The pressure thermometer temperature measurement is based on fluid expansion. Fluid expansion thermometer represent one of the most economical, versatile, and widely used devices for industrial temperature measurement applications.

Basic principle :

The basic principle in pressure thermometers are when liquids, gases or vapours are heated, they will expand, when they are cooled, it will contract. According to the temperature change, the pressure also varies in the filled system. This difference in pressure will give the temperature change.

Construction :

The fluid expansion thermometer consists of a sensing bulb, capillary tube and a bourden tube. The sensing bulb containing a liquid, gas or vapour is immersed in the environment. The bulb is connected by means of a capillary tube to pressure measuring device (bourden tube) as shown in fig.

The bourden tube’s free end is connected to a calibrated scale. The pressure change in the bourden tube is measured in the calibrated scale through a pointer.

Operation :

The sensing bulb is introduced into the medium those which temperature is to be measured. The difference of temperature between environment and medium to be measured will increase or decrease the pressure in the bulb.

This change in pressure of the fluid in the bulb is transmitted to the bourden tube. The transmission of pressure takes place through the capillary tube. Thus, the pressure change will be taken as the indication of temperature change.

Normally, before the pressure change in the bourden tube, the tube is in elliptical shape. After change in pressure, it tends to become a circle.

So, this small displacement is amplified using the links, and the pointer will reflect for small displacement. This new position of the pointer on the calibration scale is the indication of temperature.

Advantages :

1. Capillary tubes as long as (60m) may be used.
2. The response of the gauge is primarily dependent on the bulb size and properties of the fluid.
3. A small size bulb also can also be used to get the highest response.
4. Low in cost and stable in operation.
5. Accurate within ±1°C.

Limitations :

1. The error may occur in filling fluid and in tube.
2. It is not suitable for long transmitting capillary tubes.
3. The filling fluid may give the calibration drift.