BITTMANN applied technologies
Matthias Bittmann, Dipl. Phys.
27356 Rotenburg (Wümme)
Tel. +49 4261 961805
The performance of a vacuum system is strongly dependent on the leak status of that system. This is the sum of all leakages of the particular vacuum components. The ultimate pressure that a vacuum pump can achieve in a vessel is given by the overall leakrate that equals the pumping speed. It is therefor an indispensable measure to control the vacuum status when a system is taken into service.
The simplest method to detect a leakage is by recording the rate of pressure rise. Hereby, with the chamber isolated, the increase in pressure is plotted against time and related to the volume of the chamber to give the leak rate:
From the slope of the pressure rise one may distinguish between a real and a virtual leakage. Virtual leakage accur when moisture or other volatile contaminants desorp from the walls at a given vapor pressure.
A more precise method to detect and pinpoint a real leak is by means of a trace gas and a mass spectrometer system. The commonly used Helium is sprayed from outside the chamber onto potential leaking areas. Since Helium is a tiny molecule, it can easily penetrate through even the smallest pinhole. Once inside, the He molecule is drawn towards the vacuum port by the vacuum pump and may be detected and counted by a mass spectrometer. To further locate a leak and to make a distinction between particular leaks it is necessary to apply only small amounts of Helium in order to avoid flooding of the whole apparatus. In some cases it is good practice to "mask" adjacent areas with a hood of nitrogen or to apply alcohol.