Pressure decay is the standard method for leak testing sealed pneumatic assemblies. Getting accurate leak rate figures requires more than just watching a pressure gauge fall — you need temperature compensation, correct volume calculation, and proper units conversion.
The Physics
Pressure decay in a sealed volume with a small leak follows:
dP/dt = -Q_leak × P_atm / V_system
Where Q_leak is volumetric leak rate (cc/s at atmospheric), P_atm is atmospheric pressure, V_system is system volume.
But temperature changes cause pressure changes too (ideal gas: PV = nRT). A temperature rise of just 0.1°C in a 350 bar system causes a pressure increase of ~0.3 bar — easily masking a small leak.
Temperature-Compensated Leak Rate
The compensation algorithm first calculates a corrected pressure time-series by scaling each measured pressure value by the ratio of reference temperature to current temperature — effectively removing the pressure variation caused by thermal expansion in a perfectly sealed system. A linear fit is then applied to the corrected pressure data versus time, and the slope of that fit gives the true pressure decay rate in bar per second. Converting to standard cubic centimetres per minute (sccm) requires multiplying the decay rate by the system volume and dividing by atmospheric pressure. Negative slopes indicate falling pressure, confirming a leak is present. The initial temperature reading serves as the reference baseline, so any subsequent temperature drift is automatically corrected throughout the test window.
Typical Aerospace Leak Specifications
| System | Max Leak Rate | Standard |
|---|---|---|
| Aircraft oxygen system | 1 sccm | MIL-PRF-27210 |
| Pneumatic brake line | 5 sccm | OEM specification |
| Missile actuation | 2 sccm | MIL-PRF-27422 |
| Aircraft tyre valve | 10 sccm | ASME |
Test Duration for Statistical Confidence
For very small leaks, the required test duration depends on the pressure sensor's resolution. If the leak rate is so slow that it would produce less than three times the sensor resolution's worth of pressure change per minute, the test duration must be extended until at least ten times the resolution worth of pressure change has accumulated — giving statistically distinguishable signal above noise. Leaks large enough to produce clearly detectable pressure changes within 60 seconds can use a standard one-minute measurement window. This approach prevents false passes on marginal leaks where thermal noise dominates a short test window.
Neometrix High Pressure Air Test Systems implement this compensation and calculation in the PLC for real-time leak rate display and automated pass/fail.
→ https://neometrixgroup.com/products/high-pressure-air-test-system
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