However water quality and suspended particles greatly degrade the water meter error curve and in some cases produce a definitive blockage of the meter. In contrast, velocity meters are affected by many different factors like flow distortions, environmental and working conditions or changes in the internal dimensions of the measuring chamber or the inlet nozzle (for example those caused by calcium depositions) that may interfere with the flow velocity passing through the meter. The errors of velocity meters, especially for low flows, are sensitive to any increment of the drag torque on the sensor element. Consequently, many are the variables that can affect the accuracy of these meters. The combined action of these variables can have unpredictable consequences depending on the meter construction and materials. This circumstance makes it very difficult to predict in advance, for a given water supply, the behaviour of a specific type of velocity meter.
In Spain, the most used and less expensive metering technology for domestic users is single jet. Different manufacturers compete in this market offering models within a wide range of prices and metrological performances. The construction characteristics and quality diversity of available meters are considerable. Quite often, different models even from the same manufacturer are used simultaneously in the same water supply.
This scenario makes it more complex to analyse the real performance of the meters unless a long term strategy has been prepared for such a task. As said before, since many interrelated variables can simultaneously affect meters performance, a considerable sample has to be draw from the field. Therefore, only a long term study can provide with enough information to clearly identify and quantify the parameters with most influence on the error curve for each meter type.
This paper describes some of the problems that may arise when carrying out a research on this subject and illustrates it with examples of a real case study.