A fan is a constant volume device. The blades of a fan wheel can be compared to “buckets of water”, each carries a finite volume of air. Dependent on temperature and altitude, a cubic foot of air has a variable mass.
The fan industry has established a standard for listing performance data on a specific fan. That standard is based upon the air / gas density of 0.075 lbs. / ft³. This is the density of air at 70 °F at sea level (29.92” Hg).
