Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Need help from the Big Brains

  1. Sep 26, 2005 #1
    Hey all, :confused:

    I have a few questions....

    1. Is Helium the lightest inert gas?

    2. If not what is the lightest inert gas?

    3. If so, then how much weight can 1 cubic foot of Helium keep afloat without sinking below the surface?

    4. Since fresh water and salt water have different characteristics, what is the difference in the amount of weight that can be kept afloat with the same 1 cubic foot of helium?

    IE: If 1 cubic foot of helium can keep 5 lbs afloat in fresh water, how many pounds can it keep afloat in salt water?

    5. How many cubic feet of Helium are compressed into each of the standard sized Gas bottles?

    Any help is greatly appreciated :smile:
  2. jcsd
  3. Sep 26, 2005 #2


    User Avatar

    Staff: Mentor

    Fresh water weighs 62.4 lb/ft^3 and helium is an insignificant fraction of that. You may as well assume that the buoyancy is all of that unless there is a reason you need more precision. Salt water varies, but the weight density is around 64.0lb/ft^3.
  4. Sep 26, 2005 #3
    Sorry I should have known that, so 1 cubic foot of helium will keep almost 64 lbs afloat in salt water, and a little less than 62.4 lbs in fresh water?

    Is there a more bouyant gas that would be more effective?

    Does compressing the gas make it more bouyant?

    In other words if I have a balloon with a higher PSI than in another one and they both take up the same 1 cubic footof space, the one with the higher PSI will be more bouyant and be able to keep a heavier object afloat, right?

    If that is the case, what is the formula to figure out what the optimal PSI would be to give me the greatest amount of bouyance for each cubic foot of space that the gas occupies?

    Thanks again for your help :smile:
  5. Sep 27, 2005 #4
    Highest buoyancies are in order:
    1.Hard Vacuum
    2.Hydrogen plasma (Single Proton plasma state, Protius)
    3.Hydrogen Isotopes
    4.Helium plasma
    5.Helium Isotopes (True inerts)

    The amount of space between particles, The greater the space the more buoyant it is, The closer the space the more dense it is and is less buoyant.

    You can do some research on (Specific Gravity). try Google.com and type in Specific Gravity within the search engine.
  6. Sep 27, 2005 #5
    Points to ponder. What happens to the density of a gas when compressed? Why are pontoons pressurized initially and depressurized when they have to lift something up?

    Once you know the water volume of a helium bottle and pressure, you can calculate the quantity by gas law.
  7. Sep 27, 2005 #6


    User Avatar

    :uhh: It increases!
    Last edited: Sep 27, 2005
  8. Sep 27, 2005 #7
    You got it :biggrin: What happens when the density increases is already covered in the above posts.

    PS: I am just helping the OP work himself on the solution. No pun intended.
  9. Sep 27, 2005 #8


    User Avatar

    Oh, lol. I knew that! :biggrin:
  10. Sep 27, 2005 #9
    First thanks for all the replies :smile:

    Now let me ask you this then........ Using an inert gas, what is the best way to create the maximum amount of bouyancy?

    Is Helium my best bet? And if not, what readily available inert gas should I use to get the best bouyancy?

    And if I understand correctly increasing the PSI in a balloon will NOT make it more bouyant, right? It will make it less bouyant because the particles are closer together, and there are more of them, Right?? :bugeye:

    Say I wanted to lift a boat off the ocean floor using airbags filled with an inert gas, how would I determine the amount of cubic feet of gas needed to lift the boat to the surface? Lets assume the boat is 40 feet long and weights 40,000 pounds on dry land.

    If my calculatons are correct I should be able to raise the boat to the surface with 635 cubic feet of an inert gas, right? or do I have to compensate significantly for the weight of the water that is inside the boat?

    Sorry if these are dumb questions but if you don't know the answer ask someone, right? :blushing:
  11. Sep 27, 2005 #10
    If the gas is allowed to expand in a balloon then it will become very buoyant, If the balloon does not expand then the gas will only condense as the PSI is increased. Liquify.

    Fill a balloon with water and put it in a bath tub of water, it won't float above the water tension because it is at equalibrium with the water pressure, So it floats benieth the surface.

    and expanded gas balloon will float on the water surface tension because it has less specific gravity contain within it.

    You have to keep the gases from condensing.

    Helium I believe is the Gas you are looking for. it is the only true inert Gas that I know of.

    If you could make a bubble with a Hard Vacuum inside then it would be even more buoyant as long as the shell of the bubble was thin, light weight, strong with a very large area of evacuated space.
  12. Sep 27, 2005 #11
    Is the density of the medium really significant(when compared to water density) to float something in water? I will do a sample calculation to check that.

    For simplification, I am considering 40000 as mass of water. Suppose, the mass of the boat is 40000 pounds and with water inside, say, the total mass is 60000 pounds. To bring that up, you have to force in a water volume of 60000/64 = 937.5cu.ft(generally any figure greater than this). Ignoring the mass of pontoons(for simplification again), density of air at STP is 0.075lb/cu.ft and that of helium is 0.011lb/cu.ft, If you use air, then additional mass will be 937.5 x .075 = 70.31 lb, so the water volume should be 70.31/64 = 1.09 cu.ft more than what we calculated(or with respect to full vacuum). With helium, it is 937.5 x .011 = 10.31 i.e 10.31/64 = 0.161 cu.ft more than what we calculated.
  13. Sep 28, 2005 #12
    I hope my calculation is not too tedious.
  14. Sep 29, 2005 #13
    My Brain hurts :bugeye:

    Can you be a little less specific?

    So what your saying is yes I have to add additional bouyancy to overcome the effects of the extra weight added by the water inside the boat?
  15. Sep 29, 2005 #14


    User Avatar

    Yeah, that's right.
  16. Sep 29, 2005 #15
    Yes and it matters little if you use helium or or vacuum or compressed air.
  17. Sep 29, 2005 #16


    User Avatar
    Gold Member

    If this is a practical problem that you are going to attempt, as opposed to a mental exercise, I'd suggest that you might try sealing the boat and blowing the water out with compressed air.
    At the very least, put your balloons inside the boat to displace as much water as possible.
    Last edited: Sep 29, 2005
  18. Sep 29, 2005 #17


    User Avatar

    Danger, what the hall does your signature mean? I don't get it.

    Full flaps? Tennis court?
  19. Sep 30, 2005 #18


    User Avatar
    Gold Member

    Hey, Mk;
    It just popped into my head one morning. It's an exaggeration of a bit of a flashback I had to the time I parked a Cessna in some guy's back yard. Flaps are the little tabs on the back of the wings that allow you to have a much steeper approach and thus less landing distance. :biggrin:
  20. Sep 30, 2005 #19
    Cool, Thanks everyone :smile:
  21. Sep 30, 2005 #20


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I keep seeing a jet pilot approaching the deck of a carrier for th first time.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook