More than "Advanced", but not really "Technical"

[ianr33]

Can I take Tec 40 in Eagles Nest ?

 

[lowviz]

Yes, I believe that you can. the depth is good...

 

[DevonDiver]

...all I need to do is take TDI AN and PADI Tec 40.

I'm not sure why you'd need both. There's a lot of duplication.

PADI Tec40 gives you 10min deco, tech rig, planning, gas management and deco gas to 50%.

TDI AN gives you zero deco, tech rig, planning, gas management and deco gas to 100%.

Basically, doing the Tec40 and AN only progresses you to use 50-100% O2.

This is just my opinion, but there's really no need whatsoever to use rich deco gas (typically 80-100%) for the sort of dives we are talking about. There's plenty of advantages to 50%. Plug it into VPM and you'll get shorter deco than if you use 100%, for the dive parameters we're discussing here. The ox-tox risk is less whilst buoyancy remains untested to perfection. It's cheaper... and you can get higher fill pressures in larger tanks more easily available.

Most importantly, from a 'tecreational' perspective, 50% also provides a 'bailout' option from 21m. In the event of a catastrophic gas loss etc, it is accessible much deeper.

50% is with you throughout the shallow ascent profile, providing it's benefits over a longer period. It's the optimal ascent gas IMHO. 100% just comes into it's element when your deco obligation is very substantial and you will be carrying two gases (50% and 100%).

Compare, these light deco dives (36m / 20min):

With no deco gas (whole dive on air) - 44 minute run-time:

With 100% deco gas (available at 6m) - 32 minute run-time:

With 50% deco gas (available at 21m) - 34 minute run-time:

So... two minutes difference 50% versus 100%....


If you want that extra 50-100% O2 capability, you could do Tec45 rather than AN...and get so much more (end state of Tec45 equates to AN + DP). But that's... beyond the scope of this thread ('advanced recreational').

 

[lowviz]

I am curious I am looking at NAUI dive tables and maybe I dont understand - but at 120 ft at 12 mins you are clear. At 120 ft at 15 mins or less you owe 5 mins of deco stop at 15 feet but I like 20 feet. At 120ft at 25 mins or less you owe 6 mins deco stop at 15 feet. At 120 ft at 30 mins or less you owe 14 mins deco stop at 15 feet.
How much further did you want to go and call it lite deco? Are you asking beyond 30 mins? If so I agree with the others take the class and be the wiser.

Sorry, my oversight. Missed your post somehow.

Yes, for a no-load first dive your analysis is spot on. However, my concern is also with the aggressive 1-week vacation diver that doesn't realize that a slow compartment is now leading. This threat is covered in any intro to tech course.

...//... 50% is with you throughout the shallow ascent profile, providing it's benefits over a longer period. It's the optimal ascent gas IMHO. ...

Certainly better than 100% for the type of divers we are discussing.

...//...100% just comes into it's element ...//...

On the surface.

 

[uncfnp]

I'm not sure why you'd need both. There's a lot of duplication.

PADI Tec40 gives you 10min deco, tech rig, planning, gas management and deco gas to 50%.

TDI AN gives you zero deco, tech rig, planning, gas management and deco gas to 100%.

Basically, doing the Tec40 and AN only progresses you to use 50-100% O2.

This is just my opinion, but there's really no need whatsoever to use rich deco gas (typically 80-100%) for the sort of dives we are talking about. There's plenty of advantages to 50%. Plug it into VPM and you'll get shorter deco than if you use 100%, for the dive parameters we're discussing here. The ox-tox risk is less whilst buoyancy remains untested to perfection. It's cheaper... and you can get higher fill pressures in larger tanks more easily available.

Most importantly, from a 'tecreational' perspective, 50% also provides a 'bailout' option from 21m. In the event of a catastrophic gas loss etc, it is accessible much deeper.
...//...

So... two minutes difference 50% versus 100%....

Thanks Andy, once again a very insightful answer. How would 80% change the ascent profile? Just curious...

 

[DevonDiver]

An 80% mix on that specific profile gives you a 31 min run-time. You still need to get shallow to access the gas though. No real benefit over 50% still, with more drawbacks. A little 'safer' than 100% if precision buoyancy control isn't a given certainty.

 

[dumpsterDiver]

So you guys are suggesting 50% be used as a bail out gas option for planned "lite" deco dives? And one benefit of 50% is that you can carry a bigger tank of it because you will be using more of it since you will be on it longer and deeper?

I have no training in any of this technical dive stuff or decompression etc. but I think I would prefer to a have a SAFE redundant back up gas and a smaller deco bottle, rather than a large one (for convenience). When does this come into consideration?

 

[DevonDiver]

DD, the concept is of an ''optimum ascent gas'. It isn't a 'bail-out' gas (that term, used correctly, only applied to rebreather diving). I used the term very casually earlier...which I tried to indicate by using apostrophes.

50% has many benefits, within certain parameters. One of those benefits is increased off-gassing efficiency over a longer period of the ascent profile. Another is the ability to access that gas at a deeper depth (in comparison to 80% or 100%).

We're not talking 'redundant gas' here... because (as per this thread and the courses being discussed) redundant back-gas is already a given factor...

No gas is unsafe, if you're trained to use in competently.

Cylinder size is a matter of preference. One of those preferences is economy. I pay the same for an AL40 as I do for an AL80. The option to have 50% at 200bar in an AL80 makes prudent economic sense. If I dove 100% it'd be at ~100bar (no booster around here) and in an AL40. An AL80 of 50% at 200bar will be sufficient for several dives, depending on the extent of the deco (based on the ''lite' deco discussed here... one cylinder would last for several days of heavy diving). Of course, there's nothing to stop you using smaller cylinders... or even lower pressure, if that's what you wanted.

 

[maniago]

DD, the concept is of an ''optimum ascent gas'. It isn't a 'bail-out' gas (that term, used correctly, only applied to rebreather diving). I used the term very casually earlier...which I tried to indicate by using apostrophes.

50% has many benefits, within certain parameters. One of those benefits is increased off-gassing efficiency over a longer period of the ascent profile. Another is the ability to access that gas at a deeper depth (in comparison to 80% or 100%).

We're not talking 'redundant gas' here... because (as per this thread and the courses being discussed) redundant back-gas is already a given factor...

No gas is unsafe, if you're trained to use in competently.

Cylinder size is a matter of preference. One of those preferences is economy. I pay the same for an AL40 as I do for an AL80. The option to have 50% at 200bar in an AL80 makes prudent economic sense. If I dove 100% it'd be at ~100bar (no booster around here) and in an AL40. An AL80 of 50% at 200bar will be sufficient for several dives, depending on the extent of the deco (based on the ''lite' deco discussed here... one cylinder would last for several days of heavy diving). Of course, there's nothing to stop you using smaller cylinders... or even lower pressure, if that's what you wanted.

Devon, I think you've single-handedly turned the corner on this thread - from "just drink the friggin Kool-aid!" to "ok here's kinda how it works". The 50% gas discussion was an unknown at least to me, and I already understand a bit more than in the last 4 pages...

Thanks for your recent series of posts! Mani.

 

[rhwestfall]

To supplement Andy's great assistance here, I suggest reading Powell's "Deco for Divers".....