The pressure drop through a particle bed can be used to determine the external sur- face area and the average particle size. Data for a bed of crushed ore particles show Δp/L = 84 (lb_ƒ/in.²)/ft for airflow at a superficial velocity of 0.015 ft/s. The measured void fraction is 0.47, and the estimated sphericity Ф is 0.7. Calculate the average particle size and the surface area per unit mass if the solid has a density of 4.1 g/cm³. How sensitive is the answer to an error of 0.01 in ε?