Vortex Flow Meter Calculator: Flow from Shedding Frequency, K-Factor and 4-20 mA Output (ISO 12764)
Convert the frequency of a vortex flow meter to flow rate, or find the frequency and mA output for a flow.
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How this calculator works
Enter the inside diameter of the pipe, the width of the bluff body and its Strouhal number, then the measured frequency (or the flow to find the frequency for) and the flow at 20 mA.
The calculator finds the flow area and the meter factor K = A d over St (the volume of fluid for each vortex). The velocity is u = f d over St, and the flow is Q = K x f.
It also gives the frequency at the top of the range and the 4-20 mA output for the flow.
Formulas used
- f = \frac{St\,u}{d}
- K = \frac{A\,d}{St}
- Q = K\,f
- I = 4 + 16\frac{Q}{Q_{max}}\ \mathrm{mA}
Worked example
With the default numbers in the calculator above:
- Flow area of the pipe: 0.0079 m2
- Meter factor: the volume of fluid for each vortex: 0.81 litres per pulse (1227.77 pulses per cubic metre)
- Velocity in the pipe, from the shedding frequency: 6.22 m/s
- Flow rate (Q = K x f): 175.93 m3/h (48.87 L/s)
- Frequency at the top of the range: 204.63 Hz at 600.00 m3/h
- Analogue output for this flow (4 mA at no flow, 20 mA at the top of the range): 8.69 mA
Change the numbers in the calculator to see the full working, every step, for your own case.
A second worked example
A different case, with these numbers: What do you want? The frequency (and the mA output) for a given flow.
- Flow area of the pipe: 0.0079 m2
- Meter factor: the volume of fluid for each vortex: 0.81 litres per pulse (1227.77 pulses per cubic metre)
- Velocity in the pipe: 7.07 m/s
- Vortex shedding frequency at this flow: 68.21 Hz
- Frequency at the top of the range: 204.63 Hz at 600.00 m3/h
- Analogue output for this flow (4 mA at no flow, 20 mA at the top of the range): 9.33 mA
Common questions
Which standard applies?
ISO 12764:2017 Measurement of fluid flow in closed conduits: flowmeters using vortex shedding. It covers the installation, the straight pipe lengths, the calibration and the uncertainty.
What is the Strouhal number?
The dimensionless number St = f d over u. For a vortex meter it is nearly constant over a wide range of flow, so the frequency is proportional to the velocity.
What are the limitations?
It does not work at very low flows, needs a straight pipe before and after it, and can be disturbed by strong vibration and very viscous fluids.
What K-factor should I use?
The one on the calibration certificate of the meter. The value calculated here is a guide for checking.
Related calculators
Got a question about this? Ask an engineering question and get a plain-language answer with references.
Further reading: ISO 12764:2017 Measurement of fluid flow in closed conduits: flowmeters using vortex shedding; ISO 5167 Measurement of fluid flow by means of pressure differential devices.
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