Pitot Tube

12 Key excerpts on "Pitot Tube"

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  Instrument Technology

  Measurement of Pressure, Level, Flow and Temperature

  • E. B. Jones(Author)
  • 2013(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  186 MEASUREMENT OF FLOW aircraft relative to the air. It actually measures the velocity of the stream at a point, but by traversing the stream and measuring the velocity at several points it is possible to obtain the average velocity of the stream, and hence, by multiplying this by the area of cross-section, the volume flowing. Principle. If a tube is placed with its open end facing into a stream of fluid, then the fluid impinging on the open end will be brought to rest, and its kinetic energy converted into pressure energy. Thus, the pressure built up in the tube will be greater than that in the free stream by the 'impact pressure' or pressure produced by the loss of kinetic energy. This increase in pressure will depend upon the square of the velocity of the stream. The difference is measured between the pressure in the tube and the static pressure of the stream. The static pressure is measured by a tapping in the wall of the main, or by a tapping incorporated in the pitot static tube itself. The difference between the pressure in the tube and the static pressure will be a measure of the 'impact pressure', and therefore of the velocity of the stream. Consider the small stream of liquid flowing on to the tip of the Pitot Tube. Originally it will be moving with a velocity V y and finally it is at rest. In equation 3.8, h, the pressure differential or impact pressure developed, is given by h = f g ~fi where F * = 0 V 2 therefore h = V 2 i.e. the pressure increases by — I The negative sign indicates that it is an increase in pressure and not a decrease. V 2 Increase in head h = —- or V 2 = 2gh y x = sfigh (3.36) As the whole of the stream flowing on to the end of the tube may not be brought to rest, since some may be deflected around the edge, this value of V x may vary from the true velocity. This variation will depend upon the design of the tube. To compensate for this variation, a coefficient C, called 'the Pitot Tube coefficient', is intoduced.

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  Solved Practical Problems in Fluid Mechanics

  • Carl J. Schaschke(Author)
  • 2015(Publication Date)
  • CRC Press

    (Publisher)

  Again, the total flow is the sum of the elemental flows in each area. While the Pitot Tube is usually slim in design, it is important that in their use within narrow pipes or tubes they do not provide an unnecessary obstruction. This would otherwise influence the velocity around the device and not give a true indication of the rate of flow. 37 Flow Measurement What Else Is Interesting? As with all flow measurement devices, the Pitot Tube should be ideally located away from disturbances such as bends. The device was devised by the Italian-born French engineer Henri de Pitot (1695–1771). Commercial and military aircraft and Formula 1 motor racing cars (Figure 2.7) use Pitot Tubes to measure the velocity of the vehicles. A differential pressure transducer is used to provide an electronic signal to indicate velocity. Problem 2.5: Venturi Flume Water flows along a flume at a depth of 2 m. The flume features a 10-cm-high obstruction that causes the surface of the water to dip by 8 cm. Determine the rate of flow of the water per unit width of flume. Solution The Venturi flume (Figure 2.8) is a commonly used method of measuring the rate of flow in open channels and flume. The use of an obstruction at the floor causes a rise of the water over it, which reduces the flow area and increases the velocity. According to the Venturi principle, there is a corresponding decrease in pressure at this point. This is manifested as a decrease in the level at the surface. FIGURE 2.7 Pitot Tube on the Nose of a Formula 1 Car (Photo from C.J. Schaschke.) Flow h h 1 h 2 FIGURE 2.8 Venturi Flume 38 Solved Practical Problems in Fluid Mechanics Applying the Bernoulli equation at an upstream location and at the obstruction itself gives an average velocity in the flume as U gh h h 1 1 2 2 2 2 1 2 9 81 0 08 2 1 82 =       -= × ×       . . . -= -1 2 75 1 . ms (2.13) The rate of flow per unit width is therefore dotnosp Q W U h = = × = -1 1 3 1 2 75 2 5 5 .

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  Engineering Fluid Mechanics

  • William Graebel(Author)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  501 chapter 10 Measurement of Flow and Fluid Properties Chapter Overview and Goals In previous chapters we have looked briefly at several flow-measuring devices such as the venturi meter and the Pitot Tube. In our study of laminar viscous flows we also studied several flow cases that were suited to the measurement of fluid properties. In this chapter we elaborate on some of the details of these devices to better understand their use, and also introduce other types of measuring instruments. The devices are divided into the following categories: measurement of velocity, measurement of mass rate or volume rate, measurement of pressure, measuremen t of viscosity. In this chapter we give a representative sampling of some of the measuring instrumentation currently in use. The list is by no means complete, since the list of commercially available devices is continually changing with advances in technology. The theory developed in previous chapters finds considerable use in giving simple theoretical results that are then refined through calibration procedures. The list of references at the end of the chapter should be consulted for further details and methods. 1. Velocity-measuring Devices a. Pitot Tubes Pitot Tubes, also called pitot-static tubes, can be used for measurements of steady velocities at a point. They are named after Henri de Pitot, a French engineer of the eighteenth century. Pilot’s idea was an important contribution to flow measurement, even though his analysis of the theory of his device was in error. Pitot Tubes are designed with either rounded or blunt noses. (See Figure 10.1.) An opening at the nose of the Pitot Tube measures stagnation pressure, and openings on the circumference of the tube downstream of the stagnation point read the static 502 Measurement of Flow and Fluid Properties Figure 10.1.

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  Process Plant Instrumentation

  Design and Upgrade

  • Miguel J. Bagajewicz(Author)
  • 2000(Publication Date)
  • CRC Press

    (Publisher)

  c Panametrics, Inc., 1996. Flow Rate Instrumentation 39 A B d v a FIGURE 3.8. Schematics of ultrasonic meter. The Pitot Tube ( Figure 3.9 ) consists of a small tube inserted horizontally in the pipe. This tube creates a stagnation point in the fluid flow. At this point, the pressure exerted by the fluid is larger than the pressure at static holes located in the wall. Applying the Bernoulli equation between the wall and the static holes one obtains P s -P 0 = ρ v 2 s 2 (3.17) where P s and v s is the pressure and velocity at the stagnation point, whereas P 0 is the static pressure. From this equation, one can derive an expression for the velocity: v s = s 2( P s -P 0 ) ρ (3.18) In the case of gases (typically above 200 ft/sec), compressibility becomes important and the fluid is assumed to be isentropically compressed at the point (a) Side-Wall Static Tap (b) Static Tube D P Flow Flow Low pressure side High pressure side Openings for static p ressure FIGURE 3.9. Pitot Tube schematics. 40 INSTRUMENTATION of impact. Thus, the following equation is derived: v s = C v u u t 2 k k -1 P 0 ρ 0 ¶ P 0 ρ 0 ¶ ( k -1) / k -1 # (3.19) The coefficient C is close to 1. We now express the mass flow rate as a function of the pressure drop and density for the incompressible case as follows: Q = v s A = √ 2 K pitot 4 Pitot s ( P s -P 0 ) ρ 0 (3.20) where K pitot is the pitot coefficient, which takes care of the departures from testing conditions, and 4 Pitot is the compressibility factor. Because of the form of this equation and the fact that pressure difference is measured, pitot meters are sometimes also classified as differential pressure meters. Positive Displacement or Volumetric Meters Positive displacement (PD) meters, also called linear meters, directly measure volumetric flow rates by letting the flow pass through compartments of known volume. The fluid passage is then counted and multiplied by the individual volume of each segment.

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  Measurement and Instrumentation Principles

  • Alan S. Morris(Author)
  • 2001(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Another advantage of the Dall flow tube is its shorter length, which makes the engineering task of inserting it into the flow line easier. The Dall tube has one further operational advantage, in that the permanent pressure loss imposed on the measured system is only about 5% of the measured pressure difference ( P 1 P 2 ). The flow nozzle is of simpler construction still, and is therefore cheaper than either a Venturi or a Dall flow tube, but the pressure loss imposed on the flowing fluid is 30–50% of the measured pressure difference ( P 1 P 2 ). Pitot static tube The Pitot static tube is mainly used for making temporary measurements of flow, although it is also used in some instances for permanent flow monitoring. It measures the local velocity of flow at a particular point within a pipe rather than the average flow velocity as measured by other types of flowmeter. This may be very useful where there is a requirement to measure local flow rates across the cross-section of a pipe in the case of non-uniform flow. Multiple Pitot Tubes are normally used to do this. The instrument depends on the principle that a tube placed with its open end in a stream of fluid, as shown in Figure 16.6, will bring to rest that part of the fluid which impinges on it, and the loss of kinetic energy will be converted to a measurable increase in pressure inside the tube. This pressure ( P 1 ), as well as the static pressure of the undisturbed free stream of flow ( P 2 ), is measured. The flow velocity can then be calculated from the formula: v D C 2 gP 1 P 2 The constant C , known as the Pitot Tube coefficient, is a factor which corrects for the fact that not all fluid incident on the end of the tube will be brought to rest: a proportion will slip around it according to the design of the tube. Having calculated v , the volume flow rate can then be calculated by multiplying v by the cross-sectional area of the flow pipe, A . Pitot Tube Flow P Fig. 16.6 Pitot Tube.

