The thest 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
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
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