Pitot static tubes

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. 

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. 

ISO 3966. Measurements of fluid flow in closed conduits—Velocity area method using Pitot static tube. 1977, p. 39. 

I he origins of the above two errors are chfferent in cause and nature. A sim ple example is, when the mass of a weight is less than its nominal value, a systematic error occurs, which is constant in absolute value and sign. This is a pure systematic error. A ventilation-related example is, when the instrument faaor of a Pitot-static tube, which defines the relationship between the measured pressure difference and the velocity, is incorrect, a systematic error occurs. On the other hand, if a Pitot-static tube is positioned manually in a duct in such a way that the tube tip is randomly on either side of the intended measurement point, a random error occurs. This way, different phenomena create different ty pes of error. I he (total) error of measurement usually is a combination of the above two types. 

The question is often asked. How often should calibration be carried out Is it sufficient to do it once, or should it be repeated The answer to this question depends on the instrument type. A very simple instrument that is robust and stable may require calibrating only once during its lifetime. Some fundamental meters do not need calibration at all. A Pitot-static tube or a liquid U-tube manometer are examples of such simple instruments. On the other hand, complicated instruments with many components or sensitive components may need calibration at short intervals. Also fouling and wearing are reasons not only for maintenance but also calibration. Thus the proper calibration interval depends on the instrument itself and its use. The manufacturers recommendations as well as past experience are often the only guidelines. 

The absolute, barometric pressure is not normally required in ventilation measurements. The air density determination is based on barometric pressure, but other applications are sufficiently rare. On the other hand, the measurement of pressure difference is a frequent requirement, as so many other quantities are based on pressure difference. In mass flow or volume flow measurement using orifice, nozzle, and venturi, the measured quantity is the pressure difference. Also, velocity measurement with the Pitot-static tube is basically a pressure difference measurement. Other applications for pressure difference measurement are the determination of the performance of fans and air and gas supply and e. -haust devices, the measurement of ductwork tightness or building envelope leakage rate, as well as different types of ventilation control applications. 

This states that the sum of the velocity pressure 0.5pv plus the static pressure / the total pressure, is constant along a streamline. In the case of standard air density (1.2 kg m ), 0.5pv becomes 0.6v. When a Pitot-static tube is immersed into the flow, as in Fig. 12.19, the velocity at the stagnation point at the tube nose is f = 0 and the local static pressure equals the total pressure p,. The flow static pressure p, is measured a short distance downstream from the surface of the tube. The flow velocity is obtained by applying Eq. (12.27) ... 

By connecting manometer hoses to both output pressures given by the tube, the pressure difference Ap can be measured directly. The barometric pressure and the fluid temperature are required for the determination of the fluid density. The Pitot-static tube is not a suitable instrument for measuring low velocities. It can be applied in cases where the flow velocity is high... 

When the axis of the Pitot-static tube is not aligned to the main flow direction, an error of inclination occurs known as yaw. It is not more than 1% for the measured pressure difference if standard tubes are used and the devia tion from the flow direction is less than 11-13°. 44 (hg further... 

In measuring the local velocity in ducts, the sensor will obstruct a part of the duct cross-section. This results in accelerated flow by the sensor and an error occurs. In a Pitot-static tube, this is called stem blockage. If the ratio of the tube diameter to the duct diameter is smaller than 0.02, stem blockage can be neglected. Otherwise a correction has to be applied. 

The blocking effect does not apply to the Pitot-static tube alone. Any sensor/instrument immersed into a duct has a similar effect the larger the sensor is, the greater the problem. For other types of instruments an analysis must be made, so as not to block large proportion of the duct cross-section with the meter. A good rule of thumb to avoid corrections is to keep the cross-section of the meter less than 5% of the duct cross-section. 

Constriction measurement devices constructed to standards do not necessarily require calibration. One idea of strict standardization is to define the manufacturing, tolerances, and other features in such a way that the instruments made according to these rules require no calibration. The properties are so well known that a certain accuracy can be guaranteed. If the accuracy specified in the standard is inadequate, additional calibration procedures are required. The same applies to Pitot-static tubes made according to standard specifications." ... 

ISO Standard 3966. Measurement of Fluid Flow in Closed Conduits—Velocity Area Method Using Pitot Static Tubes. International Organisation fot Standardisation, 1977. 

For isokinetic sampling, the doer sampling-point gas velocity has to be measured, and the corresponding sample gas flow calculated and adjusted. " Normally, a Pitot static tube is used for the measurement of duct gas velocity. 

Pitot-static tube A measuring device consisting of two concentric tubes used to measure the total and static pressures in a duct run, known as a Prandtl tube. 

Air at 323 K and 152 kN/m2 pressure flows through a duct of circular cross-section, diameter 0.5 m. In order to measure the flow rate of air, the velocity profile across a diameter of the duct is measured using a Pitot-static tube connected to a water manometer inclined at an angle of cos-1 0.1 to the vertical. The following... 

To measure the internal flow velocity in the duct, dust sampling was taken at various points along the vertical diameter. A pitot static tube and magnehelic gauge, shown in Figure 1, was the equipment used for these measurements. The duct humidity, tempertaure, and static pressure were measured to calculate the gas density. In determining the humidity, the wet and dry bulb temperature of a continuous sample stream was used. To prevent dust buildup on the wet bulb thermometer, an inline metal filter was inserted into the line. 

The combined pitot-static tube shown in Fig. 10-12 consists of a jacketed impact tube with one or more rows of holes, 0.51 to 1.02 mm (0.02 to 0.04 in) in diameter, in the jacket to measure the static pressure. Velocity Vq m/s (fl/s) at the point where the tip is located is given by... 

Where conditions are such that it is impractical to measure static pressure at the wall, a combined pitot-static tube may be used. This is actually two tubes, one inside the other. The outer tube is plugged off at the end facing the flowing fluid, but small holes are drilled through it to receive the fluid pressure. These holes open into the annular space between the tubes. The static pressure is measured through two or more of these holes, and it is assumed that the flow follows along the outside of the tube in such a way that the true static pressure is obtained. 

The pitot-static tube includes an annular tube surrounding the stagnation tube, with holes in the sides through which the static pressure is measured. For this configuration, errors introduced by... 

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