Part IV: How differential pressure flowmeters are used for volumetric flow measurement

Editor’s Note: This is Part IV in a five-part series discussing the types of technologies that can be applied to measure the flow of raw materials.

READ ALSO: Part I—Considering the Pros & Cons of Volumetric Flow Measurement

READ ALSO: Part II—The Pros & Cons of Velocity Meters for Volumetric Flow Measurement

READ ALSO: Part III—The Pros & Cons of Mass Flowmeters for Volumetric Flow Measurement

Positive displacement flowmeters that measure the actual volume of the fluid passing through the flowmeter (volumetric flow), flowmeters that measure fluid velocity, and flowmeters that measure mass flow were discussed in previous articles. These technologies measure volumetric flow, infer volumetric flow, and measure mass flow (respectfully).

Other flowmeters do not measure volume, velocity or mass but rather infer the flow through the flowmeter from other measurements. Differential pressure flowmeters are commonly used to infer the flow rate in a pipe by measuring the differential pressure produced across a restriction. Differential pressure flowmeters such as orifice plate, Venturi, nozzle, variable area, and target flowmeters generate differential pressures that are proportional to the square of the flow through the flowmeter.

The differential pressure produced by differential pressure flowmeters is proportional to the velocity head — one-half of the fluid density times the square of the fluid velocity. Stated differently, the flow measurement is proportional to the square root of the product of the fluid density and differential pressure. Therefore, the inferred flow measurement is affected by the density of the fluid where the effect is approximately one-half that of flowmeters that measure volumetric flow or fluid velocity.

Differential pressure flowmeters have been widely applied to measure fluid flow throughout industry for decades yet it is somewhat ironic that these flowmeters infer flow and do not measure the volume, velocity or mass of the fluid.

To be continued…

David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes, LLC, offering engineering, training, strategic marketing consulting, distribution consulting, and expert witness services for manufacturing and automation companies. Spitzer and Boyes is also the publisher of the Industrial Automation INSIDER. Mr. Spitzer has more than 40 years of experience and has written more than 10 books and 300 articles about flow measurement, instrumentation and process control. David can be reached at 845-623-1830 or via www.spitzerandboyes.com. Click on the "Products" tab to find his Consumer Guides to various flow and level measurement technologies.