Five questions to ask when choosing a mag meter

Oct. 1, 2013

The ideal starting point for choosing an electromagnetic flowmeter is not with a manufacturer’s specifications, but with the details of your unique application.

The ideal starting point for choosing an electromagnetic flowmeter is not with a manufacturer’s specifications, but with the details of your unique application. The electromagnetic flowmeters, or mag meter, is often the meter of choice when considering cost, accuracy and longevity. Here are some tips that can assist with ensuring that the electromagnetic flowmeter is the right choice.

It’s important to note that all electromagnetic flowmeters work with conductive liquids only, and that the flow of compressible fluids (gasses) cannot be measured with electromagnetic flowmeters.

1. What is your budget?

How much have you been allocated in your organization’s budget? Typically when choosing a flowmeter, the higher the accuracy, the higher the cost. So, when considering an electromagnetic flowmeter, cost and accuracy go hand in hand.

The reliable electromagnetic flowmeter has proven its ability to provide a very high accuracy rate with a very reasonable cost. Return on investment (ROI) is typically less than six months, and the accuracy achieved can be as high as 0.25 to 0.5 percent of flow rate. Considered one of the best choices when it comes to flowmetering, this type of meter can be applied in a wide range of industrial and municipal applications.

The primary advantage of electromagnetic flowmeters is that they have no moving parts. As a result, maintenance is typically minimal. Expected service life is 30 years. Depending on your fluid media or water quality, the electrodes may need to be periodically cleaned according to the manufacturer’s recommendations.

When looking at different electromagnetic flowmeters be sure to compare the cost of installation and maintenance. The next step is to amortize the cost of the flowmeter over its lifespan.

2. What are the application requirements?

Documenting and reviewing application requirements includes identifying the fluid media, media temperature, flow range and pressure.

Depending on whether the fluid media is drinking water or wastewater, the type of electromagnetic meter that would work best for the application varies. It depends on the electrode material and the sensor liner or coating that comes in contact with the media. Knowing the media being measured is only part of the understanding of the overall application. Some electromagnetic flowmeters are affected by fluid temperature and operating pressure. The same electromagnetic flowmeter used for a 150-psi application may not work for a 300-psi application. Most electromagnetic flowmeter specification sheets indicate the specific pressure and temperature rating of the meter in question. Be sure the meter selected has specifications that meet the application needs.

What are the characteristics of the fluid being measuring? In this regard, things to consider include the following:

  • Is the fluid being measured conductive? If the fluid is not conductive, an electromagnetic flowmeter will not work.
  • What is your line size? This will determine the cost of the meter.
  • Do you have sufficient room to install this meter? Typically an electromagnetic flowmeter has minimum upstream and downstream straight-run requirements in order to measure the fluid flow accurately. Flow disturbers must be considered as well — modulating valves, chemical injection points and anything that would compromise the fluid flow before it’s measured by the electromagnetic flowmeter.
  • What is the fluid being measured? Is the measured fluid corrosive? Is the measured fluid clean water or wastewater? These are relevant details when evaluating the type of lining in the electromagnetic flowmeter. Common liners that are available with today’s electromagnetic flowmeters are hard rubber, neoprene, Teflon, Tefzel and ceramic. However, these types of liners do come with a risk of delamination. An electromagnetic meter with a fusion-bonded liner is recommended to avoid the possibility of delamination issues altogether.

3. How important are accuracy and range?

How accurate does your flow measurement need to be? Electromagnetic meters, in general, are highly accurate flowmeters. Accuracy for an electromagnetic flowmeter is based on the percent of reading accuracy. Most of them offer +/-0.5 percent accuracy with no head loss. Whether using a full-bore mag meter or a full-profile insertion mag meter, be sure the meter chosen offers precisely the accuracy needed.

What is meant by percent of reading accuracy? It is the degree of precision with which the meter can measure the indicated flow rate at that flow rate.
An example of this accuracy statement is as follows:

If the meter’s percent of reading accuracy is described as 0.5 percent at 100 GPM, this means that while the meter is reporting the flow rate being 100 GPM, it could actually be anywhere from 99.5 GPM (-0.5 percent) to 100.5 GPM (+0.5 percent).

In addition, range, or turndown, must be considered in the selection process for an electromagnetic flowmeter. Turndown is the flow range of the meter, from the highest rate of flow it can measure accurately to the lowest rate of flow it can measure accurately, within the meter’s percent of reading accuracy specification. The typical turndown ratio for an electromagnetic flowmeter is 160 to 1.

An example of turndown: If the maximum flow rate is 100 GPM, this electromagnetic flowmeter can read down to 0.625 GPM and maintain a specification accuracy of +/-0.5 percent of flow rate.

4. What about upstream and downstream piping?

If a flowmeter is placed too close to pumps, valve elbows or other obstructions, unstable or irregular flows can impact performance. Electromagnetic flowmeters typically require minimal upstream and downstream straight-run pipe. These requirements can depend on whether the meter is part of a new piping project, a retrofit or a line expansion. Installation requirements also vary depending on whether a full-bore mag meter or a full-profile insertion meter is chosen. Nearly all major electromagnetic flowmeter technologies require a manufacturer’s specified pipe diameter straight-run upstream and downstream from the meter to ensure a stable flow profile. Failure to comply with the manufacturer’s installation requirements for upstream and downstream straight-pipe run often leads to either poor accuracy or inconsistent performance.

Always check the mag meter specification sheets to verify the upstream and downstream straight-run requirements. Most manufacturers provide these requirements. Be sure the meter selected will fit in the spot it’s intended for.

5. What makes for ease of installation?

The ease of installing an electromagnetic flowmeter depends greatly on the line size, type of meter chosen and the location in your application.

For smaller line sizes (12 inches and under), a full-bore electromagnetic flowmeter may be the best choice. For larger line sizes, the heavier weight of full-bore meters typically requires use of a crane, along with the necessary extra personnel, working space and installation time. A full-profile insertion meter, on the other hand, can provide an economical exception to costly installation on larger line sizes. While some meters require installers to shut the line down to install flanges to accept a traditional full-bore type flowmeter, a full-profile insertion meter can be installed via hot tap while under pressure, thus avoiding taking the line out of service.

Katie Englin is a senior marketing specialist with McCrometer. McCrometer is a flow instrumentation specialist in flowmetering solutions. McCrometer develops innovative, high-quality, precision flowmeters for the most demanding liquid, steam and gas measurement applications. McCrometer says its FPI Mag meter represents the next generation of development of magnetic flowmeter technology and employs a ground-breaking configuration whereby the coils and electrodes are assembled in a tube that inserts into a pipe perpendicular to the full profile of the flow stream. The advantages of this configuration are numerous in a wide variety of applications. In fit and form the FPI Mag is unique; in function the FPI Mag meter is just that — a mag meter.

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