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Leak
Testing
The Basic Priciples
What
is a Leak?
A leak is a flow of gas (or liquid) through
the wall of a vessel (via an imperfection such as a hole, crack
or bad seal).
Leaks require a pressure difference to generate
the flow; they always go from higher pressure to lower pressure.
Leaks are pictured as going from positive
pressure (inside an object) to outside (at atmospheric pressure).
This is not always the case (a leak could
be from atmosphere to inside an evacuated object), but it helps
to think about it this way because the units and terminology are
based on this model.
Leak
Testing - what units do you use?
For leaks of air into atmosphere, units are
usually expressed as mm3 or cm3 (cc) per second
or minute. So 16.6 mm3/sec = 1 cm3/min.
A bubble under water is about 30 - 50 mm3,
so 1 bubble per second is about 30 mm3/sec or 2 cm3/min.
A standard unit of leakage which takes account
of air pressure is the mbarl/sec. (Millibar-litre per second). A
leak into atmosphere of 1 mbarl/sec is equivalent to a volume leak
of 1000 mm3/sec.
Leak
Testing - what technique should be used?
Key questions at the start
of any leak test requirement are:-
| What size is the component
and what is it’s internal volume? |
Is there access to inside
or is it a sealed unit? |
| What is the leak limit?
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Is it rigid or flexible? |
| Does it have hidden internal
volumes that may affect leak measurements? |
Are parts at ambient temperature? |
| Are the parts clean and
dry? |
What is the surface finish
of any sealing surfaces? |
Based on the acceptable
leak rate limit (shown in ml/sec) alone the following test method
can be used
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Tracer Gas
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Flow Rate
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Water Submersal / Dunk Tank
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High Vacuum Helium
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Air Decay
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10-9
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10-8 |
10-7 |
10-6 |
10-5 |
10-4 |
10-3 |
10-2 |
10-1 |
1 |
5 |
Leak
Testing - generic systems
pressure/vacuum
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The test piece and the reference volume are
simultaneously pressurised (or evacuated) to a preset pressure.
The air in the system is then allowed to stabilise, with the
supply valves all closed. The Differential Pressure Transducer
is automatically zeroed.
After this stabilisation time, the pressure
change in the test piece is compared to the pressure change
in the reference volume, using the Transducer. If the test
piece is leaking, the difference will increase and be measured,
an alarm limit may be set for a pass/fail decision.
The sequence is fully automatic, the accuracy
and sensitivity of the system is defined by the method of
setting the preset pressure together with the quality and
type of control valves and Differential Pressure Transducer.
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helium systems
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A vacuum pump evacuates the test chamber and
test piece simultaneously to a preset vacuum. At this preset
level, the chamber and the test piece are isolated and the
chamber evacuated further to a very low pressure. A positive
pressure variation is therefore created between the test piece
and the chamber.
Helium gas is then introduced into the test
piece, often in a 10% concentration. A Mass Spectrometer analyses
a sample from the chamber as the vacuum continues to be drawn.
The Mass Spectrometer measures the helium leakage and sets
the pass/fail decision.
The test piece pressure is often compared to
the chamber pressure before dosing with helium, to avoid saturating
the Mass Spectrometer in the event of a gross leak
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For more information on leak testing solutions please visit our
leak test machines
page
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