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While components e.g., of a combustion engine may only need to be watertight or “oil-tight”, modified atmosphere packaging for food or pharmaceuticals must not lose any gas. Accordingly, depending on the intended use, leakages on different scales can be tolerated – a component or packaging is considered leak-tight as soon as it fulfils a defined and thus verifiable requirement for its leak-tightness.
The measure of leak-tightness is the leakage rate. It is quoted in the units mbar l/s and abbreviated with the formula symbol qL. Less frequently, the unit of mass flow sccm (standard cubic centimetres per minute), which is independent of pressure and temperature, is used to indicate the leakage rate.
A leakage rate of 1 mbar l/s corresponds to a gas flow of 1 litre per second at a gas pressure of 1 mbar or describes a situation in which in a closed 1 litre container the pressure rises or falls by 1 mbar within one second. With reference to the leakage rate, leaks can be quantitatively recorded, and the following leak tightness classification defined:
Leakage rate qL in mbar l/s | Leak-tightness classification |
10-6 | viral tigth |
10-5 | petrol- and oil-tight |
10-4 | bacteria-tight |
10-3 | vapour-tight |
10-2 | watertight (drop) |
The definition of permissible leakage depends largely on the nature of the object. At what point does a leak disrupt the function or up to what point is a leak negligible? A look at comparable applications and empirical values from the industry, which provide indications for determining a leakage rate, might help. Once a limit value has been defined, a suitable method for leak testing can be selected on this basis.
Multi-stage leak testing procedures can also be an economical solution: First, a rough leakage test determines whether the test item is approximately leak-tight. If it is, then, a fine leakage test can follow, in which the leakage rate can be determined precisely. This saves unnecessary leak tests and conserves the use of this measurement technology.