In this example ☓% of reading is more accurate below a flowrate of 100 kg/h, however, as the flowrate increases beyond 100 kg/h, then ☐.3% of FSD gives a more accurate result in terms of actual flowrate.

For this example a value of ☐.3% FSD will be used.Īs in the previous case, the maximum flow = 1 000 kg/h.ĥ0 kg/h + 3 kg/h = 53 kg/h an error of +6%Īs the flowrate is reduced, the percentage error increases.Ī comparison of these measurement terms is shown graphically in Figure 4.2.1įigure 4.2.1 demonstrates why flowmeter manufacturers state their accuracy as a combination of both a percentage of FSD and actual reading. Error stated in percentage FSD tends to be smaller than the error as a percentage of actual reading. Percentage of full scale deflection (FSD)Ī flowmeter’s accuracy may also be given as a percentage of full scale deflection FSD, which means that the measurement error is expressed as a percentage of the maximum flow that the flowmeter can handle.Similarly, at an indicated flowrate of 500 kg/h, the error is still ☓%, and the ‘uncertainty’ is between: Percentage of measured value or actual readingįor example, a flowmeter’s accuracy is given as ☓% of actual flow.Īt an indicated flowrate of 1 000 kg/h, the ‘uncertainty’ of actual flow is between:.The following two methods used to express accuracy have very different meanings: The subject of accuracy is dealt with in ISO 5725. This is a measure of a flowmeter’s performance when indicating a correct flowrate value against a ‘true’ value obtained by extensive calibration procedures. Most would be much better, but a few, 5% could be worse. For example, it may be shown that with a large population of flowmeters, 95% would be at least as good as the uncertainty calculated. It is important to recognise that it is a statistical concept and not a guarantee. However ‘uncertainty’ can be estimated and an ISO standard exists offering guidance on this matter (EN ISO/IEC 17025). This is because accuracy cannot be established, as the true value can never be exactly known. The term ‘uncertainty’ is now becoming more commonly referred to than accuracy. However, this does not dilute the importance of accuracy under any circumstances. Good repeatability is important, where steam flowmetering is required to monitor trends rather than accuracy. its repeatability may be excellent in that it shows the same value for an identical flowrate on several occasions, but the reading might be consistently wrong (or inaccurate). It should not be confused with accuracy i.e.

This describes the ability of a flowmeter to indicate the same value for an identical flowrate on more than one occasion. When discussing flowmetering, a number of terms, which include Repeatability, Uncertainty, Accuracy and Turndown, are commonly used.

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