Influence of humidity on hot-wire measurements

Abstract

When applying hot-wire anemometry to velocity measurements in air, it is standard practice to neglect the effect of humidity. In this paper the influence of the thermodynamic and transport properties of humid air on hot-wire measurements is examined on the basis of the correlations for Nusselt number proposed previously by other researchers. Experimental results at controlled levels of relative humidity between 30% and 90% at 30 • C, 50 • C and 70 • C are reproduced satisfactorily by the theoretical approaches of two of these equations. A corrective term is defined to expand formulae designed for dry air to work in a humid environment. The error in velocity by omitting the influence of humidity is estimated in terms of temperature and relative humidity.

LITOS – a new balloon-borne instrument for fine-scale turbulence soundings in the stratosphere

“…In our case, i.e. for stratospheric conditions, the density varies between 1.2 kg/m 3 and ∼ 1.0×10 −2 kg/m 3 , the (relative) wind velocities are up to 2 m/s and the temperature decreases to ∼200 K. The water vapour mixing ratio in the stratosphere amounts to ∼5 ppm and therefore the influence on our measurements is negligible (Durst et al, 1996).…”

“…The crucial point is that the calibration coefficients are only valid if the ambient conditions do not differ from those during the calibration. This concerns not only the wind conditions, but also the density, temperature and humidity (Cimbala and Park, 1990;Cardell, 1993;Durst et al, 1996;Hugo et al, 1999). Thus the calibration should be performed under ambient conditions similar to the conditions during the measurements.…”

“…Re, Gr, the Knudsen number Kn, the Prandtl number Pr and the Mach number Ma. There exist several approaches in the literature for the empirical estimation of the Nusselt number for specific flow conditions (e.g., Collis and Williams, 1959;Cimbala and Park, 1990;Durst et al, 1996). The advantage of those formulations of Nu is that the heat loss from the wire and the dependence on the flow velocity can be obtained without calibration.…”

Integrated sensor cross-sensitivity analysis

“…For the anemometer, sensitivity increased slightly as humidity increased and then decreased by 28% at high humidity levels. Durst et al derived theoretical calculations for the thermal conductivity of air and found that the highest conductivity occurred at medium humidity levels (30 to 40 %RH depending on air temperature) [11]. We think that the initial increase in sensitivity is due to the change in thermal conductivity of the moist air.…”

Design Document for Passive Bioventing

“…Although most commercially available thermal anemometer sensors automatically compensate for changes in air temperature, there is no correction for changes in barometric pressure or air humidity. The error in air velocity due to changes in barometric pressure can be upward of 12% (Johnston and Fleeter, 1997) and upward of 13% due to changes in air humidity (Durst et al, 1996).…”