Time-of-flight ultrasonic flowmeters have been widely used these days in industry. It is however in suspicion if its high accuracy is tracable to the national standard. It was made clear why traceability cannot be guaranteed from a fluid mechanical point of view. The main reason is a difference of flow configuration between the flow standard and the measurement position on-site. The concept of ‘Facility Factor’ is introduced and it is concluded that the ‘Profile Factor’ is not sufficient for correcting the meter reading. It is discussed that measurement of velocity profile on-site is essentially required.

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