A gas leak is a non-expected release of gas that can create a potentially dangerous situation - either because the released gas is poisonous or because it can ignite and create an explosion. When traditional gas detection systems detects the leaking gas, the gas detector will in most cases convert the reading into either a concentration readout (ppm - parts per million) or a LEL (Lower Explosive Level) readout.
The physical size of the leak and the gas pressure decide how fast a potentially dangerous concentration is reached. A leak occuring indoors will reach a dangerous level much faster than a leak outdoors. In outdoor installations the leaking gas will simply dilute or drift away with the wind before it is picked up by any gas detector.
The ultrasonic (acoustic) gas leak detectors do NOT detect ppm level or LEL level - instead it detects the ultrasound generated by the escaping gas. The ultrasonic sound is proportional to the leak rate (mass flow rate), meaning that the higher leak rate (large leak), the more ultrasound is generated from the leak. The ultrasound from the leak is not affected by the wind or dilution, and thus the ultrasonic gas leak detection technology is very reliable in outdoor installations.
In the oil/gas industry hydrocarbon gas leaks have been divided into the following categories:
Minor release: 0.1 kg/sec and duration 2 min
Significant release: 0.1-1 kg/sec and duration 2-5 min
Major release: > 1 kg/sec and duration > 5 min