Varian 959D, Operation Manual

Page: 73 / 82

959D Leak Detector
A-5
A.5.4 Facts About Leak Rates
Visualizing Leaks in
Everyday Terms 10 −
5
atm cc/sec: approximately 1 cc/day
10
−
7
atm cc/sec: approximately 3 cc/year
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.