Variable area flow meters operate on the principle that the variation in area of flow stream required to produce a constant pressure differential is proportional to the flow rate. The flowing fluid enters the bottom of the meter, passes upward through a variable area metering tube, and around the float, exiting at the top. The flow rate is read by noting the position of the float against the calibrated scale etched on the tube.
Variable area flow meters are generally available as direct-reading or correlated meters. The direct-reading meter allows users to read the flow rate in units printed directly on the tube, by aligning the top of the float with the marks on the flow tube but is restricted to a particular fluid. By contrast, correlated meters employ a unitless scale and are supplied with a separate correlation chart to provide flow rate data, but can be used for various fluids applications and changing pressure or temperature conditions.
Where calibration has been carried out for defined media conditions, deviations in the density, pressure or temperature of gases, or in the density or viscosity of liquids, result in measurement errors. It is essential to observe the calibration conditions and it is essential to provide data on the medium, density and viscosity at the operating temperature and pressure. With gases, it is additionally necessary to specify the exact reference point for the pressure (pressure above atmospheric or absolute pressure).
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