The enhanced sensitivity of pitot tubes at low Reynolds number

Pitot tubes are commonly used for measuring flow velocity in air and other fluids. The aim of this experimental study was to link flow conditions and geometry to the differential pressure response (zip) of pitot tubes. Based on outer diameter the pitot tube Reynolds number (ReD) ranges from less than 1 (molecular regime) to more than 100 (inertial regime). This large range in Reynolds number was achieved by varying the pitot tube outer diameter in the range 0.4-10 mm, the air density from 0.006 to 1.2 kg/m3 and the air velocity from 0.25 to 30 m/ sand by operating pitot tubes within a recirculating low pressure wind tunnel down to a pressure of 0.5 mbar as well as performing comparative observation in an open circuit wind tunnel. For the smallest ReD, viscous forces enhance zip giving the smallest micro-pitot tubes superior functionality especially at low pressure. The small diameter of the micro-pitot tubes can also allow measurements close to a wall within the boundary layer. Measurements of zip for ReD between approximately 0.08 and 1000 agree well with an analytical model derived by Muriel Barker in 1922 while they agree less well or even poorly with more recent models. At the lowest ReD the maximum enhancement of zip was observed to be a factor of 65. The enhancement became undetectable for ReD larger than approximately 70. At the lowest air density and the smallest pitot tube inner diameter possible effects due to slip were observed.