Example Calculation of Air Velocity from Pitot-static Tube Measurement:
Given: A pitot-static tube via a manometer measures a dynamic pressure of 0.201 in H2O in a
wind tunnel. The ambient temperature and pressure are 72.0 ˚F and 14.55 psia.
Find: Calculate the air velocity in ft/s
Assume: Steady, incompressible, isentropic flow of air; good alignment of pitot-static tube
(streamlines parallel to stagnation tap and perpendicular to static tap); air as an ideal gas
Analysis:
Convert pressure from in H2O to lbf/ft2 using Equation 1-6 and Table A.7* for ρ H 2O at 72.0 ˚F.
( pT − p ) = ρH O g h
2


( pT − p ) = 1.93
€
 1 ft   1 lbf€ 
slug  
ft 
32.174
0.201
in



(
)


2
ft 3  
s2 
 12 in  1 slug ⋅ ft/s 
( pT − p ) = 1.040 lbf /ft 2
which is the dynamic pressure
€ Calculate density of air at patm = 72.0 ˚F, Tatm = 14.55 psia using ideal gas law where R is the
universal gas constant for air from Table A.6*.
€
p
ρ air = atm
R Tatm
2
ρ air
f
f
€
2
 1 slug
(14.55 lb /in ) (12 in 1 ft)
=

(53.33 ft ⋅ lb lb ⋅ R) [(72.0 + 459.69) R]  32.174 lb
m




m
ρ air = 0.002297 slug/ft 3
€ Calculate air velocity using Equation 1-5.
€
V=
2 ( pT − p )
ρ air
2 (1.040 lbf /ft 2 ) 1 slug ⋅ ft/s2 
V=


(0.002297 slug/ft 3 )  1 lbf 
€
V = 30.1 ft/s
€*
€
Fox, R. W., Pritchard, P. J., and McDonald, A. T., Introduction to Fluid Mechanics, 7th edition, John Wiley & Sons,
Inc., 2009.