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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