Industrial applications of Helium Leak Test

Leaks are special types of defects which can have a major importance in systems where they have influence on safety and performance. Many objects will have a reduced reliability if they contain leaks.

Leak testing is a non destructive examination method which is used for detection and localisation of leaks and for measurement of leakage in systems or objects which is under vacuum or pressure.

Before a leak test examination is performed it is necessary to determine if the examination is to ascertain whether leaks are present or not, overall leak detection, or if the examination is to determine the location of a leak, localising leak detection. In some cases an examination for overall leak detection is performed first, and if leaks are detected, the localising method is applied for pinpointing of the leak. This is however not always required nor possible.

Secondly it is necessary to determine the leak rate which can be tolerated, as no objects are 100% tight. That is the requirements to tightness of the object. If for example the object have to be watertight, a leak rate below 10-4 mbar l/s will be sufficient. But if the object for example is to be used in the chemical industry the requirements can be a leak rate below 10-6 mbar l / s.

In leak testing a pressure difference between the outer and the inner side of the object to be examined is produced. Subsequently the amount of gas or liquid which is passing through a leak is measured. In helium leak test helium is used as a search gas, from this the nomination of the method.

In principle two methods are applied for leak testing and localisation of leaks, the "Vacuum method" and the "Overpressure method".

At the "Vacuum method" the object to be examined for leaks is evacuated and sprayed from the outside with a search gas, in this case Helium. The gas enters through any leaks present in the object and is detected by a sensor connected to the leak test instrument.

At the "Overpressure method" the object to be examined for leaks is filled with a search gas, Helium, under slight overpressure. The search gas escapes through any leaks present to the outside and is detected by a detector probe. This detector probe is in most cases called a "sniffer" acting as a gas sampling probe.

For both methods specially developed leak detectors are available.

The object under test should, if possible, be tested according to its final mode of use, i.e. if it is used under vacuum, the vacuum method should be applied, if it is finally pressurised, the overpressure method should be adopted.

Examples of testing with the two methods are illustrated below. The Vacuum method is illustrated with the Hood Test and the Tracer Probe Test and the Overpressure method with the Hood test, the Bombing test and the Detector Probe or Sniffer test.