Kurz 454FTB - Velocity Probe & the Pitot Tube

KurzHardwareReferenceGuide D–15
Calibration
Velocity Probe & the Pitot Tube
TheclassicPitottubeisavelocity‐sensingdevicebasedontheprincipleofconvertingkineticenergyofa
flowstreamtopotentialenergyasapressurebystoppingtheflow.Ithasanimpactportandastaticport
thatdonotreceiveanykineticcontribution.TheclassicPitottube
hasavelocity‐dependentcorrection
factorKof1.0bydefinition.
TheS‐TypePitottubehasanupstreamimpactportanddownstreamportunderslightvacuumthat
generatesahigherdifferentialsignalandimprovestheturn‐downratio.Thisincreaseddifferential
pressure(dP)comesatthepriceofacorrectionfactorK(whichisvelocity‐dependent),sothedevicemust
becalibratedagainsttheclassicPitotdesign.
ThebasicPitottubeequation(inactualvelocity)isasfo llows:
v=k(2gdP÷)
0.5

where:
k isthecorrectionfactor,1.0forclassicPitotand0.84forS‐typePitot.
g istheaccelerationofgravity,32.2ft/s
2
.
dP isthedifferentialpressurebetweentheimpactandstaticportsinlb/ft
2
.
 isthedensityofthegas,lb/ft
3
.
AdditionalcalculationsinvolveconvertingthedPunitstoinchesofwaterandconvertingthegasdensity
toafunctionofpressureandtemperature.Theresultsareconvertedtostandardvelocityinsteadof
actualvelocity.
Thefollowingformulaconvertsgasdensitytopressureandtemperature:
=
s(Ts÷Ta)(Pa÷Ps)
where:

s isthestandardgasdensity,0.07387lb/ft
3
.
MW isthegasmolecularweight,MW=28.96.
Ts is77°For25°Cinabsoluteunitsso460+°For273.15+°C.
Ta istheactualtemperatureinabsoluteunits.
Pa istheactualpressureinabsoluteunits,inPSIAorHg.
Ps isthestandardpressurewhich
is101.325kPa,14.69PSIA,760mmHgor29.92
inHg.
Thefollowingformulaconvertsthedifferentialpressure(dP)frominchesH2Ointolb/ft
2
:
inchesH
2O÷(27.68inH2O÷lb÷in
2
)(144in
2
/ft
2
)=(144÷27.68)lb/ft
2
Usethefollowingformulatomakethefinalcalculationtoconvertstandardvelocitytoactual velocity:
V=k[dP(144÷27.68)(2)(32.2)÷(MW0.07387÷28.96)(Ts÷Ta)(Pa÷Ps)]
0.5
(60)