Abstract
This study investigated the effect of the speed and direction of air movement and the presence of the human body on sampling rate by changes in the boundary layer mass transfer resistance of samplers. The mass transfer resistance of a modified commercial diffusive sampler was measured mounted on a mannequin and compared with the mass transfer resistance of a sampler in an unobstructed airstream. The latter condition is the normal manner in which diffusive samplers are calibrated, while the former is more representative of field use. Results of the analysis are generalized in terms of dimensionless parameters. The presence of the mannequin produced boundary layer mass transfer resistances from 0.8 to 10 times that of samplers in unobstructed airflow for the range of air speeds and incidence angles studied. Illustrating the impact of the mannequin's presence on sampling performance, the sampling rate for m-xylene at 25°C and 1 atm would range from 0.390 to 0.514 cm3/sec, or ± 13.7% of the midrange value. The range of the boundary layer mass transfer resistance was most pronounced at low air speeds. A speed in excess of 70 cm/sec was necessary to keep the difference between the sampling rates of the mannequin-mounted sampler and the sampler in unobstructed airflow less than 10%.These results suggest that the mass transfer resistance during sampler calibration should be as close as possible to that encountered during sampling.