The sensitivity of differential-pressure flow meters to the quality of the approaching flow continues to be a cause for concern to flowmeter manufacturers and users. Distortions to the approaching velocity profile generated by pipe fittings and installations located upstream of a flowmeter, can lead to considerable errors in flowmetering. This cannot be ignored because of the likely cost and process efficiency implications. This paper describes the effects of various entrance flow velocity profiles on the performance of an orifice flowmeter with and without flow conditioning. Asymmetric swirling velocity profiles were generated by a ball valve. These caused significant shifts to the meter's calibration, The use of a vaned-plate flow conditioner, consisting of six vanes attached to a 70% porosity plate, greatly improved the performance of the meter. Thus, the device can be used as part of a flowmetering package that will have considerably reduced installation lengths. The less sophisticated NEL plate proved to be a good flow straightener, i.e. a good swirl remover, but was not an efficient flow conditioner.