What is Flow Meter Accuracy?

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.

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 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.

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.

• Send it to the factory or an independent provider to verify the accuracy. KOBOLD tests their flowmeters for accuracy verification before shipping them to the customer.
• Use or rent a flow meter with markedly better accuracy than the one that you have and compare the readings from the far more accurate flow meter. Clamp-on ultrasonic flow meters are ideal for this application because they are simply strapped to the existing pipeline and there is no system shut down or cutting into pipes. Clamp-on ultrasonic flow meters can be rented by certain companies for this purpose.

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.

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.

• Percentage of the Full Scale (abbreviated as %FS) which is a fixed inaccuracy based off of the full-scale value applied though out the measuring range.
• Percentage of the Flow Rate (abbreviated as %R or RD) which is a variable inaccuracy of a fixed percentage applied against the instantaneous rate.
• Calibrated Span (abbreviated as %CS)
• Upper Range Limit (abbreviated %URL)
• Percentage of Measured Value (% MV)

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.

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

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

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

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 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, 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 meter accuracy can be good. It typically ranges from 0.1% to 2.5%.

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%.

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%.

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%.

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.