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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.
The limited rangeability of this type of meter can be overcome by adding a bypass leg around the Orifice, which has a pressure drop equal to the drop across the Orifice and bypassed flow is directly proportional to the mainline flow. This results in a meter, which has a linear function and not a square root. Since the flow meter has a 10 to 1 range, the Orifice meter will now have a 10 to 1 linear flow range, which is as easy to read at the bottom as the top of the scale. This principle can be used virtually any pipeline size, handling almost any flow rate and metering either liquid OR gas. It is one of the most versatile meters available. Its only limitation is that the solid particles OR slurries can clog the Orifice and prevent proper operation.