Measuring Principle - Bell VFM60 Series User Introduction Manual

VFM-60
Vortex Flowmeter

1.5 Measuring principle:

Vortex flowmeter measures the flow by sensing the vortices in the flow according to "Karman Vortex
Street". When put an shedder in the path of flow, vortices are alternately shed on each side (see picture
1.2)
1.2 Von Karman vortices
The frequency of vortices (f) is in direct ratio with velocity of flow (v) and in inverse ratio with width
of obstacle (d).
f=St*v/d (formula 1)
v=fd/St (formula 2)
St is Strouhal Number, is a dimensionless constant related to shape of the shdder, which can be get by
testing. St is Strouhal Number, is a dimensionless constant related to shape of the shedder, which can
be get by test.
Because d and St is constant, flow velocity (v) and average velocity (v0) also have certain relationship
( v0=v/(1-1.25d/D) ), so, you could get v0 by having the frequency of vortices shedding (f) , and then
get the mass flow. The ration between quantity of vortex in a certain period of time and the volume of
the flow pass by is called coefficient of the instrument (K)
K=N/V (formula 3)
VFM60 series digital vortex flowmeter is designed to provide most reliable performence. This series of
vortex flowmeter is designed on PA60 plateform. Every part utilized is universal for all VFM60 series
products. The circuit boards use signal isolation and self-diagnose technology. VFM60 series utilize
spectrum analizing signal process technology, which ensured lower under measuring limit and better
turndown ratio. The enhanced version uses unique dual-sensor design and vibration signal analyzing
technology to improve its anti-vibration cabability and provide with more stable reading. VFM60 also
has density calculation function as option, which means it can calculate the density and measure mass
flow rate of air/satureated steam/superheated steam without secondary device. It also has AGA-NX-19
and AGA-8 algorithm to measure natural gas directly.
Rev1.3
Date:24/04/2020 User's Manual
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