Huge helium discovery 'a life-saving find'

Re:Huge helium discovery 'a life-saving find' by dschuler on Tuesday June 28, 2016 @03:12PM (#11996617)

If your cylinder is an airship, and you put that on a scale, you'll find that it has negative weight :)

I would think measuring the correct weight would be:

(weight of empty cylinder) + (weight of air volume that the cylinder could contain at atmospheric pressure) - (measured weight - positive or negative)

If you measured e.g. an empty cylinder, you'd end up with only the weight of air volume in cylinder at atmospheric pressure, which would be correct.

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Re:Huge helium discovery 'a life-saving find' by thaumasiotes on Tuesday June 28, 2016 @03:32PM (#11996801)

This is nearly correct. Rather than the weight of air volume the cylinder could contain, you want the weight of a volume of air equal in volume to the cylinder (including its walls) -- and you seem to have the wrong sign on the measured weight and the cylinder weight. Your equation says that the measured weight is equal to the buoyancy force, plus the weight of the cylinder, minus the weight of the helium, which would be weird.

(weight of helium) = (measured weight) + (buoyancy force, which is the weight of an equal volume of air at the appropriate pressure) - (weight of cylinder)

source: https://en.wikipedia.org/wiki/Buoyancy

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Re:Huge helium discovery 'a life-saving find' by lhecker on Tuesday June 28, 2016 @04:04PM (#11997069)

As I've written in another comment:

> 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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