The test specimens were exposed to helium flowing through thin pipes leading outwards. A differential pressure of exactly 200 mbar was adjustable by means of a special valve cycle and pre-evacuated intermediate vessels. This made it possible to determine the leak rate on red-hot components. However, leaks of this magnitude are not permissible for almost any technical application. As the leaks become smaller, the amount of liquid penetrating through them also decreases.
Leakage is stopped when the leak is clogged by large liquid molecules.
The leak rate is measured in mbar l s -1 . Large amounts of gas can still escape even through a leak whose size is only about one millionth of the fluid-tight leak. In many cases, however, even these low leak rates are still much too high for special applications. Helium is used as the tracer gas for detecting these small leaks. Helium is nontoxic, does not react with other elements and can even penetrate through the smallest leaks due to its very small atoms.