Implementing reliable CNG leak detection requires separating the leak signal from thermal noise. Here's the engineering approach for pressure decay and differential pressure methods.
Pressure Decay with Temperature Compensation
A raw pressure decay reading mixes two effects: the pressure drop caused by an actual leak, and the pressure drift caused by ambient temperature change (via the ideal gas law). To isolate the leak component, the system logs pressure and temperature together throughout the hold period, then computes what the pressure would have been at a constant reference temperature, using the gas law to scale out the thermal contribution. Subtracting that thermally-corrected baseline from the actual decay curve leaves a signal that's attributable almost entirely to leakage rather than ambient drift. In practice this means continuous temperature sampling alongside pressure sampling, not a single temperature reading at the start of the test.
Differential Pressure Measurement
The differential approach pressurizes the test component and a sealed, leak-free reference volume at the same time, in the same environment, then tracks the pressure difference between the two channels rather than either absolute value. Because both volumes see identical ambient temperature swings, the common-mode thermal effect cancels out automatically in the subtraction — there's no need for a separate compensation calculation. What remains is a cleaner, faster-settling signal that reflects real leakage from the test component, which is why differential pressure testing supports shorter cycle times in high-volume production.
Putting It Together
A production leak-test evaluation typically combines both: a temperature-compensated decay measurement for absolute leak-rate figures, and a differential check as a fast pass/fail gate. The result is classified against a leak-rate threshold (informed by ISO 15500 and AIS requirements) to produce a pass, fail, or borderline-retest decision.
The Neometrix CNG Vigilant implements both temperature-compensated pressure decay and differential pressure methods in its Vigilant Ver. 0.30 configuration, with helium mass spectrometry available in Ver. 0.34 for the highest-sensitivity applications.
→ https://neometrixgroup.com/products/cng-vigilant
