10
Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit
armstrong
international.com for up-to-date information.
How the Vortex Mass Flow Meter Operates
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.
Flow Velocity Range
To ensure trouble-free operation, vortex flow meters must be correctly sized so that the flow velocity range through the
meter lies within the measurable velocity range (with acceptable pressure drop) and the linear range.
The measurable range is defined by the minimum and maximum velocity using the following table.
Gas
Liquid
Vmin
Vmax
0.5 in
0.75 in
1 in
1.5 in and greater
5
√
density (Lb/ft
3
)
175 ft/s
250 ft/s
250 ft/s
300 ft/s
1 ft/s
30 ft/s
30 ft/s
30 ft/s
30 ft/s
English
p
(lb/ft
3
)
Vmin
Vmax
DN15
DN20
DN25
DN40 and greater
6.1
√
density (kg/m
3
)
53 m/s
76 m/s
76 m/s
91 m/s
0.3 m/s
91 m/s
91 m/s
91 m/s
91 m/s
Metric
p
(kg/m3)
The linear range is defined by the Reynolds number. The Reynolds number is the ratio of the inertial forces to the viscous
forces in a flowing fluid and is defined as:
Where:
Re = Reynolds Number
p = mass density of the fluid being measured
V = velocity of the fluid being measured
D = internal diameter of the flow channel
µ = viscosity of the fluid being measured
p V D
µ
RE =
ft/s
m/s
