Gas flow metering apparatus for obtaining flow measurements in respect of gas in a conduit, the apparatus comprising: an ultrasonic mass flow meter including: a first, upstream ultrasonic transducer and a second, downstream transducer, each transducer being mounted in a peripheral wall of said conduit, at an angle to the flow of gas therethrough; and a first calculation module for receiving data representative of an ultrasonic transit time between said transducers (10a, 10b) and calculating, using said data, a first flow velocity of said gas; at least one measurement device (22, 36, 47, 48) 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 including gas composition, wherein said verification module is configured to compare said first and second flow velocities with historical flow velocity data and selecting said preferred flow velocity based on said comparison; and an output module for calculating, using said selected preferred flow velocity, a mass flow in respect of said gas flow; the apparatus further comprising a calibration module configured to utilise said selected preferred flow velocity as a master input for cross-calibrating said first and second flow velocities. an ultrasonic mass flow meter including: a first, upstream ultrasonic transducer and a second, downstream transducer, each transducer being mounted in a peripheral wall of said conduit, at an angle to the flow of gas therethrough; and a first calculation module for receiving data representative of an ultrasonic transit time between said transducers (10a, 10b) and calculating, using said data, a first flow velocity of said gas; at least one measurement device (22, 36, 47, 48) 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 including gas composition, wherein said verification module is configured to compare said first and second flow velocities with historical flow velocity data and selecting said preferred flow velocity based on said comparison; and an output module for calculating, using said selected preferred flow velocity, a mass flow in respect of said gas flow; a first, upstream ultrasonic transducer and a second, downstream transducer, each transducer being mounted in a peripheral wall of said conduit, at an angle to the flow of gas therethrough; and a first calculation module for receiving data representative of an ultrasonic transit time between said transducers (10a, 10b) and calculating, using said data, a first flow velocity of said gas; at least one measurement device (22, 36, 47, 48) 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 including gas composition, wherein said verification module is configured to compare said first and second flow velocities with historical flow velocity data and selecting said preferred flow velocity based on said comparison; and an output module for calculating, using said selected preferred flow velocity, a mass flow in respect of said gas flow; the apparatus further comprising a calibration module configured to utilise said selected preferred flow velocity as a master input for cross-calibrating said first and second flow velocities.