Palladium Leak Detector *)

Every physicist working with vacuum installations is well aware how much their operation is complicated when leaks appear. If the leak is large, it is usually not difficult to determine its location and to eliminate the causes producing it as quickly as possible. Particularly troublesome are “high-vacuum leaks,” i.e., leaks detected only at high vacuum and manifested in the fact that it is not possible to obtain a pressure below \(10^{-6}\) mm Hg—the pressure of reaching “sticking vacuum.” In this case it is extremely difficult to find the place where the installation is leaking. Among a number of instruments and devices that help locate leaks, the palladium leak detector stands out advantageously for its simplicity, both in design and in operation. It is a metallic ionization manometer with a heated cathode and an ion collector. Its design feature is the presence of a tube, one end of which opens to the outside and the other (inner) end of which has a palladium bottom. The outer end of the tube is connected to the vacuum installation. The palladium bottom of the tube is a barrier to air and gases when the manometer is not operating. If, however, the manometer is switched on, the ions emitted by the cathode bombard the palladium plate and heat it. Heated palladium then becomes permeable to hydrogen.

In manufacturing the manometer, it is pumped down to a high vacuum. If a leak appears in the installation, the vacuum in the instrument is not spoiled, as it is in ordinary ionization manometers connected to the system.

In order to discover where the system is leaking, the suspected place should be blown with a stream of hydrogen. When it strikes a target, a crack, or some other place through which leakage occurs, the hydrogen in turn penetrates into the system and, through the heated palladium partition, enters the manometer. An increase in the ion current immediately shows that the place where the installation is leaking has been found. Attempts to find leaks in a similar way using ordinary ionization manometers are not very effective, since these manometers have a considerably poorer vacuum than the one described. This occurs because a leak that appears even before the search for it is begun will already spoil the vacuum in the system, and consequently in the manometer as well, whereas in the case described the vacuum in the manometer begins to deteriorate only after blowing the system with hydrogen. Owing to this, the palladium leak detector registers the slightest increase in hydrogen pressure (\(10^{-7}\) mm Hg). Such high sensitivity makes it possible to detect the smallest leaks. The simplicity of its design and operation permits the described leak detector to be used in any vacuum installation, from laboratory apparatus to large industrial units.

*) RCA — J. Sci. Instr. 25; 9; p. 325 (1948).

V. V. Fedorov