The only ones I see that are significantly higher than nitrogen/oxygen are methane, hydrogen, and helium.
Helium is mondo-expensive. It's not cheap even in welding-tank quantities, much less in the amounts you'd need here.
Hydrogen is flammable and/or explosive (depending on mixture) and is a huge pain in the butt to keep confined (so is helium).
Methane is cheap (but not as cheap as air :-)), and we have lots of experience with keeping it confined in big pipes. However, it's also flammable or explosive.
Using anything but air is going to require some sort of complicated replenishment/purging system and an air lock at both ends, whereas air leaks just require pumping it out again.
The only ones I see that are significantly higher than nitrogen/oxygen are methane, hydrogen, and helium.
Helium is mondo-expensive. It's not cheap even in welding-tank quantities, much less in the amounts you'd need here.
Hydrogen is flammable and/or explosive (depending on mixture) and is a huge pain in the butt to keep confined (so is helium).
Methane is cheap (but not as cheap as air :-)), and we have lots of experience with keeping it confined in big pipes. However, it's also flammable or explosive.
Using anything but air is going to require some sort of complicated replenishment/purging system and an air lock at both ends, whereas air leaks just require pumping it out again.