Sierra INNOVA-MASS 241-VTP - Manual (Page 10)

Chapter 1 Introduction Series 24-HP Instruction Manual
1-4 IM-24-HP
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 oth-
er wake rotate counterclockwise. Vortices generate one at a time, alter-
nating from the left side to the right side of the shedder bar. Vortices in-
teract 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 Innova-Mass™ Flow Meter computes the total fluid volume.
Vortex shedder bar
Flow
Vortices
Velocity sensor
Constant
wave length
Figure 1-2. Measurement Principle of Vortex Flow Meters
Vortex Frequency Sensing
The velocity sensor incorporates a piezoelectric element that senses the
vortex frequency. This element detects the alternating lift forces pro-
duced by the Von Karman vortices flowing downstream of the vortex
shedder bar. The alternating electric charge generated by the piezoelec-
tric element is processed by the transmitter’s electronic circuit to obtain
the vortex shedding frequency. The piezoelectric element is highly sensi-
tive and operates over a wide range of flows, pressures and temperatures.