Gas flow metering apparatus for obtaining flow measurements in respect of gas in a conduit is provided. The apparatus includes an ultrasonic mass flow meter including a first, upstream ultrasonic transducer, a second, downstream transducer, and a first calculation module for receiving data representative of an ultrasonic transit time between said transducers and calculating, using said data, a first flow velocity of said gas. The apparatus including at least one measurement device for measuring a flow parameter of said flow of gas through said conduit, a second calculation module for calculating, using said flow parameter, a second flow velocity of said gas, a verification module configured to select a preferred flow velocity from said first and second calculated flow velocities dependent upon expected accuracy in current gas flow conditions, and an output module for calculating, using said selected preferred flow velocity, a volumetric flow in respect of said gas flow.