Super-critical atmospheres that we could tolerate?

[Nik_2213]

Other than the arguably supercritical Heliox mix used by 'Technical Divers', are there any other atmospheres that could be tolerated by humans, perhaps after some acclimatisation ??

( Per essential weeks' stay at eg Everest 'Base Camp', and the proportion who cannot get past the onset of 'Altitude Sickness'... )

I've only found mention of 'industrial strength' supercritical media, such as CO₂ and, yes, 'super' steam, both way beyond human tolerance...

 

[.Scott]

I am not at all clear on what problem you are trying to solve.
There are lots of websites that provide advice on altitude sickness.
I am not clear on what effect you are hoping for with by using a breathing mixture.

 

[TeethWhitener]

https://en.m.wikipedia.org/wiki/Critical_points_of_the_elements_(data_page)
Helium and hydrogen are pretty much the only tolerable ones here. Above about 3-4 atmospheres, nitrogen narcosis sets in, which is actually a pretty general effect with breathing gases (except hydrogen and helium). Above 10-15 atmospheres, helium causes high-pressure nervous syndrome, and above about 40-50 atmospheres, hydrogen narcosis starts to set in. In any case, one needs to carefully calculate the partial pressure of oxygen to avoid oxygen toxicity.

 

[Nik_2213]

Thank you both for prompt replies.

Scott, per topic title, I'm hunting for a super-critical atmosphere mix humans could tolerate. Perhaps, after some acclimatisation, by analogy with climbers' obligatory stay at Everest Base Camp, with its proportion of 'drop-outs' who cannot adjust....

TW, I've already checked the elements, which is why I mentioned Heliox mix and CO₂...

Provided supercritical compound or mix is benign up to a dozen Bar, occupants could breathe Heliox mix. Potentially, 'nervous' syndrome may be medicated if great need arose...

Sadly, I've been unable to find supercritical data on more than a few compounds. Possibly because a lot of proprietary extraction processes use 'tame' options...

 

[hmmm27]

Why do you think that "supercritical" is critical ?

 

[Nik_2213]

Super-critical media compress smoothly from 'gas' to 'dense gas' to 'sorta-liquid' without phase change.

( We're in SciFi & World Building forum: Surmise upon such environment... )

 

[.Scott]

Super-critical media compress smoothly from 'gas' to 'dense gas' to 'sorta-liquid' without phase change.

( We're in SciFi & World Building forum: Surmise upon such environment... )

So you're picking out an atmosphere for your world.
You want it to be "supercritical" like Helium.
But you also want it to be humanly breathable - at a range of altitudes.

Is that it?

 

[Nik_2213]

I'd settle for tolerable atmosphere --Won't dissolve you !!-- plus Heliox for breathing.

With the proviso that, like altitude sickness, there will be a proportion who cannot tolerate such...

I appreciate that sustained elevated helium pressure causes nervous problems, but there's been no great 'imperative' to medicate such, mitigate / resolve. Again, there will be a proportion who cannot tolerate such...

 

[.Scott]

The Terrestrial sea level O2 partial pressure level is 160mmHg. Some can get altitude sickness at about 3000meters of elevation, about 130mmHg partial pressure. That drop from normal to marginal in 3000 meters would apply to any world with Earth-like gravity. With higher gravity, the drop would occur more rapidly.
Similarly, you will get higher O2 levels at lower altitudes. At 1G, every drop of 5500 meters will approximately double the O2 partial pressure.

If your atmosphere includes bilogically inert gase such as nitrogen and helium, then your travelers may need to be concerned with decompression sickness.

 

[Hornbein]

Golly I had no idea such things existed. And my Dad did research in the area for years He had a grant from a fossil fuel company(s?) to study the properties of natural gas near the critical point. They wanted that for accurate metering of pipelines.

 

[Nik_2213]

Per original question: Are there any super-critical mixes other than divers' Heliox that we could tolerate ?
Doesn't need to be breathable, that's for the Heliox. And, by analogy with altitude sickness, there will be some who cannot tolerate it and/or hypothetical meds used to mitigate the high-pressure helium's nervous effects...

Per 'technical diving', lengthy 'decompression' would be needed to return to 'STP'. Again per 'technical diving', stays in that super-critical environment would be lengthy. Think underwater habs...

 

[snorkack]

So, to repeat, the critical pressures are:
He - 2,2 bar
H₂ - 13 bar
Ne - 27 bar

How efficient would oxidation of hydrogen by oxygen be in nature?

 

[Algr]

above about 40-50 atmospheres, hydrogen narcosis starts to set in. In any case, one needs to carefully calculate the partial pressure of oxygen to avoid oxygen toxicity.

So you can mix hydrogen and oxygen and it won't spontaneously explode? The name Heliox implies helium. I can see why. Grabbing a quick smoke break during scuba diving was a bad idea even without H₂.

 

[snorkack]

So you can mix hydrogen and oxygen and it won't spontaneously explode? The name Heliox implies helium. I can see why. Grabbing a quick smoke break during scuba diving was a bad idea even without H₂.

Two factors:

1. Hydrogen-oxygen mixture will not spontaneously ignite at 300 K at any concentration - unlike H₂/F₂, or air/trialkylborons, or air/white phosphorus. Hydrogen/air actually autoignites only at 570 Celsius. Then again, hydrogen has a quite low minimum ignition energy. In the flammable range, very small sparks can cause the hydrogen flame to propagate.
2. Hydrogen/oxygen does have flammability limits. They are wide compared to hydrocarbons - from 4% hydrogen to 94%. But you can go to rich mixtures. So 95% of hydrogen and 5% oxygen just won´t burn. Too much fuel, not enough oxygen. Sure, you cannot breathe it at 1 bar either - 5 centibars is not enough for a man. But at say 4 bars total... 20 centibars oxygen and 380 centibars hydrogen. Excess fuel simply will cool down any flame and extinguish it (and unlike hydrocarbons, burning hydrogen in shortage of oxygen will form no smoke, carbon monoxide or anything else noxious - just unaltered hydrogen). Only stuff with extra oxidant inside can burn as long as the extra oxidant lasts - like the chlorate of a match head. Strike a match in the pressurized, low oxygen environment and the match head burns but nothing else will sustain fire. Not the wooden matchstick, not the hydrogen atmosphere, not the paper and tobacco of the cigar. Of course, the moment your oxygen concentration approaches or exceeds the atmospheric range, fire will get highly dangerous!