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  Engineering Fluid Mechanics

  • Donald F. Elger, Barbara A. LeBret, Clayton T. Crowe, John A. Roberson(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  Static Tube A static tube, as shown in Fig. 12.1b, is an instrument for measuring static pressure. Static pressure is the pressure in a fluid that is stationary or in a fluid that is flowing. When the fluid is flowing, the static pressure must be measured in a way that does not disturb the pressure. Thus, in the design of the static tube, as shown in Fig. 12.3, the placement of the holes along the probe is critical because the rounded nose on the tube causes some decrease of pressure along the tube, and the downstream stem causes an increase in pressure in front of it. Hence, the location for sensing the static pressure must be at the point where these two effects cancel each other. Experiments reveal that the optimum location is at a point approximately 6 diameters downstream of the front of the tube and 8 diameters upstream from the stem. LEARNING OUTCOMES VELOCITY AND PRESSURE (§12.1) • Describe common instruments for measuring velocity and pressure. FLOW RATE (§12.2) • Calculate flow rate by integrating velocity distribution data. • Calculate flow rate for an obstruction flowmeter (i.e., an orifice, venturi, flow nozzle). • Calculate flow rate for a rectangular or triangular weir. Measuring Velocity and Pressure 431 431 Pitot-Static Tube The Pitot-static tube, Fig. 12.1c, measures velocity by using concentric tubes to measure static pressure and dynamic pressure. Application of the Pitot-static tube is presented in Chapter 4. Yaw Meters A yaw meter, Fig. 12.4, is an instrument for measuring velocity by using multiple pressure ports to determine the magnitude and direction of fluid velocity. The first two yaw meters in Fig. 12.4 can be used for two-dimensional flow, where flow direction in only one plane needs to be found. The third yaw meter in Fig. 12.4 is used for determining flow direction in three dimensions. In all these devices, the tube is turned until the pressure on symmetrically oppo- site openings is equal.

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  Instrumentation Reference Book

  • Walt Boyes(Author)
  • 2002(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  1.5.2 Hot-wire anemometer The hot-wire anemometer is widely used for flow studies in both gas and liquid systems. Its princi- ple of operation is that a small electrically heated element is placed within the flowstream; the wire sensor is typically 5pm diameter and approxi- mately 5mm long. As flow velocity increases ii tends to cool the heated element. This change in temperature causes a change in resistance of the element proportional to flow velocity. 1.5.3 Pitot Tube The Pitot Tube is a device for measuring the total pressure in a flowstream (i.e., impacthelocity 38 Measurement of flow Sfafic I’ presgure-Yn] taPPml !I Figure 1.52 Single hole Pitot Tube. pressure and static pressure) and the principle of operation is as follows. If a tube is placed with its open end facing into the flowstream (Figure 1.52) then the fluid impinging on the open end will be brought to rest and its kinetic energy converted into pressure energy. The pressure build-up 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 (1.15) h the pressure differential or impact pressure developed is given by h = (V2212g) - (V;/2g) where V, = 0. Therefore, h = - V:/2g, Le., the pressure increases by V:/2g. The negative sign indicates that it is an increase in pressure and not a decrease. Increase in head: h = V:/2g or V: = 2gh Le. VI = &@$ (1.51) However, since this is an intrusive device not all of the flowstream will be brought to rest on the impact post; some will be deflected round it.

