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The fact that the pressure reading at vacuum gauges is sensitive to the type of gas involved (see page on Indirect pressure measurement) can, to a certain extent, be utilized for leak detection purposes. Thus, it is possible to brush or spray suspected leaks with alcohol. The alcohol vapors which flow into the device – the thermal conductivity and ionizablity of which will vary greatly from the same properties for air – will affect and change pressure indication to a greater or lesser extent. The availability of more precise, easy-to-use helium leak detectors has, however, rendered this method almost completely obsolete.
The pressurized test specimen is submerged in a liquid bath. Rising gas bubbles indicate the leak. Leak detection will depend greatly on the attentiveness of the person conducting the test and involves the temptation to increase the “sensitivity” by using ever higher temperatures, wherein the applicable safety regulations are sometimes disregarded. This method is very time-consuming for smaller leaks, as Table 5.3 shows. It references leak testing on a refrigeration system using type R12 refrigerant. Here the leak rate is specified in grams of refrigerant loss per year (g/a). Water is used as a test liquid (which may be heated or to which a surfactant may be added) or petroleum-based oils. The surface tension should not exceed 75 dyn/cm (1 dyn = 10‑5 N).
In many cases pressurized containers or gas lines (including the gas supply lines for vacuum systems) can be checked quite conveniently for leaks by brushing or spraying a surfactant solution on them. Corresponding leak detection sprays are also available commercially. Escaping gas forms “soap bubbles” at the leak points. Here, again, the detection of smaller leaks is time-consuming and will depend greatly on the attentiveness of the inspector. The hydrogen gas cooling systems used in power plant generators represent a special case. These are indeed sometimes tested in the fashion described above but they can be examined much better and at much higher sensitivity by “sniffing” the hydrogen escaping at leaks using a helium leak detector which has been adjusted to respond to H2 (see the page on local leak detection).
As a variation on the spray technique mentioned above, in which the escaping gas causes the bubbles, it is possible to place a so-called “vacuum box” with a seal (something like a diver’s goggles) on the surface being examined once it has been sprayed with a soap solution. This box is then evacuated with a vacuum pump. Air entering from the outside through leaks will cause bubbles inside the box, which can be observed through a glass window in the box. In this way it is also possible, for example, to examine flat sheet metal plates for leaks. Vacuum boxes are available for a variety of applications, made to suit a wide range of surface contours.