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Ultrasonic Flowmeters
(Part 1 of 4)

By David W. Spitzer

E-Zine May 2009

In general, vortex shedding and fluidic flowmeter applications include liquids, gases and vapors where it is acceptable to not measure low flow rates because the flowmeter turns off at low fluid velocity and at low Reynolds number. This should be well understood and quantified because failure to measure low flow rates can adversely affect many processes. Viscosity tends to be relatively low in many applications because Reynolds number must be sufficiently high so as to not violate its constraint. Typical applications include water, gas and steam flow measurement.

Saturated steam flowmeters often use pressure compensation to correct for varying pressure. Measurement of the vapor phase of saturated steam with condensate (wet steam) can usually be done with reasonable error because the volume of liquid represents but a small percentage of the volume measured. Superheated steam applications usually use pressure and temperature compensation to infer mass flow.

Typical vortex shedding and fluidic flowmeter construction is such that the wetted parts include the body and sensor that can usually be made from materials that can withstand corrosion and do not contaminate the fluid. These flowmeters can be applied to measure corrosive fluids, and fluids where contamination is an issue, such as in sanitary applications. However, some vortex shedding and fluidic flowmeter designs do incorporate internal seals that should be considered for compatibility with the process. All-welded designs are should be considered for applications where corrosive or hazardous fluids are present.

Vortex shedding and fluidic flowmeters usually require straight run upstream and downstream of the flowmeter. Straight run requirements can reduce the usability of some vortex shedding and fluidic flowmeters in some applications. The supplier’s detailed mounting recommendations should be examined to evaluate suitability for a particular installation.

Vortex shedding and fluidic flowmeter technology has Reynolds number constraints, so it can be applied where the liquid viscosity is relatively low. The viscosity of the fluid can vary as long the Reynolds number constraint is not violated. However, as fluid viscosity increases, the pressure drop across the flowmeter will increase and may limit the applicability of the flowmeter.

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Excerpted from Excerpted from The Consumer Guide to Vortex Shedding and Fluidic Flowmeters

ISSN 1538-5280

Spitzer and Boyes, LLC
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