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Here is a table of some important properties of most common (and some less common atmospheric gases) I've compiled, which could give you some idea when trying to "mix" an atmosphere o terraformed or imaginary worlds. Values for standard Earth air and some common breathing gases are also provided.
https://www.dropbox.com/s/jye086s9xpq5whc/SubProp.pdf
A few remarks:
1. Mole mass (M), heat capacity (cp and cv) and narcotic factor (NF) values for an arbitrary gas mixture may be computed as weighted averages of individual components based on mole fractions.
2. If NF × pressure (in atm) value is below 2.5, it sould normally be safe for long-term exposure for humans. This value approximately corresponds to the depth of 60 fsw while diving on air, at which point SCUBA divers might start to experience the first symptoms of narcotic effects.
3. NF values I provide are my personal estimates, as explained in footnotes. Those values may differ from values you might find on the net - most authors seem to use the so-called Meyer-Overton rule (relation of narc factor to lipid solubility) to estimate narcotic factor of a gas, but this is not very accurate. First of all, it has been empirically proven that Meyer-Overton rule does not work for biologically active compounds (i.e. O₂ and CO₂). Secondly, while narcotic factor does seem to be related to lipid solubility for other substances, data obtained from actual experiments clearly demonstrates that this relation is not, in fact, linear, as most authors seem to presume. While inerts gas narcosis still remains a relatively poorly studied phenomenon and I do not pertain to absolute accuracy of my estimates, those mostly appear to correspond to actual data of experiments according to numerous spreadsheet simulations I have run. Values I provided for H₂ and Ne are likely somewhat overestimated (due to insufficient data) and value I provide for CO₂ may have as much as ± 15% error margin (due to the fact that the value assumed for O₂ could have an important impact on the outcome of my simulations in this particular instance). Value assumed for O₂ is basically a mean value of what most experts and divers seem to claim (which would, at least, minimize possible error margin) as the actual scientific data currently available is insufficient to compute a more accurate estimate. Thus far, I was unable to find any data which would allow me to predict (even very roughly) the NF values of C₂H₆ (ethane) and C₃H₈ (propane).
4. While helium does not appear to produce any effects of inert gas narcosis at depths up to at least 2000 fsw (over 60 atm) and no reliable data is currently available for higher pressure environments, it is believed to cause the condition known as HPNS at partial pressures above 13 atm. While the symptoms of HPNS caused by helium and symptoms of inert gas narcosis caused by nitrogen or hydrogen appear to cancel each other out to some extent, we cannot presume that human body can safely tolerate such situation for prolonged periods of time. Until the further research on the nature of HPNS and inert gas narcosis has been conducted, it is best to assume that prolonged exposures to partial pressure of helium above 13 atm may be hazardous for human health and that "compensating" such effects with effects of inert gas narcosis could present additional long-term risks.
5. GWP and NFPA 704 values are only provided as indicative, since those values are strongly based on the Earth environment and may not be accurately applied to a world with radically different atmospheric conditions. However, GWP values may provide some indication as to relative greenhouse effect the particular gas could have. Likewise, NFPA 704 "H" column may give some indication regarding potential health risks a gas may present for humans (you probably would not want any significant amounts of gases with index values above 0 in a breathing mixture). Since non-zero values in "F" column mean that those compounds would readily produce an exothermic chemical reaction (a.k.a. "fire" or "explosion") with compounds marked as "ox" in "S" column, it is probably not a very good idea to mix such substances with each other at any significant ratios (e.g. mixing hydrogen and oxygen at ratios greater then 4:96 is known to be extremely unwise).
Last edited by agent009 (2014-06-27 16:47:09)
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Many thanks, agent009. This will be quite useful!
You might be interested in this: Minimally Martian Terraformed Atmospheres. It was a post by one of our users (Midoshi), which was unfortunately lost in the crash but reposted on that website. He gives an upper limit of 70-90 mb of CO2.
-Josh
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Josh: I've actually found Midoshi's article on Wikispace while I was doing my research - that's how I found my way here. But, as I commented back on Wikispace, I believe his upper limits for human CO2 tolerance are greatly overestimated, based on various research data I have studied. My estimate is that 25 hPa ppCO2 would be the maximum, provided ppO2 was around the minimum safe levels (~ 160 hPa) and there would be no more then trace amounts of any inert gases other then helium or neon.
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Well, I certainly don't claim any expertise on this matter. I think this is the kind of thing where, like it or not, there's no substitute for human testing, but to be safe I think we could all agree on an upper limit of at least 25 mb as giving plenty of room for the use of CO2 as a greenhouse gas while not threatening human function.
-Josh
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