[PSUBS-MAILIST] Inert gas

[CWall@swri.edu]

Hi Craig - This may be a little off topic, but in all of this talk about
propane, I began to wonder - do you know if anyone is using other chemically
inert gases for ballast purposes? I'm a little removed from my chemistry,
but how about nitrogen or the like?
I know you won't be able to use it as fuel, which would defeat your
purpose - just wondering.
Thanks,
Greg

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  Greg, I'm probably the least qualified person here to address this, in terms
of what is actually out there, and the
only answer I can give is that I don't know.

  While I can imagine that liquid nitrogen would have some advantages, it
obviously wouldn't be plausible for durations beyond a couple days. Another
disadvantage is the other obvious point concerning the risk to the crew of
asphyxiation, requiring plumbing isolation.  All these points are no-brainers,
of course, so I'm sure I'm not telling you anything you didn't already know.

  I do think, however, that there is a chance that it might have been used on
unmanned submersibles or possibly wet subs.   I just flat don't know. 

For those who haven't  thought about this, the choice of gases does include the
 possibility of phase changes, which is a two-edged sword:  nitrogen and argon
are still gases when compressed, and require strong pressure containers for
anything beyond small amounts- and while weight penalties in a sub are
relatively moot, there is still a practical limit to the quantity that can be
carried.  A  liquid state would required cryogenic storage.   

For that you get much higher available pressures- warm the material or open the
 valve, and you are able to blow ballast at significant depths.

The problem with CO2 and propane, or other liquids that undergo a phase change
by liquifying, is that while you can carry much more quantity, the available
pressure is limited- for propane it's around 90 psi, and for carbon dioxide up
around 900psi.   If you try to use propane to blow ballast much below 150ft, it
 will simply stay liquid. With CO2 the same thing occurs, but deeper. In both
cases you can overcome this by heating the liquid to get the vapor pressure up,
 but that takes a lot of heat....and if it cools off again, it condenses.

Of course, you can blow a LOT of ballast from a liquified gas--- but it has to
turn into gas to be useful......so I suspect that most applications that would
use an inert or unreactive gas would be either one-shot (emergency type
applications) or shallow water (to cope with the pressure limit to
vaporization). And in most cases I suspect (but I don't know) that these would
be for unmanned vehicles.

Hope that is useful- it's nothing anyone who has had chemistry wouldn't know,
so it's probably fluff at this point.

Craig Wall