The simplest & most common flow meter is a fixed Orifice in a pipeline. This Orifice creates a pressure differential (i.e. pressure drop) which is proportional to flow. But the flow rate versus differential pressure is a square root function, thus at the high end of the scale the graduations are far apart and very easy to read. At the low end of the scale, the graduations are so close together that reading becomes impossible. The net result is a meter with only a 3 OR 4 to 1 usable range.