Working of McLeod Gauge

McLeod Gauge

McLeod gauge amplifies the low pressure and was developed to extend the range of vacuum measurement significantly.
The McLeod Gauge measures the vacuum pressure in the range between 10^-1 and 10^-5 torr. This can be used as a primary standard device for calibrating other low-pressure gauges.

Working Principle

McLeod gauge is essentially a mercury manometer in which a volume of gas is compressed before measurement. It operates by compressing a low-pressure gas of known volume into a smaller volume so that its pressure is sufficiently higher enough to be read. The resultant final volume and pressure provide the indication of applied low pressure.

Construction

McLeod Gauge - Before and After Compression

The McLeod gauge consists of a reservoir containing mercury. A plunger is attached on the top of the reservoir which is used to raise or lower the level of mercury into the reference column and bulb. Above the reservoir, there is a bulb and reference column. The point of connection of bulb and reference column is the opening or cut-off point. The other end of the reference column is open to vacuum pressure and it has a reference capillary. The reference capillary has a zero reference point up to which the mercury is raised. The mercury rises in the capillary as much as it rises in the column but only the volume differs. The reference column is attached to a measuring capillary which is a sealed chamber and from which the final volume of gas is read.

Operation

The working of McLeod Gauge is based on Boyle’s law.

pV = p_cV_c

where,

p & V – Pressure and Volume of gas before compression

p_c & V_c – Pressure and Volume of gas after compression

The unknown vacuum pressure source is connected to the reference column and the pressure is applied. The level of mercury is adjusted so that it at the opening or cut-off point. Now, the unknown pressure, p, fills the bulb and capillary. The volume of unknown pressure is the volume of bulb and capillary which is given by V.

The mercury is forced into the bulb and capillary by operating the plunger. Once the level of mercury crosses the cut-off point or opening, it stops the entry of applied pressure into the bulb and measuring capillary. The level of mercury is raised until it reaches the zero reference point. The pressure and volume of gas trapped in measuring capillary are read and unknown vacuum pressure is calculated.

The pressure of gas remaining in the measuring capillary is given by,   

p_c = p + h

The volume of gas remaining in the measuring capillary is given by, 

V_c = A_ch

Where, h – the difference in height of mercury in reference and measuring capillary

A_c – Area of the cross-section of measuring capillary

By Boyle’s Law,

          pV = (p +h) A_ch

                = p A_ch + A_ch²

p(V - A_ch) = A_ch²

             A_ch <<<<<V, so it can be neglected.

 p = A_ch²/ V

Advantages

1. McLeod gauge is an inexpensive standard that measures vacuum pressure without any electronics or sophisticated equipment.

2. It is used for calibrating other low pressure measuring gauges.

3. It is not influenced by gas composition.

4. The readings obtained from McLeod gauge do not require any correction.

Disadvantages

1. The McLeod gauge is useful for measuring pressures of true gases that obey Boyle’s law.

2. Condensable gases cannot be used.

3. The gauge is to be handled only by skilled technicians.

4. It cannot give continuous output.

5. Precautions are to be taken to remove any condensable vapors present inside otherwise the gauge will show a false low reading.

6. There are chances for the gauge to get contaminated.


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