Embodiments of a portable verification system can move from one in-field gas flow meter location to another and temporarily connect downstream of a main pipeline's meter run or station. A control valve of the portable verification system allows volume measurement at different flow velocities to be verified. In some embodiments, the portable verification system is connected to the meter run and the main pipeline by an adjustable pipeline section. This section can extend horizontally and vertically, as well as swivel to provide versatility when connecting in the field. Adaptor fittings having one flange sized for and fitted to the inlet and outlet ends of the portable verification system and another flange sized for the meter run or main pipeline connection provide additional versatility. Downtime is limited to the time required to complete a circuit between the meter run, portable verification system, and main pipeline.