# B Mixing H with He for blimps

#### DaveC426913

Gold Member
Saw the saddest thing in the world today: party balloons peeking out over the top of a dumpster.

I started thinking about the buoyancy of H versus its flammability. H is 6-7 times more buoyant than He. It would be more more economical to use H if we could eliminate the explody bit.

If you can find an expedient way of keeping them from separating, could you mix He with H sufficiently that the H could be rendered non flammable? What concentration of He mixed with it might make the H non flammable?

(Is my logic here correct? Fuels like propane gas need a sufficient concentration before they will ignite. Is it sufficient to keep the H diffused in an inert gas so that it will not explode in the presence of oxygen and a spark?)

Related Classical Physics News on Phys.org

#### willem2

I started thinking about the buoyancy of H versus its flammability. H is 6-7 times more buoyant than He. It would be more more economical to use H if we could eliminate the explody bit.
In air at Standard temperature and pressure, H is less than 10% more buoyant than He. 1.12 Kg/m3, and He 1.04Kg/3. Of course the density of H2 is half that of helium, but the difference with the density of air is what matters.

#### Nugatory

Mentor
Hydrogen burns in a very wide range of ratios with oxygen, so I don't know how effective this technique would be. But quantitative analysis always beats handwaving, so I'd be interested in hearing from someone who can caculate the lift that we'd get from a significantly less easily ignitable H/He mix.

Staff Emeritus
The lower explosive limit of hydrogen is 4%, so your gas will be mostly helium anyway.

#### DaveC426913

Gold Member
The lower explosive limit of hydrogen is 4%, so your gas will be mostly helium anyway.
Wow. OK.

In air at Standard temperature and pressure, H is less than 10% more buoyant than He.
I thought H was 7 times more buoyant!
Oh wait. Or is it 1/7th more buoyant..

Oh for Pete's Sake.

#### jbriggs444

Homework Helper
I thought H was 7 times more buoyant!
Oh wait. Or is it 1/7th more buoyant..
Back of the envelope...

Air has an average molecular weight somewhere between 28 (diatomic nitrogen) and 32 (diatomic oxygen). At an 80/20 mixture it'll be a tad under 29.

Diatomic Hydrogen comes in at 2. The net buoyancy when displacing air is 27.

Monatomic Helium comes in at 4. The net buoyancy when displacing air is 25.

So we are talking a delta of two parts in 25. [Or two parts in 27 depending on whether you are calculating "percent more" or "percent less"].

Which fits pretty well with the 10% figure suggested by @willem2

#### Dullard

The more significant advantage of Hydrogen (for a balloon application) might be that it doesn't leak quite as readily (larger molecule). It bounces around faster than Helium (at equal temps), but needs a larger hole to get out of the envelope. I think.

#### gmax137

The lower explosive limit of hydrogen is 4%, so your gas will be mostly helium anyway.
Not that it changes your conclusion, but 4% is the flammability limit; detonation is more like 17 or 18%. Interestingly, you can add quite a bit of steam to the mix (over 50%) before the 4% flammability limit changes much.

#### DaveC426913

Gold Member
The more significant advantage of Hydrogen (for a balloon application) might be that it doesn't leak quite as readily (larger molecule). It bounces around faster than Helium (at equal temps), but needs a larger hole to get out of the envelope. I think.
I think it's the other way around. He leaks less than H.

#### jbriggs444

Homework Helper
I think it's the other way around. He leaks less than H.
A quick trip to Google suggests that H diffuses through gasses faster (due to its low molecular weight and correspondingly high thermal velocity) but that He leaks through solid barriers faster (due to its small size, being monatomic rather than diatomic).

See, for instance, https://aip.scitation.org/doi/10.1063/1.1750133 where He is said to diffuse through glass 45 times more rapidly than H does.

Last edited:

#### cjl

I think it's the other way around. He leaks less than H.
Helium is a significantly smaller atom than hydrogen (same number of orbitals with electrons in them, but stronger nuclear charge to pull the electron cloud closer), and hydrogen is diatomic. Because of this, helium diffuses through solids much, much faster than hydrogen.

#### skeptic2

Why not just put a hydrogen balloon inside a helium balloon?

#### Jon Richfield

Firstly, I reckon you can forget about separation of the gases inside the balloon. They will not separate spontaneously any more than the CO2 and SO2 in the atmosphere sink down to smother us.

