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78 CHAPTER 4 1954 Guide FLOW MEASUREMENT The measurement of quantity of flow of fluids is generally accomplished either by making observations on the change of state due to flow system configuration, such as the pressure drop across a metering orifice, or by the displacement of some device, such as a rotating vane system. The selec tion of the metering system will be determined by the type of fluid, the precision of measurement desired, the cost of equipment and installation, the range of flow quantity, the ease of maintenance, and -the method of observation and recording. Fluid meters may be classified as follows: (1) Head Meters (Pressure Sensitive), which may be of Venturi, flow nozzle, orifice plate, or Pitot tube types; (2) Area Meters, which may be of gate, tapered tube, or tapered plug types; (3) Force Meiers, which may be of vane, propeller, or turbine types;' (4) Quantity Meters, which may be of weighing tank, reciprocating piston, or geared impeller types. Rate meters are generally of the first three groups, although rates are Fluid Flow 79 Fir,. 8. Dimensions for International Standards Association Flow Nozzle Note: From Reference 3. Used by permission of AJSM.E. obtainable from timed observations of quantity meters. Similar quantity measurements can be obtained by suitable integration of rate meter indications. Head Meters The head meter is of sufficient flexibility so that almost any type of flow measuring problem can be handled. For this reason, standard and refer ence measurements are usually made in this way. With proper care, extreme precision can be obtained. Also, an inexpensive installation can be made to give moderate precision. In general, a head meter requires fairly competent installation and maintenance to give satisfactory service. Among the types of head meters, the Venturi has the advantage of having a low pressure loss, but requires considerable length of space. The orifice plate reduces the required length to a minimum at the cost of considerable pressure drop. The flow nozzle represents a compromise, but must be calibrated individually, or made with extreme care from standard specifica tions, to obtain a good precision of measurement. Standard configurations, for representative orifice plates and flow nozzles, are shown in Figs. 7 and Fig. 10. Flow Coefficients, K, for Square-edged Taps in Smooth Pipe Ori fice Plates and Radius Note: From Fig. 36d of Reference 4. Fig. a Oontracta Taps, in Smooth Pipe Note: From Table 7 of Reference 2.