Answer: Helium
At room temperature, helium is about seven times less dense than air; this means that it will rise up in the atmosphere. This gives it a major advantage over nitrogen gas, which is more of an atmospheric mix. Nitrogen has to be combined with other gases before being used in general lighter operations because it needs to have heavier-than-air properties adjusted.
The flow rate of a gas is calculated by the equation RO = -ln(C/Ca), where C is the concentration of the gas in parts per million, and Ca is its concentration in air. The values can be estimated for helium to be about 20,800 liters/second at 20°C, which would be noticeably quicker than oxygen's effusion rate at 5300 liters/second.
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