Ultrasonic vs Magnetic Meters: Which Works Best For Low-Flow ...

Flow meters are an important component in water treatment systems. Operators rely on the accuracy of a flow meter(tl,kk,mn)’s readings, particularly for chemical dosing and other low-flow processes. Accuracy in metering depends on a range of factors, from the types and quality of liquid being metered to the meters themselves. Generally, the two most accurate meter technologies are static meters (i.e., those without moving parts), which includes ultrasonic and magnetic flow meters (mag meters). Both have their advantages, but the right fit will depend on the application. This makes it important to understand the pros and cons of each technology before making a purchase.

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How Meters Measure Flow

Ultrasonic meters detect and measure flow using sound waves, and Ultrasonic meters work in one of two ways. Doppler ultrasonic meters send out a signal and measure the change in frequency as it bounces back to determine the flow rate. To do this often requires bubbles, particles or a high concentration of solids that will reflect the sound wave back. By comparison, transit-time ultrasonic meters send two signals, one upstream and one downstream. The difference between them is proportional to the velocity of the liquid. However, bubbles and large solids will cause inaccurate readings.

Mag meters create a magnetic field and measure the voltage of any conductive material passing through the field. The major downside to this technology is that it requires liquids with specific properties or impurities. Mag meters work well with water with some natural mineral content (such as most drinking water), as well as acids and caustic liquids.

Comparison Factors

Accuracy. Both types of flow meters, ultrasonic meters and mag meters, have comparable accuracy under ideal conditions. The exact level of accuracy will depend heavily on the manufacturer and the application.

Impact of air bubbles. As mentioned, air bubbles can either aid or hinder a meter, depending on the technology. Mag meters, for example, will read air bubbles as flow and may read high if there is too much air in the fluid. Doppler ultrasonics actually rely on air bubbles for their reading, so the opposite is true in that the absence of air can cause inaccuracies.

For transit-time ultrasonics, air bubbles can be problematic, but there are solutions, such as including a strainer that mitigates the bubbles and installing the meter vertically rather than horizontally. This also works with substances like sodium hypochlorite, which can off-gas and cause similar problems in meter accuracy to air bubbles in general.

Impact of temperature. Both types of meters are impacted by fluid temperature. The conductivity of a fluid changes with temperature, which can cause inaccuracy for mag meters, particularly at low flow rates. Similarly, ultrasonic signals will travel faster in warmer fluid than colder fluid. Thankfully, some meters have temperature sensors and built-in algorithms that allow them to compensate for temperature changes.

Installation process. Mag meters require an invasive installation, which involves welding them in-line. Ultrasonics are often welded in place as well. Clamp-on ultrasonic meters are less invasive and easier to install, although they tend to be less accurate since the pipe itself dampens the signal.

Minimum flow range. The lowest possible detectable flow rate will depend less on the technology and more on the manufacturer and the intended application. For example, meters for administered chemicals need to have very low flow ranges. Some, such as those offered by Blue-White Industries (Figure 1), can detect as low as 10mL/min. This is ideal for municipal and industrial water treatment when operators need to be as precise as possible to avoid overdosing with a given additive.

Type of liquid. Although it has been discussed a bit already, the type of liquid and its properties matter when selecting a meter. For example, mag meters will not work well with hydrocarbons, distilled water, and non-conductive solutions, as these liquids have little or no electrical charge. Ultrasonic meters will not work with a liquid that dampens sound, which includes some polymers and dense slurries.

Cost. Mag meters tend to be more expensive than ultrasonic meters, although exact prices vary depending on the manufacturer, pipe size, application, and more.

Unique features/innovations. As with flow range, specific features will vary from product to product and manufacturer to manufacturer. Features like the above-mentioned strainer to reduce bubbles and temperature compensation algorithms are only available in some models, such as those offered by Blue- White Industries. Water treatment professionals should be sure to ask vendors about innovations that may be advantageous to the intended application before making a decision.

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Blue-White® Industries
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Flow meters measure the volume or mass of liquid, gas, or steam moving through a piping system. There are many different flow meter technologies, and each type delivers different accuracies than other technology types. The accuracy requirements for a flow meter depends a great deal on the exact application. While it may seem like it may be advantageous to gravitate towards the technology types that deliver extremely high accuracy, those meters may have technological principles or other limitations that do not work with your needs.

Ultra-high accuracy flow meters, like Coriolis flow meters, typically are more expensive than any other flow technologies. A flow meter with an accuracy of 5%, which costs significantly less than another flowmeter with 0.2%, may deliver adequate results for your process to run correctly and deliver a much lower cost. Accuracy versus budget considerations and understanding your application’s exact accuracy needs can sometimes be confusing. Our sales engineers are available to help you find the best solution for your application for free.

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Accuracy is the most common term and is sometimes used incorrectly. Accuracy is how close your instrument comes to giving you the exact value that exists in the process at that moment. It is commonly expressed as a value, or margin of error, above or below the reading that the instrument is showing.

