Pressure Measurement

Mcleod gauge for low pressure measurement

McLeod Gauge is a vacuum pressure gauge suitable for measuring pressure down to about 10-3 Pa. It works by compressing the volume of the gas by a known high factor and measuring the higher pressure; the original pressure of the gas is then found from Boyle’s law.

Principle of Mcleod gauge, Boyle’s laws:

Boyle’s law states that, for a fixed mass of gas at constant temperature, the volume of the gas is inversely proportional to the pressure. This is only strictly true for ideal gases (it can be derived from the ideal gas equation), but it remains a close approximation for real gases.


Construction and working of Mcleod gauge:

McLeod gauge work by taking a volume of gas sample from a vacuum chamber, and then compressing it by tilting it and filling it with mercury. The pressure in this smaller volume is measured with a mercury manometer and, knowing the compression ratio, the original vacuum pressure can be determined. It should be noted that McLeod Gauge is an absolute measurement device.



A mercury rod that moves in a tube is used to isolate a volume of gas at the pressure to be measured. The gas in the volume is then compressed by a known quantity, and the final pressure is obtained with a manometer.

Boyle’s law is then used to find the initial pressure of the final pressure, and the initial and final volumes. This is a relatively slow and laborious process, and a cold liquid nitrogen trap should be used to prevent the vapor pressure of the mercury from disturbing the pressure measurements.

Procedure to check the pressure:

  • Turn the McLeod meter horizontally so that the mercury is completely collected inside the reservoir.
  • Connect the vacuum pump to the nozzle of the hose provided on the back of the pressure gauge.
  • Turn on the vacuum pump and begin to absorb all the air inside the meter.
  • Turn the meter slowly so that the mercury fills the two pressure gauges. You can see that one arm of the pressure gauge is closed (end of reading) and the other arm is open to the vacuum chamber (reference end). Tilt the indicator so that the mercury level exactly matches the ZERO mark on the reference end of the pressure gauge.
  • The mercury level at the reading end will show the vacuum built into the vacuum chamber. The vacuum can be measured from 0.01 to 10 mm Hg.

Advantages of the McLeod Gauge:

  • It is independent of the composition of the gas.
  • It serves as a reference standard for calibrating other low-pressure manometers.
  • There is a linear relationship between the applied pressure and h
  • It is not necessary to apply corrections to the McLeod meter readings.

Limitations of McLeod Gauge:

  • The gas whose pressure is to be measured must obey Boyle’s law.
  • Moisture traps must be provided to prevent the entry of considerable vapor into the meter.
  • It is measured only on a sampling basis.
  • It can not give a continuous exit.


Instrumentation Engineer

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