Panametrics PanaFlow MV80, User Manual, Page: 15 / 184

PanaFlow™ MV80, MV82 & MV84 User’s Manual 3
Chapter 1. Introduction
1.3.1 Vortex Shedding Frequency
Von Karman vortices form downstream of a shedder bar into two distinct wakes. The vortices of one wake rotate
clockwise while those of the other wake rotate counterclockwise. Vortices generate one at a time, alternating from
the left side to the right side of the shedder bar. Vortices interact with their surrounding space by over-powering
every other nearby swirl on the verge of development. Close to the shedder bar, the distance (or wave length)
between vortices is always constant and measurable. Therefore, the volume encompassed by each vortex remains
constant, as shown in
Figure 2 below. By sensing the number of vortices passing by the velocity sensor, the
PanaFlow™ Flow Meter computes the total fluid volume.
Vortex Shedder Bar
Velocity Sensor
Vortices
Constant Wave Length
Flow
Figure 2: Measurement Principle of Vortex Flow Meters
1.3.2 Vortex Frequency Sensing
The velocity sensor incorporates a piezoelectric element that senses the vortex frequency. This element detects the
alternating lift forces produced by the Von Karman vortices flowing downstream of the vortex shedder bar. The
alternating electric charge generated by the piezoelectric element is processed by the transmitter's electronic circuit
to obtain the vortex shedding frequency. The piezoelectric element is highly sensitive and operates over a wide range
of flows, pressures, and temperatures.