“Ullage” means the empty volume of a gasoline storage tank system that contains liquid gasoline. For vapor recovery systems, ullage is expressed as accumulated gallons of empty volume for all of the gasoline storage tanks in a certified vapor recovery system.

Source. (See Revision Note #1 and Revision Note #2 at chapter heading for Env-Or 500) #10229, eff 11-17-12; ss by #13458, eff 10-7-22 (formerly Env-Or 502.35)