OK thinking about it more i guess it's maybe the
density of the gas that matters (
hyrdogen,
helium and
air). The density alters with temperature and pressure so say the temperature stays constant you just need to know the pressure at 1000m and 2000m (which is and respectively).
The air density thing shows you how to calculate it at altitude but i'm not the best with calculations.
Anyways ... i still need help please.