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Helium is expensive, and getting more so. Hydrogen is cheap. Unfortunately, hydrogen is deemed unsuitable material for use in airships because of its flammability.
I would expect that blending hydrogen with helium would allow one to save money compared with using helium, and yet avoid the flammability
risks. I'm not sure how much hydrogen one could use (that would depend on a number of factors) but even a 20% blend would reduce the helium requirement by 20%.
Molecular Weight of Dry Air
http://www.engineer...mass-air-d_679.html
Table for weight and composition of air [CoolSolutions, Jan 09 2008]
Helium is a dwindling non-renewable resource
http://www.physorg.com/news118491348.html
[+] [quantum_flux, Jan 11 2008]
For Doctor Remulac.
http://www.driedger...e6_v&t/CE6_V&T.html
search "all gases are miscible" [MaxwellBuchanan, Jan 12 2008]
Gas Density Distribution vs. Height
http://usera.imagec...d_Ideas/image22.gif
I cant seem to find the original website, but this graph demonstrates the point. [quantum_flux, Jan 15 2008]
Solar/ Fuel Cell H2/He Airship
Solar_2f_20Fuel_20C...20H2_2fHe_20Airship
I like blimps [BunsenHoneydew, Oct 07 2008]
[link]
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[Supercat] You and I are alike: neither of us
knows how much hydrogen could be
added to helium before it becomes
inflammable. |
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The difference is that I didn't post an idea
based on this. |
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After some googling, I found a measure called LEL/UEL; lower explosive level and upper explosive level. For hydrogen it is 4% to 74% so I believe this means a 4% mixture in air will still combust. A 4% hydrogen/helium blend would be fine on its own because (obviously) the helium isn't air. The question is what the concentration would be following mixing with air. A leak into air would perhaps halve the effective concentration of the hydrogen (a guess). So maybe you could get away with an 8% blend. |
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I'll be watching from the ground as you make the first test flight... |
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Nitrogen gas would also works. Add a little moisture and it works even better! |
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Nitrogen gas is barely lighter than air.(remember, air is already 78% nitrogen anyway). A hydrogen/nitrogen blend would be a lousy thing to fill an airship with. |
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What if you just made a hydrogen airship, but then added a thin jacket around the entire thing full of helium, and then monitor the pressure in the helium jacket? |
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If some flaming thing hits the gas bags, it will hit the helium first, and be doused, before it has a chance to ignite the hydrogen. Even if it is still hot enough, just from residual heat, when it contacts the hydrogen, the helium jacket should prevent the hydrogen from burning, because it stops oxygen from getting in, at least until the helium all leaks out (which could be a long time) In the meantime, people in the control room will see that helium pressure is dropping, and they can send over an emergency repair team to check things out, or land/evacuate/whatever is needed long BEFORE the protective helium is all gone. |
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No He or H2, just N2. H2O has a molecular weight of of only 18 (as compared to air's 29, and N2's 28). Dry air is composed of "ideal" gases, whereas water's contribution (displacement) is temperature dependent. A moist air bag could provide loft at the warmer ground level, and precipitated out at higher, cooler elevations, like a warm tent on a cool night. (I can't help myself!) The liquid water is extra ballast which could be pissed over the side for further ascent, or the condensation / evaporation equilibrium could determine the cruising altitude. Tis the principle of cloud's in a nutshell. |
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The moist air thing would NOT work. |
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Atmospheric pressure is about 101 KPa. |
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Water vapor pressure (100% humidity in your balloon) at 21 degrees C (70 Fahrenheit or so, a warm day in your lowland valley) is 2.5 KPa. |
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In short, humid air is going to have an extremely negligent effect on the volume of your balloon, since it only increases total pressure inside by a couple of percentage points. Therefore, no matter how light water vapor is, it can't possibly decrease your density in the airship by much (it might even make you heavier overall, I'm not sure about that.) At best, you may be able to have a football field sized balloon to lift a small field mouse or something. |
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Compare to helium, with a molecular weight of 4, and complete replacement of nitrogen (instead of 2.5% or so), and you can get a feel for the vast difference there in feasibility. |
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Considering that the book _Balloon and Airship Gases_ was published back in 1926, there's a good chance that somebody already thought of this. It's been years since I read it, but an internet search finds that one of its subjects is "gaseous mixtures". |
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But the flammability of hydrogen is not really a problem, despite what happened with the Hindenburg. In an airship designed for it,
hydrogen is very safe, according to several experts/advocates. Many industrial operations manage hydrogen safely, under a wide variety of conditions. |
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Some technical info: When mixed into air, hydrogen can burn at hydrogen concentrations ranging from 9% to 64%. When the opposite occurs and an otherwise pure mass of hydrogen is contaminated with air, the lower limit seems to be about 15% air, the mixture becoming "explosive" at 36% air. |
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Since most airships these days are blimps, comparisons with the problems of rigid airships aren't strictly proper. In a hydrogen-filled blimp, there would be no space for an air/hydrogen mixture to form or accumulate. Other issues could be designed around. |
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//humid air is going to have an extremely
negligent effect// Shame on that
negligent moisture! |
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[Smurfs] You are right in that nitrogen filled dirigible with a high humidity interior would need to be quite large as compared the the Helium (He) or Hydrogen (H2) equivalent. But consider that the displacement volume of a sphere is a cubic function whereas the cloth to contain it is only a square function. |
