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.
Summary of common vacuum pumps:
Pump | Lowest Attainable Pressure | Typical Use |
---|---|---|
Mechanical pump | 10-3 - 10-3 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.
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