12.4.4 Vacuum Technology
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IUPAC. Analytical Division. Compendium of Analytical Nomenclature (the “Orange Book”). Definitive Rules, 1979.
|12.4.4 Vacuum Technology|
|IUPAC Orange Book Chapter 12|
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12.4.4 Vacuum technology
Accumulation leak detection technique
A leak detection technique in which tracer gas (e.g.helium) enters the part under test and is allowed to accumulate within the part, or within a system containing the part, for a period of time. The part or system is then opened to the leak detector. The system may contain the leak detector element.
The process by which a gas or vapour is bound on a solid or liquid surface.
The additional resistance encountered by a gas flowing through a tube with an abrupt reduction in cross-section. For molecular flow it is the product of the molecular effusion impedance of an orifice with a cross-sectional area A2 and the aperture correction factor (1 - A 2/A 1), where A 1 is the cross-sectional area of the larger tube and A 2 that of the smaller tube.
A mass spectrum of residual gas species in a system. It is usually obtained before a sample of interest is analyzed in order to deduce by subtraction of spectra the true mass spectrum of the sample.
Backing space leak detection technique
A leak detection sytem in which the leak detector is connected to the forevacuum side of a pump attached to a vacuum system or an element undergoing leak test. A tracer gas is sampled at a higher pressure after compression by a diffusion pump or other pump operating at high speed relative to its backing pump.
The flow of charged and/or neutral particles emanating from a pump and moving in the opposite direction to the intended flow of the gases being pumped.
A term for a leak detection technique in which the part under test is enclosed in a bag (or other enclosure) that is filled with a tracer gas at a pressure slightly above atmospheric.
The number of molecules striking a unit area of surface per unit time. Alternatively the number of collisions between molecules or atoms in a gas per unit volume and unit time. The collision frequency per molecule is equal to the probability per unit time that a molecule will collide with a surface or another molecule.
The collision probability per unit time for an atom or molecule travelling at a specified speed through a gas.
The ratio of throughput, under steady-state conservative conditions, to the pressure differential between two specified cross-sections within a pumping system.
Critical inlet pressure
The inlet pressure of a vapour pump above which an abrupt decrease in pumping speed occurs.
A technique of producing vacuum by physical adsorption of gases on solid adsorbents cooled to a low temperature.
An opening in a vacuum punp or stage from which gases are ejected either to a preceding stage or to atmosphere.
Holding time (pump)
The time required for the forepressure of an isolated vapour pump or diffusion pump to reach the limiting forepressure.
The reciprocal of conductance.
The number of molecules striking a unit area of surface per unit time.
The ratio of the mean free path of a gas molecule to a characteristic dimension of the channel through which the gas is flowing. For a cylindrical tube a characteristic dimension is the diameter.
The quantity of gas passing through a leak per unit time.
Load (vapour pump)
The quantity of gas, not including pump fluid vapour, in mass units, flowing through the pump per unit time. It is also called mass flow.
An instrument for measuring the pressure of gases and vapours.
Mean free path
The average distance a particle travels between successive collisions with other particles
of an ensemble.
The mean distance a particle travels between successive collisions with other particles
or surfaces. When the pressure is high or the vessel dimensions are large, so that the
mean path is small with respect to the vessel dimensions, the mean path and the mean
free path are numerically equal.
The molecular flow of gas from a region at one pressure to one at a lower pressure
through an orifice, in a wall of negligible thickness, with a diameter much less than the
mean free path of the molecules.
The net number of gas molecules crossing a specified surface in unit time. Those
having a velocity component in the same direction as the normal to the surface are
counted as positive and those with a velocity component in the opposite direction are
counted as negative.
Molecular velocity distribution
The average value of the fraction of the molecules in a small volume, dr, surrounding a
given point, located by the radius vector r in a fluid medium, which have velocity
vectors lying within an infinitesimal volume dv, surrounding the point in velocity space.
