The ISS Is Leaking Air–and NASA and Russia Can't Agree Why

"Earlier this year, NASA elevated the leak to the highest level of risk as the rate of air escaping from the module had doubled from one pound of air per day to a little over two pounds."

How, in a gravity free environment, do you weigh air?

I would assume - knowing literally nothing about it - that the internal atmosphere is constantly monitored. That monitoring detects a change and a lowering of 'air' and so releases 'air' from a reserve and it is the weight of _that_ which is the measurement?