McLeod Gage

Originally the McLeod gauge was developed to increase the low pressure and significantly increase the range of vacuum measurement. Commonly MacLeod gauges measure vacuum pressures in the range of 10–1 and 10–5 Torr. It can be used as a primary standard device to calibrate other low-pressure gauges.

Working Principle of McLeod Gage

A McLeod gauge is essentially a mercury manometer in which a volume of gas is compressed prior to measurement. And the McLeod gauge operates by compressing a known volume of low-pressure gas into a smaller volume so that its pressure is high enough to read. The resulting final volume as well as the pressure indicate the low pressure applied. And the Macleod gauge function is based on Boyle's law. 

pV = pcVc 

where, 

p and V  is pressure and volume of gas before pressure 

pc and Vc is pressure and volume of the gas after compression

Construction of McLeod Gage

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

Benefits of McLeod Gage

• The McLeod gauge is an inexpensive standard that measures vacuum pressure without any electronics or sophisticated equipment. 
• McLeod gauges are used to calibrate other low pressure measuring gauges. 
• The Macleod gauge does not affect the composition of the gas. 
• The reading obtained from the McLeod gauge requires no correction.

Disadvantages of McLeod Gage

• The McLeod gauge is useful for measuring the pressure of real gases obeying Boyle's law.
• Macleod gauges cannot use condensable gases. 
• The gauge should only be handled by a skilled technician. 
• McLeod gauges cannot provide continuous output. 
• Care must be taken to remove any condensable vapors inside or the gauge will show a falsely low reading. The gauge is prone to contamination.