A flow meter’s specifications are pivotal elements in choosing which one is right for your application. Two important statistics are its accuracy and repeatability. Let’s start with explaining what these two parameters mean:

## Flow Meter Accuracy

Accuracy is how close the measurement is to the true value. In flow meters, that means how close the output of the meter is to its calibration curve. This is expressed as a percentage, e.g. ±1%. It means that any given reading can be in error 1% above or below the calibration curve. In general it can be said the lower the percentage, the more accurate the meter. However, this also depends on the specification of either FS (Full Scale) or Rd (Reading). The meaning of Full Scale and Reading will be explained later in this blog. Flow meters are becoming more and more accurate, especially with the advent of mass flow meters.

## Flow Meter Repeatability

Repeatability is producing the same outcome given the same conditions. In other words, a flow meter should produce the same readings when operated under the same variables and conditions. This, too, is expressed as a ± percentage. While accuracy usually takes the spotlight in the measurement world, repeatability is the foundation on which accuracy rests. You can have high repeatability without high accuracy but you cannot have high accuracy without high repeatability. It is not helpful if the meter is highly accurate only once in a while. If your data is unreliable, if you get different numbers under the same circumstances and setup, there is no way those numbers can all be accurate.

## Is accuracy always important?

No one wants an inaccurate meter, but not all applications require high amounts of accuracy. It may be acceptable to stray further from the calibration curve if you are only looking to get an idea of how much is flowing through a pipe. It isn’t acceptable if you are mixing pharmaceuticals for consumption or volatile elements. How accurate your meter needs to be is important when selecting a flow meter, because usually the more accurate a meter, the higher the price.

When you see an accuracy specification, it should be expressed as a percent of Full Scale (FS) or Reading (Rd or RD). The difference between those can be significant.

## What is Full Scale (FS)?

The definition of Full Scale is “Closeness to the actual value expressed as percentage of the maximum scale value.” With Full Scale, the error remains the same but the percentage changes as the flow goes up and down the flow range. If the accuracy is calibrated 1% of 200 ln/min then the error is 0.01 x 200 ln/min = 2 ln/min. If the flow is 100 ln/min, the error is still 2 ln/min or 2%, a much bigger percentage.

## What is Reading (Rd or RD)?

The definition of Reading (Rd) is “Closeness to the actual value expressed as percentage of the actual value.

With Reading, the accuracy is the percentage of what is being read. The percentage stays the same, no matter where the flow is in the flow range. If it is 1% at 200 ln/min it would be 1% at 100 ln/min. So the error for a 200 ln/min flow would be 2 ln/min but for 100 ln/min it would be 1 ln/min rather than the 2 ln/min of Full Scale. Depending on the application, the difference between Full Scale and Reading can quickly add up and have a significant impact on the end product.

## Full Scale (FS) versus Reading (Rd)

Full scale is actually a carryover from mechanical gauges when readings were dependent on physical marks on a dial. Digital meters now can give much more precise readings, so high-end meters generally use Reading rather than Full Scale.

Although you don’t want an inaccurate flow meter, not all applications require high amounts of accuracy.

In terms of mass flow, accuracy requirements can change the type of sensor being discussed. If you need very high accuracy you can have a Coriolis flow meter, if high accuracy is less important, you may need a Constant Temperature Anemometry (CTA) or other sensor type.