The ideal starting point for choosing a flow meter is not with a manufacturer’s specifications, but with the details of your unique application. Electromagnetic flow meters are often the meter of choice when considering cost, accuracy, and longevity. There are several steps to ensure the right choice is made.
What is the budget?
How much have you been allocated in your organization’s budget? Typically when choosing a flow meter, the higher the accuracy, the higher the cost.
The electromagnetic flow meter has proven that it can provide a very high accuracy rate at a very reasonable cost. Your return on investment (ROI) is typically less than six months, and the accuracy achieved can be as high as .25% to .5% of flow rate.
Electromagnetic flow meters can be applied in a wide range of industrial and municipal applications. Their primary advantage is that they have no moving parts. As a result, maintenance is typically minimal. Depending on your fluid media and/or water quality, the electrodes may need to be periodically cleaned according to the manufacturer’s recommendations. Expected service life is 30 years.
When comparing different electromagnetic flow meters, be sure to calculate the cost of installation and maintenance. The next step is to amortize the cost over its lifespan.
Documenting and reviewing your application requirements includes identifying the fluid media, media temperature, flow range, and pressure. Please note that all electromagnetic flow meters only work with conductive liquids. Flow of compressible fluids (gases) cannot be measured with electromagnetic flow meters.
The type of electromagnetic meter that would work best for your application varies, depending on whether your fluid media is drinking or wastewater. It also depends on the electrode material and/or the liner or coating of the sensor that comes in contact with the media.
Knowing the media being measured is only one part of understanding the overall application. Some electromagnetic flow meters are affected by fluid temperature and operating pressure. The same electromagnetic flow meter used for a 150-psi application may not work for a 300-psi application. Most electromagnetic flow meter specification sheets indicate the specific pressure and temperature rating of the meter in question. Be sure the meter you select has specifications that will meet your application needs.
What are the characteristics of the fluid are you measuring? There are several considerations that you must make in order to use an electromagnetic flow meter:
How accurate does your flow measurement need to be? Electromagnetic meters, in general, are highly accurate. Most of them offer +/-.5% accuracy, with no head loss. Whether using a full-bore mag meter or a full-profile insertion mag meter, be sure the meter you choose offers precisely the accuracy needed.
Accuracy for an electromagnetic flow meter is based on the percent of reading accuracy. This is the degree of precision with which the meter can measure the indicated flow rate at that flow rate.
For example, if the meter’s percent of reading accuracy is described as .5% 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 (-.5%) to 100.5 GPM (+.5%).
Turndown must be considered in the selection process for an electromagnetic flow meter. Turndown is the flow range of the meter, from the highest rate of flow 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 flow meter is 160 to 1. For example, if the maximum flow rate is 100 GPM, this electromagnetic flow meter can read down to .625 GPM and maintain a spec accuracy of +/-.5% of flow rate.
If you place flow meters too close to pumps, valve elbows, and other obstructions, unstable or irregular flows can impact performance.
Electromagnetic flow meters typically require minimal upstream and downstream straight run pipe. This 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 flow meter 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 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. Be sure the meter you select will fit in the spot for which it is intended.
The ease of installing an electromagnetic flow meter depends greatly on the line size, type of meter chosen, and the location in your application.
For smaller line sizes (12” and under), a full-bore electromagnetic flow meter may be the best choice. For larger line sizes, the heavier weight of full-bore meters typically requires the 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. With some meters the line must be shut down to install flanges to accept a traditional full-bore type flow meter. A full-profile insertion meter, however, can be installed via hot tap while under pressure, thus avoiding taking the line out of service.
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