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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 · p _{w} · g_{c} / ρ ) ]^{0.5}*

where

*V* = velocity of the fluid (ft / sec)

*p _{w}* = velocity pressure (pitot-tube manometer reading) (inches of water)

*g*= gravitation constant (32.174 lbm-ft / lbm-sec

_{c}^{2})

*C*= unit conversion factor = 136.8

*ρ*= density of air, lbm/ft

^{3}

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

*u _{v} = [ ( u_{V, bias})^{2} - ( u_{V, prec})^{2} ]^{0.5}*

Eq. 3

*u _{v} = [ ( u_{V, bias})^{2} - ( ( 0.5 u_{pw} )^{2} ( 0.5 u_{ρ} ]^{0.5} )^{2} ]^{0.5}*

where

*u _{ρ}* = density bias or variation

*u*= velocity pressure bias or variation

_{pw}

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

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Source

2013 ASHRAE Handbook - Fundamentals, ASHRAE: 2021.