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Lower Explosive Limit and Upper Explosive Limit, related to the concentration of the fuel gas in "normal" air. So, anything more than 4% Hydrogen/96% Air is explosive, up to 74% mix. Outside this the hidrogen will burn but not detonate. |
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When the "fuel" gas mixes with air, it must contain no more than 4% hydrogen. Taking an exampe of dilution with equal volumes, for 4% hydrogen from a leak in a H2/HE mix, the maximum hydrogen content of the mix must not exceed 4% (the critical factor being the oxygen/hydrogen ratio). But since the helium reduces the overall proportion of oxygen, this too has a quenching effect. |
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Atmospheric air is about 21% oxyygen, therefore the ratio of hydrogen to oxgen to get above the LEL is about 1:4 (an approximation). |
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So, in any given volume, what is the maximum percentage of hydrogen in the hydrogen-helium mix for safe operation below the LEL in all conditions ? |
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(We know the answer and will award points to the first correct solution. [baconbrain] is on the right track) |
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Are you saying that below 4% hydrogen concentration in air, self sustaining burning would occur? What is the lower limit of this? |
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No, the hydrogen will burn, but NOT explode as the mixture will be too weak. |
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See also under "stoichometry" |
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I would expect that to really evaluate things, one would have to know which concentrations of (hydrogen, oxygen, inert gas) will sustain combustion at what ambient temperatures. |
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I'm aware that hydrogen in a blimp isn't particularly dangerous, but it does have a severe public image problem. If a company could demonstrate that its blimp containing a mix of hydrogen and helium wouldn't go "poof", that would probably increase people's comfort level versus one that was pure hydrogen. To be sure, the discomfort with pure hydrogen may not be entirely rational, but it would still have to be allayed. |
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// hydrogen in a blimp isn't particularly dangerous // |
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This is true, but unhelpful, as it is hydrogen outside the blimp which becomes a problem. |
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Somewhere between 0% and 4%, burning will not occur. What is this percentage? And by burning, I understand this to mean that the reaction will sustain itself. |
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Getting back to the original idea, to what degree will the helium suppress the flammability of the hydrogen? Are we assuming that a 50/50 mix will only double the lower explosive threshold? Building on [baconbrain]'s revelation of the different behaviors of H2 leaking into the air versus the air leaking into the H2, I think that the He would push the lower explosive threshold considerably higher. |
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I'm guessing that the tiny difference of density between the two gases won't permit rapid stratification - then you'd have a flammable half and a stable half. |
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I wonder if one could fluff up aerogel with hydrogen and float a blimp with that. This should prevent catastrophic decompression and so might be a good idea in any event. The walls of the aerogel pores should serve to delay spread of ignition. |
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Unless the Hindenberg was... PAINTED WITH THERMITE!!!! |
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Every bit of helium counts. [+] |
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Also, is there a lightweight material strong enough to contain nothing inside of it? Just a thought. |
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I like the aerogel idea. Those aerogels are crazy effective insulators, and the ultratiny pores would really slow down the rate at which air could infiltrate the lifting volume. With heat expansion pushing combustion products out of the pores, I think it would be self-extinguishing. |
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// won't permit rapid stratification // |
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Correct; at any temperature much above 30 K, Hydrogen and Helium are so moblie that brownian motion is enough to prevent stratification. |
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// suppress the flammability of the hydrogen // |
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Please, please just go and read up prperly on the properties of flammable gases, and explosive atmospheres, before you make daft comments like that ..... otherwise we may be forced to disintegrate you with a plasma beam (nothing personal, mind). |
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You can't have a hydrogen/helium blend. Since hydrogen is lighter than helium they would just seperate and the hydrogen would be at the top and the helium at the bottom. |
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I suppose you could try to keep them mixed or suspended together by blowing both gasses around with fans or something, but mixing up a flammable gas with an electric motor sounds like you're asking for more problems than you're solving. |
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// You can't have a hydrogen/helium blend // |
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Seems like every damned day we have to add at least one more person to the list of those we're going to have to execute ........ READ ABOUT BROWNIAN MOTION........ you can read, can't you ? Do you know what mean free path is for gas molecules ? Helium is a noble gas with its 1s1 and 1s2 orbitals filled, do you understand how this affects its properties ? Gaaaahhhhh.... |
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How about the catagoric explanation, based on science,
that it definitely cannot go bang? And why use hydrogen,
Cheaper, available? The way forward for the airship age,
that the Hindenberg effectively stopped for years, was
helium. The Hindenberg explosion was part political as it
was designed for helium but using hydrogen due to US/
German relations at the time. Hydrogen, old airship
technology at the time, cheap but explosive, made it go
bang . A very useful and efficient form of transportation
was basically abandoned, despite a very impressive safety
record to that point in history. Too expensive for
backyard developments from then on as have occured
with winged flight. Rapid descent and tie down/
floatation capability anywhere would seem to be what
was needed to be worked on. Avoid bad weather, same as
with light planes, but takeoff, airspeed and landing issues
totally different, potentially making it a far safer form of
transport to powered wing flight, using far less energy.
