FlowMeasLab_and_Worksheet.pdf

# That if a manometer is connected to a pitot static

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that if a manometer is connected to a Pitot-static tube the dynamic pressure will be given by P = g h, where is the manometer fluid density and h is the difference in height of the fluid columns. The DP cells convert pressure force acting over the surface area of a plate to a movement of the plate to a varying electrical capacitance, which may be displayed or digitally acquired. The gages are calibrated in "inches-of-water", an antiquated but common pressure unit which corresponds to the pressure exerted by a one-inch vertical displacement of water at standard conditions. It is easy to imagine how experimenters, using water-filled manometers, chose this as a unit of pressure measurement. We can convert units of "inches-of-water" to Pascals by the following conversion: kPa = 4.019 "Inches-of-water" The dimensions of the Pitot-static probe can be important in assuring that the probe gives an accurate measure of the velocity. The diameter of the Dwyer Pitot-static probe is 1/8th of an inch. To minimize the blockage effects of the Pitot-static probe on the measured flow, the manufacturer recommends that this tube be used in ducts with an inside diameter of three inches or more. This ensures that the blockage of the probe does not significantly change the duct velocity at the probe static ports, causing an error in static pressure measurement. The length of the axial tip of the Pitot-static probe is also critical. In this tube the side ports used to sense the static pressure in the flowing air are five probe diameters from the end of the tube. This requires a smooth end design to prevent disturbances from this leading edge from altering the static pressure to be measured by the side ports. In most Pitot-static probe designs, a minimum distance of eight diameters is recommended to remove this effect. The bend in the tube is also a minimum distance from these side ports so that minimal interference will occur. In this tube, the bend is eight diameters behind the side ports. A shorter distance could produce a higher static pressure reading than is present in the stream.

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ME 4600:483 Lab Notes Revised 11/16/2015 Flow Measurement Page 4 of 18 Textbook descriptions of Pitot-static probes usually describe their use in a laminar flow. What happens when Pitot-static probes are used in time-varying turbulent flows? The pressure difference associated with the fluctuation velocity must move a mass in the pressure sensor to measure the pressure change associated with a given velocity change. The measurement devices are thus second-order mechanical systems with their own natural frequency and damping ratio. If the frequency of the velocity fluctuation is much faster than the natural frequency of the measuring system, then it will display the average value of the fluctuating signal. This will only hold true for moderately turbulent flows (less than 10% turbulence intensity) because the velocity vector must remain approximately parallel to the Pitot-static probe. Duct flows typically have
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