Pressure-type thermometer: In this type of thermometer, the temperature-sensing element is a sealed, incompressible fluid or a volatile liquid. A window-type pressure gauge serves as the indicating component, and the two are connected by a relatively long thin tube with an inner diameter ranging from 0.18 to 0.25 mm. When the pipe is connected, changes in temperature cause the fluid to expand or alter the vapor pressure; the pressure gauge’s pointer then indicates both the pressure and the temperature.
Although the temperature measured by this thermometer is not particularly high, it can indicate temperatures from a considerable distance—up to about 20 meters—and is easy to use. For this reason, it is widely employed as an instrument for monitoring the temperatures of vehicle coolant and lubricating oil, as well as serving as an instrument on control panels. As with other thermometers, it is of course important to ensure that the sensing element is in full contact with the medium being measured. In addition, the thin capillary tube must be carefully secured to prevent vibrations. The resistance values of metal wires such as platinum, copper, and nickel have a certain relationship with temperature; resistance thermometers are precisely based on this principle.
This type of thermometer is easy to implement for remote indication and can also automatically record measurement values. When platinum is used, the measurement range can reach up to 500℃; when copper is used, it can reach up to 120℃; and when nickel is used, the range can extend to 300℃. For internal combustion engines, in addition to measuring exhaust gas temperatures, this thermometer can be applied in a variety of other scenarios. However, a drawback is that the temperature-sensing component cannot be made very small. Another type of temperature sensor is the semiconductor thermistor. Due to its large temperature coefficient, it can be miniaturized at extremely low cost, making it widely used as a replacement for the wire-based thermometers mentioned above. A key characteristic of thermistors is that their temperature coefficient is roughly one order of magnitude larger than that of metal wires, and it is negative. Although recent improvements have significantly reduced the variation of resistance value and temperature coefficient over time, it is still advisable to calibrate such thermistors frequently in practical applications.