Model 979 Series Helium Mass Spectrometer Leak Detector
DRAFT 9/15/03
C-5
C.5.4 Facts About Leak Rates
Visualizing Leaks in
Everyday Terms
10
−
5
std cc/sec: approximately 1 cc/day
10
−
7
std cc/sec: approximately 3 cc/year
Audible or Visual
Detection by Observer
a. Bubbles rising
in water
10
−
4
std cc/sec or larger
b. Audible Leaks
10
−
1
std cc/sec or larger
Sizes of Leaks in
Man-Made Joints
Studies indicate that almost all leaks at joints are about
5 x 10
−
7
std cc/sec (about 1 cc/month) or larger. This is true of
ceramic-to-metal, plastic-to-metal seals, welded, soldered and
brazed joints. Some long-path leaks may be slightly smaller. Diffu-
sion of helium through glass may be as high as 10
−
8
std cc/sec per
square centimeter of surface area.
Variation in Leak Sizes Leaks unintentionally built-in at joints during manufacture may
vary from hour to hour and day to day. Breathing on a
10
−
6
std cc/sec leak provides enough moisture to close it tempo-
rarily; perhaps for several days. Atmospheric particles can close a
leak of this size. Never depend on an
accidentally made
leak to
remain constant. Manufacturing standard leaks for calibration pur-
poses requires special techniques.
C.6 Leak Detection Methods
Most leak detection methods depend on the use of a tracer gas passing through the leak and
being detected on the other side (for example, visual detection of air bubbles in water).
The mass spectrometer leak detector operates with helium as a tracer and is widely used
because it combines high sensitivity with production testing capability. The three basic
methods in common use are described below.
Summary of Contents for 979 Series
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