As with almost every facet of vacuum systems, there is no single method which fulfills every situation and every criterion. This is certainly the case with leak detection, with four main methods being employed: the bubble test; pressure decay test; pressure rise test; and helium sniffer mode/helium vacuum mode tests.
These four tests roughly correspond to the “simplistic” bubble test (for low-vacuum pressures), through to the “high-tech” helium tests (for high-vacuum pressures). There are many companies that provide several types of package leak tests.
The bubble test is best illustrated by placing a punctured bicycle tube underwater and marking where the bubbles come from, or placing washing-up liquid around the joint of an active water/gas pipe and observing whether the liquid forms a froth. Both are reliable ways of detecting a low-pressure leak. The bubble test is employed up to vacuums of 10-4 mbar.
The pump-down test is conducted by evacuating a closed vacuum vessel until a certain pressure is obtained, then closing the pump’s inlet valve. After a pre-determined period of time, the inlet valve is again opened, and the time is recorded for the pump to return the vacuum to the original evacuated level. This process is repeated a number of times.
If the time to return the vacuum to the original level remains constant, then a leak is present. If this time period decreases, this indicates reduced gas liberation (out gassing) on the inside of the system (i.e. a “virtual” leak), however, it does not exclude a leak from also being present.
Alternatively, the pressure rise test is made by plotting the vacuum level against the time after a vacuum level has been achieved, and after isolating the system, the curve will be a straight line if a leak is present. However, if the pressure rise is due to gas liberation from the system walls, the rise will gradually taper off to reach a final, stable value.