How to proceed ?

[Klopo]

I believe that SSI "Intro to deco" class does not include advanced nitrox, so 40% is the best deco mix.

Yes, thats right. According to the Deco Diver Certification your only allowed up to 40%. And for most of the dive sites it is fine. If we are plan to go to 40m ill take a 32% so it will not exceed the ppo2 max of 1.6

 

[Tom_Ivan]

This is a tech forum, I would be very surprised if anyone that has tech training, or a tech instructor, would suggest diving 25m/82ft on 40% nitrox.

IMHO, its not reasonable. But, maybe I'm in the minority here?

Yes but why?

Are you saying that its not reasonable to find 40% mixes? Or you don't like diving PO2 of 1.4? Or something else?

 

[Tom_Ivan]

i still have my normal 21% Air as a back gas in my 15l Tank at 200 Bar. Just in my 80 cuft stage is up to 40% EAN for deco.

Ah. I assumed that you were using the same gas in both tanks. Since you should be allowing for a lost deco gas in your gas planning, you should be leaving bottom with enough gas in your 15 to get your to the surface.
For this reason I'd be inclined to just put the right mix for your bottom depth in both cylinders. That way all your regs are breathable at any depth.
For example. Lets look at air on a 25m dive for 30 minutes. When you leave bottom your TTS is 19 minutes (GFhigh=70).
Now put 40% in your 15L and you have a direct ascent to the surface for exactly the same bottom time. A much safer dive plan IMO.

Pure speculation on my part after reading this thread, but it seems like a convoluted way of dealing with redundant gas in a deco dive. Because there is no twinset or side mount tanks to offer gas redundancy at depth, the deco gas is specified to be "breathable" at depth to offer some redundancy. I don't see any other reasons to limit the deco gas to <40% O2. Doubles + a real deco gas sounds more reasonable to me, but then again I'm not tech trained yet.

The OP does have redundancy because he's using an H valve. Imagine a set of manifolded 7L doubles.

Regarding your last comment. My opinion is that deco gasses are overrated at entry level tech diving. Often the correct bottom gas will result in such insignificant levels of Deco that there is no point in carrying it. Your only adding danger by carrying around a gas that is toxic for the majority of your dive.

 

[Klopo]

Ah. I assumed that you were using the same gas in both tanks. Since you should be allowing for a lost deco gas in your gas planning, you should be leaving bottom with enough gas in your 15 to get your to the surface.
For this reason I'd be inclined to just put the right mix for your bottom depth in both cylinders. That way all your regs are breathable at any depth.

The course basically allows a deco of 15 minutes in total. It is not exactly described whether this is TTS or whether it is about the effective deco. In our Swiss lakes, most of the things to see is from 30 metres and deeper. For redundancy I calculate the dive with the AVM. I have an average AMV of 16 litres per minute. But I calculate the whole dive with 20 litres per minute as a safety. So I can calculate exactly how much gas I need for a dive. I also include a lost deco gas so that with my maximum set TTS and a lost deco gas with my 15l tank, I can do deco and return to the surface and still have a third for a buddy just in case.

 

[Tom_Ivan]

So your dives are between 30-40m most of the time?
If I was in your position I'd be asking for 28% in both cylinders. Then I'd keep and eye out to exchange that AL80 for another steel and dive them sidemount.
Consider a 40m dive for 20 minutes on 28%. You have a 3 minute stop at 9m then an 18 minute stop at 6m. Total runtime 47 minutes.
Now do the same dive on air for the bottom portion and swap to 40% at 25m. You have a 3 minutes at 9m then 19 minutes at 6m. Total runtime is 49 minutes. Two minutes longer on your ascent and your deco gas was toxic for most of your dive.
Even if you put 100% in your deco gas your total dive time is still 44 minutes. Not worth the extra risk that comes with.
I believe a 28% gas in both tanks and no deco gas to be the safer option.

 

[Manatee Diver]

And I'd rather breath my back gas first than my stages, worst case scenario if I bump my back into the ceiling and damage the regs, I know that I'll have my sides to breath from. While if I had entered the water using said sides, then damaged my twins later on, I wouldn't have much gas left in my sides to bail me out.

I would rather have the gas in my redundant back gas. As in an emergency if they are a hindrance I can abandon the stages and retrieve them later. Also because of the minimum pressure required to keep a reg working, particularly with doubles it is better to have all your gas in one place, than spread out across three or four places.

 

[steinbil]

Regarding your last comment. My opinion is that deco gasses are overrated at entry level tech diving. Often the correct bottom gas will result in such insignificant levels of Deco that there is no point in carrying it. Your only adding danger by carrying around a gas that is toxic for the majority of your dive.

Hmm. It will be interesting to see if my perspective changes when I get tech training and more experience, but as of now, my thoughts are as follows: Deco diving requires different equipment, skills, planning and a different mentality. If you're not ready to use a deco gas that can get you out of the water faster and cleaner, then maybe you shouldn't incur a deco obligation. It also doesn't seem very hard to avoid breathing off a deco bottle by mistake when you only have one stage, so it doesn't intuitively seem that dangerous to me. But of course you need the skill to stay at your deco stop depth. I'll look at some profiles to see what the difference would be in using air vs oxygen for deco for some dives, to see if that changes my perspective.

 

[berndo]

Deco diving requires different equipment, skills, planning and a different mentality.

What different skills and 'mentality' is required?

 

[VikingDives]

Regarding your last comment. My opinion is that deco gasses are overrated at entry level tech diving. Often the correct bottom gas will result in such insignificant levels of Deco that there is no point in carrying it. Your only adding danger by carrying around a gas that is toxic for the majority of your dive.

First let me say that I don't consider the OP's deco class to be a tech class. It's some kind of in-between mess that blurs the lines between standard rec and real tech classes...

That being said, entry-level tech classes aim to teach skills that can be built upon. Learning to do gas switches and do them well is kind of important for deeper dives and having those skills nailed down before you start diving trimix is helpful (to put it ever so mildly).

Using recreational nitrox as a deco gas isn't ideal, but at least you're getting some practice... maybe in the OP's case, I don't know how that training went... but in a real deco class with double tanks and a deco cylinder, you can make a serious impact on time spent in the water.

28% for 60 minutes of bottom time at 40 meters (GF50/80) gives you a run time of 153 minutes, vs a run time of 115 minutes. I don't know about you, but I'd say that knocking 40 minutes off my run time might be worthwhile and significant.

 

[steinbil]

What different skills and 'mentality' is required?

Maybe it would be more accurate to say more refined skills rather than different skills. Again here's some differences off the top off my head, for what it's worth as a non-tech diver...

New/different skills:
- gas switching procedures
- planning of decompression and gas switches
- more knowledge of how to deal with equipment failures under water

Refined skills:
- buoyancy control under task loading
- positioning in the water to maintain contact with team
- free ascents in a team with precise stops

Different mentality:
- more diligence with equipment, gas analysis etc
- the surface is no longer an option for troubleshooting
- more situational awareness
- you need to plan for failures and be able to deal with them efficiently