[scubadog]

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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]

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<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.  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.  I had used 220bar of a 15ltr tank on a 23 min dive with a max depth of 25m.  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.  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.  I would advise that you get a pony just to add a safety factor to your diving.

HTH

Ian Daly

 

[Mark Chase]

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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

 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]

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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]

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<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  

ATB

Laurence

 

[Dominic]

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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]

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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]

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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]

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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  

 

[MATTBIN]

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John!
OK I'm talking cobblers  

Matt

 

[Dr Stevil]

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I agree with Dom, far too deep too fast, and AFAIK there's no agency which advocates 30m during entry level training

  Build up experience in shallower water before rushing into 30 metre dives.

As the others have said air consumption  drops with experience, but don't get so hung up on time in water on XXX sized tank, breathe what you need when you need it and come up with your 50bar reserve whatever the time, nuff said.

I've gotta say I'm a bit concerned about the quantity and quality of the instruction you've been getting, doesn't sound particularly safe or thorough to my way of thinking

Regards
steve

 

[Rupert Bear]

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The post might be a troll but if not then 30m for 30mins on the third dive is asking for trouble.
Hope he survives his "basic training" to need a bigger tank!

 

[scubadog]

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Firstly I'd like thank everyone for their imput and sound advice.
Having read all the replies I feal I should have made my first message a little clearer. The total time from leaving the boat to surfacing was 33 minutes....this included a 3 minute saftey stop at 5 metres. Clear visibility was approx 20 metres, and 35 metres was the max to make out the shape of others divers. The temp was 20 c, at 28 metres. I followed the anchor line to 12 metres, which lay at the top of the reef, Before dropping down to 28 metres. Total time at this depth could only have been several minutes, as I then followed the contours of the reef back to the anchor line to complete the saftey stop.
Thanks again lads for the imput.
scubadog .....

 

[Dominic]

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Ah, non-square profile. That makes more sense

 

[Paul Oliver]

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Scubadog

Thats still good air consumption and awareness, but please do not get carried away with depth. Please stick to your qualification depths as they are set for good reasons. When you do progress deeper please do it with someone who has the right qualifications and experience to take you there.

Its very easy to go deeper, but you will not yet have the knowledge of the problems this can cause. For instance you were getting into the Nitrogen Narcosis range which can have dire effects. What rescue skills do you have? If one of you had had a problem do you know how you could have resolved this?

You were getting into the area where a small problem can become a life threatening incident.

Enjoy your diving but please stay safe,

Regards

Paul

 

[Mark Chase]

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Quote[/b] (Laurence @ Oct. 14 2003,09:02)]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  

ATB

Laurence

I run 20/85GFs on Decoplanner (which is a DIR program produced by GUE by the way) and 30min at 30 gives 22mins of deco.

100/100 gives 10mins deco

20mins at 30 on decoplanner 20 / 85 gf gives 8mins of deco but that is three 1min stops at 15m 12m,& 9m so a slow ascent rate of 3m/min would cover that phase without actualy stoping and then its 5mins at 6m.

Now if you look at your PADI tables you will see that 20mins at 30m puts you in the black zone. Which if my OW training serves me corectly requires you to do a 3min stop at 5m if you are within three blocks of the black zone. We were told that a dive in the black zone or beyound you should do an 8min safety stop.

The PADI instructors amongst us may prove me wrong but I am sure thats what I was taught.

ATB

Mark Chase

 

[Mark Chase]

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Just for the hell of it I planned a 30m dive for 20mins and fiddled with the Gradient factors untill I got the stop down to 3mins at 6m.

Surprisingly it came out a 95low 95high GF

I thaught the PADI tables would be less agresive than that.

I also planned the dive on my Vytec computer and it gave me 7mins of stops/ascent after 20mins using 100RGB on air.

Obviously I have sod all else to do right now

Mark Chase

 

[rasilon]

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If I recall correctly, the PADI tables are straight Buhllman tables, but with slightly tweaked half times according to DSAT research.

 

[Dr Stevil]

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I was under the impression that PADI tables are only slightly modified US Navy tables, which AFAICR are different from Buhllman

Anyhoo, here's the Buhllman tables on line for comparisons with your PADI ones http://www.diving.co.za/buhlmann_dive_tables.htm

and here's an IANTD comparison because I know some of you love this kind of stuff http://www.iantd.com/comparison.html

Some interesting observations
http://www.reefscuba.com/navytables.htm

And if you're really nerdy...
http://www.ssl.umd.edu/project....es.html

Here's a selection of tables for comparison
http://www.flash.net/~table/table/p0000065.htm

 

[rasilon]

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Quote[/b] (Steve W @ Oct. 15 2003,12:18)]I was under the impression that PADI tables are only slightly modified US Navy tables, which AFAICR are different from Buhllman

In the DDPlan reference section, one can download a copy of Eric bakers article, "Understanding M-Values", in which he lists M-Values for several tables including the PADI RDP under the heading ..."Various Haldanian Decompression Algorithms".
Understanding M-Values