spring tube of the three inner cavity together constitute a closed container, which is filled with working substances. Temperature package in direct contact with the measured medium, it should be sufficient to transfer the temperature change to the internal working material. Therefore, its material should have anti-corrosion ability and good thermal conductivity. In order to improve the sensitivity, the thermal expansion of the temperature package itself should be much smaller than the expansion of the internal working material, so the coefficient of body expansion of the material should be small.
In addition, there should be sufficient mechanical strength to withstand large internal and external pressure differences on thin vessel walls. Usually, with stainless steel or brass manufacturing temperature package, brass can only be used in non-corrosive media. When the temperature package is heated, will make the internal temperature of the working material and pressure increases, this pressure through the capillary tube to the spring tube, so that the spring tube deformation, and by the drive system to drive the pointer, indicating the corresponding temperature value.
Currently produced pressure thermometer according to the closed system filled with different working substances can be divided into gas-filled pressure thermometer and steam-filled pressure thermometer.