The averaging process is carried out over a time long enough to smooth statistical
fluctuations in the molecular populations, but short compared with the time required for
significant variations in the macroscopic properties. For a gas in equilibrium at rest,
the distribution of the velocity vectors with a given magnitude is uniform over a sphere
about the origin in velocity space. The distribution known as MaxwellâˆšÃ‰â€šÃ„Â¢s law of velocity
fv dv = 4 âˆšÃ‡Â¬Ï€v 2[m/(2 âˆšÃ‡Â¬Ï€kT)]3/2 exp[-mv 2/(2kT)] dv,
where m is the mass of the molecule, T is the absolute temperature, k is the Boltzmann
constant, and 4âˆšÃ‡Â¬Ï€v 2dv is the volume of a spherical shell of radius equal to the magnitude
of v and of thickness dv equal to the increment in this magnitude and gives the fraction
of molecules having speeds between v and v + dv. The function fv is the Maxwellian
distribution function. Net speed (Vacuum pump)
The throughput across a section remote from the pump inlet divided by the pressure as
measured at that section. The net speed can be calculated, when the pump speed is
known, by adding, to the sum of all the impedances between the pump inlet and the
given cross section, the reciprocal of the measured pump speed and then taking the
reciprocal of the result.
Pascal (Symbol Pa)
The pascal is the SI base unit of pressure. It is equivalent to 1 newton per square cm.
This term replaces the Torr. (1 mm Hg = 1.00000014 Torr = 133.3323867 pascal.)
The rate of flow of gas through a unit area and a unit thickness of a solid barrier per
unit differential pressure at a given temperature.
The passage of a gas through a solid. The process always involves diffusion through
the solid and may involve surface phenomena such as sorption, dissociation, migration
An adsorption process caused by van der Waals forces between adsorbent and
The volumetric rate of gas flow across a section at the pump inlet. It can be obtained
from the ratio of throughput of a gas to the partial pressure of that gas at a specific
point. Pumping speed
Same as pump speed.
The ratio of the speed to the product of the vacuum pump inlet cross section area and
the maximum flow rate per unit area as given by the effusion law. It is also called
efficiency or speed efficiency. Speed of exhaust
The instantaneous rate of reduction of pressure in a system multiplied by its volume and
divided by its pressure.
The ratio of the number of molecules which are adsorbed on a surface for a finite
period of time to the number of molecules striking the surface.
The quantity of gas, in pressure-volume units, at a specified temperature, flowing per
unit time across a specified open cross section. Throughput may be referred to a
specific constituent of a gas in which case the partial pressure of that constituent and the
associated flow rate are the relevant quantities.
The flow of gas through a pump or system under transition flow conditions which are
intermediate between viscous flow and molecular flow. See Transition flow.
See Load (vapour pump).
The flow of gas through a channel under conditions such that the mean free path is
much greater than the transverse dimensions of the channel. At these pressures the
flow characteristics are determined by collisions of the gas with surfaces and not
significantly with other molecules.
The flow of gas through a channel under conditions such that the mean free path is of
the same order as the transverse dimensions of the channel. At these pressures the flow
characteristics are determined by collisions of the gas with surfaces as well as with
other molecules. Also called Knudsen flow. Viscous flow
The flow of gas through a channel under conditions such that the mean free path is very
small compared with the transverse dimensions of the channel. At these pressures the
flow charecteristics are determined mainly by collisions between the gas molecules, i.e.
the viscosity of the gas. The flow may be laminar or turbulent.
A hot-cathode ionization gauge in which a fine wire ion collector is positioned on the
axis of a cylindrical grid functioning as the anode. The cathode is mounted outside the
Hot-cathode ionization gauge
An ionization gauge in which the pressure is measured in terms of the current of
positive ions produced by electrons emitted from a heated cathode.
A vacuum gauge which indicates pressure by responding to the net rate of transfer of
momentum by molecules moving between two surfaces maintained at different
temperatures and separated by a distance smaller than the mean free path of the gas
molecules. Various types of Knudsen gauges differ mainly in the shape and method of
suspension of the moveable element.
A liquid level manometer in which a known volume of the gas, whose pressure is to be
measured, is compressed by the movement of a liquid column and confined in a
measurable volume. Corrections need to be made for any appreciable change in gas
pressure in the system caused by movement of the liquid.
See thermal conductivity gauge.
