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17 February 2021
Liquid flow meter performance can be expressed in several ways.
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/ turndown ratio?
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.
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.
This term is often misused and misunderstood.
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.
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).
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.
Turndown ratio is also commonly referred to as rangeability. It indicates the range in which a flow meter or controller can accurately measure the fluid. In other words, it’s simply the high end of a measurement range compared to the low end, expressed in a ratio and is calculated using a simple formula.
Turndown Ratio = maximum flow / minimum flow
For example, if a given flow meter has a 50:1 turndown ratio the flow meter is capable of accurately measuring down to 1/50th of the maximum flow. So, suppose a flow meter has a full scale rating of 20 l/min the flow meter will measure down to 0.4 l/min of flow.
Keep in mind that the maximum and minimum flow capability of a meter or controller is likely to be a greater span than the measurable and controllable range. For example, a mass flow controller with a 50:1 turndown ratio may have the capability of measuring as high as 25 ln/min or as low as 0.16 ln/min but the turndown ratio will govern the actual measurable range. In this example if the calibrated high flow is 25 ln/min, then the lowest that can be measured is 0.5 ln/min (1/50th of 25). If the application requires that the calibrated minimum flow is 0.1 ln/min, then the maximum flow that can be measured is 5 ln/min (50 times 0.1).
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