# He Canister

1. Jul 9, 2009

### Nuha99

This is a practical question. A balloon is to be filled with He gas. I know the volume of the balloon. That means I know the mass or number of moles of He gas that is needed. Question is, if a given small canister (volume ~ 150 ml) made out of aluminum can hold this gas. I am thinking of using P*V = n*R*T . Every quantity is know except P. So I can find a value for P.

How do I go about finding if this canister will withstand this pressure? What are the factors that need to be considered? I think the thickness of the wall, and the material the canister is made of are two factors. But are there any data available to exactly know if the given canister is right for this job?

Thanks.

2. Jul 9, 2009

### mgb_phys

If this is a real question then probably no - the strength will depend on the engineering regulations in your country.
As a rough guide an aluminium scuba tank can hold about 200bar and is 11.8 – 15.5 mm wall thickness.

3. Jul 10, 2009

### Nuha99

Is pressure in relation to the wall strength the only factor that I need to be worried about?

Another question I have regarding this issue is, IF, lets say, there exist a canister whose strength can stand unlimited pressures. Does this mean that I can get unlimited amount of gas in that hypothetical canister? My understanding is 'you can'. Only limiting factor would be the number of molecules (because they have finite size).

4. Jul 10, 2009

### Mapes

You may be interested in looking at Wikipedia's article on http://en.wikipedia.org/wiki/Hoop_Stress" [Broken]. The maximum pressure P for a cylinder can be estimated as

$$P=\frac{2t\sigma_y}{Fd}$$

where t is the wall thickness, $\sigma_y$ the yield stress of the material, F a factor of safety, and d the diameter.

Yes, although a complicating effect is the liquification and then solidification of the gas under increasing pressure.

Last edited by a moderator: May 4, 2017
5. Jul 10, 2009

### mgb_phys

Upto the point that they form a neutron star - yes

6. Jul 10, 2009

### Mapes

Incidently, it's interesting to run the numbers mgb_phys gives. Assuming a steel yield stress of 200 MPa and diameter of 20 cm, the factor of safety would be over 1000! I'd speculate that gas cylinders may be overdesigned to avoid failure by fatigue (crack propagation over many loading cycles), which is a more insidious mechanism than simple yield. Plus there's a corrosion aspect, of course.

7. Jul 10, 2009

### mgb_phys

Thats for an Al tank ( yield stress of 6061-T6 is around 280Mpa) a steel tank is only about 4.0 - 5.0 mm thick.

The other stupid thing about scuba tanks is that you can't take them on aircraft (even empty - in case they have some residual pressure). So they are used at 200bar but the extra 0.3bar caused by the lower ambient pressure in the plane would make them explode. Leaving aside the -50deg temperature in the hold would actually give them a much lower pressure.