From Wikipedia

Gas leak detection is the detection of gas leaks with apparatus like catalytic/infrared point sensors and IR open path sensors, more recent detection methods include Ultrasonic Gas Leak Detectors.

In order to detect gas leaks with traditional methods, the gas itself must either be in close proximity to the detector or within a pre-defined area. Outdoor environmental conditions such as changing wind directions and quick dispersion of a potential gas cloud, which can be found e.g. on an offshore platform, can result in undetected gas leaks, leading to extremely dangerous situations.

An alternative to these traditional detection methods is ultrasonic gas leak detectors. This detection method often use a microphone technology; which detect outdoor leaks by sensing the distinct high frequency ultrasound emitted by all high pressure gas leaks, meaning that the gas does not have to reach the sensor – just the sound of the gas leak.

Basic detection can be accomplished using semiconductor gas detection. The semiconductor sensor method can be used for a variety of gases including combustible, toxic, and refrigerant gases. Semiconductor gas detectors are very economical and have a life span of 5 to 8 years however they are prone to cross sensitivity.

Gas released from any environment into another may undergo chemical changes. Electrochemical gas sensors are used where gases can undergo an oxidation/reduction reaction. This method is very selective. Electrochemical gas sensors can measure at only a few particles per billion. Unfortunately the sensors themselves are subject to corrosive elements and may last only 1-2 years before a replacement must be deployed.