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1-7
1020NFM-1D
Section 1
θ
Upstream
transducer
Downstream
transducer
Beam Angle
FLOW
Wide Beam™
Axial Beam Injection
into pipe wall
Sonic Wave front
Wide Beam Transmission
As shown in the figure above, an ultrasonic transducer induces an axial sonic beam within the wall of
the pipe. These vibrations spread along the pipe wall and then enter the liquid in the form of a Wide
Beam wave front traveling at an angle to the main pipe axis. The wide beam “rains” over the receiving
transducer. The wide coverage of the receiver is necessary because the angle of the sonic beam is
related to the liquid’s sonic propagation velocity by Snell’s Law.
According to this formula, we can state that, as the liquid sonic propagation velocity changes so will the
angle between the sonic beam and the flow stream .
Therefore, a significant liquid sonic velocity shift could deflect a “narrow” beam transmission away
from the receiving transducer entirely. The upstream vs. downstream transit-time difference will also
be affected by the changing (or refracting) beam angle. This makes it necessary for clamp-on systems
to continuously compute this angle, since it is subject to varying degrees of refraction. The flow com-
puter derives the angle by knowing the fixed position of the transducers, the dimensions of the pipe and
the measured transit-time. Therefore, the flow computer computes the beam angle relative to the axis
of the pipe.
For transit-time operation, the flow computer converts a series of electronic pulses to high frequency
sonic signals. Transducers inject these sonic signals through the pipe wall into the flowing liquid. Each
transducer alternates as either a transmitter or a receiver of the high frequency signals. This causes
the direction of the sonic signal to alternate between the downstream direction (traveling with the flow)
and the upstream direction (traveling against the flow). The transit-time technique relies on the effect of
the flowing liquid on the amount of time it takes to travel “downstream” versus “upstream.”
( )
Beam Angle = Arc Sine
Liquid Sonic Propagation Velocity
Transducer Phase Velocity