Among the different ultrasonic methods used for the measurement of fluid velocity, the method of the " transit time difference " is the most accurate and the only one which permits to realize a true integration of velocity profile along the acoustic path.According to the flow hydraulic conditions and to the required accuracy, one or several chords can be used (a chord is composed of 2 probes)
There are two successive operations in the flow measurement :
• The velocity measurement on one or several chords
• the flow calculation from one or several velocity measurements
The two probes A and B are alternatively emitting and receiving.
The acoustic signal propagates in the fluid and is collected by the opposite probe.
The measurements of the propagation times in the upstream and downstream directions are measured with accuracy (typically 0.2ns).
The average velocity can be calculated from these times and from the geometrical position of probes
Advantages and strong points:
The measurement :
• only depends on the L and D geometrical parameters
• is independant of the sound velocity in the fluid
• is the average velocity along the AB line
• is bidirectional
• no provides pressure drop
This measurement principle is applicable to liquids and gas.
The used probes can be intrusive or external (clamp-on)