> Helium is very expensive. I once bough enough to fill three 1meter
> diameter balloons (total of about 1500 liters) and it cost about $25
> US. Another $20 or so for the tank rental. I'm sure it can be had for
> around 1/10th the price if you are a high-volume industrial user but
> this guy isn't.

In that case I would be tempted to use hydrogen ... 
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