A leak detection system comprising a substantially sealed accumulation chamber adapted to accommodate a single gas container, the accumulation chamber being sealed so that when a gas container is in the accumulation chamber gas leaked from that container accumulates. An optical detector is provided for detecting leaked gas, the detector including an optical cell. A controller directs a reference sample to the optical cell and subsequently directs a sample from the from the accumulation chamber to the optical cell. The optical detector is operable use both the reference sample and the accumulation chamber sample to detect leaked gas.