ONICON F-2600 - Manual (Page 14)

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F-2600 & F-2700 Vortex Flow Meter Manual 02/15 - 0808-7 / 19204 Page 14
1.2.2 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 below. By sensing
the number of vortices passing by the velocity sensor, the F-2600 & F-2700 Flow Meter
computes the total fluid volume.
Figure 2. Measurement Principle of Vortex Flow Meters
1.2.3 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 elements 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.