CORRELATION OF MASS-TRANSFER RATES FOR A DIFFUSIVE SAMPLER WITH AIR SPEED AND INCIDENCE ANGLE

An accurate measurement of a gas concentration in air by diffusive sam pling requires knowing the sampling rate. Both the boundary layer betw een turbulent ambient air and the sampler and the stagnant air layer i nside the sampler impose resistance to the transport of analyte into t he sampler. As the boundary layer mass transfer resistance is a functi on of the air speed and direction of the air movement, the sampling ra te also depends on these variables. By the procedure developed here, t he boundary layer mass transfer resistance was accurately measured as a function of wind speed and direction, and from these data a basic co rrelation with dimensionless parameters describing mass transfer was o btained. Deviation of air incidence angle and speed during sampling fr om the calibration conditions may produce a small positive bias, proba bly not in excess of 10%. Random variation of incidence angle and air speed while the sampler is in use may also contribute to the variabili ty of this sampling method.