Measure the volume of a lighter than air balloon

[psuedoben]

hello, i am trying to calculate the volume of a balloon (which is quite large). It has been filled with helium via a valve connecting helium storage tanks to the balloon. The knowns I have are the volume of the storage tanks, the intital pressure in the tanks, and the final pressure in the tanks after filling the balloon. I am confident there are enough knowns here to solve this problem and my gut feeling is that its a pretty straightforward solution too. despite this, i cannot seem to solve it.

 

[Nidum]

Measure the balloon ?

 

[psuedoben]

I'm sorry Nidum, I suppose I left out a key detail. I don't want to fully inflate the balloon. I want to inflate it to a percentage of its maximum volume and then confirm that I've achieved that percentage by doing this calculation

 

[Baluncore]

What is the pressure in balloon? Is the balloon envelope elastic and under tension with greater than atmospheric pressure, or is the balloon a bag with a slack surface at 1 atm?
What is the balloon temperature? As He is transferred it will cool, so volume will be less until it equilibrates.
Is the pressure in the cylinder measured absolute or gauge, above atmospheric?

Method 1.
You could weigh the cylinders, fill the balloon, then weigh the cylinders again. The weight reduction will give the moles of He transferred. Convert moles to volume at balloon temperature and pressure.

Method 2.
Absolute pressure in cylinders will be proportional to helium content. PV=nRT.
Pressure reduction will give moles of He transferred = volume of balloon, adjusted for pressure and temp.