Aspiration Efficiency and Inlet Wall Deposition in the Fiber Sampling Cassette

Abstract

The 25-mm diameter sampling cassette with a 50-mm long conductive inlet (cowl) is widely used for sampling fibers. Comprehensive testing of the sampling accuracy of this device has not been carried out. Several researchers have found significant fiber deposits in the inlet rather than on the collection filter. The sampler approaches the dimensions of a thin-walled tubular inlet, which has been studied extensively both theoretically and experimentally. A semiempirical model was used to calculate aspiration efficiency and inlet deposition for compact particles. The model was based on particle aerodynamic diameter and should apply with sufficient accuracy to fibers. This semiempirical model includes effects due to particle inertia, gravitational settling, and vena contracta deposition. The results of the model were compared to experimental measurements from a wind tunnel and from previous field studies. The laboratory measurements agree well with the model, but field measurements of fibers show higher inlet deposition than predicted. Some of this additional deposition not predicted by the model may be due tos extrapolation outside the model's original range, but may also be due to electrostatic effects or sample handling. According to the measurements and the model, higher sampling rates generally reduce the biases described here, while higher external wind velocities and higher angles between the ambient air motion and the sampler axis increase the biases. The effect of some mechanisms producing biased samples may be reduced by changes in the sampling protocol.