Aalborg Vortex - Manual (Page 7)

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1.3 Sensor Operation
Two pressure sensing devices are mounted in the pipe wall of the flow meter.
Inserted in its center is a stainless steel bluff body, on either side
of which alternating stresses are induced in response to the forming and
shedding of vortices. The piezo electric sensors detect the pressure pulses, and
their energy content is amplified in the preamplifier and processed to furnish an
output proportional to the rate of flow.
1.4 Calibration Factor
The frequency at which vortices are shed is a linear function of fluid velocity,
and therefore, a measure of flow. Vortex frequency is insensitive to density (tem-
perature, pressure), the total volumetric flow of compressible fluids should how-
ever be adjusted to flow conditions other then calibration conditions. In the range
covered by the particular flowmeter, vortex frequency is insensitive to specific
gravity, viscosity, and temperature of the fluid and depends only upon the width
(d) in inches and shape of the flow element, and the inside diameter (D) of the
pipeline in inches.
The frequency is: F = SV/d
where : F = Karman vortex frequency
S = constant (Strouhal Number)
V = fluid velocity at the flow element
d = face width of the element
The frequency relationship is accurately linear and reproducible, eliminating the
need for “wet calibration.” (See Figure 2.)
2. HANDLING AND STORAGE
FIGURE 2