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BULLETIN N0. H-11 AIR VELOCITIES WITH THE DWYER PITOT TUBE • AIR VELOCITY The total pressure of an air stream flowing in a duct is the sum of the static or bursting pressure exerted upon the sidewails of the duct and the impact or velocity pressure of the moving air Through the use of a phot tube connected differentially to a manometer, the velocity pressure alone is indicated and the cor- responding atr velocity determined For accuracy of plus or minus 2%,as in laboratory applications, extreme care is required and the following precautions should be observed 1 Duct diameter 4" or greater -0n o 2 Make an accurate traverse per sketch at right, calculate the velocities and av- erage the readings 3 °rovide smooth, straight duct sections a minimum of 81/2 diameters In length upstream and 11/2 diameters down stream from the pitot tube 4 Provide an egg crate type straightener upstream from the pitot tube In malting an air velocity check select a location as suggested above, connect tubing leads from both pitot tube connections to the manometer and insert to the duct with the tip directed into the air stream if the manometer shows a minus indication reverse the tubes With a direct reading manometer, air velocities will now be shown in feet per minute In other types, the manometer will read velocity pressure in inches of water and the corresponding velocity will be found from the curves in this bulletin If circumstances do not permit an accurate traverse, center the pitot tube in the duct, determine the center velocity and multi. ply by a factor of 9 for the approximate average velocity Field tests run in this manner should be ac- curate within plus or minus 5% The velocity indicated is for dry air at 70oF , 29 9" Barometric Pressure and a resulting density of 075#/ cu ft For air at a temperature other than 706F refer to the curves in this bulletin For other variations from these conditions, corrections may be based upon the following data f Air Velocity= 1096 2 V where Pv=veioc3ty pressure in inches of water D=Air density in Z/cu ft Air Density= P8 1 325 x T where P8 =Barometric Pressure in inches of mercury T=Absolute Temperature (indicated temperature 'F plus 460) Flow in cu ft per min =Duct area in square feet x atr velocity in ft per min AIR VELOCITY STAINLESS STEEL CALCULATOR PITOT TUBES z-r Computes „elocity 6aseo Test confirmed unity co on air density corrected efficient ana lifetime can- for conditions of temoero- struction of No 304 stain cure and pressure Elim. less steel I n c h graou nates t e d i au s co I cu ations show depth of .n 4' lotions Ranges from 01 sertion for tra ver sing to 10 water correspond- Campiies with AMCA and ing to 400 to 20 000 FPM ASHRAE specifications = Furnished with each pilot Sixes 12 to 60 long tuba iAl able.. Hone or f-oa m o u n t i n g types Pnntea in USA 8/92 c'Copyngttt 1992 Qwyer Insmirients Inc y •Rj a- DWYER INSTRUMENTS, INC. _ Tetlaphone 219/'879.800 P 0 BOX 373 a MICHIGAN CITY, INDIANA 48380, USA Fax 2191872-9057