Clamp-on type flow meters, with one single ultrasonic path, are very sensitive to the velocity profile in flow rate measurement and must be used, for greater metrological realiability, when the flow is completely developed. This is not the normal condition of use on oil production platforms, where small straight pipelines are usually available. Numerical and experimental studies have shown that for symmetric ultrasonic paths, the arithmetic mean of the values indicated by the flow meter is practically independent of non-axial velocity components in developing flows in straight pipeline equivalent lengths larger than ten pipeline diameters. The single path was secured by means of an introduced mechanical device that positions the sensor plane every 45 degree angle to the horizontal, thus allowing measurements at other ultrasonic paths, procedure that showed to minimize the uncertainty associated with the flow measurement. The metrological characterization of this measurement procedure, qualifies the meter to be used as a in situ traveling calibration standard of flow meters, which prevents its removal for calibration in laboratory. The alternative methodology calibration proposed was validated experimentally in loop especially built at PUC-Rio to reproduce the field measurement conditions.