The Vocabulary of Liquid Flow Meters: An Explanation of Terms
Since liquid flow meter performance can be described in a variety of terms, we have decided to explain the definitions associated with them in language that is less commercialized. These liquid flow meter terms are not meant to be published in strict standards, but rather to be understood in a more accessible language. Within the industry, the performance of liquid flow meters is sometimes expressed with imaginative specification writing. For example, an engineer may ask questions like: “What is meant by uncertainty?” “Is there any use to a repeatable liquid flow meter?” Or, “What is the real meaning of a full scale deflection (FSD) linearity?” Here we attempt to explain just a few of some of the liquid flow meter terms.
Calibration Uncertainty: The basis of all the performance claims is formed by how good the flowmeters calibration uncertainty is. The figure is quoted by certified calibration houses but, other than that, it is rarely cited. Because there is an uncertainty on every measurement, no fully traceable or untraceable calibration can be absolute. Each of the flow measurements, such as density, temperature and pressure, has an effect on the calibration result. The typical uncertainty is ±0.1%, but the best calibration houses still claim ±0.02% uncertainty.
Repeatability: Repeatability is what allows the flowmeter to have good performance. This term applies to the flow meter’s ability to give the same result on repeated runs, when under the same operating conditions. Repeatability is checked by taking multiple points at each calibration, but these are not always reported on the certificate of the calibration.
Accuracy: This term’s definition is often used incorrectly by commercial suppliers. Accuracy is the term used to describe the deviation from the absolute flowmeter reading. It can be confused as repeatability, but it is not the same thing (as demonstrated in the Figure on the right.). The figure should include the calibration uncertainty data, repeatability and linearity.
Discrimination: Also known as resolution, this term determines how small a flowmeter measurement can be made. This is not related to accuracy in any way. An example would be a flowmeter that only gives 1 pulse per liter, which may generate that pulse between 0.999 and 1.001 liters. That would mean it has 0.1% accuracy with very poor discrimination. However, when approaching it from the other end, increasing the resolution does not make any meter more accurate because adding digits to a digital indicator does not convince.
Rangeability: This term is also referred to as turn down ratio. It describes the flow ratio of minimum to maximum, which usually depends on the meter technology being used: the application and the flow rate. In a production process, some flowmeters at almost constant flow may be required to very accurately measure flow. In this case, the rangeability may only be 4:1 while the accuracy could be ±0.2%. By comparing the flowmeters as general tools in research or for use on pilot plants, the application may require a rangeability of 100:1 but only an accuracy of ±1%.
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*Stay tuned for our next blog, where we will explain the term “linearity”.