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Psi to J?

  1. May 13, 2013 #1
    Is there any way, crude as it may be, to find an equivalency between the pressure exerted on an object from layers of atmosphere and the force or energy put on an object from an explosion?

    I'm trying to categorize the durability of certain things and I've gotten to a point where instead of the yield of a nuclear explosion or its TNT equivalent I've got sea pressure.

    I know "durability" isn't exactly a scientific tool of measurement, and pounds per square inch and joules are completely different measurements, but I'm stuck.

    According to an online calculator I used for sea pressure, an object at 30,000 ft below sea level will experience 13,345.7 psi. Assuming something was that deep and could handle the pressure fine, is there a way to put a number on how much force from an explosion it could take?

    If specifics on the item itself are required I will provide them.
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  3. May 13, 2013 #2

    Simon Bridge

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    Hint: An explosion in air produces a pressure wave.

    Note: "durability" needs to be defined to be useful, but testing durability in different circumstances is something that lots of science gets directed at so it is a perfectly reasonable scientific term. The point is that you have to define what you mean somehow.
  4. May 13, 2013 #3


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    Materials behave differently when subjected to static, such as your sea pressure, or dynamically ( or from an impact) such as your explosioin. A static load can be distributed throughout the whole structure. An impact will have a focus on a smaller subsection of the structure.
    Consider you pushing on a wall with a pointy sharp object such as a knife versus shooting a bullet at the wall. Or a fabric such as Kevlar, a woven fabric, that will stop a bullet but not a sharp object such as a knife.
    What I am saying is that it is not so cut and dry that a structure that does not fail under a static load can translate to what will happen under a dynamic or impact load.
  5. May 13, 2013 #4
    I take your point about the differences. Clearly the real-life circumstances will vary based upon the specific occurrence.

    Well, I guess I'll use the most literal definition of durability. Able to resist wear, decay, etc., well; lasting; enduring.

    To give a more human answer, the ability to resist force and pressure without being noticeably damaged.

    Say I have a reinforced structure that can withstand the pressures of the deep sea, more durable than any sub or unmanned exploration device. It's resistant to most artillery fire as well. At a depth of 30,000 ft below sea level, fully operational, roughly 50ft in size it can withstand 13345.7 psi without problem.

    This doesn't have to be exact I suppose. It's been mentioned that an explosion produces a pressure wave. Is this overpressure being referenced or something entirely different? If possible I'd like to assume it's at ground zero of the structure to assume the most possible resistance to the pressure created. I'm also aware that waves move in all directions so this isn't perfect but I'd like to find some kind of measurement if possible.
  6. May 13, 2013 #5


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    Not easly. Water pressure is (almost) the same all over the object, and changes slowly over time. The pressure wave from an explosion is neither of those.
  7. May 13, 2013 #6
    Hmm. Is there a way to calculate the explosion needed to result in the same level of psi? Ignoring the omnipresent psi so deep under water, the water being everywhere, I have the starting point of the number itself. Is there a way to work backwards so to speak to find the explosive yield or TNT level needed to result in such a pressure, even if it isn't present around the entire object?
  8. May 14, 2013 #7


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    Very true. There is also the fact that, from a shock wave, the pressure on the target object will not be the same all over the surface, this can stress the target in unexpected ways.
  9. May 14, 2013 #8
    Like I said it doesn't need to be exact, it just needs to be rational. The scenario in and of itself if hypothetical.

    Can I work backwards from 13345.7 psi to get a yield or tnt equivalency? I get water pressure is present at every point while an explosion travels as waves and thus will not hit every point and will move in all directions. With that being said I'm willing to settle with the stated psi being at any part of the explosion.

    BTW I've read that the Tzar Bomba created a psi of 300. Now that's a lot of psi but it's not at much as 13345. Would I be wrong to assume Only a bomb much much bigger than the Tzar Bomb (well over 50Mt) could achieve 13345 psi at all?
  10. May 14, 2013 #9

    Simon Bridge

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    You can work out how much TNT would produce a shockwave with that pressure difference at standard temperature and pressure at a certain distance from the epicenter of the explosion.
    I don't think that helps you.

    Each kg of TNT releases a certain amount of energy.
    You can work out the internal energy of 1m^3 of ideal gas that has a pressure of 13345.7PSI (LEEAS.T?) and then express it in TNT-equivalents?

    However - you should realize that Bombs do not produce a fixed pressure.
    There is not enough information in your last questions to answer them.
    See: http://en.wikipedia.org/wiki/Blast_wave
    They are also quite different questions that what you wanted before and the answers will not relate easily to ideas about "durability" either. I think you are trying to achieve specific ends by means that are not suitable to those ends. Try another way.
  11. May 14, 2013 #10
    Well if my questions have changed it's only in response to what I'm perceiving as a lack of options. I'm desperately trying to find some equivalency here, so much so that I can live without the answer being exact. I just need a general idea at the least of what kind of yield would grant a similar strain on a hypothetical object as ocean pressure. It's why I've been willing to make exceptions for the blast's direction and total output. This isn't a school project or anything either so I'm not hung up on it being the perfectly precise answer.

    I didn't think the Tzar Bomba thing would work though. 300 psi for a 50Mt bomb, compared to 5~ for the 0.013Mt Little Boy shows me I can't scale it like that.
  12. May 14, 2013 #11

    Simon Bridge

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    That would be "none" - there is no relating the two concepts because of the different geometries of the situations.

    the 300PSI from the Tsar Bomb would put more strain on an object in it's path than to the same object in 300PSI water-pressure. A Tsar Bomb would tear a submarine apart.

    The best you can do is be able to say something like "object survives depths with a pressure equivalent to x-many kgs TNT" ... which will be totally misleading. You should be using a metric appropriate to what you want to claim.
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