Wide Beam Transmission; Flow Calibration Factor - Siemens SITRANS FUH1010PVDX Operating Instructions Manual

Section 1

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 trans-
ducer. 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 af-
fected 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 computer
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.

Flow Calibration Factor

Normally, the flow stream is in line with the axis of the pipe. On this basis, the calibration factor of a
Clamp-on ultrasonic flowmeter is proportional to the cosine of the beam angle relative to the pipe axis.
However, this reveals that if the angle of flow stream is not in line with the pipe axis, the flow calibration
factor could be compromised. This most often occurs when the transducer mounting location is within
close proximity of a bend or other pipe obstruction. This is why we recommend that whenever possible
you mount clamp-on transducers on the longest available straight run of pipe and also use Reflect Mode
mounting (see next page).
Upstream
transducer
θ
Beam Angle
(
Beam Angle = Arc Sine Liquid Sonic Propagation Velocity
Wide Beam™
Axial Beam Injection
into pipe wall
Downstream
transducer
Transducer Phase Velocity
1-5
CQO:1010PVXFM-3
Sonic Wave front
FLOW
)