QUIZ CORNER: How Flow Turndown Affects Measurement Range
What is the additional measurement range in which a flowmeter can accurately measure flow if its turndown is improved from 50-to-1 to 100-to-1?
A. 0 to 1 percent of full scale
B. 1 to 2 percent of full scale
C. 2 to 4 percent of full scale
D. 50 to 100 percent of full scale
Commentary
It appears that the turndown specification of the improved flowmeter is vastly superior to that of the original flowmeter. However, the flowmeter with a 50-to-1 turndown will measure accurately from 2 to 100 percent of full scale. The improved flowmeter with a 100-to-1 turndown will measure accurately between 1 and 100 percent of full scale.
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Both flowmeters measure accurately between 2 and 100 percent of full scale. The difference is that the improved flowmeter will also measure accurately between 1 and 2 percent of full scale. Answer B is correct.
Additional Complicating Factors
The performance provided by the improved flowmeter appears superior (and it is). However, the improvement is of little or no value if the process rarely (or never) operates at low flows between 1 and 2 percent of full scale.