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The Artist formerly known as 'John Duncan'
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<font color='#F52887'>I want to a quick roundup of what are the common mixes that people use for deco, especially if they use Air for the main dive with 2 sidemounts for deco. not a lot of science in my question but I am just interested to see if there are some mixes more in common use than others. I would like to limit this to dives in the 50metre range on Air.
 

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Concreting the world!
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I use 40% and 80% and get on them at a PPO2 of 1.6b.  I used to use 50% and O2 but it was adding about 25% more onto the CNS% total and only getting me out of the water a few minutes earlier than using the 40 and 80, I also get a bit more flexibilty on the shallow (6m) stop than with the O2.

Hope this helps

Simon
 

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A short fat well off crap cave diver. Likes wrecks
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32% and 80% for a 50 to 60m dive

Main reasion is I can bail out to the 32% in an OOA situation. At 50m. Its a PP02 of just under 2.0 which is US Navey extreme Exposure limit so your not going to tox and die unless your very unlucky.

I find 80% more flexable than 100% so I tend to use it all the time and like most people I gas switch at 1.6 so I get on the 32% at 40m and the 80% at 10m

ATB

Mark Chase
 

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Interesting mixture (32%, 40%, 60% 80% etc...)

I never dive air, but we have a standard set of gasses which we all use.  If the dive is 20m or 65m we all turn up with the same gas.  In fact the dives are planned with gas in mind, rather than depth (i.e. a 21/35 dive or an 18/45 dive).  That way, everybody knows which gas everybody else is using.

Most wreck dives are done using 50% for deco.  The longer/deeper dives are done with 50% and 100%.

Bob
 

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A short fat well off crap cave diver. Likes wrecks
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[b said:
Quote[/b] (Bob Cooper @ Nov. 02 2003,17:33)]Interesting mixture (32%, 40%, 60% 80% etc...)

I never dive air, but we have a standard set of gasses which we all use.  If the dive is 20m or 65m we all turn up with the same gas.  In fact the dives are planned with gas in mind, rather than depth (i.e. a 21/35 dive or an 18/45 dive).  That way, everybody knows which gas everybody else is using.

Most wreck dives are done using 50% for deco.  The longer/deeper dives are done with 50% and 100%.

Bob
My buddy and I will both be on the same gas. Only difference from DIR is we will plan the best bottom mix for that particular depth based on a PP02 of 1.4. Deco gases stay the same in a given depth range similar to DIR standard gas but with a smaller range.

It doesn’t matter what any one else on the boat is diving. However, God forbid I have to use their gas Andrew and I can reprogram ANY gas into the dive profile on our VR3s during the dive it's self and it will recompute the deco for us instantly.  


ATB


Mark Chase
 

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[b said:
Quote[/b] (Mark Chase @ Nov. 02 2003,18:49)]
[b said:
Quote[/b] ]My buddy and I will both be on the same gas. Only difference from DIR is we will plan the best bottom mix for that particular depth based on a PP02 of 1.4.
Mark

1.4 is too high for bottom gas, we plan for around ppO2=1.  I suspect this is why you are having issues with your CNS %. Keep the BG to around ppO2=<1 and you wil be able to use the ppO2=1.6 deco gas at 21m and 6m and not have an issue with CNS%

Bob
 

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A short fat well off crap cave diver. Likes wrecks
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Hi Bob

Generaly we plan a dive like say the Nyon with a depth of 48m at APX 1.4 at 48m so we would run 24/20 with an equivilent air depth of 30ish m But we wont be nose to the gravel for long and we will soon be at 40m on a pp02 of 1.2.

Obviously we try to keep the PP02 high and the He low to avoid extending deco. I apreciate fulley the 80% extends the deco too but as I have said before ther are other reasions why I dont use it.

