Siemens SITRANS FUP1010 Operating Instructions Manual page 19

Wide Beam Transmission
As shown in the figure below, an ultrasonic sensor 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 sensor. 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, it can be stated 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 sensor entirely. The upstream vs. downstream transit-time difference
will also be affected by the changing (or refracting) beam angle. This makes it necessary for
systems to continuously compute this angle, since it is subject to varying degrees of refraction.
The flowmeter derives the angle by knowing the fixed position of the sensors, the dimensions
of the pipe and the measured transit-time. Therefore, the flowmeter 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 sensor
mounting location is within close proximity of a bend or other pipe obstruction. It is
recommended that, whenever possible, mount sensors on the longest available straight run
of pipe and also use Reflect Mode mounting (as shown below).
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FUP1010 IP67 Portable Flowmeter
Operating Instructions, 02/2010, A5E02951522A Revision 01
Upstream Sensor
Downstream Sensor
Flow Direction
Wide Beam transmissions exchanged between the Upstream and Downstream sensors
Description
3.3 Theory of Operation
17