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Many analytical instruments require a low pressure to operate. 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.
Pressure is the force per unit area, with units of pounds per square inch (PSI), or neutons per square meter (N*m-2). In practice, presure is usually reported in atmospheres (atm), which is the atmospheric pressure at sea level, or torr, which is 1.0 mm of mercury in a Hg manometer. One atm = 760 torr.
Summary of common pressure gauges:
|Gauge||Pressure Range||Typical Use|
|Manometer||760-1 torr||systems near atmospheric pressure|
|Thermocouple gauge||1 - 10-3 torr||monitoring mechanical pumps|
|Ionization gauge||10-3 - 10-9 torr||high-vacuum systems|
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. It consists of an electrically heated filament and two electrodes. The filament (at ground voltage) emits electrons that are accelerated to the positively electrode. If the electrons collide with gas atoms or molecules they produce positive ions. Positive ions are collected at the negative electrode, creating an ion current which can be measured. The measured current on the density and on the ionization cross-section of the gas-phase species. As with thermocouple gauges, a calibration must be used for different gases in the vacuum system.
The two important parameters of a vacuum pump are its lowest attainable pressure, and its pumping speed, typically listed as liters per minute (lpm) or cubic feet per minute (cfm). The lowest attainable pressure depends on the design of the pump as listed in the table. The pumping speed of the different types of pumps depends on the physical size of the pump.
Summary of common vacuum pumps:
|Pump||Lowest Attainable Pressure||Typical Use|
|Mechanical pump||10-3 - 10-4 torr||roughing or backing pump|
|Diffusion pump||10-6 torr||vacuum lines|
|Turbomolecular pump||10-9 torr||high-vacuum systems|
Mechanical pumps consist of an inlet, and exhaust with a one-way valve, and an off-center rotating piston in a cylindrical cavity. As the piston rotates, gas is pulled into the cavity, and forced out through the exhaust port. The rotating piston has spring-loaded vanes to create a seal with the cavity walls. This seal, and the exhaust port valve, are lubricated with a low-vapor-pressure oil. A two-stage mechanical pump consists of two pumping cavities in series to achieve a lower vacuum pressure. Accessories needed when using mechanical pumps are a mist filter (or vent) to trap oil mist in the pump exhaust, and a trap to prevent oil vapor from backstreaming into the volume being evacuated.
A diffusion pump consists of a bath of boiling oil that streams through a jet-shaped volume. The oil entrains gas molecules and transports them in the direction of the oil flow. A mechanical pump can then pump away the exhaust from the diffusion pump.
A turbomolecular pump (or just turbo pump) contains a turbine that is spinning at a very high rate of revolution, typically tens of thousands revolutions per second. The turbine blades are spinning faster than the average speed of gas atoms or molecules, so that any gas-phase species that enter the turbo pump are physically forced out of the pump by the turbine blades. A mechanical pump is required to maintain a low pressure and pump away the exhaust from a turbo pump.
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