Swedish Bristolian

[Conor]

Welcome aboard,

I am interested by this tank set-up, seems to make a lot of sense as a alternative to a single 12L at 210. But I guess the question is do you manage to outlast people on twin 12's, now that would be impressive

BTW is this maths right (I have left out the bar to ATM conversion, but assuming it wouldn't make much difference)?
2x4L at 300 bar = 2400 L at 1 ATM
12L at 210 = 2520 L at 1 ATM
i.e. 5% difference in content with the benefit of redundancy etc.

[John Gulliver]

If I might stick my oar in, the maths is NOT right. The Real Gas Laws apply and not the Ideal Gas Laws, because the compressibility of gases is not unlimited. At pressures above 232 bar (for air) you get less gas than you would if the Ideal Gas Laws were correct (90% at 300 bar).
Anders twin 4 L x 300 bar tanks therefore provide 0.9 x 8 x 300 = 2160 litres of air.
Here is a link that explains it all in more detail:
http://www.babilim.demon.co.uk/pages.../gas_laws.html
By the way, Anders set-up does NOT provide redundancy as there is only one valve, if I remember correctly. Anders uses a pony bottle for redundancy.

[Conor]

I'm glad you 'stuck your oar in', I was hoping for a discussion

You are right, I was using the simpler version of the gas laws, but thats because to an accurate representation of the true state you would have to take into account all gas laws appropriately for the system, and I haven't attempted that for years.....and even then the empirical v theoretical results are still distinct.

Where does the 90% figure come from ? It seems quite low(oops said high originally, but I think you know what I mean....i.e. quite far from 100%), not disputing it just curious.

Thanks for the pointers

[John Gulliver]

For the figure of 90% (to be precise, 91.77%), look at the curves in figure 4 in the link I gave.

[Conor]

I ended up seeing if I could calculate it out, cos I never tust those graphs, and ended up at 91.8, not far off. Thanks for that it was a good mental excercise, I have learned something.

Sorry, if this was dull for everyone else, I was a chemist and its nice to see the stuff I learned all those years ago actually being applied.

Conor

[daz]

Hi Anders.

Bristol based, swedish, diving and photography...

I just happen to know of a fellow swedish diver, heavily in to photography based in bath..  Mikael uses these forums occasionally and you should be able to find him in the members lists.

I dive with Mikael quite often so if you want some contact details give me a shout and I'll dig them out.

Happy diving

Daz

[Anders Schersten]

Hi Daz,
That is great, I hope he wants a buddy, cause I'm on the lookout for a good regular.
Cheers,
Anders