One cubic foot is one cubic foot, so one cubic foot of water is the same as one cubic foot of air, one cubic foot.

But air is compressible while water, for practical purposes, is in-compressible. ■

Reminds me of "Which is heavier: a pound of feathers or a pound of lead?"■

The answer is then (based on designer's remark):

1 cubic foot of air of the same pressure as the water.

For calculating the scf, the temperature must be accounted for also.

Teus ■

Originally Posted by Teus Tuinenburg

1 cubic foot of air of the same pressure as the water.

Maybe to expand on Teus, at the same pressure as the water needs to be when it's being pumped. ■