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  Experimental Fluid Mechanics

  Thermodynamics and Fluid Mechanics Division

  • P Bradshaw, J. R. Horlock, W. A. Woods(Authors)
  • 2016(Publication Date)
  • Pergamon

    (Publisher)

  CHAPTER 3 Fluid Velocity and Shear Stress Measurements Pitot Tubes As mentioned on p. 19, the fluid velocity at a point can be found by measuring the total pressure and the static pressure, and apply-ing eqn. (4) or (6). The total pressure can be measured as the pressure at the front stagnation point of any suitable body: an obvious shape of body to choose is a tube aligned along the flow direction, known as a Pitot Tube after its inventor. The pressure recorded by the tube is closely equal to the total pressure, to an accuracy of, say, per cent of the dynamic pressure, provided (i) that the Reynolds number based on tube diameter is more than about 100, (ii) that the tube is aligned within about ±10 deg of the flow direction, (iii) that the root-mean-square intensity of turbu-lence in the stream is less than about 5 per cent of the mean velocity, (iv) that the total pressure does not change by more than 1 to 2 per cent across the tube diameter, and (v) that the probe is not too near a wall. In addition we must remember that in super-sonic flow the pressure recorded by the tube will be the total pressure behind a normal shock wave. Naturally the strict observance of all these conditions would leave very few situations in which a Pitot Tube could be used, and most of the information required for the interpretation of Pitot Tube measurements con-sists of a description of the various corrections to be applied to the readings for the effect of low Reynolds number, yaw, high turbu-lence, transverse total-pressure gradient and wall proximity. A thorough review of the Pitot Tube 26 occupied 92 pages and quoted 147 references, so it will be seen that the use of Pitot Tubes is not as straightforward as would appear from eqns. (4) and (6). The Pitot 76

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  Fluid and Particle Mechanics

  Chemical Engineering Division

  • S. J. Michell, M. Perry(Authors)
  • 2013(Publication Date)
  • Pergamon

    (Publisher)

  its kinetic energy, at the point of measurement. Let this point velocity be v x9 then the differential head produced is given by H = | j (2.20) The velocity head is usually measured by a,manometer, and this makes the Pitot Tube a practical instrument for the measurement of point veloc-ities in gases as well as liquids. From a series of measurements across a stream, the average velocity can be evaluated by a process of integration. This is often a laborious procedure, and in case of circular ducts, such as pipes, it can be simplified by taking only a single measurement at their axis, where the point veloc-ity is a maximum (v x — v max ) 9 and calculating the average velocity from the relationship v = xv max (2.21) in which the factor a may be obtained from Fig. 1.7, upon the evaluation of the Reynolds number. EXAMPLE 2.9 An oil (sp.gr. = 0-88, viscosity = 50 cp) flows in a pipe of 3 in. diameter. The flow is measured by a Pitot Tube located centrally, and the differential head is indicated by a U-tube, containing water as the manometric liquid. What is the rate of flow for a reading of 16-4 in.? 72 FLUID AND PARTICLE MECHANICS Solution Using eq. (2.3) 16-4/1-00 // = 0-187 ft From eq. (2.20) v x = v max = V(2gH) = V{(64-4) (0-187)} î^max = 3-46 fpS Assuming laminar flow, a = 0-5, and (from eq. (2.21) the average velocity is v = 1-73 fps β--«·»>(π)'(ΐ) = 0-085 cfs This is equivalent to 51 cfm, or to (5-1) (0-88) (62-4) = 280 lb/min Checking on the Reynolds number _ *>01 _ (3/12) (1-73) (0-88) (62-4) e ~ μ (50) (0-000672) Re = 707 This is less than the critical number 2000, thus proving the correctness of the assumption made. 2.8. Area Meters The Venturi tube, orifice and nozzle meters, as well as the Pitot Tube, are based on a common principle, namely that they measure the flow in terms of the differential head created in their primary elements. For this reason they are classed as differential head meters.

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  Chemical Engineering Fluid Mechanics

  • Ron Darby, Raj P. Chhabra(Authors)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  tube connected to the wall. Since there is no flow in the vertical direction, the difference in pressure between any two vertical elevations (at the same horizontal location) is strictly hydrostatic. Thus, the pressure difference measured at the DP cell is the same as that at the elevation of the probe, because the static head between point 1 and the pressure device is the same as that between point 2 and the pressure device, so that Δ P = P 2 − P 1. We usually want to determine the total flow rate (Q) through the conduit, rather than the velocity at a point. This can be done by using Equation 10.1 or Equation 10.2 if the local velocity is measured at a sufficient number of radial points across the conduit to enable accurate evaluation of the integral. For example, the integral in Equation 10.2 could be evaluated by plotting the measured v(r) values as v(r) versus r 2, or as r v(r) versus r (in accordance with either the first or second form of Equation 10.2), and the area under the curve from r = 0 to r = R (the radius of the conduit) can be determined numerically. The Pitot Tube is a relatively complex device and requires considerable effort and time to obtain an adequate number of velocity data points, especially close to the wall of the conduit, and to integrate these over the cross section to determine the total flow rate. On the other hand, the probe offers minimal resistance to the flow and hence is very efficient in that it results in negligible friction loss in the conduit. It is also the only practical means for determining the flow rate in very large conduits (such as smokestacks). There are standardized methods for applying this method to determine the total amount of material emitted through a stack, for example. III.    VENTURI AND NOZZLE There are other devices, however, that can be used to determine the flow rate from a single measurement

