Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit
armstrong
international.com for up-to-date information.
11
How the Vortex Mass Flow Meter Operates
Figure 1-3. Reynolds Number Range for the AVF and AVI vortex meters
The Strouhal number is the other dimensionless number that quantifies the vortex phenomenon. The Strouhal number is
defined as:
St = fd
V
Where:
St = Strouhal Number
f = frequency of vortex shedding
d = shedder bar width
V = fluid velocity
As shown in Figure 1-3, Armstrong vortex meters exhibit a constant Strouhal number across a large range of Reynolds
numbers, indicating a consistent linear output over a wide range of flows and fluid types. Below this linear range, the
intelligent electronics in the AVF and AVI meters automatically correct for the variation in the Strouhal number with the
Reynolds number. The meter’s smart electronics corrects for this non-linearity via its simultaneous measurements of the
process fluid temperature and pressure. This data is then used to calculate the Reynolds number in real time. Armstrong
vortex meters automatically correct down to a Reynolds number of 5,000.
Pressure Drop
The pressure drop for series AVI insertion meters is negligible. The pressure drop for series AVF in-line meters is defined
as:
∆P = .00024 PI
V
2
English units ( ∆P in psi, PI
in lb/ft
3
, V in ft/sec)
∆P = .000011 PI
V
2
Metric units ( ∆P in bar, PI
in kg/m
3
, V in m/sec).
