Limit values

Again and again there are difficulties with the metrological description of the term "tight" in leakage testing.

The comparative values listed below have been compiled from given values we received over the last appr. 30 years from our customers.

We would like to mention that these values are only general empirical values which can certainly vary for specific applications. Particular attention has to be given to the demand for "gasoline tightness" for which increasingly the overall emission regulations valid in the USA have to be taken into consideration.

Comparative values air leakage

Charakteristic Air leakage   Comment
  from to  
Water tightness 0,5 cc/min 12 cc/min  
Oil tightness 0,6 cc/min 4,5 cc/min  
Gasoline tightness 0,1 cc/min 3 cc/min older values
  0,0006 cc/min   current value for
USA export      

If this leakage is tested with overpressure, vacuum, or gas detection methods, usually depends on the operating conditions of the product.

Ideally the test should take the actual operating conditions into consideration. However, it has to be noted that with increasing pressure the reproducibility of a leakage test with air decreases.

The reason for this lies mainly in thermal processes during filling.

In leakage tests with test gas, thermal effects do not affect the quality of the testing result so that these methods allow for testing hot or thermally sensitive parts without problems. Furthermore, particulars of the design of the test specimen should be considered.

For example, high test pressure can press an elastic seal in the test piece so tightly on its seat that an existing damage is closed and therefore not detected. If this seal hardens through aging, leakages can occur at such imperfections during operation.

If test pressure has to be reduced for such reasons, it has to be noted that the limit value should be made correspondingly stricter.

For leakage rates bigger than 1E-04 cc/min a quadratic relation between test pressure and leakage rate can be achieved. This means: a test piece that has to be watertight at 5 bar and for which therefore a limit value of 6 cc/min has been set, needs to be tested for a leakage rate of 0.5 cc/min at a pressure of 1 bar.

Of course the above mentioned design particulars have also to be considered for this definition. When testing hermetically sealed parts from the outside, special attention has to be paid for the defect "gross leakage". For such testing tasks the filling process must not be carried out exclusively time-controlled because in the case of gross leakage the space behind the leakage location is already brought to test pressure during the filling time and thus no pressure changed can be determined during the test time.

Regardless of your testing problem, we offer competent advice and suggestions for solutions.

If we deem it necessary, we carry out preliminary trials with your test specimens and will be happy to be to your disposal already during the product development phase, if you wish.


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