Examples include mass spectrometry, X-ray photoelectron and Auger electron spectroscopies, and electron microscopy. This document provides background information on pressure measurement and vacuum pumps . A manometer consists of a U-shaped tube that is closed and evacuated at one end, and filled with Hg or oil. One atmosphere of pressure at the open side of the tube pushes mercury in the tube to a height of 760 mm in the evacated side of the tube. Lower pressure on the open side of the tube pushes the Hg to less than 760 mm, and provides a measure of the pressure.
Thermocouple gauges operate on the dependence of thermal-conductivity on gas pressure.
In these gauges, a constant current is applied to a metal filament to heat the filament. The temperature of the filament depends on the heat transfer to gas molecules, which depends on the pressure. The temperature of the filament is measured by making a thermocouple junction with the filament. The pressure reading is based on a calibration, which depends on the gas present in the vacuum system. The most common type of ionization gauge is a thermionic, or hot-cathode gauge.