Optimum deco gas choice

[ianfirmin]

Is there a recognised way (bit of software??) that will optimise your deco gas choice for dives of 50m or less? Say you know your planned depth. Your back gas will be the richest nitrox for that depth. Now say you have one deco stage. How would you work out what to put in it to reduce your overall ascent time to the minimum?

Any ideas out there?

[RichW]

Well, I could be inflammatory and say that the answer is 50%, but that would be less than helpful to you, I think.

They way you have asked the question does not actually allow the answer to be given, I'm afraid. If an "optimum gas" did exist for a 50m dive, then the actual gas would depend on the length of the bottom time.

As the amount of inert gas your tissues absorb increases with the time you spend on the botom, the point in the ascent at which you begin to decompress gets progressively deeper. That would lead you initially to choose a gradually weaker nitrox mix. However, the time required at the shallow stops would also be increasing, leading you to require a richer mix.

So the "ideal mix" will depend not only on the depth, but also on the bottom time.

If you calculate a range of deco schedules, you will most likely find that a gas around 50% gives you the best cover-all-bases mix. The advantage or familiarity with one deco gas also cannot be underestimated here - you get to know what deco needs to be done for the dives you do on a regular basis.

Hope that helps,

Rich

[Mark Chase]

Originally Posted by ianfirmin

Is there a recognised way (bit of software??) that will optimise your deco gas choice for dives of 50m or less? Say you know your planned depth. Your back gas will be the richest nitrox for that depth. Now say you have one deco stage. How would you work out what to put in it to reduce your overall ascent time to the minimum?

Any ideas out there?

EDIT sorry I miss read it


Decoplanner does it automaticly under the tools section. Just put in your max PP02 and your prefered narcotic depth and it punches out the correct mix.


PP02 is easy to work out.

((Max depth +10)devided by 10) X 02 content = Prefered PP02


So lets say you plan 1.4 on the bottom 45m dive


45+10 = 55

55 devided by 10 = 5.5

5.5 X 02% = 1.4


So 02% = 1.4 devided by 5.5

02% = 0.25


HTH

Mark

[Gareth Burrows]

If you run the numbers, 50% is about as good as it gets. If you do a half hour at 50 metres on 24% then you'll end up with a total runtime of about 67 minutes (vplanner). On pure o2 it goes up to 70 minutes. On 70% or 80% its 66-67 minutes again.

So what does optimal mean. Pure O2 is not a good idea for this sort of dive as you'd have to come shallow very quickly to get onto it, carry loads of backgas to get you most of the way through the ascent. 70% is more forgiving as you can get onto it earlier. 50% is the ideal (for me) as it means I can get onto it from 21M but don't really lose any advantage of the 70%.

As Rich says, there's a lot of sense in picking a deco gas and sticking to it, as you start to learn what gets you out of the water feeling a bit fizzy, and what gets you out of the water feeling great. If you are constantly swapping gases about you won't learn this, and more importantly you won't develop a regular practice of swapping to gas at an understood depth - it will change every dive. I always know that when I'm approaching 21 metres, its time to think about the switch. This makes it all very simple.

[Gareth Burrows]

Originally Posted by Mark Chase

EDIT sorry I miss read it


Decoplanner does it automaticly under the tools section. Just put in your max PP02 and your prefered narcotic depth and it punches out the correct mix.


PP02 is easy to work out.

((Max depth +10)devided by 10) X 02 content = Prefered PP02


So lets say you plan 1.4 on the bottom 45m dive


45+10 = 55

55 devided by 10 = 5.5

5.5 X 02% = 1.4


So 02% = 1.4 devided by 5.5

02% = 0.25


HTH

Mark

The OP said optimal deco gas, not bottom gas, you clown. How can you know so much about deco, yet have the reading attention of a Frenchman in a marriage guidance lecture

[Alun]

Originally Posted by Mark Chase

02% = 0.25

Effin rebreather divers and their PP02 fanatacism

That would be 25% O2 would it not? Unless you like 99.75% Nitrogen in your tanks...

[/InsufferableSmartarse]

[Mark Chase]

Originally Posted by ianfirmin

Is there a recognised way (bit of software??) that will optimise your deco gas choice for dives of 50m or less? Say you know your planned depth. Your back gas will be the richest nitrox for that depth. Now say you have one deco stage. How would you work out what to put in it to reduce your overall ascent time to the minimum?

Any ideas out there?

Ill try again:

Optimum deco gas?

Optimum in what way?

1: Shortest time for deco?

2: Safest stratagy?

3: Least task loading?

4: Most convenient?

5: Optimim for gas planning



If its shortest time in the water? Take two gases.

Safets Stratagy? Take two gases

Least task loading? Take 50%

Most Convenient? Take 50%

Optimim for gas planning? Take two gases


Personaly I always took 32% and 80% on dives avg depth 45m

Why?

Safest option, I could bailout on to 32% on the bottom if needs be and if its a shot line ascent I could end my deco anywhere from 5-9m.

Optimim for gas planning? I could virtualy empty the twin 12s before going on to my 32% and still be safe so I could do an hour at 45m on twin 12s.

Least Deco? 60mins at 45m on 25/35 and 50% gives me 74mins deco. On 32 and 80 it gives me 60mins deco

On my SACs i could do the dive on two ali 40s or two steel 5s.

I used 7s as they covered a wider range of diving.


If your not pushing bigger bottom times then the margin for using just a single tank of 50% is smaller and the hassle factor wins out, but remember you cant bailout on to 50% at 45m.

ATB

Mark

[ianfirmin]

Thanks guys. Since the original question was to minimise ascent time using one deco gas it appears 50% is a good choice. I think the concept of standardising on a deco gas is a good idea as well.

Thanks once again.

[stevechesh]

Ian,

In addition to the above, download HL Planner from here HLPlanner - FREE VPM-B based decompression software It's free and you can play around with the figures and choose whats right for you

[Paul Oliver]

Will some Mod please close this thread, i've just nodded in agreement with a couple of GUE/DIR/Chimp comments

FFS i must be pished