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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