Siemens SITRANS FUH1010 Product Instruction Manual page 29

Description
3.4 Theory of Operation
2-Path
2-Path flow meters use two measurement channels to achieve a single output via a "virtual"
third channel. The resultant data is the average of the two channels. Only clamp-on or in-line
transit-time operation is allowed. Benefits include highest available precision and enhanced
immunity to distorted flow profile conditions.
①
②
WideBeam Transmission
As shown in the figure above, 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 WideBeam wave front traveling at an angle to the main pipe axis. The WideBeam
"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 flow meter systems to continuously compute this angle, since it is subject to
varying degrees of refraction. The flow meter derives the angle by knowing the fixed position
of the sensors, the dimensions of the pipe and the measured transit-time.
28
Sensor Path 1
Sensor Path 2
③
Average= (Path 1 + Path 2) / 2
④
Pipe (front view)
FUH1010 IP65 NEMA 4X Precision Volume
Operating Instructions, 01/2013, A5E32122003-AA