Internal Invoicing (or Custody Transfer, or Metering) is often done by measuring the Heating Value by means of a Gas Chromatography (GC). Disadvantage is that the measurement should be within a prescribed range. Especially for fluctuating gas compositions such as fuel gas this could be a challenge. Also the response time of a GC in minutes is slow. For internal invoicing a fast-responding WIM COMPAS™ can be used to solve these issues. In addition, the WIM COMPAS™ can also be equipped with an GC for Components measuring as often specified in the chromatography standards for Custody transfer.