Varian 979 Series, Руководство по эксплуатации

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Model 979 Series Helium Mass Spectrometer Leak Detector
DRAFT 9/15/03
C-7
A sampling probe is connected to the leak detector. The object to be tested is filled with
helium at the desired test pressure and the probe is moved over its surface. Some of the
helium escaping from a leak is captured through the probe and enters the leak detector, thus
locating the leak.
Sensitivity of this type of testing is limited to about 10
−
7
std cc/sec, since most of the escap-
ing helium diffuses into the surrounding atmosphere. The sensitivity is also limited by oper-
ator technique and variation in ambient helium concentration in the vicinity of the testing.
An alternative to probing is to enclose the object and probe the enclosure for a change in
helium content.
C.6.3 Test Piece Already Sealed (Figure C-3)
Figure C-3 Test Piece Sealed with Helium or Mixture of Helium and Other Gases:
Bell Jar Used to Determine Overall Leak Rate
Sometimes it is necessary to leak check a completely sealed object. This may be done by
placing helium inside the object before sealing (either 100% or mixed with other gas used
for backfilling). The object is then placed in a vacuum chamber connected to the leak
detector. Helium escaping from the object into the vacuum chamber is detected by the
spectrometer tube. Sensitivity depends on the partial pressure of helium in the object.
If the presence of helium in the finished object is undesirable, units already sealed may first
be placed in a container that is then pressurized with helium for a specific time at a known
pressure. Helium will enter the object through any leaks and may be detected later, as
described in the previous paragraph. Gross leaks may sometimes not be detected, since all
helium entering through a large leak may be lost prior to testing. Also, spurious signals may
be given by helium not entering the object, but entering surface fissures and remaining long
enough to be detected.
Leak
Detector
Leak
Sealed unit
containing
Helium