• Support PF! Buy your school textbooks, materials and every day products Here!

Need help from the Big Brains

  • Thread starter ksaun
  • Start date
  • #1
5
0
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:
 

Answers and Replies

  • #2
russ_watters
Mentor
19,226
5,238
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.
 
  • #3
5
0
russ_watters said:
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.
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:
 
  • #4
270
0
ksaun said:
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:
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.
 
  • #5
231
1
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.
 
  • #6
Mk
1,984
3
quark said:
What happens to the density of a gas when compressed?
:uhh: It increases!
 
Last edited:
  • #7
231
1
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.
 
  • #8
Mk
1,984
3
Oh, lol. I knew that! :biggrin:
 
  • #9
5
0
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:
 
  • #10
270
0
ksaun said:
First thanks for all the replies :smile:


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:
:blushing:
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.
 
  • #11
231
1
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.
 
  • #12
231
1
I hope my calculation is not too tedious.
 
  • #13
5
0
quark said:
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.
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?
 
  • #14
Mk
1,984
3
Yeah, that's right.
 
  • #15
231
1
Yes and it matters little if you use helium or or vacuum or compressed air.
 
  • #16
Danger
Gold Member
9,608
244
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:
  • #17
Mk
1,984
3
Danger, what the hall does your signature mean? I don't get it.


Full flaps? Tennis court?
 
  • #18
Danger
Gold Member
9,608
244
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:
 
  • #19
5
0
Cool, Thanks everyone :smile:
 
  • #20
Integral
Staff Emeritus
Science Advisor
Gold Member
7,198
55
Full flaps, dammit! That's a tennis court!
I keep seeing a jet pilot approaching the deck of a carrier for th first time.
 
  • #21
Danger
Gold Member
9,608
244
Integral said:
I keep seeing a jet pilot approaching the deck of a carrier for th first time.
:rofl:

Dang! Why didn't I think of that? I could have hooked the dude's clothesline with the mains and stopped in half the distance. :grumpy:
 
  • #22
ZA
13
0
buoyancy

There is basically no difference in the gas you use for buoyancy in water. This is because one cubic unit of a gas weighs so much less than the same volume of liquid, that you would be affecting the difference in buoyant force by no more than the difference in the weights of the gases. The mass difference for a unit of volume between helium and xenon is so trivial compared to the difference in mass between any gas and any unit of liquid, that I wouldn't worry about in your place. The buoyant force is basically the mass of liquid displaced. the net buoyant force is the mass of liquid displaced-the mass of whatever is displacing the liquid. About the salt water question. Regardless of the salinity, you probably won't see any salt water with a density deviation of more that 30% so your buoyancy will increase proportionally to density of the liquid. Helium is the lightest inert gas. I'm too lazy to calculate the buoyant force of a cubic foot of helium, but one cubic meter of atmospheric air will give you a little more than 1000 kg buoyant force (actually, the force in Newtons would be 1000*g). I suggest that you read into some basic physics, because these are trivial problems. Again, if you are talking about buoyancy being gas and water type, IT REALLY DOESN"T MATTER WHAT GAS YOU USE. Unless you are talking about buoyancy in a gas, FORGET HELIUM it won't make any practical difference. However, if you're talking about gas in gas buoyancy, I suggest that you use the formula m*g-M*g=Force with m being mass of the gas displaced, and M being the mass of whatever displaces the gas. This will give you the net upward or downward force of the system. if you get the values to = zero, then you're in equilibrium and there is no acceleration. To find the density of a gas, you will need to use the PV=nRT formula. In SI units, R=8.314 here is what you do to find the density of a gas: d=m/v m=n*Mm PV=nRT n= PV/RT d=((PV/RT)*Mm)/v this simplifies to DENSITY=(P*Mm)/(R*T) . ALWAYS use the same units for all values. For SI, P is pressure in Pascal’s (101300=1atm) Mm is the molar (Atomic mass*Avogadro's number (6.022*10^23)) mass of your gas (look in the periodic table, and add up the masses of all the elements in your gas) to get grams, then convert to kilograms. For example, the Atomic mass of Sulfur is 32. one mole of sulfur weighs 32 grams) R is 8.314 and T is temperature in KELVINS (celcius+273.17) to get the net buoyant force, all you need to do is (D*V-M*g) D is density of your atmosphere, V is volume displaced, M is mass of the object displacing the gas, and g is 9.8 at earth's surface. Remember, ALWAYS use SI units for your calculations. If you have any questions, e-mail me at hyperspace12000@yahoo.com.
 

Related Threads for: Need help from the Big Brains

  • Last Post
Replies
3
Views
2K
  • Last Post
Replies
5
Views
3K
  • Last Post
Replies
2
Views
2K
Replies
1
Views
745
Replies
4
Views
3K
  • Last Post
Replies
9
Views
4K
Replies
27
Views
2K
Replies
3
Views
1K
Top