Arguably that is proven by the record it had when it was
utilised comparative to the early development of winged
flight. |
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Wow 8th of 7! Somebody needs to get
out a little
more. Might want to work on the social
skills before going out in public though.
Baby steps brother, baby
steps. |
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I'll dumb it down so even you can
understand it: To think that the
extremely, extremely, extremely minute
effects of Brownian motion would
somehow cancel out gravity shows an
ignorance of the very most basic laws
of physics that might be charming in a
cute 5 year old. Somehow though, I get
the distinct impression that your "cute"
days are long behind you. |
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The effects of Brownian motion can be
seen at the barrier layer of oil and
water. What happens to the mixture as
a result? Does the atomic activity at
that layer somehow turn on an unseen
mix-master that churns both the oil
and water together into a homogeneous
fluid? Does this invisible atomic
maelstrom cause the salad oil container
to dance and jump all around the table
knocking off silverware and scaring the
children? No, it doesn't. That's your job.
The effects of Brownian motion are very
minute and gravity separates the VAST
MAJORITY of the two disparate
substances and layers them according
to their relative densities. |
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This is the kind of ignorance I try to
fight all the time as a good citizen
despite any risk of "execution" I might
face at the hands of the technical
troglodytes, however minimal it may be.
A little knowledge about physics is
useless
when not applied while taking in
consideration of scale. It's this kind of
numb-skullery that brings us perpetual
motion machines, bad energy
generation schemes and lousy Star Trek
spin-offs. |
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So go back and read your Albert
Einstein again and this time pay special
attention to how much emphasis he put
on humanity and gentility. |
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// It didn't go bang, it burned rapidly as a result of
the envelope catching fire following a hydrogen leak in
the aft tanks. Read your history.// |
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Bang, vwoosh, fireball, far too toasty, whatever, it went
up, first initial major drama in the history of media and it
remains a mystery why it stopped the floating platform
concept in it's tracks, especially given it's war record.
Modern airship developments prophesising a freight
haulage revolution at this stage are actively
seeking funding via the internet so have to be considered
in that light. |
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Uh, doctor remulac? Hello? I think you
might want to apologize to 8th. You can
mix any gases you like. Where did you fail
to learn your physical chemistry? It's
usually a good idea to have at least a basic
understanding of what you're talking
about, if you really want to have a go at
someone. Otherwise you'll just embarass
yourself. |
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You can mix oil and water as well.
Lighten up. |
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By the way, just to get civil again for a
second, there was an amazingly well
done expose on the Hinderburg
flammability controversy done on the
show "Mythbusters". (the only reason to
have a tv, that and Southpark) |
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They burned ten foot models of the
airship filled with helium and hydrogen
respectively and with and without the
explosively flammable dope painted on
the skin that some say was the real
reason the Hindenburg blew up,
argument being that when Hydrogen
burns, you can't see the flame. (Which
is true) |
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I won't spoil the outcome. It's worth
watching. |
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I'm still reeling from the concept of
"immiscible gases". |
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Might want to start off with simpler
concepts like "light" and "heavy". |
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Oh, doctor, doctor. For the record, all
gases are miscible. Brownian motion
isn't the relevant factor (look up
Brownian motion). You're breathing a
pretty well-mixed gas (air) at the
moment. Divers breathe mixtures of
helium and oxygen. Would you like me
to post a bunch of links referring
specifically to hydrogen-helium
mixtures? Better yet - just Google the
phrase "all gases are miscible". |
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This is a bit like arguing with someone
who swears blind that gravity doesn't
work in a vacuum. I mean, where does
one start? |
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After a bit of rooting around, I find that the difference in density between oxygen and nitrogen gases is slightly less than the difference in density between hydrogen and helium gases. But that isn't important. |
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What might happen in the proposed idea is separation through leakage. Helium gas is made up of single atoms, and leaks like crazy. Hydrogen gas is made up of two-atom molecules, and may not leak out as quickly. Maybe. |
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That two-atom business is why hydrogen gas isn't much lighter than helium gas. I found one source that put the ratio as helium lifting .92 as much as an equal volume of hydrogen. |
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Quite. All gases (under given conditions of
temperature and pressure) contain the
same number of particles per unit volume.