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  The Measurement of Air Flow

  • E. Ower, R. C. Pankhurst(Authors)
  • 2014(Publication Date)
  • Pergamon

    (Publisher)

  In pipe flow it is often adequate to align the tube geometrically, and in con-ditions of two-dimensional flow a single rotation can suffice (about an axis perpendicular to the planes of two-dimensionality). In such cases it is often sufficient to use the instrument itself as direction indicator, e.g. by rotating a pitot-static tube until its reading is a minimum; but one or other of the more sensitive instruments mentioned below must be used if for some reason the flow direction is to be determined accurately in its own right. To specify flow direction in three-dimensional conditions it is necessary to determine two orientation angles, although in practice one of these angles is often known in advance. Various designs of pressure-type yawmeters for measuring flow direction have been designed; some of these are shown in Fig. 3.23. Those built up from open-ended tubes are easier to make than those in which pressure holes are drilled into a body. Designs with only one pair of off-centre pressure holes (types (a), (b), (c), (d)) are intended for use in conditions of two-dimensional flow. The design shown in Fig. 3.23(b) is mounted with its axis normal to the t An exploratory (unpublished) investigation has been made at the N.P.L. on Pitot Tubes undergoing forced oscillations in an airstream. Appreciable oscillation effects on the pitot reading were found; but their magnitude, and indeed their sign, varied with the type of oscillation. In general, therefore, the simple quasi-stationary theory given above is insufficient. These complications emphasize the importance of preventing probe vibration in practice. CHARACTERISTICS OF PITOT AND STATIC TUBES 53 t nT^ Axis °* rotation ' r T Ί, E C 3 (d) (e) h; (f) (b) <€ O 2 O Ϊ (c) (g) FIG. 3.23. Types of yawmeter. planes of two-dimensionality, either cantilevered from a solid boundary or completely spanning the flow.

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  Engineering Fluid Mechanics

  • Donald F. Elger, Barbara A. LeBret, Clayton T. Crowe, John A. Roberson(Authors)
  • 2019(Publication Date)
  • Wiley

    (Publisher)

  340 Flow Measurements CHAPTER ROAD MAP Measurement techniques—for example, see Fig. 13.1—are important because fluid mechanics relies heavily on experiments. Thus, this chapter describes ways to measure flow rate, pressure, and velocity. Also, this chapter describes how to estimate the uncertainty of a measurement. CHAPTERTHIRTEEN LEARNING OUTCOMES VELOCITY AND PRESSURE (§13.1) ● Describe common instruments for measuring velocity and pressure. FLOW RATE (§13.2) ● Calculate flow rate by integrating velocity distribution data. ● Calculate flow rate for an obstruction flowmeter (i.e., an orifice, venturi, flow nozzle). ● Calculate flow rate for a rectangular or triangular weir. FIGURE 13.1 This photograph shows a laminar flow element being used to measure the volume flow rate of air for testing of fans. (Photo by Donald Elger.) 13.1 Measuring Velocity and Pressure Stagnation (Pitot) Tube The stagnation tube, also called the Pitot Tube, is shown in Fig. 13.2a. A Pitot Tube measures stagnation pressure with an open tube that is aligned parallel with the velocity direction and then senses pressure in the tube using a pressure gage or transducer. When the stagnation tube was introduced in Chapter 4, viscous effects were not discussed. Viscous effects are notable because they can influence the accuracy of a measurement. The effects of viscosity, from reference (1), are shown in Fig. 13.3. This shows the pressure coefficient C p plotted as a function of the Reynolds number. Viscous effects are important when C p > 1.0. This guideline can be used to establish a Reynolds number range. In Fig. 13.3, it is seen that when the Reynolds number for the circular stagnation tube is greater than 60, the error in measured velocity is less than 1%. For boundary layer measurements, a stagnation tube with a flattened end can be used. By flattening the end of the tube, the velocity measurement can be taken nearer the boundary than if a circular tube were used.

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