Varian 990 dCLD II, Operation Manual

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Model 990 dCLD II Component Leak Detector
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 is done by placing
helium inside the object before sealing (either 100% or mixed with other gas used for back-
filling). 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 can first
be placed in a container that is then pressurized with helium for a specific time at a known
pressure. Helium enters the object through leaks that are later detected, as described in the
previous paragraph. Gross leaks are sometimes not detected, since all the helium entering
through a large leak can be lost prior to testing. Also, spurious signals can be given by
helium not entering the object, but entering surface fissures and remaining long enough to
be detected.