Flow meter measurement is of the utmost importance when determining accuracy. Typically, accuracy is measured as a percentage of full scale or percentage of reading/percentage of rate. Understanding how these two methods operate will allow you insight into how water meters operate and will enable you to determine their accuracy.
The percentage of full-scale accuracy has a fixed error band on the flow range within the meter. This type of measurement is usually used in an application with a constant flow rate in a higher degree of industrial meters. They are used in fluctuating flow applications and are usually less common in low flow applications. Other meters used include variables for meters and pressure meters.
The percentage of reading accuracy is shown as a percentage in a flow rate within the meter range. Error bands are not fixed. They are most used when projects need a higher level of accuracy. Turbine meters and magnetic flow meters are standard meters used with the percentage of reading accuracy.
The standard accuracy measurement will depend on meter class and may not be the same across all flow rates. It can be as low as +/- 1.5%. To reach this level of accuracy, flow meters must have a fully developed flow profile, including the ability to straight-run pipe up and downstream. Accuracy statements are often given with turndown values, indicating the lowest possible levels of velocity meter can maintain their accuracy statements.
Meters tend to lose accuracy when they reach the lower end of the operating range. End users that use VFDs and diverting values for both hot and cold water need to consider the way initial installation can impact performance. Knowing how meters measure accuracy can also save issues later in its lifespan.