The McLeod gauge is used for measuring pressures ranging from 0.01 micron to 1 mm of Hg.
Figure 1: McLeod Gauge.
Working Principle of McLeod Gauge
It operates on the principle of compressing a known volume of low pressure gas to a higher pressure and measuring the resulting change in volume by a mercury manometer.
Construction and Working of McLeod Gauge
The construction of McLeod gauge is shown in Fig. 1. McLeod gauge comprises a system of glass tubing made of tough glass and mercury is used to trap the known volume of gas. The gauge is connected to the unknown gas whose pressure is to be measured. The plunger moves up, lowers the mercury level to the cut off positions, entering the gas at unknown pressure through the tube.
This gas fills the tubes down to the cut off position of mercury level. Here the pressure is equal throughout the tubes.
Now the plunger is moved down to rise the mercury above the cut-off and traps the gas inside the bulb and measuring capillary.
Further pushing of plunger compresses the gas in the measuring capillary and mercury in the reference capillary reaches to zero reference line.
The pressure in the measuring capillary is higher than the measured pressure in the reference capillary. This difference in pressure causes the difference in mercury level in two tubes.
This difference in height represents the rise in gas pressure and unknown pressure is calculated.
Advantages of McLeod Gauge
- It is very simple in use.
- Measurement is independent of gas composition and it is related to physical dimensions of gauge.
- It is a very accurate pressure measuring device.
- It can be used as a standard to calibrate other low pressure gauges.
Disadvantages of McLeod Gauge
- If the gas contains the vapour, it may not give correct result.
- It is applicable to those systems where mercury is tolerable.
- It does not give continuous output.