The McLeod Gage is considered the standard for low-pressure (vacuum) measurements, where the pressure is below 10-4 torr (10-4 mmHg, 1.33×10-2 Pa, 1.93×10-6 psi). A McLeod Gage compresses a sample of low pressure gas to a sufficiently high pressure, obtains the compressed pressure from a standard manometer, and then calculates the original low pressure through Boyle's law. The compression is passed through a dense, nearly-incompressible, low vapor pressure fluid, such as mercury. A schematic of the McLeod Gage is shown below.
Typical McLeod Gage and Its Measurement
The error in typical McLeod gage measurements is usually larger than 1% and may be much larger, due to the possibility of gas to liquid (or solid) phase change during compression, and to the contamination by mercury vapors.