Secondly, The buoyancy of gases depends on how much of your lifting gas it takes to displace its own mass of air plus the weight of the intended vehicle plus cargo. One can add surprising volumes of fairly weakly buoyant gas to H2 or He (or He-3 ) before your mix becomes unfavourable. I often have wondered why they don't use H2/NH3/H2O in outer container bags around round internal bags of H2.

That should prevent casual explosions and sparking...?

I hate the idea of He in balloons only a little more than He in dirigibles.

#### David Lewis

When He leaks out of a blimp or balloon (diffusion), it's a minor nuisance because He can be topped up easily. The big problem is when air leaks in (infusion). Then you have to renew the entire volume of helium or purify it via cryogenic fractional distillation.

The small buoyancy advantage of H over He can be significant. If you have two otherwise identical blimps, and the lift of the He blimp is 100g then the gross lift of the H blimp will be about 110g. If the empty weight is 90g then the net lift will be 10g for He and 20g for H.

#### DaveC426913

Gold Member
Firstly, I reckon you can forget about separation of the gases inside the balloon. They will not separate spontaneously any more than the CO2 and SO2 in the atmosphere sink down to smother us.

We don't get smothered by CO2 or SO2 because the atmo is constantly being stirred up. Inside a closed space such as a balloon, the gases may initially diffuse, but if left long enough without disturbance, I think they should separate - like CO2 in a tank - even an open-topped tank - will pool at the bottom.

#### JT Smith

...CO2 in a tank - even an open-topped tank - will pool at the bottom.
I don't think that's true unless you have a very liberal definition of "pool" or a really tall tank. I believe that significant separation occurs on a scale of tens of kilometers.

#### DaveC426913

Gold Member
I don't think that's true unless you have a very liberal definition of "pool" or a really tall tank. I believe that significant separation occurs on a scale of tens of kilometers.
No. It literally is done in an aquarium. You put candles on the bottom, and pour in CO2. It pools on the bottom. Candles set several inches higher do not get snuffed out.
It's a common lab demo.

This one uses both candles and bubbles.

#### Jon Richfield

We don't get smothered by CO2 or SO2 because the atmo is constantly being stirred up. Inside a closed space such as a balloon, the gases may initially diffuse, but if left long enough without disturbance, I think they should separate - like CO2 in a tank - even an open-topped tank - will pool at the bottom.
Not thus, but far otherwise. Buoyancy in gases is a bulk characteristic rather than a molecular one. It is perfectly true that if I have a gas of density D over a layer of density L (for light) and they are separated by a membrane that I then suddenly remove, then there will be a great slopping and convection and temperature effects, and if the container is large, it will be some time in settling, and meanwhile there will be a lot of dense gas low down and so on. Vice versa if the gases start out with L on top, then if I remove the membrane without noticeable disturbance, they will stay put till diffusion does its work.

BUT:

Firstly diffusion does do its work eventually. Ideally as a first approximation the molecules do their work independently, and each component of the total gas content zips around as if the other gas is not there, so one would expect practically instant mixing. In practice however, we both know that this only would be true of ideal gases at low pressure, and the rate of mixing is limited by mutual interference and mean free paths and drunken walks etc. In other words diffusion, which is comparatively slow.

None the less slow or not, it is effectively one-way, and the upshot is that a mix of He, H2, Ne, Ar,Kr, and Xe will not separate out, even though the molecular masses range from 2 to about 131.

The reason that CO2 pools in say, fermentation tanks (very dangerously!) is not because it settles out, but because it was generated there. Leave your tank open for long enough, and it equilibrates with the atmosphere. Pour a mix of air and CO2 into your tank, and it remains a mix from now until the Big Crunch, as long as you do not freeze out the CO2 as dry ice (or liquid CO2 if the pressure is high enough!)

There are some cases where gases can repel or attract each other at the molecular level, which queers the pitch, but that does not affect the case at our level. The reason for the stratification in your neat little fish-tank demo is again that your bulk CO2 got generated low down. If you wait long enough your bubbles would not float at any special level. Long enough could be quite long of course, but for a proper experiment, add a little fan, run it till the gas mix is even, then leave the setup for as long as you like before introducing more bubbles. The sun will be a red dwarf before there is any settling out of the CO2 at shirt-sleeve temperatures and pressures.

