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Physics/Recommendation of Compressed Gas

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Question
Hi Steve, I am currently at the research stage of a design project concerning the personal safety of divers during marine exploration. I have been working towards a concept involving the underwater decompression of gas to create a harmless deterrent against marine wildlife. Unfortunately I have a limited knowledge of the stability of compressed gases, and was wondering if you could recommend a gas that, when decompressed from a vessel underwater, could be capable of producing the desired 'harmless explosion of bubbles', or indeed suggest any other processes or chemical elements that you feel could best replicate this effect?
Thank you, and best regards, Nick

Answer
I'm not sure what you want by an "explosion" of gases.  Compressed gas underwater is pretty much compressed gas underwater.  If you had some that could go into a liquid state at very high pressure then you could keep it at a much smaller volume.  If you could incorporate a very tiny amount (not toxic to the water, just enough to smell very bad) it might far more seriously help deter the underwater creatures (presumably sharks) that you want to deter.  It seems the problem you're looking at is getting the bubbles to the location you want them to be.  So either you would have to shoot a canister of gas to the desired location, have some sort of "diver drone" type remote delivery system, or a long pole (easiest) with a release nozzle on the end.  The particular gas is relatively unimportant.  CO2 could be used at diving depths because it can be compressed into a liquid in a much smaller tank but at those shallow depths it will expand into a gas (unlike the deep ocean miles down, where it will remain a liquid).  For convenience and cost, CO2 is probably your best bet.  The expansion won't be as explosive as super-compressed helium or argon (argon is far cheaper).  I'd say argon with a very tiny some very smelly sulfur or fluorine compound would be ideal.

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Dr. Stephen O. Nelson

Expertise

I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

Experience

I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

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Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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