Abstract: Methods and apparatus for the metering of two-component, single-phase fluid flow. A primary element of a differential pressure flow meter is used as a joint mixer and flow meter. Diagnostic techniques can be applied to detect and help determine the cause of erroneous meter readings. A volume meter can also be provided downstream of the differential pressure meter, and the respective outputs can be cross-referenced to produce a mass flow rate, volume flow rate and density output with no prior knowledge of fluid density required.