INFICON :: LEAK TEST - Best Practices for Leak Detection
 

How to Convert from Pressure Decay to Leak Rate

Most pressure decay (or air leak test) instruments display pressure in psi, mbar, Pa, or atm. In helium leak detection, a leak rate is displayed in mbar l/s, atm cc/m or Pa m³/s. How does one relate to the other?

All leak rate units consist of three components:

1) Pressure (mbar, atm or Pa)
2) Volume (l, cc or m³)
3) Time (s or m).

Note: ppm is a concentration, not a leak rate.

As you can see from the description of the units used above, a leak rate is:

  • a changing volume (2) in a given time (3) at a certain pressure (1)
  • a changing pressure (1) in a given time (3) at a certain volume (2).

The same components present for a leak rate, are also needed for pressure decay:

  • measures a pressure drop (1) in the inner volume (2) of the part in a given time (3)

Example: Let's say the part under test has an inner volume of 200 cc (or ml) and we measure a pressure drop of 30 Pa in 5 s. The corresponding leak rate can then be calculated as: 200 cc x 30 Pa / 5 s = 0.2 l x 0.3 mbar / 5 s = 0.012 mbar l/s = 1.2E-2 mbar l/s.

The pressure drop can be converted to a leak rate by multiplying the pressure drop with the inner volume of the part and by then dividing the result by the measurement time needed for the pressure drop.

 


 
 
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