Leak testing during the manufacture of sealed-off vacuum devices

The paper discusses the requirements for testing and location of leaks in the course of manufacture of various sealed-off vacuum devices. A number of methods are at present in use and others have been assessed in the course of experimental work. Their advantages, limitations and sensitivities as found in practice are discussed. The methods fall into two classes. The non-specific methods involve measuring differences in total gas pressure with time or at different points in a vacuum system. The specific methods detect a particular gas and are generally found to be better suited for the requirements discussed; they include the halogen detector, the hydrogen diffusion detector and various types of mass spectrometer. While more than one method can be used for large leaks, the mass spectrometer has been found the most useful for smaller leaks, using helium as the test gas. The relative merits of the conventional magnetic type, the omegatron and the linear r.f. spectrometer are discussed. The magnetic type is the most generally suitable, and various design features and precautions in use are outlined.