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Just not enough dive time.
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Just starting to think about twins, yes its the Nitrox course thats done it for me. Proper dive/gas management is to start the ascent with 50bar in the tank although some say come up with 50 bar remaining at the surface. I tend, rightly or wrongly to start the ascent if I'm at 20m with 50 remaining, if I was deeper I'd start a bit earlier. I usually end up at the surface with 30bar.

So I calculate my bootom time base on available gas (Fill pressure - 50bar) and multiply by tank size divide by consumption rate (based on 25sac - so 100 l/min at 30) [1]
Now if I run twins can I assume availabe gas is
tank 1 Max fill - nominal amount (say 20bar) x size
tank 2 max fill - 50 bar. x size

so for twin 10's at 220 bar I'd have
tank 1 (220 - 20) x 10 = 2000 ltrs
tank 2 (220 - 50) x 10 = 1700 ltrs
total                         = 3700 ltrs

In theory 3700/100 = 37 mins for a 30m dive, I have ignored travel time to keep it easy but would include that in calcs in the real world.

[1] My sac hasnt dropped over the last 30 dives and its still at 25l/min if I use (press in - press out) x capacity/corrected press average(i.e. divide by 4 for a 30m dive)  x time of dive
e.g. 220 in - 50 out x12 = 2040 ltrs
dive time (to start of ascent) = 33mins
av depth                             =  15m
corrected press  av.              = 2.5

2040 /(33 x 2.5)                   = 25L/min average.

maybe it will get better maybe it wont, who knows but I think my reg is partly to blame as it throws so much gas at me that most of it seems to get exhasuted away. As soon as I stop breathing in I can hear/see/feel exhaust gas coming out of it, is that 'normal'?

So many questions for a Friday, sorry guys.

Matt.
 

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[b said:
Quote[/b] ]tank 1 Max fill - nominal amount (say 20bar) x size
tank 2 max fill - 50 bar. x size
You're planning on independent twins?
 

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A short fat well off crap cave diver. Likes wrecks
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No I dont get it eather?

Whats all this tank 1 tank 2 bit?

twin 10's at 220bar is 20 X 220 4400ltrs of gas

50 bar reserve is 1000ltrs so you have 3400 for the dive.

As for personal reserves. On a 30m or less no deco dive I would plan to leave the bottom on 50bar or ascend to a safe depth (6m) for the rest of the dive if I was on a reef.

If i was decoing off of back gas I would allow 210% of the required deco gas bassed on a bail out plan GF 50/90.

That would get me and my buddy out of the water quick in an emergancy.  but i would probably suck a bit of 02 on the boat after  


ATB

Mark Chase
 

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Just not enough dive time.
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Cheers so you leave the isolator valve open and just assume you have one big tank of 20ltrs for the dive.

Matt
 

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Okay.. well.. forget all about the 50bar reserve, that's a rule of thumb that's good for beginners.
What you REALLY want to do is:
Work out how much air you'll need to ascend from bottom to surface. On a non-deco with a constant ascent rate, that's easy.
Eg 30m to surface at 10m min:
ave depth = 15m, ascent time =3mins, SAC =25 therefore 25x2.5x3 = 187.5 litres gas used.
For deco, eg a 3 min safety stop at 6, add this on afterwards:
Depth = 6m, time = 3min, SAC=25, so 1.6x25x3 = 120
So total gas usage is 187.5 + 120 = 307.5 litres

That'll get YOU to the surface.
Now assume an OOA buddy. For conservatism, we'll say that he'll be so stressed he'll be breathing twice as much as normal.
So you need 307.5 x 3 (one for you and two for him) = 922.5
So you need 922.5 litres of useable gas in your twinset.
In twin 10s you have 20 litres, 922.5/20 = 46bar
Because regs use an IP of about 10bar, I like to plan to surface with 10 bar in the cylinders - so I would plan to leave the bottom with 56 bar in my cylinders, no less.

That's how I go about planning a reserve- work out your requirements, then add an OOA ascent to it, and there you have it - how much you need to have before you head up.
Because this gives you triple your own requirements, you don't have to worry about isolating and staying isolated, as this only halves your available gas. Unless you're paranoid enough to worry about having an OOA buddy AND only one available cylinder...
 

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<font color='#810541'>Matt a lot of people to help you out here but an Advanced Nitrox course is the order of the day. You get to do the dives and dive with stages, do gas switches and shutdowns and stage drops and pickups etc etc.

Great fun while learning
 

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Just not enough dive time.
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Phil
Advanced Nitrox is at the moment not for me, what I needed to know was how I needed to plan for minimum gas requirements from bottom to surface if I used manifold twins.
I hadnt realised that both tanks are simultaneously available, unless you need to shut the isolator down due to a major problem. Dom has answered my question above. I currently run a single & pony so obviously I regard 50/60 bar in the 12l as lift-off point and I still have the 3l as emergency gas.
But thanks for the thought/comment anyway as I shall be staying in the 30m region for a while yet, oh where have we heard that before.  

Matt
 

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Grumbler-chief in Residence
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Matt,

Just a thought, Advanced Nitrox isn't really about depth, it has more to do with maximising your bottom times and bringing you into accerlerated deco. It is a good course, part of it involves kit config, which as you move into twins can be incredibly useful, the DIR fundimental course is also useful, but requires you to do the tech1 to have any lasting benifit. Plus the Dark Lord is Out There


Take care

Andrew
 

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IMO for the sort of diving you are doing at the moment, I wouldn't worry too much about getting into detailed gas planning. I might get flamed for saying that but its my view. Yes its no bad thing to learn and get into but really, its a bit of an overkill at this stage for the diving you are doing. Stick to 50 bar for the time being, its what you are taught from the beginning so stick to that. Once you get into deeper diving and deco then start worrying more about detailed gas planning.
 
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