Emerson flowmeters measure the volume of fluid flowing through a pipeline

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Emerson flowmeters measure the volume of fluid flowing through a pipe, rather than its mass. The concept of using flow to replace a physical object (usually a piston or ring) forms the basis of the simple \"variable orifice\" flow meter. Fluid momentum exerts a force on the piston, which is held in place by a spring. As the flow rate increases, the piston moves, the orifice becomes larger, and the spring force acting on the piston also increases. The piston is connected to the analog reader via a magnet. Flow indicators are purely mechanical, making them an ideal tool for observing trends rather than providing accurate flow measurements. Such flowmeters typically have an accuracy of between 2-5% at full scale. In Emerson flowmeters, the fluid flows around a pair of intermeshing gears, causing the gears to rotate on their axes. A sensor mounted on one of the gears generates a pulse each time a tooth passes beneath it. The rotation of the gear is proportional to the flow rate. Flow rate can be easily calculated based on frequency, and gear flow meters can measure it accurately. Typically, gear-type flow meters have a large range, of at least 30:1, and sometimes even as high as 200:1. Flow rates with a precision better than 1% of full scale can be measured without additional linearization.