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.

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.

•	Pros:
	-	Can serve as a reliable benchmark.
	-	Simple and reliable.
•	Cons:
	-	Limited to static measurements.
	-	Accuracy may not high enough for some applications.
	-	Cannot be used in weightless (0 g) environments.
	-	The liquid in the McLeod Gage must NOT interact with the targeted gas.
	-	Condensation of low-pressure gas to the liquid/solid phase may occur during the compression stage.
	-	Contamination by mercury vapors may occur.