Agreed, liquid is probably the way to go. Very tempting to swap nitrogen for helium, but AIUI the main downsides are that the speed of sound is higher so air w

Agreed, liquid is probably the way to go. Very tempting to swap nitrogen for helium, but AIUI the main downsides are that the speed of sound is higher so air will leak out from a breach faster, and there may be negative effects on flammability due to helium's high thermal conductivity and low specific heat.