990 CLD Autoline™ Leak Detector
6-8
DRAFT 2/2/04
Contra-Flow™
A technique that utilizes the differences in the maximum
compression ratios of the tracer gases (such as helium) and other
gases found in the air. When the tracer, gas exceeds Its maximum
compression ratio, it diffuses backwards through the diffusion
pump and is detected by the spectrometer tube.
Diffusion Pump
A vapor pump having boiler pressure less than a few Torr and
capable of pumping gas at intake pressure not exceeding about
2 milliTorr and discharge pressures (forepressures) not exceeding
about 500 milliTorr.
Fine Leak
A leakage rate of less than 10
−
4
std cd/sec.
Foreline Valve
A vacuum valve placed in the foreline to permit isolation of the
diffusion pump from its forepump.
Forepump
The pump that produces the necessary fore vacuum for a pump that
is incapable of discharging gases at atmospheric pressure.
Sometimes called the backing pump.
Gross Leak
A leakage rate never more than 10
−
4
std cd/sec.
Helium Bombing
A method of testing for leaks in which hermetically-sealed units
containing an internal volume are subjected to a helium pressure. If
leaks are present in the sealed unit, the helium pressure drives
some helium into the internal volume and this may be
subsequently detected during bell jar testing or immersion in a hot
fluid to detect bubbles (See “Bomb Test” above).
Ion
A molecule whose net charge has been altered from neutral for the
purpose of magnetically or electrostatically moving it from one
place to another.
Ion Source
That part of a leak detector tube in which the trace gas is ionized
prior to being detected.
Mass Spectrometer
Leak Detector
A mass spectrometer adjusted to respond only to the tracer gas.
Helium is commonly used as the tracer gas, and thus, the
instrument is normally referred to as a helium leak detector.
Millisecond
One one/thousandth of a second.
MilliTorr
A unit of pressure equal to 10
−
3
Torr (1/1000 Torr).
Leak Rate
The rate of flow through a leak with a specified gas (at a specIfied
pressure on the inlet and exit sides).
Preferred units:
Standard cc/sec of a specified gas
Standard gas conditions: 760 mm 66 pressure (absolute),
25 °C (77 °F) temperature
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