Thermal conductivity gauge
A vacuum gauge (also known as a Pirani gauge) containing two surfaces at different
temperatures between which heat can be transported by gas molecules. Changes in
the temperatures, or in the heating power required to maintain the temperature of one
of the surfaces constant, can be correlated with the gas pressure. Thermal conductivity
gauges differ in their method of indicating the temperature change. See thermocouple
gauge and thermistor gauge.
A form of thermal conductivity gauge in which the temperature-sensitive elements are
made of semiconducting material instead of metal.
A thermal conductivity gauge which contains a heated filament and a thermocouple for
measuring the filament temperature as a function of gas pressure.
A leak which has a known leakage rate for a specific gas under specific conditions.
A leak having a small cross-section and a length many times greater than its cross
A leakage resulting from the temperature-dependent diffusion of a specific gas through
a membrane. Examples include the diffusion of hydrogen through palladium and
helium through glass or Teflon (PTFE).
A leak which permits gas flow by permeation through a thin non-porous wall.
A leak of such a size that the flow of gas through it is predominantly molecular for a
given pressure. See molecular flow.
A leak with an adjustable leakage rate. Virtual leak
An apparent leak caused by the presence of contaminants which outgas slowly within a
A leak of such a size that the flow through it is mainly viscous. See viscous flow.
The pressure measured downstream from the outlet or foreline of a vacuum pump.
The gas pressure at the entrance to a pump.
The gas pressure at any point between the exhaust port of the low-pressure stage and
the high-pressure or roughing stage of a compound pump.
The pressure at the discharge side of a vacuum pump, at a stated throughput, above
which the pumping action of the pump rapidly deteriorates, as evidenced by sudden
increase of inlet pressure.
Maximum pressure ratio (of a vacuum pump)
The maximum value of the ratio of forepressure to inlet pressure which a pump can
maintain at zero gas flow.
(Saturated) vapour pressure
The pressure of a vapour in thermodynamic equilibrium with a condensed phase at a
fixed temperature. The definition applies to single components as well as to
multicomponent systems. In the latter case it is necessary to distinguish between the
total pressure over the condensed phase and the partial pressure of a given component.
The limiting low pressure approached in a vacuum system after sufficient pumping time
has elapsed to establish that further reduction in pressure will be negligible.
Ultimate partial pressure
The part of the ultimate pressure in a vacuum system caused by the partial pressure of a
A vacuum pump which operates by the condensation and/or sorption of a gas at
surfaces maintained at a temperature sufficiently low for the vapour pressure of the
condensed gases to be negligible.
Differential sputter pump
A sputter-ion pump having two cathodes for which materials and sputter rates are
A vapour pump in which the pumped gas flows into a vapour stream under conditions
in which molecular flow predominates. Momentum is transferred from the vapour to
the gas carrying it along in the direction of the stream. Pump fluid is heated in vacuum
to generate the vapour which is directed through a nozzle. It expands freely in the
stream before it reaches a cool wall where it condenses and is returned to the boiler to
begin a new cycle.
An ion pump containing two uniquely shaped electrodes, viz. an anode and a cathode;
two-electrode ion pumps are also referred to as diode getter pumps and diode sputter
A vapour pump in which the pumped gas enters the pump and the vapour stream under
predominantly conditions of viscous flow.
An ion pump having only electrostatic fields rather than both electrostatic and magnetic
fields to generate the ionizing discharge.
Electrostatic getter pump
An electrostatic ion pump in which a getter material is made to sublime and react with
the gas molecules being pumped.
A getter-ion pump in which the getter is evaporated from a molten surface rather than
sublimed or sputtered from a solid source.
A vacuum pump for maintaining the forepressure of another pump below its critical
A diffusion pump whose design allows the more volatile impurities in the pump fluid,
resulting from decomposition or contamination of pumping fluid, to be ejected out of
the foreline or trapped within the pump in such a manner as to reduce their chance of
backstreaming through the pump inlet.
A pump which combines the pumping mechanism used in the ion pump and the getter
An electronic device in which ionization produces a significant rate of gas removal by
reaction of the ions with the cathode material.
