Pressure-spring thermometers are essential in industries where remote temperature readings are required. Unlike glass or bimetallic thermometers, which provide point-based measurements, these thermometers operate based on the expansion of liquids, vapors, or gases.
Types of Pressure-Spring Thermometers
- Liquid-filled Thermometers
Function similarly to liquid-in-glass thermometers but operate a Bourdon tube for readings. They provide good linearity and accuracy and can measure temperatures up to 550°C.
- Vapour-Pressure Thermometers
Consist of a partially filled bulb and vapor system with liquids like methyl chloride, ethyl alcohol, ether, or toluene. The thermometer’s pressure-temperature characteristic is non-linear, evident from the vapor pressure curve for different chemicals.
- Gas-filled Thermometers
Filled with gases like nitrogen under high pressure (1,000-3,350 kPa) at room temperature, they follow gas laws to produce a linear relationship between temperature and pressure.
Function and Components of Gas-filled Thermometers:
The system’s bulb is at the measuring point, filled with materials with a high expansion coefficient, ensuring sensitive responses to temperature changes.
The metal tube terminates in a Bourdon spring pressure gauge, which provides readings in degrees.
Accuracy and Application: These thermometers can be accurate within 0.5% and are effective for remote measurements up to 100 meters. However, they require calibration because the stem and Bourdon tube are temperature-sensitive.
System Limitations: The lowest operating temperature must be above the liquid’s boiling point, and the maximum temperature is constrained by the liquid’s critical temperature. The response time of the system is slow, typically around 20 seconds.
Utility: These devices are versatile, with applications ranging from chart recorders to actuators and potentiometer wipers for electronic signals. The bulb’s pressure increases as temperature rises, and the Bourdon tube, bellows, or diaphragms detect the pressure change.
