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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.

Pros and Cons

  • 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.