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Pitot-Static Tube Air Velocity Calculator and Equation

Fluid Flow Engineering and Design

Pitot-Static Tube Velocity of Fluid Flow Measurement Calculator and Equation

The pitot-static tube, in conjunction with a suitable manometer or differential pressure transducer, provides a simple method of determining air velocity at a point in a flow field.

Eq. 1
V = C [ ( 2 · pw · gc / ρ ) ]0.5

where

V = velocity of the fluid (ft / sec)
pw = velocity pressure (pitot-tube manometer reading) (inches of water)
gc = gravitation constant (32.174 lbm-ft / lbm-sec2)
C = unit conversion factor = 136.8
ρ = density of air, lbm/ft3

The type of manometer or differential pressure transducer used with a pitot-static tube depends on the magnitude of velocity pressure being measured and on the desired accuracy. Over 1500 fpm, a draft gage of appropriate range is usually satisfactory. If the pitot static tube is used to measure air velocities lower than 1500 fpm, a precision manometer or comparable pressure differential transducer is essential.

Uncertainty estimate.

The typical bias (i.e., calibration) uncertainty of the pitot tube may be 1 - 3% and is dependant on a variety of factors.

Eq. 2
uv = [ ( uV, bias)2 - ( uV, prec)2 ]0.5

Eq. 3
uv = [ ( uV, bias)2 - ( ( 0.5 upw )2 ( 0.5 uρ ]0.5 )2 ]0.5

where

uρ = density bias or variation
upw = velocity pressure bias or variation

Source: Reprinted with permission from 2013 ASHRAE Handbook - Fundamentals, ASHRAE: 2013.

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Source

2013 ASHRAE Handbook - Fundamentals, ASHRAE: 2021.