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Vacuum Terms


The process that involves the penetration of a gas or vapour into the bulk of a solid or liquid.

The assimilation of a gas by the surface of a solid.

The positive electrode in an electron device. The most positive electrode in an ion pump. This electrode may collect electrons and negative ions.


A system of cold surfaces placed between the inlet of a pump and the region on which it is pumping to condense back streaming vapour and return it to the pump.

The heating of vacuum system components during the pumping process for the purpose of degassing; accelerates the evolution of adsorbed and absorbed gases.

Bayard-Alpert Gauge
A type of hot-cathode ionisation gauge in which the cathode is located outside the grid, and the ion collector is a thin wire in the axis of the tube.

Bell Jar
A cylindrical vacuum chamber with a removable seal supported by a base plate.


The ratio of charge to potential on an electrically charged, isolated conductor.

Capacitance Manometer
A manometer in which a pressure reading can be derived from a change in capacitance between the pressure sensitive element and another part.

The negative electrode in an electron device (the filament). The most negative electrode in an ion pump. This electrode may emit electrons and collect positive ions.

Cold Cathode Gauge
An ionisation gauge in which electrons are released from the cathode by photon and ion bombardment at ambient temperature, and for which thermionic emission can be neglected.

Condensation Conductance
The physical process by which a vapour becomes a liquid or solid.
The ratio of throughput, under steady-state conservative conditions, to the pressure differential between two specified cross-sections inside a pumping system.

A Pirani-type gauge with convection enhancement that provides a wider useful measuring range than a conventional Pirani gauge, particularly for the Atm to 1.10-4 mbar range.

Convection, Gas Cryogenic Pump (Cryopump)
The transfer of heat by gaseous motion caused by gravity.
A vacuum pump that operates by the condensation and/or sorption of gas at surfaces maintained at temperatures sufficiently low for the vapour pressures of the condensed gases to be insignificant


The deliberate removal of gas from a material, often achieved by heating the material under vacuum.

The release of absorbed atoms and/or molecules from the surface of a solid as a result of: a) atom and/or molecule impact, b) electron impact, c) ion impact, d) photon impact, or e) thermal energy.


An electrically conducting element of an electron device, usually connected to an external electric circuit, which emits, collects, or influences the flow of the current in the device.

Elementary particle bearing a negative charge and a mass approximately 1/1840 of a hydrogen atom.

Electron Beam
A stream of electrons moving with about the same velocity and in the same direction-forming a beam.

Electron Stimulated Desorption - The process of desorption of ions from surfaces as a direct result of electron bombardment.


A device for transmitting electrical current, fluids or mechanical motion through the walls of a vacuum system.

See Cathode.


A state of matter in which the molecules move freely to occupy the total volume of an enclosure.

Gauge Tube
A vacuum transducer or sensor.

Gauge Controller
The electronics that powers a gauge tube and provides a visual display and/or analog/digital output signal of the pressure.

A material that reacts readily with active gases to form stable low vapour pressure chemical compounds, so as to remove these gases from the gas phase.

An electrode inside a hot cathode ionisation gauge, which is charged positive with respect to the cathode, and serves to accelerate electrons from the cathode.


Hot Cathode Gauge
An ionisation gauge in which pressure is measured in terms of the current of positive ions produced by electrons emitted from a heated cathode.

Hot Filament Ionisation Gauge Tube
An envelope that contains a hot cathode ionisation gauge and tubulation that can be connected to a vacuum system.


An atom or molecule having attained one or more units of electrical charge due to loss or gain of electrons.

A process that results in the formation of ions. Such a process can occur by adding (or removing) one or more electrons to (or from) and atom or molecule.

Ionisation Gauge
A vacuum gauge comprising a means of ionising the gas molecules, and a means (Gauge Controller) of correlating the ion current to the collector with the pressure of the gas.

Ion Collector
An electrode in an ionisation gauge. Used to collect and measure ion current.

Ion Current
The rate of ion flow.

Ion Pump
An electron device in which ionisation produces a significant rate of gas removal. The ion pump is a “capture and hold” type of pump.


Laminar Flow
Nonturbulent flow of a fluid in layers near a boundary.

A hole or permeable element through which gas flow may occur under the action of a pressure difference.


An instrument for measuring the pressure of gases and vapours.

Mass Spectrometer
An instrument that produces a beam of ions from a sample, separates the resulting mixture of ions according to their mass-to-charge ratios, and provides output signals that are measurements of the relative abundance of the ionic species present.

Mean Free Path
The average distance a gas molecule travels without colliding with another molecule.

Mechanical Pump
A device with moving parts such as rotating vanes, a piston, or eccentric rotary members used for pumping gas or vapour.

The first miniature, all-metal ionisation gauge with dual ion collectors that provides reliable performance over a wide pressure range.

A unit of pressure defined as 10-3 millimeters of mercury (10-3 Torr)

A type of vacuum gauge where the gauge tube is provided with the control electronics in an integral package.

Molecular Flow
The movement of a gas through a channel under conditions such that the mean free path is much greater than the dimension of a transverse section of the channel. The molecules collide mainly with the surfaces rather than with each other.

A single layer of atoms (or molecules) completely covering a surface.


Nude Gauge
A vacuum gauge that does not have its own envelope; designed to be inserted into a vacuum system.


The evolution of gas from a liquid or solid under vacuum.


Partial Pressure
The pressure of a designated component of a gaseous mixture. The sum of the partial pressures of all the component species in a mixture is equal to the total pressure.

A basic unit of pressure equal to one Newton per square meter or 7.510-3 Torr. 101,000 Pascal = 760 Torr = 1 atmosphere

The passage of gas through a solid. The process always involves diffusion through the solid and may involve surface phenomena such as sorption, dissociation, migration and desorption.

