A process requires the batch addition of 100 liters of a liquid that has a specific gravity of 1.10 and a viscosity of 10 to 20 cP. The rate of addition is adjusted manually in a 2-inch pipe and takes from one and two minutes. An investigation into a quality problem using a weigh scale to verify flowmeter operation revealed actual additions in error by up to 15 liters — despite repeated totalized flow measurements of 100 liters. The flowmeter was removed, wet calibrated and correctly returned to service. Is Reynolds number a concern?
Most commonly applied flowmeters operate accurately in the turbulent flow regime and usually exhibit nonlinearity and/or turn off in the other flow regimes.
In this application, the flow rate is adjusted manually between 50 and 100 liters per minute so the flowmeter operates at Reynolds numbers between approximately 1150 and 4600 so the flowmeter could be in the laminar, transitional or turbulent flow regime depending upon where the manual valve is set. Therefore, nonlinearities of the nature described could be introduced.
Additional complicating factors
Cold weather in the winter could cause the liquid to further cool, which would increase its viscosity, reduce Reynolds number and tend to operate more often in the laminar flow regime where it is likely less accurate.
David W. Spitzer is a principal at Spitzer and Boyes, LLC, which offers engineering, focused market research, writing/editing white papers, strategic marketing consulting, distribution consulting, seminars and expert witness services for manufacturing and automation companies. Spitzer has written more than 400 technical articles and 10 books about flow measurement, instrumentation and process control. He can be reached at 845-623-1830 or via spitzerandboyes.com.
