On-line Chromatographs are commonly used to determine the individual components of a natural gas stream. The individual components are then used to calculate Btu and Specific Gravity. The chromatographic data is combined with flow rate data to calculate a total energy value. The energy value is then used for custody transfer. Accurate chromatograph data is a critical part of the gas measurement process. Understanding the operation and principles of the chromatograph allows system optimization for individual field conditions. Planning is an essential element of a successful chromatograph installation and continued operational accuracy