To measure, the component must be placed under a „bell”, i.e. in a closed volume. The measurement is made to see how the pressure in the bell drops after filling. 

The measurement consists of two steps. First, the large leaks are tested. A given amount of air is added to the „bell”. If air can flow into the component, the pressure level will be lower after filling. For a reliable measurement, the „bell” must be as close as possible to the component and its volume must be as small as possible in relation to the internal volume of the component. 

The second step is the traditional pressure drop test, where small leaks can be detected.