# Buoyancy - (helium in a sealed vessel)

• men5j2s
In summary, adding mass to a container increases the mass inside the container and total mass of container and contents.
men5j2s
TL;DR Summary
1atm of air + 1atm of HE in a 17.5ml vessel = a mass gain of ~2.9mg
Hi All,

I'm trying to answer a question once and for all that has caused more debate than it ever should have (talking about an internal debate)...

If I fill a sealed vessel (say an aluminium vessel with appox. 17.5ml of internal volume) with varying amounts of helium ( 1bar, 2bar, 3bar ... 10bar), will I see a net mass gain or loss?

My current understanding is as follows:
• The sealed vessel is displacing a volume of air (assuming a vacuum), and therefore will have some buoyancy! (approx -2.3mg)
• The weight of the vessel is larger, so it won't float, but the buoyancy effect can be measured!
• Adding air to 1atm (~1bar absolute) inside the vessel, means density equilibrium inside and outside the vessel, so no buoyancy effect!
• Adding helium to 2atm (on top of the 1atm of air) will simply add more moles to a closed system that is already overcoming any buoyancy effect, by virtue of the 1atm of air that was in there, therefore there will be a net mass gain.
• I should end up with a weight gain of ~2.9mg

men5j2s said:
If I fill a sealed vessel (say an aluminium vessel with appox. 17.5ml of internal volume) with varying amounts of helium ( 1bar, 2bar, 3bar ... 10bar), will I see a net mass gain or loss?
Yes, adding mass to a container increases the mass inside the container and total mass of container and contents.

I have occasionally heard where people think buoyany is itself a property of a gas like helium, so adding helium to a container adds buoyancy. Nope.

Bystander

men5j2s said:
My current understanding is as follows:
• The sealed vessel is displacing a volume of air (assuming a vacuum), and therefore will have some buoyancy! (approx -2.3mg)

Correct. The buoyant force is the weight ##mg## of the displaced air, which I'll take your word for it has mass 2.3 mg.

Note that this depends only on the volume of the vessel. It doesn't matter what's inside it: vacuum, helium, air, or solid iron.

men5j2s said:
• The weight of the vessel is larger, so it won't float, but the buoyancy effect can be measured!

The buoyancy effect can be measured by comparison to the weight measured in other media. In vacuum, you should get a value with is 2.3 mg heavier.

men5j2s said:
• Adding air to 1atm (~1bar absolute) inside the vessel, means density equilibrium inside and outside the vessel, so no buoyancy effect!
There will always be a buoyancy effect in the sense of the weight when immersed in air appearing to be 2.3 mg less than the weight in vacuum. I think you are thinking about neutral buoyancy, where the buoyant force is equal to the weight of the object. But that won't be true here with a steel vessel. It's still a lot heavier than the displaced volume of air.

men5j2s said:
• Adding helium to 2atm (on top of the 1atm of air) will simply add more moles to a closed system that is already overcoming any buoyancy effect, by virtue of the 1atm of air that was in there, therefore there will be a net mass gain.
• I should end up with a weight gain of ~2.9mg

Whatever the buoyant force was, adding mass will cause a net mass gain.

russ_watters
RPinPA said:
I'll take your word for it has mass 2.3 mg.
I'm pretty sure that is off by a factor of 10. Sea level air density is ~1.225 kg/m3 which corresponds to 1.225 mg/ml; times 17.5 ml would be 21.4 mg.

Not that that affects anything said, really.

russ_watters
gmax137 said:
I'm pretty sure that is off by a factor of 10. Sea level air density is ~1.225 kg/m3 which corresponds to 1.225 mg/ml; times 17.5 ml would be 21.4 mg.

Not that that affects anything said, really.

1.3 g/L * 0.0175 L (or 17.5ml) = 0.02275g (or 22.75mg)

You are correct, I made an error in transposing (that's why we Verify ), well spotted!

Yeah, I have spent nearly half my working career checking calculations done by colleagues (nuclear power is all about verifying). I've gotten pretty good at picking out "that doesn't look right" values.

men5j2s and russ_watters

## 1. How does helium affect buoyancy in a sealed vessel?

Helium is less dense than air, so when it is contained in a sealed vessel, it creates a positive buoyancy force that causes the vessel to float. This is because the upward force of the helium is greater than the weight of the vessel and its contents.

## 2. How does the size of the sealed vessel affect the buoyancy of helium?

The size of the sealed vessel does not affect the buoyancy of helium. As long as the vessel is completely sealed and airtight, the buoyancy force of helium will remain the same regardless of the vessel's size.

## 3. Can other gases besides helium be used to create buoyancy in a sealed vessel?

Yes, other gases such as hydrogen and methane can also create buoyancy in a sealed vessel. However, these gases are more flammable and potentially dangerous than helium, so it is not commonly used for this purpose.

## 4. Is there a limit to how much helium can be used in a sealed vessel for buoyancy?

Yes, there is a limit to how much helium can be used in a sealed vessel for buoyancy. This is because the buoyancy force of helium is dependent on its volume and density, so if too much helium is added, it can cause the vessel to become unstable and potentially burst.

## 5. How does temperature affect the buoyancy of helium in a sealed vessel?

Temperature does not have a significant effect on the buoyancy of helium in a sealed vessel. However, if the temperature changes, it can cause the density of the helium to change, which can in turn affect the buoyancy force. This is why it is important to maintain a constant temperature when using helium for buoyancy in a sealed vessel.

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