For hydrogen, the "particle" is a biatomic
molecule with a mass of 2. For helium,
the "particle" is a lone atom with a mass of
4 (I'm ignoring exotic isotopes). Hence,
helium is twice as dense as hydrogen. The
"lifting power" is simply the difference in
density between air and the gas, divided
by the density of air. |
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Well, at least there's consensus that the
Brownian motion theory that was the
original reason I was to be "executed" is
bunk. One down. |
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Show me what force is overcoming
gravity to keep two gasses of different
weight bonded together in an enclosed
envelope that is never opened at any
time. In other words, tell me why when
you take a container of helium and you
spray hydrogen into it, the hydrogen
won't rise to the top. That's a real
specific question. Closed container
mind you. No opening it up as any time
and squirting everything out together. |
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Basically, remulac, it's because all gases
consist of huge volumes of space with
sparse atoms (or molecules, depending on
the gas) whizzing around. Sorry, no more
physics 101 - go read something. Do
some googling (see above). And stop
flaming people until you have a rough idea
of what you're talking about. |
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No on both counts. Your science and
your spelling. |
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From Webster's Dictionary: |
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Pronunciation:
gas\
Function:
noun
Inflected Form(s): |
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plural gas·es also gas·ses |
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Etymology:
New Latin, alteration of Latin chaos
space, chaos |
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1: a fluid (as air) that has neither
independent shape nor volume but
tends to expand indefinitely etc. |
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So you "correct" my spelling incorrectly,
nice. Might want to get one of those
books you want me to read. Start with a
dictionary. |
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And I was flamed by you and 8, not the
other way around. |
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Be nice or right. Pick one. |
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And let's lighten up before they kick us
both out of this (virtual) bar for
disturbing the peace. There are lots of
nice people here that aren't obnoxious
smart-asses like us that just want to
browse in peace. |
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//No on both counts. Your science and
your spelling. // As for spelling,
Shorter OED does not allow "gasses".
However, if Webster says it's OK, then
by all means go ahead. I stand
enlightened and impressed by the
diversity of foreign languages. |
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As for science, you went to great
lengths to ridicule [8th] and to "dumb it
down" so that "even [he] can
understand", based on your confident
assertion that helium and hydrogen are
immiscible. This naturally pissed of
[8th], and you then went on to insult
me. The bottom line is that your
original premise was a spectacular error
stated confidently and with scorn. |
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Anyway, I'm now lightening up, but
remain fully miscible. Just out of
curiosity, what's your doctorate in? |
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What's my PhD in? You want me to "whip it out" so we can compare eh? |
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Well Max, with all due respect, I stated very nicely that gasses stratify according to their weight when under gravity and especially when in an enclosed envelope. This caused 8 to basically call me a moron and say I should be executed, THAT'S when I got snippy back. If you go back and read it, you'll see that your timeline is backwards. Defense is moral, offense if immoral. Someone that seeks to attack and humiliate shouldn't freak out when somebody does the same thing right back. |
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As far as what's become the secondary subject matter, I'm not sure if you don't understand what I'm saying because you keep saying that gasses mix like I said they don't. Again, go back and read it. |
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Anyway, enough. Science is about the joy of learning about the universe around us and sharing that knowledge without being nasty. It's about assimilating and processing information as best as we can to the best of our respective abilities towards a useful end, not some contest to see who has the biggest dong. And it's certainly not a spelling bee, luckily for some. |
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The real bright people, and I'm not saying I'm necessarily one of them, are the ones who delight in sharing their knowledge with others, even those who might not be as smart as themselves, but administering it like a spoonful of sugar, not a spiked anal probe. |
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That being said, sorry if I offended you. Ok? |
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P.S.
By the way, some of the ideas you've posted are quite clever. Yea, I went and read them. Gotta know who it is you're arguing with eh? |
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Hey supercat, I gave you a bun for putting up with us blowhards arguing about your invention with these endless annos. Plus I like airships. I actually have a dirigible memorabilia collection in fact. |
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(PPS, for post-postscriptum - It's about all I can advise on since the last gas related physics I studied was at the level of Charle's law.) |
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Hey, doc. OK, truce. Regarding the
original subject, you said "You can't
have a hydrogen/helium blend. Since
hydrogen is lighter than helium they
would just seperate and the hydrogen
would be at the top and the helium at
the bottom.", which isn't actually the
case. If you introduce two gases very
gently through seperate nozzles into a
container, the heavier gas will sink (just
as saltwater will sink below fresh). But
leave them a while to diffuse, or stir
them, and they will mix perfectly, and
will stay that way for ever (just as,
though for slightly different reasons, if
you stir salt water and fresh water you
get brackish water which won't seperate
out again). |
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Let me suggest a thought experiment
that might help. Take big glass box,
and put a dozen ping-pong balls in it;
they represent hydrogen molecules.
