How often does helium have to be added?
The ships lose very little helium in normal operations, although the gas does have to be purified about twice a year by a Goodyear designed purifying machine. As the envelopes age and have a tendency to allow gas diffusion. The crew might have to add 10,000 cubic feet of gas per month. They buy the gas along the tour and add it as needed since none is carried along.
