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It's buoyancy rather than weight. A full HP100 is about -8.4 lbs and at 500 psi it is -0.6 lbs. A full AL80 is about -1.8 lbs and at 500 psi it is about +3.2 lbs. That approximate 4 lb difference can be taken off your weight.They have the same buoyancy shift per CF drop. The difference is the weight of the steel tank material being inherently more negative than AL. You still use ~8 lbs of air but the steel tank remains neg where the AL gets floaty.
Faber steel 100 weighs 35lb empty
Catalina AL 80 weighs 31lb empty