Large-particle entry efficiencies of the MRE 113A gravimetric dust sampler

The deposit in the elutriator of the MRE sampler is sometimes used a sa rough-and-ready measure of human upper respiratory tract deposition. Entry efficiencies for larger particles have therefore been studied in winds up to 4.5 m/s, mainly by exposing the instrument to monodisperse aerosols in a dust tunnel and examining the elutriator deposit; these efficiencies are compared with those reported earlier for a human head. In calm air, entry efficiency with the standard nosepiece falls below 90% for particles larger than 8 μm aerodynamic diameter, and without a nosepiece efficiency falls to this at 20 μm. With the instrument facing the wind, efficiency falls with increasing particle size and with increasing windspeed, although the change is small beyond 2 m/s, at which speed entry efficiency is about 40% for 7 μm particles. This decline is the opposite of what is usually seen with a blunt symmetrical sampler facing the wind, and is due to the off-centre entry. Efficiencies are higher with the older, close-fitting nosepiece or with no nosepiece at all, and with the instrument side-on to or back-to the wind. It appears that these changes do not greatly affect the respirable dust sample collected on the filter. In the moderate wind speeds found in coal mines, the elutriator deposit is a rough measure of human tracheo-bronchial deposit.