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t% ULLETIN N0. M-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 pitat tube connected differentiaily to a manometer, the velocity pressure alone is indicated and the cor responding air velocity determined For accuracy of plus or minus 2 a, as in laboratory applications, extreme care is required and the following precautions should be ooserved 1 Duct diameter 4" or greater 2 Make an accurate traverse per sKetch at right calculate the velocities anct ov- erage she readings .aaeai,c—� ^I Claa 3 "rovide smooth strataht duct sections C3 minimum of 8'z diameters in length upstream and 1 'z diameters down- I stream from the pitot tube 4 Provide an egg crate type straightener i upstream rrom the picot tube , In making an air velocity check select a location as suggested above, connect tubing leads from both aitot tube connections to the manometer and insert in 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 Ue shown in feet per minute In other types, the manometer will read velocity pressure in inches of wafer and the corresoonaing velocity will be found from the curves in this bulletin If circumstances do not perm,, an accurate traverse, center the phot tube in the duct, determine the center velocity and muiti. . ply by a factor of 9 for the approximate average velocity Field tests run in this manner should be ac. curate within Pius or minus 5'a The velocity indicated is for dry air at 70°F , 29 9" Barometric Pressure and a resulting density of 075;1 cu it For air at a temperature other than 70°F refer to the curves in this bulletin For other variations from these conditions, corrections may be Dosed upon the following data Air Velocity= 1096 2 Pv D where Pv=veiocity pressure in inches of water D=Atr density in r.Icu ft Air Density= l 325 x Pe T where PB =Barometrtc Pressure in inches of mercury T=Absoiute Temperature (indicated temperature 'F plus 460) Flow In cu ft per min =Duct area in square feet x air velocity in ft per min AIR VELOCITY e CALCULATOR PITOTSTAINLESS STEEL lTOT TUBES _— Computes "ioc,ty based a Test conf,rmea unity ca_ on air density corrected efficient anis Irfenme can for conditions of temoera- structlon or N rss�� tore ano ar.ssure Ehm, less sreei I n ch sta+n- gradu- i atas tedious cal cu lat+ons snow death ar �n latrans Ranges from 01 sertion war Prove,s n q to 10 water correseona CamatweS w,e, AMCA anq ,ng to 400 to 4 000 FPM ASHRAM specirrca tions Furnrshea with each aitot S,xes 12 'o a0 ,ny to ae � ffano ar f,aeq m o u n t r n q Yp.s rmtBa in U S A 8 92 Copyright 1992 Owver instruments int ® W V E R INSTRUMENTS , INC. Telephone 2191879.8000 P 0 BOX 373 • MICHIGAN CITY, INDIANA 46360, USA Fax 2191872-9067