Real leakages

to the overview - back - next

One difficulty for planning, design and control within a production is in terms of leakage setting meaningful limits. Are the tolerances to small, the number of not ok pieces will be rising up. Are the tolerances too broad, this can lead to late complications such as oil and / or water leakage during the final test or even at the consumer. In this case, the consequences would be catastrophic for a production. Today, more customer accepts no leaks under his new car.

The result is that designers tend to set the tolerances very closely or even make the call for "absolute density parts" of such a test station. The reality, however, shows that there can be no "absolute proof parts." Each part has at some point, albeit a small leak on. There is always a question of measuring time and the medium. For these reasons, leakage can be specified only for a certain period with the parameters of the test pressure and test medium.

In the automotive sector have in recent years, crystallized the technical limits for leak out. Below are listed some of them. Technological progress and the rising technical requirements will correct this limitation in some places but in the future further down.

 Test volume  Test pressure Limits 
 Pressure oil chamber  200 to 300kPa  5 to 10 cm3/min
 Pressure less oil chamber  20 to 50kPa  30 to 50 cm3/min
 Water chamber  100 to 200kPa  5 to 10 cm3/min
 Water pumps  100 to 200 kPa  5 to 10 cm3/min
 Fuel rail  400 to 500 kPa  0,5 to 2 cm3/min
 Complete transmission  20 to 50kPa  10 to 30 cm3/min

It depends very much on what stage of construction is achieved and whether assembled specimens were previously tested in pieces once before.

For all testing facilities should be ensured that the effective direction of the audit and the actual direction of action corresponds to the operation of the inspected part. Otherwise it could easily happen that such seals are sealed in a pressure-assisted sealing but not under the real conditions!

Work may need to be held under positive pressure in some places with vacuum as the test medium.

Also interesting is the observation of a leak to the diameter of the resulting leak rate. In this approach a test pressure of 100 kPa is assumed.

 Leakage in
 mbar * l / s
of the leak
 Leakage description
 10+2  1,0mm  Water is running out
 100 = 1  0,1mm  Dripping faucet
 10-2  0,03mm  "Waterproof" (not dripping)
 10-3  30µm (diameter of hairs)  1 bubbles (1mm3) per second
 10-6  ~0,1µm (Aperture)  ~ 1cm3 Gas loss in 12 days
 10-8  ~0,4µm x 2mm Wall thickness  ~ 3cm3 Gas loss in 1 year
 10-11  <0,1µm  ~ 1cm3 Gas loss in 3000 years

These data leakage, the volume always 1000 cm3 (= 1 liter). In order to arrive at a specific flow rate for a given test volume, the corresponding information (eg differential pressure = 1 mbar in a measurement time of 1 second in a test volume of 500 cm3) are entered into the conversion formula. For the example results in a flow rate of 30 cm3/min.