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20. Sampling Probe - A device used in pressure testing and so designed
as to collect tracer gas from a restricted area of the test object and
feed it to the leak detector. Also called pressure probe or sniffer.
21. Hood Test - An overall test in which an object under vacuum test is
enclosed by a “hood” which is filled with tracer gas so as to subject
all parts of the test object to examination at one time.
22. Isolation Test - A method of determining whether a leak is present in
a system, or of obtaining an estimate of its magnitude, by observing
the rate of rise of pressure in the evacuated system when the system
is isolated from the pump. Also called rate of rise test.
23. Masking - The covering of a section of a test object so as to prevent
tracer gas from entering leaks that may exist in the covered section.
24. Flooded System - A system that, while being leak tested under
vacuum, becomes so filled with tracer gas as to make impracticable
further leak detection by means of a probe.
25. Helium Drift - In the case of leak detection with a helium probe, the
drift of helium to a leak or permeable gasket located at a point
sufficiently remote from the end of the probe to mislead the operator
into suspecting the area near the probe.
26. Standard Leak
a. A device that permits leakage through it, at a specified rate, of
a specified gas, with atmospheric pressure at one end of the
device and a pressure on the other side sufficiently low to have
negligible effect on the leak rate.
b. A capillary or porous wall leak, usually in a glass or metal tube,
whose dimensions have been adjusted to give a conductance
within specified limits for a specified gas at a standard reference
temperature with specified inlet and exit pressures. Standard
leaks for attaching to vacuum test manifolds with air at
atmospheric pressure exposed to the inlet are usually protected
by filters to avoid clogging by dust particles. Standard leaks for
calibrating mass spectrometers are usually fused to a glass
reservoir containing the specified gas at a known high pressure.
c. A device providing a known throughput into a vacuum system.
Also referred to as a calibrated leak.
a-iii
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