> I can weigh a chicken breast sitting on top of a plate, because I know how much a plate weighs and I can just subtract it from the total weight.

Not only is this math, we know that it's incorrect for the problem under discussion. The fact that it follows "I understand, thanks!" does not fill me with confidence in the original explanation.

Are you implying that helium weighs nothing? That it's unaffected by gravity? What's actually going on is helium is lighter than air, so the pressure of the air around it pushes it up. Just because it rises at ground level does not mean it weighs nothing. I can jump, but when my feet are off the ground it does not mean gravity has stopped pulling me down, it just means I have moved upwards with enough force to overcome the effects of gravity. Eventually my upward momentum will slow enough for gravity to pull me back down. Same thing with helium, eventually it gets high enough that the neutral air molecules are sparse enough that it can float on top. It doesn't continue to rise past that point because gravity is still pulling it down.

So what is the weight (that is, what is the force of gravity pulling down the helium molecules without ambient air pushing it up) of helium? If you have a cylinder of a known weight, you can fill it with helium and subtract the weight of the cylinder. What's left is the weight of the helium. To measure this effect without air pressure pushing the helium up, you might need to do this in a vacuum, but the logic is there and I think my analogy fits. I weigh the chicken on a plate and subtract the plate to find the weight of the chicken.

What I was asking is "how do you weigh a gas without it spreading around and mixing with the air?" "Putting it in a cylinder" is the answer I was looking for.

> What I was asking is "how do you weigh a gas without it spreading around and mixing with the air?"

That's a rather different question than you actually asked, "how do you weigh something that is lighter than air?" You might notice that not a single response takes the perspective that what makes the problem difficult is that helium will spread out or mix with the atmosphere. Lots of gases (not to mention liquids!) are heavier than air while still being fluid; weighing them doesn't pose the same problems.

> Same thing with helium, eventually it gets high enough that the neutral air molecules are sparse enough that it can float on top. It doesn't continue to rise past that point because gravity is still pulling it down.

This isn't true at all. https://en.wikipedia.org/wiki/Atmospheric_escape

>You might notice that not a single response takes the perspective that what makes the problem difficult is that helium will spread out or mix with the atmosphere.

Well that's not true. The very first response, the one that started this conversation, did exactly that. "Put it in a cylinder". Boom, done. That's why I said "good answer". That's the answer to the question I had asked. You can weigh hexafluoride by putting it in a bowl because it is so dense it will not float away. Helium is the opposite. That's what I wanted to know. Because obviously helium will just float away.

This is a complicated question obviously, and goes deeper than what I asked. But you and I are asking two different questions, and for some reason you seem hell bent on just shitting on everyone else's answers with poor science and snark instead of a simple Google search that takes like 5 seconds and gives you literally all the answers you could ever want.

> Earth is too large to lose a significant proportion of its atmosphere through Jeans escape. The current rate of loss is about three kilograms (3 kg) of hydrogen and 50 grams (50 g) of helium per second.

Seems true to the tune of whatever exists minus 50 grams per second.

Earth is too large to lose a significant portion of its atmosphere. It can (and does!) easily lose a significant portion of its helium.

50g of helium per second would exhaust the current level of helium in the atmosphere in about 2.3 million years. (Atmospheric total mass 5.15e18 kg from wikipedia, composition of the atmosphere by volume from http://eesc.columbia.edu/courses/ees/slides/climate/table_1.... .) That may sound like a long time, but consider that the earth is 4500 million years old.

Maybe I'm not understanding your proposal with the cylinder, but IIRC, it takes about a cubic foot of helium at standard temperature and pressure to lift an ounce. Let's call it 3.5 cubic meters per kilogram in Euros. That means your cylinder has an inner volume of 7 cubic meters, which is silly, so it sounds like you're talking about a normal sized cylinder with compressed helium in it, which is heavier than air and which you'll be able to weigh on a scale just like anything else. (More or less.)

Those numbers were made up to illustrate a point. I have no idea how buoyant helium is at STP and didn't intend to conform to any realistic scenario. "7 cubic meters at STP" is a perfectly fine empirical answer, and I guess you can get the mass of 7 cubic meters of helium from the ideal gas law.

The point is, it makes no sense to say "the weight of the helium must be equal to the weight of the helium+cylinder system minus the known weight of the cylinder, just as the weight of a chicken on a plate is equal to the weight of the chicken+plate system minus the weight of the plate". You've got to bring in some extra information.