Re:Huge helium discovery 'a life-saving find'
by lhecker on Tuesday June 28, 2016 @04:03PM
(#11997057)
That's because the weight of the gas cylinder is relative and you got the point of reference wrong: The base weight is when it's a vacuum inside the cylinder, because otherwise it doesn't even displace the air which you don't want to measure for your reference weight, making it appear heavier. Whatever you add inside the gas cylinder now will make it heavier and the difference you get is the weight of the gas.
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Re:Huge helium discovery 'a life-saving find'
by thaumasiotes on Tuesday June 28, 2016 @04:14PM
(#11997147)
While I appreciate the answer, it's not a great explanation of the balloon example, since a balloon starts out effectively containing nothing. The problem there is that as you add helium, the volume of the balloon increases.
I'm a little more comfortable with saying we do some weighings in a vacuum (say, to determine the density of air at a given pressure) than with saying we'll start with a cylinder that contains a vacuum. For example, our 40kg cylinder, when airtight and containing vacuum, will weigh less than a 40kg object should, and I think that muddles the example.