 

[Nik_2213]

Heliox mix has those 'nervous; issues. CO₂ supercritical range is far from 'benign'...
Are there any 'benign' super-critical compounds ??

I've done open-literature searches, found only a limited range of compounds with supercritical properties listed. IMHO, none could be classed as 'benign', as either toxic/aggressive or their supercritical 'zone' is far beyond human tolerance...

 

[snorkack]

What do you mean as "supercritical"?

 

[Nik_2213]

quote:
A supercritical fluid (SCF) is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist, but below the pressure required to compress it into a solid.
/
https://en.wikipedia.org/wiki/Supercritical_fluid

 

[snorkack]

quote:
A supercritical fluid (SCF) is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist, but below the pressure required to compress it into a solid.
/
https://en.wikipedia.org/wiki/Supercritical_fluid

Which means that He-3 at 25 Celsius and 1200 mbar is a supercritical fluid. It is above its critical temperature (298 K>3,35 K) and critical pressure (the critical pressure is 1,18 bar!).
Pure He-3 is a simple asphyxiant, but heliox... cool it at constant pressure and 200 mbar oxygen will reach dewpoint at 77 K. As the gas keeps shrinking, the partial pressure of He increases to keep the total constant, so you duly cool the He around the critical point.

The only gases which have critical pressures below 25 bar seem to be He isotopes (1,2 and 2,2 bar) and H₂ isotopes (13 to 16 bar). So how do you expect to handle 25+ bar pressure?

 

[TeethWhitener]

Echoing @snorkack’s excellent response, I’ll point out (as I did in my earlier post) that helium induced nervous syndrome only kicks in above about 10 atmospheres, and hydrogen narcosis at around 40 atm. So your question has been answered multiple times.

 

[Nik_2213]

Heliox is an elemental mix and its hazards are known, but are there any 'benign' super-critical compounds ??

I've done open-literature searches, found only a limited range of compounds with supercritical properties listed. IMHO, none of those examples could be classed as 'benign', as either toxic/aggressive or their supercritical 'zone' is far beyond human tolerance...

 

[snorkack]

Heliox is an elemental mix and its hazards are known, but are there any 'benign' super-critical compounds ??

I've done open-literature searches, found only a limited range of compounds with supercritical properties listed. IMHO, none of those examples could be classed as 'benign', as either toxic/aggressive or their supercritical 'zone' is far beyond human tolerance...

In terms of what?

The moment you are looking outside He and H2, "supercritical" means 25+ bar.
Of elements:
F₂ and Cl₂ are aggressive.
O₂ has critical pressure 50 bar, temperature -118 C - and is toxic at these condition (irritates to death at 1 bar already)
N₂ has critical pressure 34 bar, temperature -147 C... and already has narcotic effects before that.
Of the noble gases:
Ne has critical pressure 27 bar, temperature 44 K. Weaker narcotic properties than hydrogen... neox is maybe tolerable.
Heavy noble gases:
Ar has pressure 49 bar, temperature -122 C, and is stronger narcotic than nitrogen. Still, if you can breathe heliox inside pressurized argon environment...
Kr has pressure 54 bar, temperature -64 C.
Xe has pressure 58 bar, temperature +16,6 C - and is so narcotic that 0,8 bar Xe, 0,2 bar oxygen already causes unconsciousness.
To search for compounds, how about looking for help through this:
https://en.wikipedia.org/wiki/List_of_gases
It has boiling point not critical points. But look for a rule of thumb at compounds boiling below -78 C... which have a prospect of having critical point under +30 C.
Elements have been already addressed. So the compounds:

1. CO −191.5 poisonous
2. CH₄ −161.5
3. NO −151.74 poisonous
4. F₂O −144.3 aggressive
5. NF₃ −128.74 poisonous
6. CF₄ −127.8
7. SiH₄ −111.9 reactive
8. trans-N₂F₂ −111.45 aggressive
9. O₃ −111.35 aggressive
10. cis-N₂F₂ −105.75

Out of the first 10 boiling between -192 and -104 C, just 2 are not aggressive or poisonous. From -104 to -78 C, I count further 31. But start with the two benign ones of the first ten:
Of simple alkanes, note that higher alkanes are narcotics. Pentane-hexane isomers are benzine main components - and they´re popular for sniffing. Already propane is narcotic enough to be good for sniffing at 1 bar.
Methane and ethane? Maybe less narcotic. Still, I am not sure how safe it is to sniff high pressure methane, or ethane. Methane boils at -161 C as quoted above, and has critical point at 46 bar, -82 C. Ethane boils at -88 C, with critical point at 49 bar, +30 C.
And then you have the nethermost fluoroalkanes (perfluoromethane, perfluoroethane) and a few hydrofluorocarbons. These actually have slightly lower critical pressures... but still high. Tetrafluoromethane has critical pressure 37 bar, at -46 C; perfluoroethane 30 bar at +20 C.
How narcotic are lower perfluoroalkanes?

 

[Nik_2213]

Thank you.
So, barring some serendipitous discovery, no.
That's sad, but a bit of a relief, as I was afraid I'd overlooked an 'obvious' candidate...