HEAD FLOW METERSFlow DetectorsPitotTubeThe pitot tube, illustrated in Figure 5, is another primary flow element used to produce adifferential pressure for flow detection. In its simplest form, it consists of a tube with an openingat the end. The small hole in the end is positioned such that it faces the flowing fluid. Thevelocity of the fluid at the opening of the tube decreases to zero. This provides for the highpressure input to a differential pressure detector. A pressure tap provides the low pressure input.Figure 5 Pitot TubeThe pitot tube actually measures fluid velocity instead of fluid flow rate. However, volumetricflow rate can be obtained using Equation 4-4.(4-4)VKAVwhere= volumetric flow rate (ft3/sec.)A = area of flow cross-section (ft2)V = velocity of flowing fluid (ft/sec.)K = flow coefficient (normally about 0.8)Pitot tubes must be calibrated for each specific application, as there is no standardization. Thistype of instrument can be used even when the fluid is not enclosed in a pipe or duct.IC-04 Page 6 Rev. 0
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