Tanks' weight and buoyancy

[okvij]

Hello,

Can anyone tell me the precise weight of an 11-liter aluminum tank at 200 bar and 50 bar?

I saw someone on quora posted images of the scales reading for such a tank with 3000 psi and 1000 psi. 3000 was around 17.7 kg and 1000 psi was around 16.2 kg.

The reason I'm asking this is because I can't make sense of a positively buoyant tanks. For an 11-liter tank to be positive, it's mass should be less than the mass of the displaced water (around 11 kg).

 

[saxman242]

Don't forget that 11 liters is the enclosed air volume of the tank, not the displaced volume of the tank. Walls have thickness, etc, that displace more water.

 

[stepfen]

The way to handle this kind of things is this:
- Find the weight of the empty tank (surprise surprise there is no single 11.1lt tank out there - each model has its' weight, even within a brand there are numerous versions - BTW the empty weight is written on the tank itself).
- Then calculate the weight of the air you want to add
- Don't forget to add valve and other accessories you might have.

For a "normal" Al80/11.1lt the empty weight including the valve should be around 15kg.
Add 2,7kg for 200 bars of air or 0.7kg for 50 bars.

You can have a look at the tool in this thread: Optimal Buoyancy Computer
Open the excel file and go to the 4th tab (Tanks). There you can find an extensive list of scuba tanks and you can try different amounts of air to see the result.

Remember the 11.1lt is the internal volume of the tank. To calculate the buoyancy you need to know the overall volume (internal volume + volume of the metal) i.e. how much water the tank displaces.

 

[stepfen]

Here is a bit more of maths:
Assuming 14,2kg total (empty) weight and that at 1 ATM the weight of air inside the tank is negligible all these 14,2kg are Aluminium. With a density of 2.7gr/cm^3 (assuming pure aluminium) this give us a volume of aluminium of about 5,26lt.
Hence the overall volume of the tank is: 11.1+5.26=16,36lt.
With a weight of 14,2 we are about 2.16kg positive (less if you include the valve).

 

[okvij]

Here is a bit more of maths:
Assuming 14,2kg total (empty) weight and that at 1 ATM the weight of air inside the tank is negligible all these 14,2kg are Aluminium. With a density of 2.7gr/cm^3 (assuming pure aluminium) this give us a volume of aluminium of about 5,26lt.
Hence the overall volume of the tank is: 11.1+5.26=16,36lt.
With a weight of 14,2 we are about 2.16kg positive (less if you include the valve).

Wow, 5.26L from the material, seems a lot. But it makes sense. Thanks.

 

[stepfen]

Wow, 5.26L from the material, seems a lot. But it makes sense. Thanks.

Yeap. That's one of the disadvantages of aluminium compared to steel. You need more aluminium to achieve the same pressure rating compared to steel