I agree re the O2D2, very much, but it does depend on the hassle involved in getting a refill. If you have easy refills, then the constant flow system, with oxymiser cannulas, is actually really good. The 48 cu ft cylinder will probably do 2 people at FL150 all the way down to Crete and back. With the O2D2 it would be 1/3 used. The other advantage of the O2D2 is that if you forget to shut off...