Flow Meter Accuracy

How accurate are flow meters?

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.

Can a flow meter have perfect accuracy?

In a word, “no”. In an ideal world, the flow rate reading from your process instrumentation would be perfectly correct, with no type of deviation. Unfortunately, this is not the case and the margin of error that is inherent in measurement must always be identified, accounted for, and minimized when possible.

Accuracy vs Repeatability vs Resolution

There are many terms that are thrown around interchangeably and incorrectly when it comes to talking about how accurate your instrument’s results really are. Some of them are accuracy, repeatability, and resolution. Let’s visit each in detail and clarify the unique meaning of each term.

Accuracy, Repeatability, Resultion of Flow Meters

Flow Meter Accuracy Definition

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.

What is the repeatability of a flow meter?

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.

What is flow meter resolution?

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.

How do you measure the accuracy of a flow meter?

When you buy a flow meter, it will usually provide the accuracy of the flow meter within the product specifications. To verify that the meter is operating within the factory stated specification you can do the following.

What is flow meter accuracy class?

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 3512, 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.

VDI/VDE Accuracy Chart

Flow meter accuracy in full scale: what does that mean?

VDE/VDI accuracy classes are not the only way to state flow meter accuracy. As mentioned above, the “percentage of the full scale” or abbreviated frequently as %FS, is a frequently used way to state accuracy for many flow meters. The more common ways the accuracy of a flow meter can be expressed are as follows.

Which flow meter is the most accurate?

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.

Flow Meter Accuracy Comparison

While there can be variations in the range of accuracy within a single flow meter technology type, there are some broad generalizations that can be made regarding accuracy for each flow meter technology type. It is also worth noting that the accuracy of a single flow meter is also subject to whether the meter is measuring a liquid or a gas. To learn more about each technology type and how they operate, visit our “What is a flow meter” article.

Flow Meter Technology Type Accuracy Comparison

Coriolis Flow Meters

Coriolis flow meter accuracy is among the highest for flow meter technologies. It is often used for verifying the accuracy of other flow meters by running them simultaneously and referencing the measured values between the two. Typical Coriolis flow meter accuracy ranges from 0.1% to 0.5%. Their exceptional accuracy makes them ideal for applications requiring precision like custody transfer.

Ultrasonic Flow Meters

Ultrasonic flow meter accuracy is quite high. Typical ultrasonic flow meter accuracy ranges from 0.7% to 1%.

Magnetic Flow Meters

Magnetic flow meters deliver high accuracy for conductive liquids. Typical magnetic flow meter accuracy is from 0.2% to 2%.

Vortex Flow Meters

Vortex flow meters can also deliver high accuracy. Typical vortex flow meter accuracy ranges from 0.7% to 2.5%.

Thermal Dispersion Flow Meters

Thermal mass flow meter accuracy is usually not quite as high as Coriolis, Magnetic, Ultrasonic, and Vortex flow meter technology, but still offers an above average accuracy which typically ranges from 1% to 3%.

Differential Pressure Orifice Flow Meters

Differential pressure orifice flow meter accuracy is generally not as high as some other technology types but delivers significant advantages for particular application fields. It typically ranges from 3% to 5%.

Variable Area Flow Meters (Rotameters)

Variable area flow meters, also known as rotameters, deliver a wide variety of accuracies, depending on the individual flow meter. Typical ranges can be from 1.6% to 5%.

Positive Displacement Flow Meters

Positive displacement flow meter accuracy can be good. It typically ranges from 0.1% to 2.5%.

Paddle Wheel Flow Meters

Paddle wheel flow meter accuracy is somewhat average as it typically ranges from 2.5% to 5%. Pelton wheel flow meters, which are a certain type of paddle wheel flow meter, deliver higher accuracy than standard overshot paddle wheel flow meters, with a typical range of 1.5% to 3%.

Turbine Flow Meters

Because the actual principle of operation can vary between this category of flow meters, the range is also wider than some other technologies. A typical range is 1.5% to 5%.

Oscillation Flow Meters

Being a unique and specialized principle of operation developed by KOBOLD for gas measurement applications, there isn’t really an industry standard for oscillation flow meters. Our DOG oscillation flow meter for gases delivers an accuracy of 1.5%.

How do you improve the accuracy of a flow meter?

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.

How to Improve Flow Meter Accuracy

Accuracy in Selecting a Flow Meter

While accuracy is a key specification to be aware of in choosing a flow meter that will adequately meet your measurement needs, there are many more factors that determine the right flow meter for your application and your budget. Certain technology types will not work in certain applications and certain types of flow meter technology may be overkill for your application and your budget.

Our team of expert engineering staff is ready and waiting to provide you free assistance in finding the best solution for your application. Call us now for knowledgeable help to eliminate headaches down the road from incorrect flow meter selection.

Learn more about why you should partner with KOBOLD for your application.

Select and Accurate Flow Meter
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DOG
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About KOBOLD

KOBOLD USA is a subsidiary of KOBOLD Messring GmbH, a world-leading instrumentation engineering business founded in Germany in 1980 by Klaus J. Kobold. With patented technology and superior service, the company quickly established itself as one of the global leaders in sensor and control systems with high quality products. The KOBOLD brand name became synonymous with superior quality and technological advancement in instrumentation engineering.

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