Tank size

[Carl T]

I'm sure this must have been covered before but I can't find it.
What holds more air, a 10 litre 300 bar or a 12 litre 232 bar?
Currently have a 15 litre 232 and it's killing me! Too heavy and not enough air left for a second dive so it gets wasted. Oh, and I'm not the tallest of people.
Most people I dive with have 12 litre 232 bar but I would like to switch to Din anyway if only for the enclosed O ring (I dive wrecks and am paranoid about smacking the A clamp). I'm avoiding twin purely because of set-up and service cost and want to keep it simple (like me).

Any knowledge gratefully received.

 

[tbone1004]

at what pressure?

If we ignore the fact that the air compression is non linear after about 230-250 bar ish *can't remember where it starts getting weird*, then the 10l holds 3000l of gas, the 12l holds 2784l of gas, and 15l holds 3480l at working pressure. Realistically the 10/300 and 12/232 are going to be comparable in realistic scenarios, especially since I'm not sure how often the 10l which actually be at a full 300bar, vs the 12l likely being able to be slightly overfilled.

 

[Carl T]

Thanks tbone, my maths got all scrambled after the third pint of beer! I need to check with my local centre regarding their 300 bar fills. It looks like the added weight of the reinforced 300 bar cylinder would offset any benefits of going smaller for higher pressure. Might just go 12 litre 232 bar with DIN.
Great community, so easy to gain the advantage of other divers skills, experience and knowledge.

 

[tbone1004]

keep in mind buoyancy characteristics of all of them. I'm not familiar with euro tanks, but depending on how much total ballast you need, the heavier tanks may pose advantageous because you carry a significantly less amount of lead...

 

[DA Aquamaster]

at what pressure?

If we ignore the fact that the air compression is non linear after about 230-250 bar ish *can't remember where it starts getting weird*...

You start to see it at about 200 bar and it's at 300 bar where the effects start to be a significant factor for diving purposes.

For partial pressure gas blending purposes it can be more of a concern as the mix will start to lean out slightly at 250 bar and it'll lean out a lot at 300 bar.

The good news is that most gas blenders doing partial pressure blending don't do the math manually anymore more, they use a gas blending program instead, and the good ones take non linear compression into account.

------

At 1 bar and 70 degrees F (about 294 degrees K) the compressibility factor for air is .9999.
At 60 bar it drops to a low of .9904, which is not enough to worry about.
At 150 bar it will be 1.0074 - This is low enough to ignore as well (+11 liters in a 10L tank).
At 200 bar it is 1.0326 - In a 10L tank (2000 liters under ideal has laws) the difference is +65 liters
At 250 bar it is 1.0669 - A difference of +133 liters
At 300 bar it is 1.1089 - A difference of +217 liters
At 400 bar it is 1.2073
At 500 bar it is 1.3163

 

[tbone1004]

damn I was close! 3000-133=2867 vs 2784. So yeah, they're nominally identical at working pressures.

 

[Gareth J]

Two other points to consider:

1. How likely is it that you can get 300 bar fills when you are away. On a boat, at dive shops?
2. 300 bar cylinders are heavier than 232 bar cylinders.

Potentially I would be OK with 300 bar twin 7's, but I'm not even certain of that. If 232 10's where to small I would go for 232 12's in preference for 300 bar 10's.

If a single 12 is to small I would consider switching to twin 10's in preference to a 15.

Gareth

(All sizes quoted are litres for our cousins in the USA, and bar for pressure).

 

[DA Aquamaster]

It looks like the added weight of the reinforced 300 bar cylinder would offset any benefits of going smaller for higher pressure.

3AA steel tanks are certified with a test pressure equal to 5/3rd the service pressure while exempt/special permit tanks are certified with a test pressure that is 1.5 times the service pressure. Those are US standards but tank makers anywhere usually ensure they meet them as they want to sell to the US market.

Higher service pressures require the same safety factor with the 1.5x or 1.67x multiplier for the test pressure forcing increasingly thicker walls, which increases tank weight.

Size is the other non linear factor driving tank weight. The larger the diameter and the longer the straight section of the tank, the higher the hoop stress, and the thicker the walls need to be to resist those stresses. Consequently, fatter and/or longer tanks are disproportionately heavier than thinner, shorter tanks.

 

[BoltSnap]

Size in L---------Pressure in Bar-----------Size in ft3---------------Pressure in PSI
10.0-----------------300--------------------------106------------------------4351

10.0----------------- 232-------------------------- 82------------------------3365


12.0-----------------232--------------------------98------------------------3365



12.0----------------- 300--------------------------127------------------------4351


15.0----------------- 232--------------------------123------------------------3365

 

[Patoux01]

The 10l 300b holds roughly the same amount of air as the 12l 230b... Except one is much easier to fill. Then you get differences in tank buoyancy. I can't see which area you live, but be careful: there's "long" 12l and "short" 12l. The short ones weigh more than some 15l tanks and I would definitely avoid those, except if by "not very tall" you mean "1m30".