Said of a fan with an orifice comparable to the equivalent orifice of a mine; i.e., the area in a thin plate that requires the same pressure to force a given volume of air through as is required to force the same volume through the fan. Orifice of passage O = 0.389 Q/w.g.f., where Q = volume of air passing in thousands cubic feet per minute; w.g.f.= loss of pressure in the fan in inches of water gage.