Now shake the box violently (to
simulate thermal motion), and you'll see
that the balls are bouncing around all
over the place. Now take a second
glass box, and put a dozen table-tennis
balls in it. They represent the more
massive helium atoms. Again, shake
violently, and the balls will be bouncing
around all over the place. |
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Now take a third glass box, and put in
six ping-pong balls and six tennis balls.
Again, shake violently. If you watch, the
balls will all bounce around randomly
and will not be stratified. Once in a
while there will be collisions between
the balls (as between gas particles), but
this doesn't prevent things from mixing. |
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The only way to stratify the "gases"
would be to stop them bouncing, by
slowing the shaking until they all sit on
the floor of the box; this is equivalent
to liquifying the gases. Even then,
many liquid gases are freely miscible
(like water and ethanol). |
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The entire argument was completely miss-able. As for the stratification of gasses at ambient air temp, you would need Maxwell's Daemon, unless you are going *really* high. |
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Well, the two are different. If you
centrifuge a helium / hydrogen mixture
for instance, they separate. Centrifuge
the sea water-salt combination and you
will achieve no separation because the
salt and water are in ionic bond, not so
the mixture your model compares it to.
Helium, being a noble gas, does not
enter into any chemical reaction with
hydrogen at normal temperature and
pressure. |
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That being said,the main thing I went
off on 8 about his thesis that Brownian
motion was at play, which you also
don't believe, and that I needed to be
executed which I hope you don't
believe. |
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Bottom line, things can be discussed
civilly without saying stuff like "read a
book moron" or "we should all kill this
guy". I think that just makes the
antagonist look stupid, and it's the guy
who throws down first who deserves
whatever he gets. But even being right
is no excuse for being a jerk if you're
addressing somebody who was being
polite to you such as was the case with
8. (Who, again, was wrong) |
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That concludes, (hopefully) this episode
of "Angry Science Geek Slap Fest". Tune
in next
week. |
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//If you centrifuge a helium / hydrogen
mixture for instance, they separate.//
Well, no, they don't, at least not at any
sensibly attainable centrifugal force.
They honestly don't, really and truly. In
theory, you will get a very, very slight
enrichment of the lighter hydrogen
toward the outside. What you are trying
to accomplish has very little to do with
the density of the gases, but rather with
the mean velocity of the particles. |
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And the point I wanted to make was
that the statement "You can't have a
hydrogen/helium blend. Since hydrogen
is lighter than helium they would just
seperate and the hydrogen would be at
the top and the helium at the bottom."
was just plain and totally wrong, end of
story. Nothing wrong with an honest
mistake. |
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Having spent a certian amount of time in studying the detailed behavious of low molecular weight gasses during separation processes (The gases being Hydrogen, Deuterium, Helium 3, Helium 4 and Neon) with a view to producing useful quantities of, guess what, Lithium Deuteride (at an acceptable level of puirity) we do know a little about the subject, and can assure you that the research - which sadly could not be published, otherwise I would be happy to cite the relevant papers - shows conclusively that given the physical behaviour of such gases over a range of temperatures, pressures and ratios (which were analysed using a specially modified VG Instruments high-speed Quadrupole mass spectrometer), at temperatures over 30 Kelvin, the natural motion of the gases within a closed vessel ensured sufficent mixing that no useful stratification could be achieved, and oh, oh, how we longed for stratification, as it would have been ever so much better than having to selectively condense each gas and then do the bloody isotopic separations ...... at such low nuclear masses (unlike the gas centrifuges used for UF6 enrichment) the G-forces required to achieve preferential separation are practically unattainable. |
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In your analogy of salad oil and water, the molecules are significantly larger, the relative densities markedly different, and hydrogen bonding between molecules has a significant effect. Also, you are describing a liquid-liquid interface. Taking two ionic solvents of comparable densities, or better, two organic solvents - we offer the instances of propanal and acetone - and place them in the same closed vessel; at normal temperatures, and in a relataively short time, the two liquids will become inextricably mixed and can be physically separated only by fractional distillation. |
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Does that answer your question ? |
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PS we do not value social skills. |
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Can I say "I rest my case"? I've always
wanted to say that. |
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P.S. What are 'social skills'? |
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Guess I'm surrounded and outnumbered.
Somebody going to offer me a blindfold
and a last cigarette? |
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Are you nuts? With all this hydrogen? |
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I was thinking about what would happen in a still room if atmospheric gases did layer. As [UB] observes, we would be at the bottom in the CO2. I like the layer cake metaphor. |
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I do not understand why nitrogen dioxide does layer. This is the gas responsible for "silo fillers disease" and occurs when NO2 formed by new silage forms a layer along the bottom which excludes air. I have read about chain reaction deaths where individuals sequentially go into the silo to rescue those who preceded them and are overwhelmed. |
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If NO2 can layer, why not CO2? |
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//If NO2 can layer, why not CO2?// |
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It's simply a matter of mass and kinetic energy. If the temperature is hot out, there will be less layering of NO2 because the molecules have more kinetic energy to counteract gravity with on average. However, there always is a stratification of gas layers in our atmosphere depending on molecular mass and temperature (as you rise up higher through the atmosphere the gasses become less massive and temperature decreases), it's just a statistical normallity that this is so. Another thing that influences stratification of gases is molecular light absorption frequencies and day/night variations. |
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Thank's for the question [Bungston]. I like fluids and statistics! |
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//Guess I'm surrounded and outnumbered. Somebody going to offer me a blindfold and a last cigarette?// |
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If the firing squad are dumb enough to surround you, you might just escape with a well-timed duck. Wait! |
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<distant> ticktockquaack... ticktockquaack...</distant> |
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Just for the record, I'd like to see whether I've got this straight. |
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1. If you have a single gas, then the density of that gas (in terms of molecules per unit volume) is determined by temperature and pressure?
