Principle of McLeod Vacuum Gauge

A known volume gas is compressed to a smaller volume whose final value provides an indication of the applied pressure. The gas used must obey Boyle’s law given by;

P1V1=P2V2

Where, P1 = Pressure of gas at initial condition (applied pressure).
P2 = Pressure of gas at final condition.
V1 = Volume of gas at initial Condition.
V2 = Volume of gas at final Condition.

Initial Condition == Before Compression.
Final Condition == After Compression.

A known volume gas (with low pressure) is compressed to a smaller volume (with high pressure), and using the resulting volume and pressure, the initial pressure can be calculated. This is the principle behind the McLeod gauge operation.