Deco on the above dive 45min bottom time 15 @ 48 15 @ 45 15 @ 40

21/35    50% & 100% 20/85gf total deco 42mins CNS 72%

24/20    32% & 80%   20/85gf total deco 36mins CNS 70%

ATB

Mark Chase
 

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[b said:
Quote[/b] (Mark Chase @ Nov. 02 2003,19:12)]
[b said:
Quote[/b] ]Obviously we try to keep the PP02 high and the He low to avoid extending deco
Hmmmmm.....This is the wrong way round.  You want to keep the PPO2 low and the Helium high.  Why do you want to keep the PPO2 high? That just adds to your CNS loading, and gives you no real benefit.

You can pretty much ignore the helium as far as the deco software goes.

48m is on the cusp of 21/35 and 18/45.  I'd probably do it on 18/45.  The CNS would be <40%.

Using 24/20, you are denying yourself the benefit of the He and potentially exposing yourself to a higher CNS loading.

Come and do a dive with me and I'll show you how we do it.

Hope the weather is better next weekend than it was this weekend!

Bob
 

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[b said:
Quote[/b] (Pierre Farrugia @ Nov. 02 2003,19:14)]When I dive we also use same mixes, as this makes a lot of sense. Bob would a Ppo2 of 1 increase a lot deco penalties?

Regards

Pierre
[b said:
Quote[/b] ]Bob would a Ppo2 of 1 increase a lot deco penalties?
Pierrre

No, the advantages outweigh the disadvantages.  The deco uses PPO2=1.6, that's where the advantage is, not in a high ppO2 bottom gas.  A high ppO2 bottom gas is a potential disadvantage.

Bob
 

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A short fat well off crap cave diver. Likes wrecks
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[b said:
Quote[/b] (Bob Cooper @ Nov. 02 2003,19:28)]Come and do a dive with me and I'll show you how we do it.

Hope the weather is better next weekend than it was this weekend!

Bob
I would like that very much. I have looked at ignoring the He in dive profiles but I have not found suficient data to make it worth the risk of a bend having a go.

Peter Kemp had a He bend a few weeks ago and he reckoned it was prety damed painful.

WL has bailed on the 8th November dive  
but it's still on. Hopfully I will see you on the 15th and we can sort somthing out.

ATB

Mark Chase
 

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The Artist formerly known as 'John Duncan'
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Discussion Starter · #15 ·
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<font color='#F52887'>As usual, these topics diverge into different sideroads. I was just interested in the AIR dives (Im not into Trimix yet) and to see what the common mixes people use. I know all about planning the best mix for the dive, and gas switching at the PPO2 that gives you the best advantage for that mix, and all that. I sometimes find it advantageous to have my (O2 clean) twins filled with air (as I cannot always get to an air fill during the week) if I do not know the diving I will be doing and that way it will be covered for a wider range of dives. I may also have my sideslungs filled with 40% and 75%or80% and then I can plan the dives based on those mixes, that way my prefilled cylinders will be able to dive from 5m down to 60m. If I had the luxury of knowing what dives I will be doing and then being able to get the fills suited for those dives then that is the way I would do it, as that would be the best way.
 

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A short fat well off crap cave diver. Likes wrecks
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Regardless of whether your diving Air or Trimix, you chose your deco gas based on the best deco profile or a compromised deco profile with safety built in.

A typically good deco profile will have you on a PPo2 of 1 or more for all stops above 1min deep stops. This can end up at three deco gases so at that point you have to compromise.

If your looking specifically at safety you have to consider PP02 and CNS and the possible use of deep deco as bail out gas. For me I want a deco gas that I can get on within 15m of max depth of the dive. I can comfortably get up that far without gas and without breathing in. Then I want a rich deco mix that will allow for problems at the shallow stops. Cluttered shots, wrongly set deco stations, choppy sea conditions free hang drift deco etc. This generally means 80%. So the 80% is fixed and all other deco gas is fixed by depth.

The only difference between Trimix and air is that mix is adding He to the deco equation and you theoretically want to be off the He ASAP. That said the DIR boys WANT he and they even use it in their deco mix but this is not a generally accepted view. I unfortunately don’t know enough about He to debate the point.

So long and short of it is look at the profiles and make your own mind up.

ATB

Mark Chase
 
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