Veeco MS40 Operation And Maintenance Manual page 124

3.1.4 Helium Background
Background due to helium released from the walls of the leak detector or
leak detection system.
3.4 LEAKS
3.4.1 Leak (n)
In vacuum technology a hole, porosity, permeable element, or other
structure in the wall of an enclosure capable of passing gas from one side
of the wall to the other under action of a pressure or concentration
difference existing across the wall.
Also, a device which can be used to introduce gas into an evacuated
system.
3.4.1.1 Channel Leak
A leak which consists of one or more discrete passages that may be ideally
treated as long capillaries.
3.4.1.2 Membrane Leak
A leak which permits gas flow by permeation of the gas through a
nonporous wall. For helium, this may be a wall of glass, quartz, or other
suitable material.
3.4.1.3 Molecular Leak
A leak through which the mass rate of flow is substantially proportional to
the reciprocal of the square root of the molecular weight of the flowing
gas.
3.4.1.4 Viscous Leak
A leak through which the mass rate of flow is substantially proportional to
the reciprocal of the viscosity of the flowing gas.
3.4.2 Calibrated Leak
A calibrated leak for which the rate of leakage is known under standard
conditions, namely, 23±3°C, a pressure of 760 Torr ±5% at one end of the
leak, and a pressure at the other end so low as to have a negligible effect
on the leak rate.
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