The unit of measure for the specific volume of air is:

• A. lbs / ft3
• B. btu / lb
• C. lbs /btu
• D. ft3 / lb

Answer: (D)

Specific volume is the amount of space one unit of mass occupies. It is the inverse of density, which is mass per unit volume. For air, density is about 0.075–0.077 lb per cubic foot under typical conditions, so the specific volume is the reciprocal of that, roughly 13 ft3 per pound. That makes the appropriate unit for specific volume ft3 / lb.

Why the others don’t fit: lbs / ft3 is density (mass per volume). btu / lb is energy per mass, a measure of specific energy. lbs / btu is mass per energy. None of these represent volume per unit mass, which is what specific volume describes.