Combine it with CO2 extraction (you'll get CO2 anyway so you might as well do something good with it) and the money to do it might be there.

If we let the market price of helium go up (paired with the carbon credits), could CO2 extraction from the atmosphere become an economically feasible way of counteracting global warming?

I don't know the answer to that, but just wanted to mention you would want the market price to include the cost of storing the captured CO2.

EDIT: never mind. if anyone was buying this at market value, it doesn't make sense for the buyer to store it indefinitely.

How many tonnes of CO2 would you get per ton of He?

Market price for pure CO2 is only about $160 per ton. Market price of helium is like $45,000 per ton or something like that.

Serious question: how do you weigh something that is lighter than air?

I don't think you would actually weigh it. You would put it in a container, then measure the pressure and knowing the density of the gas and the volume of the container you can deduce the mass.

If you want to actually weigh using a scale, then put it in a container, measure the total mass accurately and then subtract the mass of the container and the buoyancy due to the atmospheric pressure gradient. The remainder is the mass of the gas.

Just put it into a gas cylinder (it doesn't even need to be compressed or anything).

ELI5 version: It might be lighter than air, but that just means that it will "float" ontop of heavier things - like oil ontop of water. But all those things still obviously have a certain weight. If you put it into a cylinder it will thus under the influence of gravity still weigh down onto it, which then can easily be measured.

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.

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

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.

Oh I understand, thanks! So like 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.

Great explanation, thank you.

Suppose you have a 40 kg cylinder that you've filled with helium. The cylinder now weighs (g × 38kg) newtons. How much helium do you have in there?

The answer is obviously not "negative two kilograms", and it's not obvious to me that the answer is "two kilograms". After all, if we weighed the same cylinder in a vacuum rather than in the atmosphere, it would presumably be heavier, but the amount of helium inside wouldn't change.

I was told there would be no math.

> 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.

What's the mechanism? It can't just be separation from the atmosphere -- if you fill a balloon with helium, it will get lighter, not heavier. Same thing if you fill a cylinder that sinks in water (perhaps because it was already full of water) with atmospheric air; it may well start to float.

Suppose you have a balance scale with a "cylinder" full of air on one side. You pump out the air and put in helium. What happens to the scale?

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.

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.

Only the difference between vacuum cylinder and helium cylinder matters. (Since the buoyancy in air only depends on the volume of the cylinder, not what's inside.)

An empty cylinder should measure exactly as much lighter than the air as the air weighs. That point is your zero point.

Put in helium and you'll eventually reach and exceed the parity point as you keep filling up (same weight as the air), and the ratio of how much it moves for x volume of additional filled gas tells got the additional weight.

Soon enough you'll reach 2x air weight and you'll have tipped the scale exactly.

Aren't you confusing buoyancy and weight?

What's weight? Mass is well-defined; you could call weight "the force of gravitational attraction between the object and the earth" or you could call it "the force acting upon the object, when at rest". Since we're talking about measuring weight, I was using the second one, which is easy to measure directly.

Granted, I was also using the first sense when talking about the weight of the helium specifically. I guess in that sense my terminology could have been better.