There are many technologies that can be applied to measure the flow of raw materials such as positive displacement, turbine, vortex shedding, differential pressure, and Coriolis mass flowmeters. Which technology would you select, assuming all of these technologies will operate reliably in the service?
One dimension of the answer is to consider what each technology measures in the context of what you want to measure. Phrasing the problem in this way may seem a bit strange because you know that you want to measure flow — so it would be logical to use a flowmeter.
Some flowmeters measure the actual volume of the fluid passing through the flowmeter. As such, they measure the volumetric flow of the fluid. Positive-displacement flowmeters repeatedly entrap fluid and are the only flowmeters that measure the actual volume of the fluid. There are many variations (geometries) such as helical gear, nutating disk, oval gear, piston, and rotary positive-displacement flowmeters. Selecting the appropriate positive-displacement flowmeter geometry is dependent on the actual application.
Measuring the actual volume of the fluid passing through the flowmeter can pose measurement issues when the fluid density varies with pressure, temperature and/or composition. Measurement accuracy can be adversely affected because the volume of a given amount of gas can vary significantly with seemingly minor pressure and temperature variations. Therefore, most positive-displacement flowmeters are applied to liquid applications. In liquid applications where precise measurement is required, positive-displacement flowmeters often compensate for temperature variations that affect the flowing density.
Despite these apparent drawbacks, positive-displacement flowmeters can and do measure the actual volume of the fluid accurately and are commonly applied in many applications. Consideration should be given to what positive-displacement flowmeters measure (actual volume) and how they perform that function.
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 +1.845.623.1830 or via www.spitzerandboyes.com. Click on the "Products" tab to find his Consumer Guides to various flow and level measurement technologies.