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VACUUM TECHNOLOGY

VACUUM TECHNOLOGY 7.9

detectors. Electrical feedthroughs also are used for transmission of data such as the signal from a
thermocouple located inside an evacuated vessel to an electronics package located outside the vac-
uum vessel.

7.5 LEAK DETECTION

7.5.1 Techniques for Locating and Quantifying Large Leaks

Rate of Rise Test. This test measures the combined effects of gas loads to a vacuum system from
outgassing sources internal to the vacuum system, gas loads due to permeation through vacuum
seals, and leaks in the vacuum system. To measure the rate of rise, one typically evacuates a vacu-
um vessel to its base pressure, isolates the vacuum pump from the vessel, and records the pressure
as a function of time. If a leak is present, the rate of change in pressure is linear with time. If out-
gassing is the major source of gas load to the system, the pressure will rise and eventually stabilize
at the equilibrium vapor pressure for the materials internal to the vacuum system.

7.5.2 Techniques for Locating and Quantifying Small Leaks
Ultrasonic Leak Detection. In this technique, a microphone sensitive to the sound in the ultrason-
ic range is used to survey the external surfaces of an evacuated vacuum vessel. Leaks present may
generate sound in the ultrasonic range that the microphone can detect.
Helium Mass Spectrometer Leak Detector (HMSLD). A mass spectrometer tuned to detect only
helium gas is used to precisely locate leak paths in vacuum vessels. Helium is used as the tracer gas
as it is in low abundance in the atmosphere, relatively inexpensive, nontoxic, nonflammable, and has
a high average velocity relative to other gas molecules in the atmosphere.
There are two modes of operation of HMSLD—tracer and sniffer probes. In the tracer probe
method, the HMSLD is connected to an evacuated vessel at a pressure of less than 10 −4 torr. The
point of connection of the HMSLD to the vessel may be at a port of the vessel or the foreline of a
secondary vacuum pump. Helium gas is carefully applied using a wand to locations on the external
surface of the vessel, where leaks are suspected. Helium gas entering the vacuum vessel through a
leak will likely be detected in the HMSLD, which will give a visual or auditory signal to the opera-
tor. As an alternative to using a probe to search for leaks as described, one may enclose the vessel in
a plastic bag and flood the bag with helium. This method will test all of the vacuum system that is
enclosed in the plastic bag at once.

7.6 DESIGN OF A VACUUM SYSTEM

7.6.1 Flow Modes for Gases
For the following discussion it is assumed the gas being pumped is atmospheric in composition
(approximately 79 percent nitrogen and 19 percent oxygen, the balance being trace gases), the con-
ductance elements through which the gases flow is circular in cross section, and the internal surfaces
of these conductance elements are relatively smooth. Gases move under the influence of pressure dif-
ferentials. As a vacuum pump operates, it creates a reduced pressure environment at its orifice. Gas
molecules in a vacuum vessel connected to the pump by a conductance element move almost instan-
taneously to equilibrate pressure within their confined environment. Some resistance to the flow of

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