When dealing with small, hermetically sealed parts where the enclosure is leaky, krypton 85, a gaseous, radioactive isotope, can first be forced into the device by applying pressure from the outside. Once an exactly measured holding period has elapsed the pressure will be relieved, the component flushed and the activity of the “gas charge” will be measured. In the same way it is also possible to use helium as the test gas (see the bombing test on the Integral leak testing page).
The so-called high-frequency vacuum tester can be used not only to check the pressure in glass equipment but also to locate porous areas in plastic or paint coatings on metals. This comprises a hand-held unit with a brush-like high-frequency electrode and a power pack. The shape and color of the electrical gas discharge can serve as a rough indicator for the pressure prevailing inside glass equipment. In the case of the vacuum tester – which comprises primarily a tesla transformer (which delivers a high-voltage, high frequency AC current) – the corona electrode approaching the apparatus will trigger an electrode-free discharge inside the apparatus. The intensity and color of this discharge will depend on the pressure and the type of gas. The luminous discharge phenomenon allows us to draw conclusions regarding the approximate value for the pressure prevailing inside the apparatus. The discharge luminosity will disappear at high and low pressures.
When searching for leaks in glass equipment the suspect sections will be scanned or traced with the high-frequency vacuum tester electrode. Where there is a leak an arc will strike through to the pore in the glass wall, tracing a brightly lit discharge trail. Small pores can be enlarged by these sparks! The corona discharge of the vacuum tester can also penetrate thin areas in the glass particularly at weld points and transitional areas between intermediate components. Equipment which was originally leak-free can become leaky in this fashion! In contrast to the actual leak detector units, the high frequency vacuum tester is highly limited in its functioning.
Occasionally leaks can also be located or detected by means of chemical reactions which result in a discoloration or by penetration of a dye solution into fine openings. The discoloration of a flame due to halogen gas escaping through leaks was used earlier to locate leaks in solder joints for refrigeration units.
A less frequently employed example of a chemical effect would be that of escaping ammonia when it makes contact with ozalid paper (blueprint paper) or with other materials suitably prepared and wrapped around the outside of the specimen. Leaks are then detected based on the discoloration of the paper.
An example of a dye penetration test is the inspection of the tightness of rubber plugs or plungers in glass tubes, used for example in testing materials suitability for disposable syringes or pharmaceutical packages. When evaluating tiny leaks for liquids it will be necessary to consider the wettability of the surface of the solid and the capillary action; see also Table 5.1. Some widely used leak detection methods are shown – together with the test gas, application range and their particular features – in Table 5.4.
*) As opposed to vapor, it is necessary to differentiate between hydrophilic and hydrophobic solids. This also applies to bacteria and viruses since they are transported primarily in solutions.