In Vapour Pressure Thermometers, a filling substance may be liquid, gas or vapour. Vapour pressure thermometer falls in the category of filled system thermometer.
Working and Construction of Vapour Pressure Thermometer
Figure 1: Vapour Pressure Thermometer.
As seen in the figure 1 the bulb contains capillary tube and Bourdon tube. An indicator is attached to the free end of a Bourdon tube. Capillary tube is very long about 5 meter. When the thermometer works, the free end of the Bourdon tube moves and due to this motion the indicator moves on the scale which is calibrated to read the temperature directly.
Materials filled in the bulb:
The bulb is partly filled with liquid while the capillary and Bourdon tube are filled with vapour. The lower side of the bulb is filled with non-volatile fluid, the middle part by volatile fluid and upper space occupy the vapour.
There is a free surface between vapour and volatile liquid. In the operation of the thermometer, the liquid boils and vaporizes which creates vapour inside the capillary tube and Bourdon tube. When the temperature the substance surrounding the bulb drops, the liquid and vapour inside the bulb also cools.
Due to this some vapour condenses and pressure inside the system decreases. The action goes on till the pressure drops to vapour pressure of the boiling liquid. Change in pressure creates a motion on Bourdon tube and its movable end moves. Due to the movement of Bourdon tube, the pointer moves on the scale. The scale is calibrated so that temperature is indicated by the pointer on the scale. In some cases instead of a pointer and scale, the strip chart recorder is used which records the temperature continuously and a graph is obtained for variations of temperature for a period of time. Following sketch shows the construction of vapour pressure thermometer.
Advantages of Vapour Pressure Thermometer
- Cheaper in cost.
- Compact and rugged construction.
- Good time response.
- Maintenance is very less.
- Very sensitive.
- Good accuracy ± 1%.
- Operation is stable.
- No electric power requirement.
Disadvantages of Vapour Pressure Thermometer
- Span of temperature is much less (compared to electrical type).
- Not repairable. On failure the entire system has to be replaced by new unit.
- Limited maximum temperature.
- Errors are produced due to:
(i) Radiation effect, (ii) Immersion effect, (iii) Barometric effect, (iv) Elevation effect and (v) Ambient temperature effect.
Applications of Vapour Pressure Thermometer
- Temperature of wide ranges can be measured.
- Temperature can be recorded by pen and strip chart recorder.
- Used in clinical centres.