Acoustic Leak Detector

There exist a number of different methods for finding the location of leaks in a vacuum installation. However, all these methods are either expensive, or cumbersome, or unreliable. One attempt to create an instrument free of these shortcomings is the use, described in the reviewed papers,¹–³ of an acoustic leak detector.

Its operating principle is as follows: the worker searching for the location of a leak blows air (or gas) over the parts of the vacuum installation. When the air (or gas) reaches a poorly sealed area, the pressure in the installation rises, and at the same time the tuning of a special oscillatory circuit connected with the vacuum installation changes. The oscillations produced in this circuit are fed to a loudspeaker. Thus a change in the pressure in the installation affects a change in the amplitude or frequency of the sound. Such a change in the sound is perceived by the worker searching for the leak as a signal that its location has been found.

Without dwelling on the method of sound modulation by amplitude,¹ let us consider the case of frequency modulation,²˒³ since the human ear is more sensitive to changes in frequency than to amplitude.

For finding leaks in high-vacuum installations, it is proposed² to use a device consisting of a sound generator and an ionization manometer. The ionization manometer is connected to the vacuum installation. To blow over it, the authors used illuminating gas. A change in pressure in the installation, caused by the penetration of gas through a slit in the glass or metal parts of the fittings, leads to an increase in the ion current in the ionization manometer. In order that the worker can search for the leak without looking away to observe the instrument, an oscillation generator is connected in parallel with the input of the electrical circuit for measuring the ion current. In the work described, a thyratron was used...

tron of type 2050. The frequency of its oscillations was modulated by changing the grid voltage, proportional to the change in the ion current in the ionization manometer. The greatest frequency is obtained when the voltage on the grid is zero, which corresponds to the absence of ion current—that is, to the presence of a high vacuum. In this case, high-pitched signals are reproduced in the loudspeaker. As gas penetrates through the slots, the ion current increases, the voltage on the grid increases, and the oscillation frequency decreases. Elimination of the leak is characterized by a new decrease in the ion current—an increase in the pitch of the sound signals. With the aid of such a device, very small leaks can be detected. According to the authors, they detected an in-leakage because of which the pressure in the system increased by only \(4 \cdot 10^{-8}\) mm of mercury.

Another article³ reports on an instrument that makes it possible to find leaks in rough-vacuum installations (in the range from 0.1 to 10 mm of mercury). For this purpose a special two-electrode gas-discharge tube is soldered into the vacuum system. It is used as the variable element of the simple relaxation circuit shown in the figure.

The generated frequency depends on the conditions under which the discharge develops, including the pressure in the tube, determined by the degree of in-leakage.

With the tube parameters selected in the work carried out (tube diameter 12.7 mm, tungsten-wire diameter 0.2 mm), oscillations arise at a pressure of 10 mm Hg, reach maximum frequency at 0.4 mm Hg, and, decreasing, break off at a pressure of about 0.05 mm Hg.

V. Fedorov

CITED LITERATURE

1. J. H. Kuper, Rev. Sci. Instr. 8, 131 (1937).
2. W. M. Brubaker, V. Wouk, Rev. Sci. Instr. 17, No. 3 (1946).
3. J. T. Lloyd, Journ. Sci. Instr. 27, 3 (1950).