Helium in a tank?


My payload went up at a slower rate that I had forecasted last time and I know I just got kind of lazy and sloppy.  But even with me adding weight to the payload and not accounting for it the helium should have caused the milk jug to go neutrally buoyant with the amount of gas that I got from the burst calculator.  We ended up emptying the bottle into the balloon which should have been 19 cu ft more than required.  I am guessing the burst calculator was correct.  Is there any reason for me to doubt the amount of helium in the tank as long as I verify the pressure is correct?  Should I ask the welding supply shop about the purity of their helium?

I am trying to be more careful this time


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