When liquids and gases are concealed in a pressure vessel or some sort of sealed system, internal pressure is created. To measure this pressure, pressure gauges are utilized. Depending on the gauge, the devices can read between 10 and 10,000 psi. Vacuum gauges are a sub-category of pressure gauges that are used to measure sub-atmospheric or vacuum pressures within a contained system. Perhaps the most sensitive vacuum gauge is the ionization gauge, which is utilized to measure very low pressures.
The ionization gauge measures electrical ions to indirectly sense pressure within a vessel. These ions are produced after the gas inside the vessel is bombarded with electrons. Lower density gases will produce fewer ions, and greater density gases will produce more ions. This makes the ionization gauge reliant on the nature of the gas being measured for calibration. This nature is not always known. However, the gauges can be calibrated against the more stable and independent of gas chemistry McLeod gauge. Electrons generated by thermionic emission collide with gas atoms to generate positive ions. These ions are attached to the collector whose inside correct is proportional to the rate of ionization.
There are two main types of ionization gauges; hot cathode and cold cathode. A third type exists called the spinning rotor gauge. However it is more expensive and sensitive. A hot cathode produces an electron beam from an electrically heated filament. All throughout the gauge the ions travel, ionizing surrounding gas molecules. A negative electrode will collect the resulting ions. A cold cathode is essentially the same as a hot cathode, except the discharge of a high voltage produce the electrons. For gases at high vacuums the composition is unpredictable. This makes it necessary for a mass spectrometer to be used alongside the ionization gauge to ensure accurate measurements.