leak testing - basic principles
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
Key questions at the start of any leak
test requirement are:-
|What size is the component and what is
its internal volume?
||Is there access to inside or is it a sealed
|What is the leak limit?
||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?
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
Water Submersal / Dunk Tank
High Vacuum Helium
Leak Testing - generic systems
||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.
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
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.
||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
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
For more information on leak testing solutions please
visit our leak test machines page