Why is the leak rate AH according to ISO 15848-1 measured with vacuum?

ISO 15848-1 aims to record an actual leak flow, not just a gas concentration in the air. In the vacuum method, the inside of the valve is under a known overpressure with helium, while the outside is connected to a helium leak detector in vacuum mode. This evacuates all released helium and directly converts the signal into a leak rate (for example Pa·m³/s or mbar·l/s), which is then compared to a calibration leak.

In a sniff test you mainly measure concentration around the leak, strongly influenced by distance, drafts, and turbulence. The vacuum method is much more sensitive, better calibrated, and less dependent on the operator. As a result, leak rates between different laboratories are reproducible and well comparable, exactly what the standard intends.

A leak rate in class AH is so small that it can practically only be reliably measured with the vacuum method.