Last edited:

#### Jon Richfield

The small buoyancy advantage of H over He can be significant. If you have two otherwise identical blimps, and the lift of the He blimp is 100g then the gross lift of the H blimp will be about 110g. If the empty weight is 90g then the net lift will be 10g for He and 20g for H.
Significant for what? Yes, every gram of added mass affects the buoyancy, but the value of the light gas compared to the not-so-light gas is not the 2:1 difference of H2e, but the amount of air it displaces minus the load of the lifting gas. To pay for that is necessary, but can be done by enlarging the volume of your bags, which can be done cheaply. Think of hot air balloons; their lifting gas is nearly as dense as air, eppur si muove!

If we replaced He with enough H2 plus adulterant to match the He buoyancy, but without the difficulty of He losses, we could get a very good improvement just by heating the gas with our engines' waste heat.

#### epenguin

Homework Helper
Gold Member
Are there no materials/engineered structures, possibly with self-resealing properties, light and strong enough that you could just pump away the air and create low-density inside the balloon without filling it with any other gas ?

#### Jon Richfield

In a word, no. Not a hope ever. If it were light enough, it would not be strong enough. And if there were, it would only be slightly better than H2. H2 can lift about 15 times its own mass.

Actually, if we could create and contain an electron gas, we could make a balloon that could lift practically as much as the same volume of vacuum would, but if we could do that, I doubt we would bother with balloons etc. And it would be like flying in a bomb.

#### DaveC426913

Gold Member
The reason that CO2 pools in say, fermentation tanks (very dangerously!) is not because it settles out, but because it was generated there. Leave your tank open for long enough, and it equilibrates with the atmosphere. Pour a mix of air and CO2 into your tank, and it remains a mix from now until the Big Crunch, as long as you do not freeze out the CO2 as dry ice (or liquid CO2 if the pressure is high enough!)
So, lab demos where people are pouring CO2 out of a beaker over a candle to snuff it out are ... what?

#### jbriggs444

Homework Helper
So, videos where people are pouring CO2 out of a beaker over a candle to snuff it out are ... what?
Irrelevant.

The claim was that gravity trumps diffusion and leaves as a result a stratified arrangement. The fact is otherwise. The supposed demonstration otherwise is irrelevant. It only serves to demonstrate that diffusion is not immediate.

That is not to say that stratification does not occur. However it is not relevant in room or acquarium-sized settings. Here is a link.

Last edited:

#### Jon Richfield

So, lab demos where people are pouring CO2 out of a beaker over a candle to snuff it out are ... what?
They are the pouring of bulk cold CO2; try it on warm CO2 someday
Remember what I said about buoyancy being a bulk effect.
If you make it cold and plentiful enough, you could get the same effect with nitrogen,
and remember that N2 is marginally less dense than air at the same temperature and pressure.

#### DaveC426913

Gold Member
The claim was that gravity trumps diffusion and leaves as a result a stratified arrangement.

OK I see the nuances.
I was thinking that gases starting off mixed would eventually stratify.
It seems that gases will diffuse - even if they start off stratified, for whatever reason.

They are the pouring of bulk cold CO2; try it on warm CO2 someday
T̶h̶e̶ ̶C̶O̶2̶ ̶i̶n̶ ̶t̶h̶e̶ ̶v̶i̶d̶e̶o̶ ̶w̶a̶s̶ ̶f̶o̶r̶m̶e̶d̶ ̶f̶r̶o̶m̶ ̶m̶i̶x̶i̶n̶g̶ ̶b̶a̶k̶i̶n̶g̶ ̶s̶o̶d̶a̶ ̶a̶n̶d̶ ̶v̶i̶n̶e̶g̶a̶r̶.̶ ̶U̶n̶l̶e̶s̶s̶ ̶i̶t̶'̶s̶ ̶a̶n̶ ̶e̶n̶d̶o̶t̶h̶e̶r̶m̶i̶c̶ ̶r̶e̶a̶c̶t̶i̶o̶n̶,̶ ̶i̶t̶'̶l̶l̶ ̶b̶e̶ ̶r̶o̶o̶m̶ ̶t̶e̶m̶p̶.̶ ̶A̶c̶t̶u̶a̶l̶l̶y̶,̶ ̶I̶ ̶t̶h̶i̶n̶k̶ ̶i̶t̶'̶s̶ ̶e̶x̶o̶t̶h̶e̶r̶m̶i̶c̶,̶ ̶s̶o̶ ̶i̶t̶ ̶s̶h̶o̶u̶l̶d̶ ̶b̶e̶ ̶h̶o̶t̶t̶e̶r̶ ̶t̶h̶a̶n̶ ̶r̶t̶.̶

nm, it's endo.

"Mixing H with He for blimps"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving