Please sir 'more air'

[scubadog]

Just completed my third dive, and trying to control my breathing. My boat dives are being cut short cos I'm going through a 12ltr tank in 30 min's on a 30 metre dive . Me and my buddy, who is also my girlfriend are about to buy our first kit. I'm thinking of twin 10's or a 15 ltr dumpy at 300 bar. First problem. My local dive centre can only go too 200 bar, second problem, my partner doesn't want anything bigger than a 12ltr. ( our air consumption is about the same ) so even if I have the larger tanks, I will still have leave the water at the same time as my buddy. Do I need the larger tanks at the moment, having only done three dives? And will more experiance increase the amount of dive time on the same size tank as I'm using now?
Hope I haven't waffled to much

[Fee]

<font color='#0000FF'>Hi there 'dog,

Take your time and don't worry about it everyone SAC comes down eventually, mine used to be way on up there; about my 15th dive I sucked a 15ltr practically inside out (reaching for his log book), actually it was my 11th dive and I was diving the Bretagne. &nbsp;I quite vividly remember the look on my buddies face as we started the safety stop and I showed him that i only had 10 bar, I completed the rest of the stop and accent on his pony. &nbsp;I had used 220bar of a 15ltr tank on a 23 min dive with a max depth of 25m. &nbsp;Beside if you are getting a 30min dive out of a wreck at 30m then ur doing ok, and you would have to come about then anyway if you are on air. &nbsp;Like you say your buddy doesn't want to get larger capacity cylinders so really you will be governed by her gas supply and does it really matter if you come up with 50 or 120 bar. &nbsp;I would advise that you get a pony just to add a safety factor to your diving.

HTH

Ian Daly

[Mark Chase]

Originally Posted by [b

Quote[/b] (scubadog @ Oct. 14 2003,00:22)]Just completed my third dive, and trying to control my breathing. My boat dives are being cut short cos I'm going through a 12ltr tank in 30 min's on a 30 metre dive .

Gosh 30m on your 3rd dive &nbsp;30m for 30mins would put you into deco by 22mins so I asume you didnt do that. 20mins at 30 m would give you 8mins of deco total run time including ascent =30mins. That would alter the math a little:

OK that said asuming you had a 220bar fill and came up with 50 bar in reserve then:

You used 170 bar in a 12

170 X 12 = 2040ltrs

Depth 30m = 4 ATM for 20min and ascent calc at 2 atm for 10

Asuming 75% of gas used on the botom

1530 devided by 20 = 76.5

76.5 devided by 4 ATM = 19ltrs /Min SAC

That is very good for a third dive. My SAC was more like 25 at first and my SAC now is planned on 15 at depth. So your breathing is not going to get that much better you might get as low as 12 if your starting point is as good as 19 but it's unlikley.

If your doing 30m dives then a 15ltr and pony is a good idea. My wife is 5.0ft and 7 stone and she uses a 15ltr for deeper dives so I doubt if your girlfreind would notice much diferance. Just remember to take lead off to compensate. Ling uses 2kg less with a 15ltr tank and a 5mm semi dry. Then the diferance in weight is unlikley to notice much. A 300bar 12 is going to be about the same weight as a 15ltr for about the same gas.

Twin 10s is not a bad choice either but it is expensive to rig up and will quickley become redundent when you start to get into longer bottom times and a bit of deco. Twin 300 bar 7s isnt bad either but strangly will cost even more than the 10's

Bearing in mind your fill problem I would stick to 235 bar tanks

ATB

Mark Chase

[MATTBIN]

300 bar is only usefull if you can guarantee you will get a decent fill, even then gas laws dictate the effective pressure is more like 260bar. I recently thought about twin 7's but a report in Diver indicated gas loss at 30l/sec if you lose a first stage to free flow. The amount of gas loss would need to be planned for in case of an emergency during ascent. When I calculated what I would need to start my ascent with in case of this gas loss I ended up with very little for the dive, especially if I could only get 220 bar fills in some remote location. It might be possible to run as independants and still use them though.
I'd echo what everyone else is saying 30mins at 30mtr from a 12l is good going, so dont worry about your gas consumption too much. Get a pony at the very least for both you and your buddy, if you lose your main tank at 30m its not going to be easy getting to the surface on your own if you cant get to your buddy quickly.
My SAC is STILL 25l/m (after almost 60 dives), it has stayed the same since I started diving and I agree it is a bit embarrasing to be first back on the boat, life sucks sometimes, so I normally try and get in last instead. This give me more time to kit up, I can help others (which they appreciate) and my buddy understands my gas consumption and if I'm back first I'll make the tea/coffee and help the other back in. Usually though if I'm last in I'm not first out.
Plus it gives me a reason to go back and dive the site again sometime - everyone's a winner.
Additionally it will keep you away from NDL by a margin and that can only be a GOOD thing in my view, in fact I've just done Nitrox to keep me even further away from any limits.
Enjoy.
HTH
Matt

[Laurence]

<font color='#0000FF'>Hi Mark, dont know which table your using but i would change it matey, the PADI table gives him 20 mins at 30m with no deco at all. And thats conservative.

You sure your not using a DIR table &nbsp;

ATB

Laurence

[Dominic]

30mins at 30m is good going... but WTF are you doing at 30m after only three dives?? Am I missing something here..?

Your air consumption will go down as you get more experience. The best way of doing it is to stay shallow where a high consumption doesn't matter and work your way deeper as you use up less air.

[Dominic]

Originally Posted by [b

Quote[/b] ] dont know which table your using but i would change it matey, the PADI table gives him 20 mins at 30m with no deco at all. And thats conservative.

I suspect that'd be Buhlmann with GFs that MC's using - a quick check on DW confirms that 20 @ 30 is a deco dive by any conservative approach (unless you're on Nitrox).

Either way, 30mins at 30m is a deco dive by any standards - even the horribly liberal BSAC tables call for 3mins. No novice with 3 dives should be at 30 for 30 - I strongly suspect (and hope for) a typo.

[MATTBIN]

Hey guys, just noticed the original poster (Scubadog) is located in Spain, suppose that makes a bit of a difference i.e. viz and temp.

Matt

[John Gulliver]

Originally Posted by [b

Quote[/b] (MATTBIN @ Oct. 14 2003,10:01)]300 bar is only usefull if you can guarantee you will get a decent fill, even then gas laws dictate the effective pressure is more like 260bar.

No, Matt! The Real Gas Laws dictate that you will get 90% of the VOLUME of air you would get if the Ideal Gas Laws were correct. The pressure will still be 300 bar (if you're lucky!). For example:
My 10 L x 300 bar cylinder provides me with 0.9 x 10 x 300 = 2700 litres of air.
Sorry to be pedantic &nbsp;

[MATTBIN]

John!
OK I'm talking cobblers &nbsp;

Matt