Liquid Flow Meter Terms – Cutting out the ‘Marketing Speak’
The following explanation of terms commonly associated with liquid flow metering are not necessarily the strict definitions to be in published standards but are more an attempt to describe the terms in more accessible ‘less commercial’ language.
Liquid flow meter performance can be expressed in several ways and within the industry there is some imaginative specification writing. What is meant by performance? What does a full scale deflection (FSD) linearity really mean? Is a repeatable liquid flow meter ever any use? What about discrimination, uncertainty and rangeability?
Liquid Flow Meter Calibration uncertainty
How good a liquid flow meter’s calibration uncertainty is forms the basis of all the performance claims. It is a figure rarely quoted other than by certified calibration houses. No calibration, even fully traceable ones, can be absolute as there is an uncertainty on every single measurement all the way back to the National standards. Liquid flow is a complicated product depending on a large number of variables including pressure, temperature, density, compressibility etc. Each of these measurements has an effect on the ultimate calibration result. The very best calibration houses claim an uncertainty of ±0.02% but more typical is ±0.1%. This is the base value on which all the other accuracy statements are founded. If the uncertainty quoted is ±0.2% the meter cannot be specified as being more accurate than that even if the repeatability and linearity are less than ±0.1% – the basic calibration uncertainty is the overriding number.
Liquid Flow Meter Repeatability
The ability of a liquid flow meter to give the same result on repeated runs under the same operating conditions, should not to be confused with accuracy or linearity. Without excellent repeatability a flowmeter cannot have good performance. Normally multiple points are taken at each calibration point to check the repeatability although these are not always reported on the calibration certificate. A highly repeatable liquid meter that can be calibrated in-situ may be perfect for situations like a batching application where any offset can be reliably accounted for.
Accuracy: This term is often misused by commercial suppliers. Accuracy is the deviation from the absolute liquid meter reading, this figure should include linearity, repeatability and calibration uncertainty data. Accuracy and repeatability are not the same thing as is demonstrated in Figure 1 below.
Liquid Flow Meter Linearity
The linearity of a liquid flow meter is the ability of the device to remain within defined limits over its entire specified flow range. The standard way of expressing linearity is “off reading” this is where the percentage error at every flow rate within the operating range is specified. An alternative linearity definition, used in some sections of the industry, is percentage of full scale deflection or FSD linearity (see Figure 2).
The linearity of this liquid meter initially looks good as the plot of the indicated flow versus the actual flow is almost a straight line. This liquid flow meter is specified as ±2% of full scale accuracy so a ±2 litre per minute tolerance applies over the whole operating range, even minimum flow. If we now plot the number of pulses per litre for the same flowmeter against the flow rate (Figure 2a) a different picture of the flowmeters linearity is evident.
Consequently a ±1% FSD specification can be likened to using an indicator with only 0-100 digital display. All of the readings are in 1 unit steps so at full flow you are ±1 litre per minute the same is true at 1 litre per minute which could be 0 or 2 i.e. ±1 LPM which is equivalent to 1% FSD linearity. One litre per minute accuracy at both 100 or 1 L/Min.
Obviously people do not normally claim a 100:1 flow range and a 1% FSD linearity but this a good illustration of the potential problem. Even a 10/1 flow range with 1% FSD would give a 10% permissible error at the specified minimum flow i.e. 10 LPM ±1.
By comparison if we now plot the same flowmeter data in a standard ‘off reading’ linearity plot (Figure 3). In this linearity performance graph the error lines are shown as percentage of reading. It is clear that the flowmeter “drops out” of the required accuracy at lower flows as the dashed line crosses the solid red lower limit line.
The linearity percentage off reading graphs (Figure 3 above) better illustrates the true situation. The meter is close to the maximum acceptable limit at full flow but drops outside the ±2% of reading specification at around 20 litres per minute.
Figure 4 is a typical calibration graph issued by a flowmeter manufacturer showing the permissible errors. In this case the flowmeter specification is 0 to 1 litres per minute, ±0.5% of reading. In practice this meter exceeds the specification having a linearity of +0.28% -0.1% and this includes both flow meter and calibration rig repeatability.
Liquid Flow Meter Discrimination
This term is the same as resolution. A Flowmeter’s quoted discrimination determines how small a measurement can be made. Quoted discrimination (or resolution) has nothing to do with accuracy. For instance a flowmeter which only gives 1 pulse per litre may do it between 0.999 and 1.001 Litres for every pulse 0.1% accuracy but poor discrimination.
Whereas another flowmeter may give 100 pulses per litre but only within 1% accuracy i.e. 99 to 101 pulses for each litre the discrimination is better but the accuracy is not.
Liquid Flow Meter Rangeability
This is the ratio of minimum to maximum flow and is usually dependant on the flow meter technology used, the flow rate and the application. Some flowmeters in a production process may be required to measure flow very accurately at almost constant flow. In this case the range of the meter may only be 4:1 but the accuracy over that range could be ±0.2%. By comparison flowmeters for use on pilot plant, or as general tools in research, may require a 100:1 range but only a ±1% accuracy.
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