How much air are you moving?
When it seems that there isn't enough air in your pneumatic conveying system to move your material, you'd like to measure the system's air volume to help you identify the problem: Is there air leakage in the system? Is something worn? Was the system designed correctly? If you could check how much actual air volume the system is using, you'd know the answers!
But measuring the air volume in a pneumatic conveying system is difficult. You can't just collect the air for 1 minute and say you have a certain weight of air, because its weight per unit volume (or density) changes greatly with pressure and temperature. A liquid, on the other hand, is much easier to measure because its density barely changes with pressure and temperature changes, allowing the liquid to be collected, weighed, or measured volumetrically.