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The pitot tube, illustrated in Figure 5, is another primary flow element used to produce a differential pressure for flow detection. In its simplest form, it consists of a tube with an opening at the end. The small hole in the end is positioned such that it faces the flowing fluid. The velocity of the fluid at the opening of the tube decreases to zero. This provides for the high pressure input to a differential pressure detector. A pressure tap provides the low pressure input.
Figure 5 Pitot Tube The pitot tube actually measures fluid velocity instead of fluid flow rate. However, volumetric flow rate can be obtained using Equation 4-4.
where
Pitot tubes must be calibrated for each specific application, as there is no standardization. This type of instrument can be used even when the fluid is not enclosed in a pipe or duct. Summary Head flow meters operate on the principle of placing a restriction in the line to cause a pressure drop. The differential pressure which is caused by the head is measured and converted to a flow measurement. The basic construction of various types of head flow detectors is summarized below. Head Flow Meter Construction Summary Flat plates 1/16 to 1/4 in. thick Mounted between a pair of flanges Installed in a straight run of smooth pipe to avoid disturbance of flow patterns due to fittings and valves Venturi tube Converging conical inlet, a cylindrical throat, and a diverging recovery cone No projections into the fluid, no sharp corners, and no sudden changes in contour Consists of a short, straight inlet section followed by an abrupt decrease in the inside diameter of the tube Inlet shoulder followed by the converging inlet cone and a diverging exit cone Two cones separated by a slot or gap between the two cones Pitot tube Consists of a tube with an opening at the end Small hole in the end positioned so that it faces the flowing fluid
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