We use velocity, because Magnum is first and foremost a velocity measurement instrument. Flow = Velocity X Area. The flow processor is able to produce an information about flow because it has information about the pipe size (area) and combines it with the velocity information it has observed. Consequently it is very important that the user enter accurate information about pipe diameter in the Magio software in order to get an accurate flow measurement. It is of course also important that the pipe is actually filled otherwise the information about the area covered by water would be incorrect also.

When we calibrate the instrument it is done based on velocity alone. This is what makes it possible for the user to set up the instrument for vastly different applications, pipe sizes and engineering units. They all have one unchangeable property in common - Velocity. 

Does this mean that the flowmeter will necessarily be off by at least some amount? Not at all! It could in fact be right on - to the litre or gallon. Although if you choose a unit like MGPD (million gallons per day) it is unlikely that the meter can really distinguish reliably between individual gallons. "Accuracy" merely defines the possibilty of a discrepancy between measurement and reality based on human and technical limitations during manufacture and calibration.

There are also mathematical limitations of course. For example a flowmeter with an accuracy of +/-1% would mathematically have a possible error of 100% if it were measuring at only 1% of the selected range. This is not really a limitation of the flowmeter, which may in fact not be off at all, but rather a limitation of the "Accuracy statement" itself. Nevertheless it is true that a flowmeter used only at a portion of its range is more likely to be inaccurate then if it were used at 100% capacity.