Results of recent research on the accuracy of orifice metering indicate that commercially available devices can be used accurately to determine system effects. The research by Chevron Oil Field Research Co. was aimed at reducing orifice metering errors. Measurement of natural-gas flow rate by orifice meters has long been an established practice. Considerable interest has been generated in recent years concerning the extent to which various aspects of the measurement process affect measurement accuracy. Among such parameters, fluid velocity distribution immediately upstream of an orifice plate is receiving increasing attention. Recent data by the U.S. National Bureau of Standards (NBS) showed that swirling flow can propagate a substantially longer distance in the piping system than was previously thought.