The "Zero" leakage does not exist and even if it existed, it should not be possible to measure it.
So it is always good to define prior the permissible leakage of the own piece, basing on tables given by the norms.
After this first test it is necessary to study the fluid of exercise ( gas or liquid ) and the operative pressures to which the element to test is subjected.
In case of gas components, where does not exist a "watershed" between the molecular dimensions of the fluids (test/operative), we have to follow the only danger evaluation: for example the same element for city gas can have two allowable leakage levels completely different in case it is applied to a domestic environment (kitchen) or through open-wire transmission lines.
Some examples of leak rates established by norms for gas components are:
In case of components for liquids (water/blood/fuel/oil, etc..) and also referring to the danger level in leakage case, there are normative parameters of leakage which is measured in air where the liquid will not certainly draw, thanks to the molecular link between air and a particular fluid.
Examples of leak rate for liquids are (measured in air, 1 Bar)
In reality, where it is possible, it is better to apply more elevated pressures, in the limits of 1 to 6 Bar at most.
Thanks to this solution, the test costs can be reduced and the test performances can be sensitively improved.
Improving the test pressure, we obtain a leakage amplification, which generically is not linear to the pressure: if for example we measure 1 nCC/minute at 1 Bar test, the same leakage measured at 5 Bar can be more major than 5 nCC/minute.
Moreover, a major pressure amplifies the eventual defect, if elastic, chipping the meat us as, for example, in case of welding on plastic or cracks.
In opposition it is necessary to value the negative aspects of major pressures, such as major settlement times in case of plastic elements, "under mask" leakages in case for example of brim gaskets where the elevated pressure increases the leak of a defective element and problems connected with the safety for people and surrounding environment.
So the right test pressures must be sought with the collaboration of professional men with an experience which is matured in years and above all with the instrumentation useful to execute the initial tests of the case.