The primary method for custody transfer measurement has traditionally been orifice metering. While this method has been a good form of measurement, technology has driven the demand for a new, more effective form of fiscal measurement. Ultrasonic flowmeters have gained popularity in recent years and have become the standard for large volume custody transfer applications for a variety of reasons. Most users require flow calibrations to improve meter performance and overall measurement uncertainty. Although AGA Report No. 9, Measurement of Gas by Multipath Ultrasonic Meters [Ref 1], currently only recommends flow calibration for ultrasonic flowmeters, the next revision will likely require flow calibration for all ultrasonic custody transfer applications