All DTI ultrasonic flowmeters measure flow by the transit time difference method. The jet lag method is based on a simple physical fact. Imagine two boats crossing a river facing each other along the same straight line: one boat goes down and the other goes up. Obviously, it takes less time for a boat traveling downstream to reach the opposite bank than a boat traveling upstream.
Ultrasound behaves exactly the same way as a boat. Sound waves travel faster in the direction of the medium than in the opposite direction (vAB> vBA).
The transmission times tAB and tBA are continuously measured. The time difference (tBA-tAB) between the two ultrasonic transmissions is proportional to the average flow velocity (vm) of the medium.
The volume flow rate of a fluid per unit time is the average flow velocity of the medium times the cross-sectional area of the pipe.
Different liquid media can be identified by directly measuring the transmission time of the ultrasound. For example, suppose that through the same path length (L), the transit time in water is shorter than in crude oil.
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