Models 947, 948 and 960 Leak Detectors Operations and Service Manual
G-5
Audible or Visual Detection
by Observer
a
. Bubbles rising in water
10
−
4
atm cc/sec or larger
b
. Audible Leaks
10
−
1
atm 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 are slightly smaller. Diffusion
of helium through glass can 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 can
vary from hour to hour and day to day. Breathing on a 10
−
6
std cc/
sec leak provides enough moisture to close it temporarily; perhaps
for several days. Atmospheric particles can close a leak of this
size. Never depend on an
accidentally made
leak to remain
constant. Manufactured standard leaks for calibration purposes
require special techniques.
G.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. There are three
basic methods in common use.
