Looking at the theoretical buoyancy change of these 4.4x200 tanks compared to a steel 3x232: ali 4.4 200b = -1.45lb full to +0.42lb at 50bar steel 3 232b = -2.3lb full to -0.74lb at 50bar So the buoyancy change is less for the ali tanks and they are a less negative when full. They won't be pure Al so they might not quite match these figures. Peter