Yokogawa digitalYEWFLO (DY) - Manual (Page 7)

INTRODUCTION
IM 1F2B4-01-YIA
Page 3
1.2.3 Qmin
Those of you who haven’t used many vortex flowmeters may be wondering, ‘Why do we need to know
viscosity, density, pressure and temperature?’. While the K-factor is unaffected by changes in
viscosity, density and pressure, the velocity at which vortices begin to be created and become stable
enough to measure accurately will vary. We refer to this velocity as Qmin, stated in desired flow units
GPM, SCFH, etc. Here’s an example to help you understand. Let’s go back to the flag example.
We’ve all seen the flag flapping in the breeze; however, on some days we can feel the breeze blowing,
but the flag isn’t flapping. Why not? For the flag to flap, there must be enough breeze blowing, or
energy, to lift the flag and create fully developed vortices. This is the same thing that happens in the
vortex flowmeter.
The higher the fluid viscosity, the higher the velocity (more energy) required to start vortex shedding.
On the other hand, the higher the density, the lower the velocity needed to start vortex shedding. In
gases, viscosity and density can vary with pressure and temperature. Sounds complicated, but
compared to an orifice plate it’s quite simple. By using the YEWFLO sizing program, vortex meter
selection is simple. Simply enter the process conditions, the program will prompt you for them, and
presto, a performance table for all meter sizes is generated. This performance table will help you
select the best YEWFLO for the application.
1.2.4 Uniquely vortex
Vortex shedding flowmeters measure flow digitally. This means, amplitude of the vortex signal is
unimportant. As long as the flow is above the Qmin threshold, only the presence or absence of a
vortex is important. Just like digital electronics, as long as the voltage is above or below a threshold
value, it is either on or off. Digital flow measurement means no zero drift or span shift . Orifice plate
flowmeters, for example, cannot make this claim, even if they are using microprocessor-based digital
D/P transmitters, they still measure the small amplitude of deflection caused by differential pressure,
and changes in temperature or pressure can shift zero and span.
1.2.5 Vortex frequency
The YEWFLO uses piezoelectric crystals embedded in the shedder bar . Note that they are 1)
hermetically sealed , and 2) surrounded by a heavy wall thickness , to protect them from the
environment and the process. The positioning of the crystals is important. Although one crystal
primarily measures flow frequency, it unfortunately picks up some pipe vibration noise. The other
crystal is positioned such that it picks up primarily the pipe vibration noise. By electronically
subtracting these two signals, we are able to obtain a high signal to noise ratio for the flow signal .
The new Style "E" body design also improves the signal to noise ratio, by stiffening the shedder bar
mounting in the measurement plane, further isolating it from pipe vibration.
1.2.6 Available outputs
After processing the digital vortex frequency as described above, what outputs can you get? You can
select either 4-20 mA output or voltage pulse, digital output. Output is selected by setting jumpers on
the amplifier board, and the setting the software for pulse or analog output. Analog output is two-
wire, and pulse output is a three-wire connection (for details see the wiring section). The pulse output
can be scaled over a range of 0-6000 Hz, down or up to maximize pulse resolution. Scaling up the
frequency output can be done to improve resolution. The pulse output is also capable of driving many
electromechanical totalizers directly without additional power.