Which of the following units would be appropriate to measure a specific feed flow to a chemical reactor?
A. Liters (gallons) per unit time
B. Kilograms (pounds) per unit time
C. Cubic meters (feet) per unit time
D. Standard cubic meters (feet) per unit time
Chemical reactions occur using the mass of the reactants, so measurements that accurately reflect that mass would be appropriate. Liters (gallons) and cubic meters (feet) are units that reflect the volume of the reactants — not their mass. Answer A and answer C are not correct.
Kilograms (pounds) is a mass unit. Answer B is correct.
A standard cubic meter (foot) represents a mass because it describes the size of a specific fluid at known conditions. Answer D is also correct.
Additional complicating factors
Flowmeters such as Coriolis mass and thermal flowmeters actually measure mass flow.
Other flowmeters measure or infer velocity from which the mass flow can be calculated and displayed. Observing a display in mass units does not provide information as to whether the flowmeter is a mass flowmeter or not. As such, the observer does not know whether varying process conditions can introduce additional measurement error — even during normal operation.
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.