2. If you mix in a second gas, which is denser than the first in terms of mass per molecule, then the two gases will tend to mix evenly?
3. And the reason why, under the influence of temperature and pressure, the more massive molecules will *not* tend to displace the less massive molecules from the bottom of the mixture is ... (this is the bit I'm really unclear about) ... that, in the random, billiard-ball-like molecular collisions, the lighter molecules are just as likely to be buffeted downward as upward?
3a. By the way, supposing that you can keep the temperature and pressure constant while mixing the gases, will the mixed gas have the same number of molecules per unit volume as the original gas? And hence, a greater mass per unit volume?
3b. Are all these question marks becoming at all annoying? |
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1. Assuming that you have one mole (gram molecular weight) of the gas at STP, then yes ..... look up Avogadro. |
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2. This is dependant on the reactivity of the gases, the temperature (which affects molecular kinetic energy, indeed which IS same...), and the relative densities of the two gases. A very heavy gas and a very light gas will indeed stratify, but two gasses of relatively similar masses will mix. |
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3. Yes. if a "big" molecule hits a "small" molecule, and transfers energy, then the small molecule takes off like a scalded cat (conservation of momentum). And being small it is likely to go a long way into the "big" molecules before hitting something. |
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3a. Avogadro again. No, it won't. |
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3b. No, not yet, since your questions are fair and reasonable, but you're getting there. |
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If I remember correctly, the temperature of a gas is a proportional to the kinetic energy per molecule, which is in turn proportional to the molecular mass and velocity^2. At a given temperature, CO2 molecules (44 g/mol) will travel at less than 1/3 the speed of He atoms (4 g/mol), and less than 1/4 the speed of H2 molecules (2 g/mol). The heavier molecules will thus be much more prone to stratification than the lighter ones. |
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Pertinax - you've got it about right.
Basically, under constant temperature
and pressure, gas molecules (or atoms,
for helium) will spread out to the same
number per unit volume. (A mole of
any gas, or 6x10^23 particles, occupies
about 24 litres at room temperature
and pressure). All the particles are
bouncing around like crazy, colliding
with eachother and the walls, and they
don't "settle out"; hence, gases of
different densities (different molecular
masses) comingle freely. |
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If you mix equal amounts of two gases,
but keep the pressure and temperature
the same, they will occupy twice the
original volume: the number of
particles per unit volume will be the
same as always. If you mix heavy and
light gases, you still get 6x10^23
particles per 24 litres, but the average
weight of the particles will be mid-way
between the two original gases, and
hence the overall density (mass per unit
volume) is also midway between the two
originals. |
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Again, think of the ping-pong ball/
tennis ball analogy (above). Just
remember that collisions between the
balls mean that they will tend to
equilibrate to a given, constant, number
of balls per unit volume, given constant
pressure and temperature. The same
analogy also explains things like why
helium leaks out of the earth's
atmosphere faster than oxygen or
nitriogen do. |
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Oh, and the reason gases can "pool" in
inspection pits etc is that they're not
stirred. A denser gas "en masse" will tend
to sink, sure. But once it's mixed with a
lighter gas (or, once you've left it long
enough to diffuse), it won't settle out
again. Think salt-water/fresh-water. |
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We beat that salt water / fresh water question to a quivering pulp on the supersimple reverse osmosis thread. I conclude: |
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1: Restratification of gases after mixing is slow and is easily opposed by other forces. |
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2:Stratification of gases can be maintained in the absence of any mixing. |
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But [MaxwellBuchanan], I thought that, even without stirring, the gas molecules were zooming energetically in all directions (including up and down), so I'm still not grasping why they would stay in the 'pool', and why some of the lighter gas molecules would not join them down there. |
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I can picture the scenario [Unabubba] describes, with a 'fog' of CO2, and in that scenario I imagine that the CO2 is at a low temperature (lower than the temperature of the air above it), and hence has less energy than normal for overcoming gravity. But that's a special case, isn't it, resulting from that temperature difference? |
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[8th], please feel free to ignore the question above, in order to save yourself annoyance; I was just trying to keep you talking long enough to save Dr. Remulac from certain death. Oh, I think that's my duck now - must fly! |
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pertinax: If gases are essentially motionless, then both mixing due to random molecular motion and settling due to gravity will be very slow processes. If anything disturbs the gases, the mixing effects of that disturbance will dominate. |
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If you had a sealed tank of air with a small quantity of dry ice on the bottom, the dry ice would give off pure gaseous CO2 which would accumulate at the bottom of the tank. Its greater density would discourage its mixing with the air above. Given enough time, it would eventually mix with the air above until the difference in CO2 concentration between the top and bottom of the tank was very slight, but such mixing would not occur very quickly. |
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If there were a different reaction at the bottom of the tank which gave off hydrogen, the areas of pure hydrogen would want to rise. As the gas rose through the air, it would be in much greater contact with it and thus mix more quickly. |
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//I thought that, even without stirring,
the gas molecules were zooming
energetically in all directions (including
up and down), so I'm still not grasping
why they would stay in the 'pool',//
Pertinax, it's true that gas molecules are
in constant (and rapid) motion.
