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6.5Point Velocity Measurement > 6.5.3Pitot Tube - Pg. 64

64 part | II Mechanical Measurements 6.4.2.3 Ultrasonic Method As for closed-pipe flow, two techniques are available: single-path and multipath, both relying on time-of-flight techniques, as described in Section 6.3.4.2. Transduc- ers capable of transmitting and receiving acoustic pulses are staggered along either bank of the river or channel. in practice the acoustic path is approximately 60° to the direction of flow, but angles between 30° and 60° could be utilized. The smaller the angle, the longer the acoustic path. Path lengths up to 400 meters can be achieved. New spool piece designs have included corner targets and other devices to improve the accuracy of the signal. Recently, clamp-on transit-time flow sensors have been adapted to work directly on the high-purity tubing used in the semi- conductor manufacturing and in pharmaceutical industries. Correlation flowmeters have also been constructed using these new techniques. the flowstream; the wire sensor is typically 5m diameter and approximately 5 mm long. As flow velocity increases, it tends to cool the heated element. This change in temperature causes a change in resistance of the element proportional to flow velocity. 6.5.3 pitot tube The pitot tube is a device for measuring the total pressure in a flowstream (i.e., impact/velocity pressure and static pres- sure); the principle of its operation is as follows. if a tube is placed with its open end facing into the flow- stream (Figure 6.52), the fluid impinging on the open end will be brought to rest and its kinetic energy converted into pressure energy. The pressure buildup in the tube will be greater than that in the free stream by an amount termed the impact pressure. if the static pressure is also measured, the differential pressure between that measured by the pitot tube and the static pressure will be a measure of the impact pressure and therefore the velocity of the stream. in Equation (6.15), h, the pressure differential or impact pres- sure developed, is given by h = ` V 2 / 2 g j - ` V 2 / 2 g j , where 2 1 V 2 = 0. Therefore, V 2 / 2 g , that is, the pressure increases by 1 V 2 /2 g . The negative sign indicates that it is an increase in 1 6.4.3 dilution gauging This technique is covered in detail in section 6.6 on flow cali- bration, but basically the principle involves injecting a tracer element such as brine, salt, or radioactive solution and esti- mating the degree of dilution caused by the flowing liquid.