Document DvryayyreGkD2vko0y2pDZR15

96 CHAPTER 4 1949 Guide downstream m any degree, a suction effect is produced to falsify Idle reading... In instruments having the.static pressure opening pointed down stream, the. coefficient may be as low as 0.86. Consequently, for accurate work, Pitot tubes should be calibrated, and the pertinent coefficient should be applied to Equation 72. In a sense, this coefficient is advantageous, since it results in a higher differential reading, which, in turn, enables more accurate readings at low flows. , In using Pitot tubes, it is generally necessary to make.a traverse of the pipe or duct to determine the course of the velocity pattern. In a pipefor. instance, one of the profiles shown in Fig.-5 would be obtained. . Near a valve or fitting, however, the profile might-be. quite distorted, a. fact which would be revealed by the traverse:- If:the-pipe or;duct is divided 97 VARIABLE AREA FLOW METERS In the routine measurements of many, liquids and gases, satisfactory results may be obtained by the use of variable area flow meters, frequently called rotameters. In their_ most common form these devices consist essentially of a float which is free to move vertically in a transparent tapered tube. The fluid to be metered enters at the bottom, narrow end of the tube and moves upward, passing at some point through the annulus formed between the float and inside wall of the tube. At any particular rate of flow the float assumes a definite position in the tube, its location J:--K" of a calibrated scale on the tube. - into equal areas, and a determination of /t.w is made for each, the average . velocity would be' obtained by using the' average of* the square roots of /la, in Equation 73. ", ' 'Y INSTALLATION OF NOZZLES AND ORIFICES ,,.,'A- filial , note should be made of the installation of orifices and nozzles. Generally/speakihg, the orifice or nozzle, together with a holding arrangeinent, including pressure taps, is available from-the manufacturer. In making the installation, the user should make certain that the flow api preaching the nozzle or orifice is.steady and.evenly distributed, i.e., with velocity profiles similar to those shown in Fig. 5. Fittings and valves, which tend to direct the'flqWj.to one side and which in.some cases cause.it to rotate as it advances, must be far enough upstream from the orifice or nozzle to. permit the disturbed stream to straighten out to the even form before'.'reaching the".niteri''; Minimum conditions 'in the installation for avoiding trouble, from fittings and vdlyes are shown in Fig. 21. If'neces sary,' straightening vahes mAy be'used upstream from the orifice or nozzle at a distance of not less than 8 pipe diameters. I The position of the float is established by a balance between the fluid pressure forces across the annuliis and the weight of the float itself. The buoyant force which must support, the float, vt(pi -- p), is balanced by the pressure difference acting on the cross section area of the float, AtAp, where pi, At, o(, are respectively the float density, float-cross section area, and float volume. Accordingly the difference in head across the annulus is given by . . ,. . ap . t(pf -- p) ,(74) The volume flow follows from-equation (51) as Q..= KAV2pei(pi -- p)/pAi (75).