The present invention is a method and apparatus for monitoring in real time the mass and energy flow rate of a gas through a pipeline. The invention determines the mass flow ratio of a pipeline gas flowing through a pipeline compared to sample gas tapped from the pipeline line when the volumetric flow of pipeline gas through the pipeline is measured by a linear flow meter. Sample gas tapped from the pipeline is flowed to a chamber having a section with a fixed volume until the pressure in the chamber section is substantially equal to the pipeline gas pressure. The sample gas is maintained at substantially the same temperature as the gas in the pipeline while the sample gas is in the chamber section. A timer measures the time interval for the sample gas to flow from the chamber section at a selected rate for a calculated pressure drop the selected rate being controlled by a flow controller. The mass flow ratio is computed using the measured time interval and a signal from the linear flow meter. The energy flow rate of the pipeline gas is determined by measuring the energy flow rate of the sample gas and relating that value to the mass flow ratio of the pipeline gas compared to the sample gas.