| Vol. 6 No. 2 Subject: Flow Measurement News Date: 4-2-04 From: McCrometer, "The Flow Measurement Specialists" ........ http://www.mccrometer.com ........ **************************************************************** This newsletter is e-mailed by McCrometer monthly to subscribers worldwide. It consists of flow measurement information of interest for professionals in this field. For More information on our complete line of flowmeters go to: <http://www.mccrometer.com> **************************************************************** McCrometer Newsletter - March 2004 Special Notice - McCrometer is pleased to announce the addition of guest authors for our monthly newsletter. This month we feature David W. Spitzer, PE. David has over 25 years experience in many facets of instrumentation, process control, electrical, and utility engineering, including conceptualization, estimating, design, construction, startup, operation, troubleshooting, and teaching. He is a principle in Spitzer and Boyes LLC, serving both the profit and not-for-profit business sectors. Magnetic Flowmeter Performance Claims (excerpt from The Consumer Guide to Magnetic Flowmeters) By David W. Spitzer For the most part, the claims made regarding magnetic flowmeters are likely true statements, even though they may seem extraordinary. The problem is that the statement may be incomplete, and not include certain facts and information that clarify the statement. Sometimes claims are simplified for convenience and easier understanding. However, in many cases, further investigation often reveals other motives for doing so. Turndown is defined as the relationship between the maximum flow that can be measured within the stated accuracy (usually the full scale flow), and the minimum flow that can be measured within the stated accuracy. For example, a flowmeter that can measure from 5 to 100 units within 1 percent of rate would have a 20:1 turndown. For example, consider a magnetic flowmeter claim to have a reference accuracy of 0.25 percent of rate and a turndown of 1000:1. The implication is that the flowmeter can measure within 0.25 percent of rate over a 1000:1 range of flow. Taken individually, both parts of the claim are likely true statements. Yet when combined, they can be misleading by omission. Further investigation will show that the reference accuracy of 0.25 percent of rate applies only within a range of flow rates. Below the minimum flow rate in the range, the reference accuracy becomes a fixed absolute error, so as the flow rate decreases, the accuracy expressed as a percentage of rate will increase. For example, assuming that the reference accuracy of 0.25 percent of rate applies between 5 and 100 percent of meter capacity (assumed 10 m/s), and that between 0.1 and 5 percent of meter capacity, the reference accuracy is fixed at the absolute error at 5 percent of meter capacity. The following table calculates reference accuracy throughout the range of flows. Velocity(m/s) Statement Absolute Error (m/s) Rate Statement 10 0.25% rate 0.25% of 10 = 0.025 0.025/10 = 0.25% 1 0.25% rate 0.25% of 1 = 0.0025 0.0025/1 = 0.25% 0.5 0.25% rate 0.25% of 0.5= 0.00125 0.00125/0.5 = 0.25% 0.1 +/-0.00125 m/s 0.00125 0.00125/0.1 = 1.25% 0.05 +/-0.00125 m/s 0.00125 0.00125/0.05 =2.50% 0.01 +/-0.00125 m/s 0.00125 0.00125/0.01 = 12.5% under 0.01 undefined undefined undefined The above table illustrates that above 0.5 meters per second, the reference accuracy is 0.25 percent of rate (as claimed) and that the turndown is 10/0.01 or 1000:1 (as claimed). What is not stated in the claim is that the reference accuracy is degraded below 0.5 meters per second and can approach 12.5 percent of rate. Also not stated is that in actual installations, flows near meter capacity would rarely be encountered, so the 1000:1 turndown would rarely be achieved. Assuming a more reasonable full scale calibration range of 0-2 meters per second, this flowmeter would achieve a 0.25 percent of rate reference accuracy from 0.5 to 2 meters per second, or a 4:1 turndown, and only a 200:1 (2/0.01) turndown when the stated performance at low flow rates is included. In summary, be careful not to be misled by assuming that all of the performance claims apply simultaneously under all operating conditions, and especially at all flow rates. More information with tabulations of magnetic flowmeter performance by supplier and model is available in The Consumer Guide to Magnetic Flowmeters by David W. Spitzer and Walt Boyes (available at <www.isa.org>). About the Author David W. Spitzer has published a number of books concerning the application and use of fluid handling technology, including “The Consumer Guide to…” series that can be used to compare flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes, LLC. <www.spitzerandboyes.com>, offering consulting services for the process industries in addition to product development, marketing, and distribution consulting for manufacturing and automation companies. **************************************************************** To find out more about McCrometer's line of flow meters, including the V-Cone®, Wafer-Cone®, the V2-Municipal™, Water Specialties Propeller Meter™, Mc® Propeller, Ultra Mag®, and SK® Variable Area Meters, please contact McCrometer at 951-652-6811 or visit our website at www.mccrometer.com **************************************************************** Copyright 2004 McCrometer If you have comments or suggestions for newsletter topics you would like to see, please address them to: Bob Peters or Richard Steven <<mailto:newscomment@mccrometer.com>> **************************************************************** Know anyone looking for a steady dose of flow measurement information? **************************************************************** To subscribe send e-mail to<mailto:news-request@mccrometer.com> with a keyword SUBSCRIBE in the subject field. 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