The on-line gas chromatograph (GC) has been widely used for natural gas quality analysis and energy measurement at custody transfer locations since the early 80’s. The energy measurement and relative density measurement provided by the GC can have a large effect on unaccounted for Gas because the gravity effects volume calculations and the heating value effects energy rates. The gas chromatograph measures component concentrations first and then calculates physical properties of the gas such as heating value and relative density. Units of energy measurement can be British Thermal Units (BTU) Mega Joules (MJ) or Kilo Calories (KJ). This paper will refer to BTU for energy units, ISO (KJ) or SI (MJ) metric units can be used interchangeably