An ion pump, usually with multiple anode cells immersed in a magnetic field parallel to
the cell axes and with two cathode end plates of reactive material spaced from the ends
of the anode cells which terminate the discharge space.
A device with moving parts, such as rotating vanes, a piston or eccentric rotary
members, used for pumping gas or vapour.
Noble gas pump
A magnetic ion pump with novel cathode geometries to enhance the pumping of noble
Positive displacement pump
A mechanical vacuum pump in which the pumping action is provided by displacement
of a trapped volume of gases, typically by a rotating or reciprocating piston, a sliding
vane or intermeshing lobes.
A getter-ion pump in which the getter surfaces are contiuously removed by sputtering.
An ion pump, usually of the sputter-ion type, containing three uniquely shaped
electrodes, an anode, a sputter cathode and an ion collector electrode.
Triode getter pump
A triode pump in which gettering plays a significant role in the pumping.
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 pump inlet
to the outlet.
A seal that can be baked at elevated temperatures.
A seal between two sections of a vacuum system that can be broken to connect them.
A seal between two elements designed for dis- assembly without resort to cutting,
fracturing or melting.
A demountable seal in which the seal is made by pressing a gasket of deformable
material between two members of harder material. The gasket may be of metal or
A gasket seal in which the gasket is a toroidal ring of circular cross section.
A joint between two elements of a vacuum system which can maintain leakage at or
below the required level.
A trap that includes a chilled surface or other means to prevent surface migration of oil
from a source into the vacuum system.
A trap with a refrigerated surface used to condense vapours present in a vacuum
Molecular sieve trap
A trap containing molecular sieve material that has a high specific surface area and that
adsorbs hydrocarbon and water vapours at or below room temperature.
A device used in a vacuum pumping line to reduce vapour pressure in a vacuum system
or prevent backstreaming and migration of vacuum pump fluids such as oil or mercury.
A trap in the form of a U-shaped tube immersed in a coolant.
Extreme ultrahigh vacuum
A vacuum in which the pressure is less than 10
-10 Pa (7.5 x10
A vacuum in which the pressure is less than or equal to 10-1 Pa (7.5 x 10-4 Torr) and
greater than 10-4 Pa (7.5 x 10-7 Torr).
A vacuum in which the the pressure is less than 105 Pa (750 Torr) and greater than 3.3
x 103 Pa (25 Torr).
A vacuum in which the pressure is less than or equal to 3.3 X 103 Pa (25 Torr) and
greater than 10-1 Pa (7.5 x 10-4 Torr).
A vacuum in which the pressure is less than or equal to 10-7 Pa (7.5 x 10-10 Torr) and
more than 10-10 Pa (7.5 x 10-13 Torr).
The condition of a gaseous environment in which the gas pressure is less than
Very high vacuum
A vacuum in which the pressure is less than or equal to 10-4 Pa (7.5 x 10-7 Torr) and
greater than 10-7 Pa (7.5 x 10-10 Torr).
A valve in which the ports are not in line, as, for example, a right angle valve.
A valve which is opened or closed by rotating a disc 90âˆšÃ‡Â¬Â° about an axis through the
centre of the disc.
A valve in which the valve stem is mounted in bonnet that is isolated from the rest of
the valve by using a diaphragm to divide the space inside the valve body.
A thin spring steel plate, fastened at one end to the pump housing, which seals the
exhaust port of a mechanical pump from the oil reservoir. During the exhaust cycle,
gas pressure is sufficient to deflect the plate from its seat and gas is discharged from the
A valve for admitting air or gas at a precisely determined rate into a vacuum system.
A valve in which a tapered needle is moved along its axis against a seat that may also
A valve which will automatically open when the pressure on the seat side rises above a
specific preset value.
Sealed bellows valve
A valve, usually for high vacuum applications, in which the stem is sealed by a flexible
metal bellows. One end of the bellows is attached to the valve body and the other end
to the disk part of the valve stem.
A valve in which the moveable member is actuated electrically by an electromagnet.
Straight through valve
A valve in which the parts are in line, or coaxial, and for which the internal
construction is such that line-of-sight flow occurs when the valve is open.
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.
A valve containing a shield, which remains in line with the valve port and can thus act
as a baffle.