However, at normal pressure, collisions
between gas molecules mean that any
one molecule doesn't go very far in one
"go". If you follow the path of one
molecule as it collides with its
neighbours, you'd see that it is a
random walk in space. The *average*
length of each step (the "mean free path
length") is quite short. |
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Because of these collisions and the
resultant random walk, diffusion over
large distances is quite slow. However,
once it has happened (or if you mix the
gases by stirring), the same random-
walk process ensures that the gases
remain mixed. |
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Similar things apply to miscible liquids
(eg alcohol and water), except that the
mean-free-path is very much smaller,
so mixing by diffusion is very much
slower. |
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(The temperature thing is a bit of red
herring. Other things being equal, the
cooler gas is denser because the
molecules are moving more slowly and
hence wind up closer together. ) |
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I saw the Mythbusters episode mentioned and was disappointed. To properly bust the myth, they should have inflated a blimp made out if relatively inelastic rubber cloth so that it contained pure hydrogen and then lit that and compared its behavior with a hydrogen-filled blimp made out of Hinden-doped cotton. Their "dirigible" test was unrealistic because the hydrogen was mixed with a generous amount of air. |
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Further, it's clear that the doped cotton has two modes of combustion; until things get hot enough, it will burn slowly, but once they do get hot enough it will burn quickly. Many factors could affect how long it would take for rapid combustion to begin, but the time of importance is that required for the fire to finish spreading once the thermite reaction takes off. |
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// Was it a thermite reaction? I thought it was merely aluminised paint on the fabric, but no ferrous oxide? // |
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The upper part of the Hindenberg had one layer containing ferrous oxide topped with three layers containing aluminum powder. This would not have yielded the right conditions for a pure thermite reaction, but once combustion got hot enough to reduce the ferrous oxide I expect it played a role. |
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First, the idea is a good one...to a point. The two gasses are immisible...that is they would not combine permanetly..they would separate into areas of more hydrogen and areas of more helium, since hydrogen has more lifting capacity (is lighter) than helium...thus you would see a kind of stratification within the container, much like oil and water in a glass separate. This could be overcome by "mixing fans" within the gas containers, forcing the two gasses to always be in a rather chaotic state...they might resemble the motion in a lava lamp if you could see them. The introduction of the helium would allow a significant degree of safety. I would think a percentage of something like fifty-fift would be practical and result in significant savings in a really large airship design. |
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The problem with hydrogen..in a pure state as a lifting gas is very much a psychological one. Let us not forget the Graf Zepplin, a pre Hindenburg airship, operated quite safely from 1928 through 1940 and never did have an accident ...the Graf Zepplin was eventually broken up for the amuminum she contained and truned into german bombers and fighters. I still do not believe the Hindenburg disaster was precipitated because of the hydrogen...the hydrogen contributed catastrophically very much as jet fuel contributes catastrophically to an airliner disaster...it certainly was flammable, but was not the prime reason for the event (I know, I am running counter to convention here...but I have my reasons for my belief on this)...but we should also notice how many people survived the disaaster...some were right in the inferno up to nearly the very end...and survived. It was the graphic horror of the news reels that forever damaged the public perception of hydrogen as a lifting gas. |
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Modern materials have dramatically changed how hydrogen could be used as a lifting medium in an airship...carbon fiber, synthetic plastics, heat and ultraviolet resistant coatings would be used to build some utterly fantastic air ships today and hydrogen could be used in them quite safely making very econmical transport. We also now have very low weight to power engines that would make such a ship fast and highly maneuverable, eliminating much of the overhead costs of such an operation. |
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The mixing of gasses helim, nitrogen and hydrogen among others, would lend to the overall safety of such a ship. I for one, am bunning this as a pretty good idea. |
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[8th] {Maxwell], I don’t want to start this up again, and asking this question may get Homeland security knocking, but one question, if Brownian motion will keep the premixed gases mixed in opposition to the effects of gravity, then why do Uranium centrifuges work? Is the gas right at the vapor point so the energy is low, or is Brownian motion less significant at the higher atomic weights? |
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// may get Homeland security knocking // |
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We ARE Homeland Security ... |
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Calatrons and gas centrifuges work differently; calatrons on differential diffusion, and gas centrifuges on differential atomic mass. At low temperatures and high G-forces, preferential concentration of the heavier fractions occurs. Brownian motion is significant, but these are both multistage processes with each stage increasing the relative enrichment and feeding back the depleted gas to previous stages for reprocessing. |
