Operating Principle

SITRANS F X vortex flowmeters measure flow rate by detecting the frequency at which alternating vortices are shed from a bluff body inserted into the flow stream. This principle of measurement is derived from the Karman phenomenon of vortex shedding. The frequency of the alternating vortices is proportional to the flow rate.
The passage of a vortex causes a slight stress on a pick-up sensor placed downstream of the bluff body. The stress is detected by piezo-electric crystals placed inside the pick-up sensor.

1 = Bluff Body, 2 = Pick-up

The flowmeter calculates the flow velocity using the following equation:

Q = A · V = A · d / St · f = 101.93 · f / K [m³/h]

Where:

Q = flow rate [m³/h]

f = vortex shedding frequency [Hz]

K = calibration constant [pulses/m³]

d = width of the bluff body [m]

St = Strouhal Number

A = cross-section area [m²]

V = flow velocity [m/s]

Requirements

In order to generate the vortex streets, the medium must have a minimum velocity:

• For steam and gases, the flow velocity must be 2 to 80 m/s (6.6 to 262 ft/s)
• For liquids the flow velocity must be 0.4 to 10 m/s (1.3 to 32.8 ft/s)