McLeod gauge

The McLeod gauge is a sensitive instrument used to measure the pressure of very low-density gases. It operates by compressing a sample of the gas within a closed capillary tube and applying Boyle's law, which states that for a fixed amount of gas at constant temperature, the product of pressure and volume is constant.12

The process involves raising mercury in the gauge to seal off a portion of the gas. Further mercury elevation compresses this gas. The difference in mercury levels between the trapped gas and the system being measured directly indicates the gas pressure in torr. Because it relies on measurable volumes and pressures, the McLeod gauge is considered an absolute gauge and is often used to calibrate other pressure-measuring instruments.2