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But it's still hard work. Only a propellerhead would bother with enriching uranium to weapons grade ..... you just burn up some uranium in a reactor, slush it in some conc. Nitric and do a chemical extraction on the Plut, it's a piece of cake. There's all the buggering about with explosive lenses, but it's 1940's technology ..... pretty soon everywhere except Liechtenstein and the Vatican will have nukes (and we're not so sure about the Vatican). |
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In 1945 there was one nuclear power. Now there are many. In 1953, it took a huge expedition to get to the top of Everest; now, people wander up there all the time. In 1903 there was one heavier that air flying machine ..... now there are tens of thousands. |
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We see no reason for optimism. |
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At a given temperature, larger molecules will move more slowly than smaller molecules. By my undestandiong, an absolute density difference of 1g/mol will affect the behavior of a large slow-moving molecule much more so than that of a small fast-moving molecule, even though the relative difference in density would be much smaller in the latter case. |
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//The two gasses are immisible...that is
they would not combine permanetly//
Dear Blisterbob, you may want to read
the foregoing annotations before
making spectacularly incorrect
statements. |
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//I don’t want to start this up again...//
Fair question. Uranium enrichment
centrifuges work for several reasons.
For one, the molecules (of uranium
hexafluoride, if memory serves) are
extremely massive and hence relatively
slow-moving (for a gas). Second, the
equivalent G-forces involved are huge.
Third, despite both of these factors, the
enrichment in a single-pass is very
small. As [8th] pointed out a while
back, centrifugal separation of lighter
gases doesn't work at all. |
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//If ever there was a HB thread to attract official attention I'm guessing it would be this one.// |
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Yup. I love the HB when it gets a good bit of genuine science/engineering talk going. |
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// a HB thread to attract official attention // |
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It would be reassuring if it did; sadly, officialdom is not that bright or alert. |
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Given the choice of relying on governments to protect their citizens, or the do-it-yourself approach, we reccommend buying lots of sandbags and putting your money into canned food and shotguns .... |
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The Homeland "internet keyword search engine"
down at Langley, VA or wherever it is never
stopped me from writing freely about International
Border Crossing Stealth Civilian Flying-Wing Blimps.
In fact, a 2004 Yahoo Private Chat chatroom
conversation with a lady from China helped me
discover some vital information. Seems she knows
an American pilot who teaches English north of
Beijing, China [ China, close allies with North
Vietnam from 1959-1975 ]. At that time in 2004,
she said his age was 63. It's 68 or 69 now. |
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Besides all of the above, there's Yin, Yan & those
expatriot military deserters [ those who simply
walked away rather than kill or be killed ]. Here's a
direct-quote from one OTHER 2004 internet web-
site I saw only once: |
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"We are a group of Americans living in Saigon
[Vietnam]. We rebuild Vespas". |
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You know, those 50cc, 100cc, 150cc and 200cc
mainstays of the Peoples' Republic of many many
People economy. |
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Age 62 - 72 POW's & MIA's. |
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Call 480 528 0632 if you'll help |
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Who knows .... ... .. . We could get a focus group
together and study Individual Freedom vs. Group
Dynamics whilst we use Stealth Aviation Technology
for locating former soldiers, thwarting military junta
regimes in Burma & Darfur and especially for UAV
drone & piloted finding persons who are lost at sea
or in bad weather. |
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Hydrogen with a fire-suppresive outer jacket of
helium sounds perfect! |
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Mix hot helium with water vapor. H2O is also (much)
lighter than air, as long it stays as a gas. |
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//Nitrogen gas is barely lighter than air.// Just
noticed that. How big would a nitrogen-filled
balloon have to be to lift a man? |
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OK, average molecular mass of air (assuming 80%
nitrogen, 20% oxygen) is 28.8. Molecular mass of
nitrogen is 28.0. So each 25 litres (molar volume)
of nitrogen will give you 0.8g of lift, so a 100kg
person needs about 3.1 million litres of air,
equivalent to a cube about 15m on a side (or a
sphere with a radius of, I dunno, 9 or 10 metres). |
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Given the cheapness of nitrogen, it's not such a
daft idea. Given a thousandfold expansion on
going from liquid to gas, and given that bulk liquid
nitrogen is about £0.10 per litre, you'd need about
3000l of liquid nitrogen, or about £300-worth, for a
man-lifting non-flammable balloon. |
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