A state of matter; solid, liquid or gas.

The smallest unit of electromagnetic energy; with respect to its particle characteristics, generally regarded as a discrete particle having zero mass, no electric charge, and an indefinitely long lifetime.

Pirani Gauge
A thermal conductivity gauge containing a heated filament having a large temperature coefficient of resistance. Because heat dissipation from the filament is a function of the gas pressure in a certain pressure range, filament resistance or power required to maintain a constant filament resistance can be correlated with gas pressure.

An ionised gas containing approximately equal numbers of positive and negative charge carriers. A state of gaseous matter in which electrons, ions, atoms and molecules may coexist in an equilibrium distribution with overall, long-range electrical neutrals.

The average normal force per unit area exerted by gas molecules impacting on a surface.

Pressure Differential
The difference in pressure between two regions in space that are at different absolute pressures.

Primary Electron
Electrons emitted from the cathode of an ionisation gauge.


Residual Gas
Gas remaining in the vacuum chamber after pumpdown.

Residual Gas Analyzer
A device for measuring the amounts and species of various gases present in vacuum chamber.

Resistance Heating
Heating a material by passing an electric current through it.


An electron produced by the bombardment of a material by a primary electron.

A measure of the ion generation and ion collection efficiency of an ionisation gauge. It is the ratio of ion current to the product of the ionising electron current and the corresponding pressure in an ionisation gauge. Its value depends primarily on the geometry of the gauge, the grid voltage, and the ionisation cross section of the gas.

A generic term used to describe the uptake of a gas or vapour by a solid without distinction as to whether the process occurs by adsorption and/or absorption.

When an electrical discharge is passed between electrodes at a low gas pressure, the cathode electrode is slowly disintegrated under the bombardment of the ionised gas molecules. The disintegrated material leaves the electrode surface either as free atoms or in chemical combination with the residual gas molecules. Some of the liberated atoms are condensed on surfaces surrounding the cathode; the remainder are returned to the cathode by collision with gas molecules.

A Bayard-Alpert type ionisation gauge tube, which is designed and manufactured to exacting specifications in order to provide exceptional measurement accuracy and stability.

The process of transition directly from the solid to vapour phase without passing through the intermediate liquid phase.

A support or carrier for a partial layer, layers, films or thin film component. Commonly a glass slide or polycrystalline alumina are used for thick film circuits and single crystals for single crystal film deposition.


Thermistor Gauge
A form of Pirani gauge in which the temperature sensitive elements are made of a semiconducting material instead of metal.

Thermocouple Gauge
A thermal conductivity gauge that contains a heated filament and a bimetallic thermocouple junction for the measurement of filament temperature as a function of gas pressure.

The flow rate into a vacuum pump or through an orifice. Pumping Speed times Pressure; or Conductance times Pressure Differential.

A unit of pressure; 1/760th of a standard atmosphere; 1 mm Hg

Transition Flow
The flow of gas through a channel under conditions such that the mean free path is on the same order as the transverse dimensions of the channel. In this pressure range, the flow characteristics are determined by collision of the gas molecules with the surfaces as well as with other gas molecules.

A device used to capture and retain vapours and gases on cold and/or adsorbent surfaces.

Turbomolecular Pump
An axial flow turbine for operation in the molecular flow range consisting of a series of alternate circular rotor and stator disks both of which have inclined blades designed to impart momentum change to gas molecules in a preferential direction from the pump inlet to the outlet.

Turbulent Flow
Gas flow at high pressures and velocities, where the flow is not laminar.


The condition of gaseous environment in which the gas pressure is below atmospheric pressure. Generally classified in six pressure ranges:
Low Vacuum 25 mbar to Atm
Medium Vacuum 10-3 to 25 mbar
High Vacuum 10-6 to 10-3 mbar
Very High Vacuum 10-9 to 10-6 mbar
Ultrahigh Vacuum 10-12 to 10-9 mbar
Extreme Ultrahigh Vacuum below 10-12 mbar

Vacuum Chamber
The container or enclosure in a vacuum system that is evacuated and in which the process or experiment is performed.

Vacuum Gauge
An instrument for measuring gas pressure below atmospheric pressure-usually consists of a transducer (gauge tube) and its associated electronics (controller).

Vacuum Pump
A device for reducing and maintaining the gas pressure in a vessel below atmospheric pressure, either by transferring the gas molecules out into the atmosphere or by capturing and holding them on surfaces within the pump.

Vacuum System
A complete assembly consisting of the vacuum chamber, pumps, lines, valves and monitoring instruments used to conduct a vacuum process or experiment.

A mechanical device by which the flow of gas or vapour may be started, stopped, or regulated by a moving part that opens or obstructs a passage.

The gaseous phase of a substance that is normally a solid or a liquid at room temperature and atmospheric pressure.

Vapour Pressure
Usually means saturated vapour pressure; a fixed value at a particular temperature for all substances.

Viscous Flow
The flow of gas through a channel under conditions such that the mean free path is very small in comparison with the smallest dimension of a transverse section of the channel. At these pressures the flow characteristics are determined mainly by collisions between the gas molecules. The flow may be laminar or turbulent.


X ray Limit
The theoretical lower measurement limit of an ionisation gauge.
A small current measured at the collector of an ionisation gauge, which is independent of pressure and caused by photoelectrons leaving or arriving at the collector. This current results from x ray irradiation of the ion collector electrode or of the envelope of the gauge and/or other electrodes, respectively. (The x ray radiation results from the electron emission of the cathode impacting on the grid and the gauge tube envelope.) This current limits the ability of the gauge to measure pressures that correspond to small ion currents.