Pitot tubes

Pitot tube Pitot tubes Pitressin Tannate Pituitary... 

Pitot Tubes. The fundamental design of a pitot tube is shown in Eigure 9a. The opening into the flow stream measures the total or stagnation pressure of the stream whereas a wall tap senses static pressure. The velocity at the tip opening, lA can be obtained by the Bernoulli equation ... 

Fig. 4. (a) Type-S pitot tube and thermocouple (b) side view of the correct pitot tube configuration when used ia conjunction with a sampling nozzle, where A is the static pressure opening plane and B is the impact pressure opening plane and (c) bottom view. 

Dynamic pressure may be measured by use of a pitot tube that is a simple impact tube. These tubes measure the pressure at a point where the velocity of the fluid is brought to zero. Pitot tubes must be parallel to the flow. The pitot tube is sensitive to yaw or angle attack. In general angles of attack over 10° should be avoided. In cases where the flow direction is unknown, it is recommended to use a Kiel probe. Figure 10-3 shows a Kiel probe. This probe will read accurately to an angle of about 22° with the flow. 

Special Tubes A variety of special forms of the pitot tube have been evolved. Folsom (loc. cit.) gives a description of many of these special types together with a comprehensive bibhography. Included... 

Chile [Prog. Aerosp. Sc7, 16, 147-223 (1975)] reviews the use of the pitot tube and allied pressure probes for impact pressure, static pressure, dynamic pressure, flow direction and local velocity, sldn friction, and flow measurements. 

A reversed pitot tube, also known as a pitometer, has one pressure opening facing upstream and the other facing downstream. Coefficient C for this type is on the order of 0.85. This gives about a 40 percent increase in pressure differential as compared with standard pitot tubes and is an advantage at low velocities. There are commercially available very compact types of pitometers which require relatively small openings for their insertion into a duct. 

The pitot-venturi flow element is capable of developing a pressure differential 5 to 10 times that of a standard pitot tube. This is accomplished by employing a pair of concentric venturi elements in place of the pitot probe. The low-pressure tap is connected to the throat of the inner venturi, which in turn discharges into the throat of the outer venturi. For a discussion of performance and application of this flow element, see Stoll, Trans. Am. Soc. Mech. Eng., 73, 963-969 (1951). 

Pitot Tubes The combination of pitot tubes in conjunction with sidewall static taps measures local or point velocities by measuring the difference between the total pressure and the static pressure. The pitot tube shown in Fig. 10-5 consists of an impact tube whose opening faces directly into the stream to measure impact pressure, plus one... 

There are certain limitations on the range of usefulness of pitot tubes. With gases, the differential is very small at low velocities e.g., at 4.6 m/s (15.1 ft/s) the differential is only about 1.30 mm (0.051 in) of water (20°C) for air at 1 atm (20°C), which represents a lower hmit for 1 percent error even when one uses a micromanometer with a precision of 0.0254 mm (0.001 in) of water. Equation does not apply for Mach numbers greater than 0.7 because of the interference of shock waves. For supersonic flow, local Mac-h numbers can be calculated from a knowledge of the dynamic and true static pressures. The free stream Mach number (MJ) is defined as the ratio of the speed of the stream (V ) to the speed of sound in the free stream ... 

With liquids at low velocities, the effect of the Reynolds number upon the coefficient is important. The coefficients are appreciably less than unity for Reynolds numbers less than 500 for pitot tubes and for Reynolds numbers less than 2300 for pitot-static tubes [see Folsom, Trans. Am. Soc. Mech. Eng., 78, 1447-1460 (1956)]. Reynolds numbers here are based on the probe outside diameter. Operation at low Reynolds numbers requires prior calibration of the probe. 

The pitot-static tube is also sensitive to yaw or angle of attack than is the simple pitot tube because of the sensitivity ofthe static taps to orientation. The error involved is strongly dependent upon the exac-t probe dimensions. In general, angles greater than 10° should be avoided if the velocity error is to be 1 percent or less. 

The effect of pulsating flow on pitot-tube accuracy is treated by Ower et al., op. cit., pp. 310-312. For sinusoidal velocity fluctuations, the ratio of indicated velocity to actual mean velocity is given by the factor /l + AV2, where X is the velocity excursion as a fraction of the mean velocity. Thus, the indicated velocity would be about 6 percent high for velocity fluctuations of 50 percent, and pulsations greater than 20 percent should be damped to avoid errors greater than 1 percent. Tne error increases as the frequency of flow oscillations approaches the natural frequency of the pitot tube and the density of the measuring fluid approaches the density of the process fluid [see Horlock and Daneshyar, y. Mech. Eng. Sci, 15, 144-152 (1973)]. 

Pressures substantially lower than true impact pressures are obtained with pitot tubes in turbulent flow of dilute polymer solutions [see Halliwell and Lewkowicz, Phys. Fluids, IS, 1617-1625 (1975)]. 

Anemometers An anemometer may be any instrument for measurement of gas velocity, e.g., a pitot tube, but usually the term refers to one of the following types. 

Once these traverse points have been determined, velocity measurements are made to determine gas flow. The stack-gas velocity is usually determined by means of a pitot tube and differential-pressure gauge. When velocities are very low (less than 3 m/s [10 ft/s]) and when great accuracy is not required, an anemometer may be used. For gases moving in small pipes at relatively high velocities or pressures, orifice-disk meters or venturi meters may be used. These are valuable as continuous or permanent measuring devices. 

In most source tests, the measurement of velocity is made with a pitot-static tube, usually referred to simply as a pitot tube. Figure 32-6 illustrates the two types of pitot tubes in common use. 

The standard type of pitot tube shown in this figure does not need to be calibrated, but it may be easDy plugged in some high-effluent loading streams. The type S pitot tube shown in Fig. 32-6 does not plug as easily, but it does need calibration to assure its accuracy. The type S pitot tube is also more sensitive to alignment with the gas flow to obtain the correct... 

Fig. 32-6. Pitot tubes for velocity determination. Source "Annual Book of Standards" (3). Note English units were used by the American Society for Testing and Materials.

Total pre.ssure is the pressure of the gas brought to rest in a reversible adiabatic manner. It can be measured by a pitot tube placed in the flow... 

When the veloeity pressure is more than 5% of the pressure rise, it should be determined by a pitot-tube traverse of two stations. For each station, the traverse consists of 10 readings at positions representing equal areas of the pipe cross section, as shown in Figure 20-2. The average velocity pressure Py is given by... 

Sensitive differential pressure Instrument connected to Pitot tube... 

Aneroid gauge A gauge used for the measurement of static, velocity, or total pressure with a pitot tube. 

Traversing The process of moving across a grid line in a duct or on a hood with a Pitot tube in order to determine the velocity or pressure distribution. 

Pistole,/. pistol gun torpedo fuse. Pistolengeblaae, n, hand-operated blast, Pitehanf, Pitahanf, m. pita hemp, pita, Pitot-rohr,n, -rfihre,/., Pitotsches Rohr. Pitot tube. 

Figure 12-129. Pressure readings from a pitot tube.

Pressure-reducing stations, orifice plates, pitot tubes, and steam meters may all suffer from excessive mechanical wear and may therefore malfunction. 

The velocity of the liquid varies over the cross-section and is usually a maximum at a depth of between 0.05 D and 0.25 D below the surface, at the centre line of the channel. The velocity distribution may be measured by means of a pitot tube as described in Chapter 6. 

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