a sealed housing evaporative determination apparatus and method for practically zero emission vehicles provides periodic sampling of ambient air within a substantially air - tight testing structure enclosing a vehicle under test . pressure drops within the testing structure are compensated by injecting air substantially free of hydrocarbons into the testing structure . pressure increases within the testing structure are reduced by withdrawing ambient air from the testing structure . hydrocarbon content of any withdrawn ambient air is determined from the latest sample , and cumulative evaporative emissions are adjusted accordingly .