For example, let’s say that your magnetic flow meter is showing a result of 1 GPM, with an accuracy of ± 10%. The exact value of the flow in the meter is more than likely not exactly 1 GPM because of the inherent deviation. More than likely, the actual flow rate is somewhere in between 0.9 GPM and 1.1 GPM. This is accuracy. When accounted for, in relation to the value being expressed by the meter, it gives you the range that the actual value falls between.

Repeatability is when close to identical results are produced after multiple measurements and there is no change in the conditions for all the results. In essence, it is the ability of the instrument to “group” the results, as in target shooting or darts. A highly repeatable instrument doesn’t necessarily mean that it is then accurate. For example, a temperature sensor could consistently be reading 5 degrees off every single time a measurement is taken. But if it is 5 degrees off every single time, calibration can come into play and turn a highly repeatable instrument into one that is highly accurate after the identified and consistent degree of separation from the actual temperature is accounted for.

Resolution is the smallest increment that can be measured by an instrument. In a sense, it is the smallest part of whatever scale is being used. For example, the resolution of a pressure transmitter could be 0.1 PSI or 1.0 PSI. How does this play into accuracy? While the importance of resolution may not seem as obvious as accuracy and repeatability, it does come into play.

Imagine that you have a process that demands that you know down to the tenth of a PSI to operate correctly. If you install an instrument that only can only give you a reading to the nearest 1 PSI, then that instrument will not deliver enough resolution for you to accurately know what the true reading is, as the instrument is, in essence, rounding up or down. In a sense, the instrument delivering a resolution of 1 PSI will not be accurate or finite enough for your process, even though it may be accurate in its actual reading.

Flow meter accuracy can be stated in many ways and sometimes the way a specific instrument’s accuracy is stated is driven by the geographical area where it was produced and how accuracy is commonly stated or classified there. Certain flow meter technology types lend themselves to the accuracy being stated in a particular way that may be different from other flow meter technologies.

The ways that flow meter accuracy is stated are not always an apples-to-apples comparison, where one could essentially be converted into another. The essence of the way that the accuracy is stated may be telling you about a different element of the inherent accuracy. When choosing a flow meter, it is helpful to understand exactly what level of accuracy the flow meter will deliver.

Sometimes the accuracy will be stated specifically as an “accuracy class”. For example, a variable area flow meter may be listed as having an “accuracy class of 4 according to VDI”. VDI specifically applies to variable area flow meters and is assigned by the VDE/VDI Guideline , where a range of accuracy is designated to each accuracy class. VDI Class 4 would more typically be stated in the US as 2.5% to 4% of Full Scale (FS), as this is the actual accuracy range assigned to Class 4. For reference, the accuracy ranges for the VDE/VDI classes are below.

The most accurate flow meters are Coriolis mass flow meters. However, these are not appropriate for many applications because they are extremely expensive, usually large, and are complete overkill for most applications.

Magnetic flow meters, ultrasonic flow meters, and positive displacement flow meters generally deliver higher accuracy than flow meters that employ a more mechanical means of measurement like variable area flow meters.

However, for your exact application needs, a simple variable area flow meter may deliver sufficient accuracy at a significant cost savings. Magnetic and ultrasonic flow meters are generally more expensive than variable area flow meters but deliver many features that variable area flow meters cannot. Their technology types do not contain moving parts that experience wear or tear which can equate to lower maintenance and a longer service life.

Choose the correct flow meter that will meet all the exact needs of your application profile.

There are many elements of an application that can affect whether or not a flow meter delivers the factory stated accuracy. For example, if you choose a flow meter that requires full pipes and no bubbles to operate correctly and you run the pipe half full and it has bubbles and foam, it will not deliver the accuracy that it is built to. It may not even work at all. Running flows much lower than the stated minimum flow range for the meter can also cause the meter to suffer accuracy or can cause the meter to not work at all. To ensure full pipes for correct operation, install the flow meter vertically, with the flow running upwards.

Install the flow meter correctly.

Certain flow meters require that the flow profile in the pipe be uniform and non-turbulent. Not accommodating for those needs can cost you significant accuracy. For example, some flow meters require straight, uninterrupted pipeline with no impediments, bends, or valves so much distance before and after the flow meter. Not adhering to these requirements will cause your accuracy to suffer as the meter cannot properly function under those flow conditions.

Make sure nothing is broken.

For flow meters that measure by mechanical means, if the accuracy begins to suffer, verify that the functioning elements of the flow meter have not been compromised. Some meters are simple enough that they can be easily repaired by the end user while others must be sent back to the factory to be repaired.

Calibrate your flow meters in line with manufacturer recommendations.

Certain flow meter technologies require calibration more often than others and some may not require any calibration at all during their service life. Make sure that you are aware of the calibration needs of your meter and adhere to the maintenance schedule. Some meters are simple and can essentially be calibrated in the field and some require removal from the system and are then sent to a company that can perform the necessary calibration and return it to you.