Any leak present will allow Helium to pass through and be detected by the instrument. There are some important practical rules to be followed to allow accurate and reliable leak detection:
- Helium should be sprayed slowly across the test piece
- The flow rate of the gas should be small: I bubble per second in water is a good rule of thumb
- Leak check from top to bottom to allow the gas to disperse vertically
The leak detector generally has a small backing pump ranging from 2.5 m3h-1 to 20 m3h-1. For large vacuum chamber applications, this pump would be incapable of pumping the item down to a level of 10-3 mbar needed to allow the unit to reach its most sensitive level. As in the illustration, the most effective method is to attach the leak checker to the backing line of the system above a forevacuum pump. This will inevitably have a higher pump speed than the leak checker, and the unit will not detect the complete He flow. To give a true leak rate, the observed leak rate needs to be factored by the pumping ratio of the competing pumps.
In other instances, such as hermetically sealed electronic packages, high vacuum valves require the total leak rate to be determined to ensure that the expected lifetime or specification of the component is met. The diagram below demonstrates how the unit operates in a quantitative manner. The component is placed in a suitable vacuum container which, depending on its volume, might require additional pumps to allow the correct inlet pressure to be achieved.