About Leak Detection
What is Leak Testing?
Even with today's complex technology, it is, for all
practical purposes, impossible to manufacture a sealed enclosure
or system that can be guaranteed leak proof without first being
tested. Through the use of modern leak testing techniques, leak
rates in the 10-11 std cc/sec range can be reliably detected. The
following provides a brief summary of specific information pertinent
to the subject of leak detection.
Classes of Leak Detection
There are four general classes of leak detection:
1) Hermetic Enclosures (or parts thereof)
These are tested to prevent entrance of contaminants or loss
of fluid that would affect performance of the enclosed unit. Examples:
electronic devices, integrated circuits, sealed relays, motors,
ring pull tab can ends, and multi-pin feedthroughs.
2) Hermetic Systems
These are tested to prevent loss of fluid or gas within. Examples:
hydraulic systems and refrigeration systems.
3) Evacuated Enclosures (or parts thereof)
These are tested to prevent too-rapid deterioration of vacuum
with age. Examples: electron tubes, TV picture tubes, bellows
sensing elements, full-panel opening can ends, etc.
4) Vacuum Systems
These are tested to minimize in leakage and allow attainment
of better vacuum or higher gas removal ability at any given vacuum
(absolute pressure).
Terminology
1) Flow
std cc/sec one cubic centimeter of gas per second at a pressure
differential of one standard atmosphere (760 torr at 0 degrees
centigrade.) atm cc/sec one cubic centimeter of gas per second
at ambient atmospheric pressure and temperature (used interchangeably
with "std cc/sec" because the difference is insignificant for
leak testing purposes.
2) Rate of Rise
In vacuum systems, this is defined as the rate of increase of
absolute pressure per unit time, with the vacuum pump isolated
from the system and is the sum of actual leakage and internal
out gassing. Rate of rise is usually expressed in torr or microns
(millitorr) per hour. The flow rate should be expressed in torr
liters/second.
3) Conversions:
| 1
std cc sec |
0.76
torr-liter/sec |
| 1
torr-liter sec |
1.3
std cc/sec |
| 1
std cc/sec |
9.7
x 10-4 micron cubic feet per hour |
| 1
mcfh |
practically
10-5 std cc/sec |
|
4) Numerical Notation Exponential System
Most leak rates of commercial significance are very small fractions
of a std cc/sec. Therefore minus powers of ten are used as a convenient
system of numerical shorthand.
The following table shows the relationship of exponents and multipliers
to the arithmetic form, and the equivalent result.
| Multiplier |
Arithmetic
Form |
Result |
| 1
x 102 |
1
x 10 x 10 |
100 |
| 1
x 101 |
1
x 10 |
10 |
| 1
x 100 |
1 |
1 |
| 1
x 10-1 |
1x1/10 |
.1 |
|
|
