Lift

I'm currently working on a better method of determining lift based on the psi differential in the He tanks. I have figured out how to use the tank gauge correctly and determined that we used 2600 psi of He for the last launch. From a discussion with a local He tank supplier I found out that a full tank is pressurized at 2200 psi and contains 219 cu ft of He. So roughly 1 cu ft per 10 psi. I'm assuming that the equivalence is constant as the tank's pressure becomes reduced. I did some rough calculations and determined that the diameter of the balloon should have been around 8 ft. This is supported by the pictures of the balloon during full inflation. I'm still exploring the effect the elasticity has on the pressure of the He in the balloon. Our lift was 12.8 lbs but this does not take into account the fill nozzle attached to the balloon. I need to weigh it keeping in mind that one side is connected to the tank. I am also going to shorten the nozzle. I want to get the determination of the actual lift to within a few 10ths of  a lbs. I also want to measure lift at regular psi increases to determine if there is a linear relationship or not with psi and lift. Hopefully this will get the lift calculations exact.