Zero point is the gas concentration at which the gas analysis unit registers zero.
Source: IMO MSC.1/Circ.1343, amendments to the International Aeronautical and Maritime Search and Rescue (IAMSAR) Manual, 24 May 2010, International Maritime Organization. Regulatory Guidance
Source: IMO MSC.1/Circ.1370, Guidelines for the design, construction and testing of fixed hydrocarbon gas detection systems, 22 June 2010, International Maritime Organization. Regulatory Guidance