Understand the Attraction of Magmeters

Magnetic flowmeters provide accuracy and can be used in a variety of applications and environments

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Other considerations
The market for magnetic flowmeters is competitive, so prices are relatively low. Magnetic flowmeters for water and wastewater service can be economical due to the economies of scale and the relatively low cost of liners and electrodes for this service. However, applying magnetic flowmeters to corrosive or abrasive services can significantly increase the cost of the meters.

Magnetic flowmeters for use in the chemical industry are typically more expensive than vortex shedders. In some applications, the cost can rival that of turbine flowmeter or orifice-plate flowmeter systems. Magnetic flowmeters are typically more economical than Coriolis mass flowmeters.

Flowmeter performance
The purpose of installing a flowmeter system is to accurately measure flow in a reliable manner. Issues related to physical properties, process parameters, electronic features and interconnections are often given much consideration. Relatively little emphasis, however, is given as to how well the flowmeter will perform its intended purpose. Adding to the confusion are the differences in how performance is expressed and the incomplete nature of the available information. Nevertheless, the quality of flow measurement should be a concern.

The performance of a flowmeter is quantified by its accuracy statements. The reader must understand not only which parameter is being described, but also the manner in which the statement is expressed. In flow measurement, parameters are commonly described in terms of a percentage of the actual flow rate, a percentage of the full scale of rate, or a percentage of the meter capacity. These terms are mathematically related, so it is possible to convert one to another (Table 1). Note that when compared on a common basis, such as percent of rate, these statements describe significantly different performance.

Other terminology may be used to express these concepts. When this occurs, confirm exactly what the other terms mean so they can be understood.

Performance statements apply to a range of flow or, stated differently, between a minimum and maximum flow velocity. It is important to identify the range in which the statement applies because performance can be significantly degraded or undefined when the flowmeter operates outside of this range.

Complicating the issue are some flowmeters that have different performance statements for different measurement ranges. For example, a flowmeter may have a reference accuracy of 0.25% of rate from velocities of 1 to 10 m/s, and an absolute error of 0.0025 m/s from 0.1 to 1 m/s. Performance is undefined below 0.1 m/s. Table 2 describes this performance using the above information. Note how performance degrades at low flows.

 

Performance claims
For the most part, the claims made by suppliers regarding magnetic flowmeters are true statements, even though they may seem extraordinary. The problem is that the statement may be incomplete, and may 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 may reveal other motives for doing so.

For example, consider a magnetic flowmeter that has a reference accuracy of 0.25% of rate and a turndown of 1,000:1. The implication is that the flowmeter can measure within 0.25% of rate over a 1,000:1 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% of rate applies only within a range of flow rates. Below the minimum flow rate of 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.

Assuming that the reference accuracy of 0.25% of rate applies between 5% and 100% of meter capacity, and that between 0.1% and 5% of meter capacity, the reference accuracy is fixed at the absolute error at 5% of meter capacity. Table 3 calculates reference accuracy throughout the range of flows.

This illustrates that above 0.5 m/s, the reference accuracy is 0.25% of rate and that the turndown is 10/0.01 or 1,000:1, both as claimed. What is not stated in the claim is that the reference accuracy degrades below 0.5 m/s and can approach 12.5% of rate. Also not stated is that in actual installations, flows near meter capacity would rarely be encountered, so the 1,000:1 turndown would rarely be achieved. Assuming a more reasonable full-scale calibration range of 0 to 2 m/s, this flowmeter would achieve a 0.25% of rate reference accuracy from 0.5 to 2 m/s, or a 4:1 turndown, and only a 200:1, or 2/0.01, turndown when the stated performance at low flow rates is included.

In addition to high turndown, some suppliers claim that their flowmeter operates at extremely low flow rates. Consider a claim to measure velocity as low as 0.01 m/s. For a meter with a capacity of 10 m/s, this corresponds to a 1,000:1 turndown. Although the flowmeter may operate at this flow rate, Table 3 shows that it does so with a reference accuracy of 12.5% of rate.

Statements about magnetic flowmeters often claim high reference accuracy. What often is not stated is that it may apply over a range of higher flows, and much of this range may not be encountered in actual operation. Furthermore, the reference accuracy as a percentage of rate generally degrades or is undefined at lower flow rates (see tables). When the calibrated full-scale is low, and the high reference accuracy statements are limited to a small range of high flow rates, the stated reference accuracy may not be achieved.

In general, reference accuracy should be clearly and completely stated for all flow rates prior to performing any analysis. The range of applicability of the high accuracy statement and the actual operating flow range should be compared.

Magnetic flowmeters are among the most versatile of flowmeter technologies. However, the user should be aware of the manner in which their application and operation are described in order to ensure that the proper magnetic flowmeter is selected and installed.

David W. Spitzer has more than 25 years of experience in specifying, building, installing, commissioning and troubleshooting process-control instrumentation. Spitzer is a principal in Spitzer and Boyes LLC, which offers consulting services for the process industries in addition to product development, marketing and distribution consulting for manufacturing and automation companies.

 

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