Pressure-spring thermometers are used where remote indication is required, as opposed to glass and bimetallic devices, which give readings at the point of detection. The pressure system can be used to drive a chart recorder, actuator, or a potentiometer wiper to obtain an electrical signal
The pressure-spring device has a metal bulb made with a low coefficient of expansion material along with a long metal narrow bore tube. Both contain material with a high coefficient of expansion. The bulb is at the monitoring point. The metal tube is terminated with a Bourdon spring pressure gauge.
As the temperature in the bulb increases, the pressure in the system rises. Bourdon tubes, bellows, or diaphragms sense the change in pressure. These devices can be accurate to 0.5%, and can be used for remote indication up to a distance of 100m, but must be calibrated, since the stem and Bourdon tube are temperature-sensitive.
There are three types of pressure-spring devices. These are:
• Liquid filled; • Vapor pressure; • Gas filled.
The liquid-filled thermometer works on the same principle as the liquid in glass thermometer, but is used to drive a Bourdon tube. The device has good linearity and accuracy, and can be used up to 550°C.
The vapor-pressure thermometer system is exactly as shown in Figure above. The bulb is partially filled with liquid and vapor, such as methyl chloride, ethyl alcohol, ether, or toluene. In this system, the lowest operating temperature must be above the boiling point of the liquid, and the maximum temperature is limited by the critical temperature of the liquid. The response time of the system is slow, being of the order of 20 seconds.
A gas thermometer is filled with a gas, such as nitrogen, at a pressure of between 1,000 and 3,350 kPa, at room temperature. The device obeys the basic gas laws for a constant volume system, giving a linear relationship between absolute temperature and pressure.