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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 wi

by lhecker 10y ago

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.

thaumasiotes 10y ago
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?
- lhecker 10y ago
  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.
  - thaumasiotes 10y ago
    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.
    - eru 10y ago
      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.)
- Natanael_L 10y ago
  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.
- wang_li 10y ago
  Aren't you confusing buoyancy and weight?
  - thaumasiotes 10y ago
    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.
freehunter 10y ago
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.
- thaumasiotes 10y ago
  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.
  - freehunter 10y ago
    I was told there would be no math.
    - thaumasiotes 10y ago
      > 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.
      - freehunter 10y ago
        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.
dschuler 10y ago
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.
- thaumasiotes 10y ago
  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 https://en.wikipedia.org/wiki/Buoyancy
- lhecker 10y ago
  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.

by lhecker 10y ago

thaumasiotes 10y ago

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?

lhecker 10y ago
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.
- thaumasiotes 10y ago
  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.
  - eru 10y ago
    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.)
Natanael_L 10y ago
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.
wang_li 10y ago
Aren't you confusing buoyancy and weight?
- thaumasiotes 10y ago
  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.

freehunter 10y ago

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.

thaumasiotes 10y ago
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.
- freehunter 10y ago
  I was told there would be no math.
  - thaumasiotes 10y ago
    > 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.
    - freehunter 10y ago
      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.

dschuler 10y ago

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.

thaumasiotes 10y ago
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 https://en.wikipedia.org/wiki/Buoyancy
lhecker 10y ago
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.