The leak test, also called the tightness test or leakage test, is a non-destructive test method with which components and assemblies are tested for tightness against gaseous or liquid media. Since there are basically no absolutely leak-proof parts, only a leak rate can be determined. Various units of measurement are commonly used to indicate the leakage rate.
Often, a rough leakage test is carried out first to determine whether the test item is at all approximately leak-proof and whether a further test makes sense and can also serve to protect the measurement technology. The subsequent test to determine the leak rate is called a fine leak test. In principle, there are two types of tests: The integral leak test to determine the leak rate as well as leak location, which is used to locate leaks on the test item.
Gaseous or liquid substances serve as test media. The most common test medium is air. The detection of very small leak rates is carried out with the detection of test gases such as helium or hydrogen. Tests with very high pressures are realised as hydraulic tests, e.g. with the test media water or oil. The criteria for selecting leak test methods and procedures are described in the standard DIN EN 1779:1999.
Often the leak test technique is combined with other pneumatic or hydraulic test methods that can be attributed to the functional test, such as flow measurement, measurement of opening and closing pressure and cavity volume measurement. The passage test is used to determine whether or not there is a passage for certain media.

Automated leak testing always requires the selection of a suitable procedure for the respective application, whereby the operating medium is often replaced by another test medium. The procedure must be suitable for the required test pressure, for detecting the maximum permissible leak rate or for leak location. Furthermore, the selection of the method can depend, for example, on the operating and test conditions as well as on the material properties of the test item, etc.
The method most frequently used in practice for measuring medium and small leak rates is pneumatic testing. For integral leak testing, the test item is pressurised with test pressure or evacuated and disconnected from the media supply. The pressure change over a defined test time is measured and then evaluated to determine whether it corresponds to the permissible leakage rate. The underwater test is used to detect small leak rates. The test item, which is pressurised pneumatically, is immersed in a water bath in which the rising air bubbles are typically detected visually or automatically. This method is also used for leak localisation.
For measuring very low leak rates, methods with different test gases such as hydrogen are used. For the highest requirements, helium is used as the test gas. When using test gases, the leak is located by moving over the test object with a sniffer probe that can detect escaping test gas.
Leak tests with high and very high test pressures, which are usually also safety-oriented, are practically only realised by hydraulic test methods with liquid test media.