Good point, but why would you switch from an AL80 to a steel 133 (mega of the usual HP steels), if you did not plan on using a lot more gas? There are smaller and/or lighter cylinders if you don't need that much gas. I guess he can add back 0.0807 lbs for every cubic foot not used
Quick estimate: breathe his Catalina AL 80 down to 500 psi, 12.95 cf of gas left, cylinder will be around 3 lbs positive. Breathe a HP 133 down 64.4 cf, you would have 55.6 cf left, cylinder will be about 3 lbs negative, a difference of 6 lbs.
So I revise my weight estimate for
@Diesel_Diver He will be able to take off somewhere between 2 1/2 to 6 lbs, depending on how much gas he uses relative to his AL80.