Tech 1 Cylinder Choices

[cmalinowski]

How much contingency gas do you have with 30mins of deco on that 40? (I'm too tired to work it out myself right now)

You really can't have "contingency" deco gas because you only have one regulator (one person breathing at a time). So, you use Limeyx's lost gas plan.

As for other gas plan:

Gas Plan (170 for 30)
Max Depth 170ft / 51.5m (6.2ATA)
Bottom Time 30 min
SAC Rate 0.6
Time from surface to Depth (170ft) @60ft/min 3 min
Gas needed to travel to Depth 6cf
Gas needed for bottom time 112cf
First Stop 130ft / 39.4m (4.9ATA)
Time from 170ft to the First Deep Stop (130ft) @30ft/min 2 min
Gas needed to get from depth to first deep stop 12cf
Time to Solve an Emergency at Depth 2 min
Gas needed for Emergency 13cf
Average ATA traveling from depth to first deep stop 5.6
Average ATA for deep stops and gas switch 4
Time to Complete Deep Stops and do Gas Switch 7 min
Gas needed for deep stops and gas switch 28cf
Minimum Gas Reserve (MGR) 106cf
Total back gas needed 224cf
Back gas used if all goes perfectly 158cf
Deco Time on 50% 40 min
Deco SAC Rate 0.5
Average ATA of Deco based on profile below 2.2
Total Deco gas (50%) needed 44cf
Total run time of dive 82 min


vs 150 for 30
Max Depth 150ft / 45.5m (5.5ATA)
Bottom Time 30 min
SAC Rate 0.6
Time from surface to Depth (150ft) @60ft/min 3 min
Gas needed to travel to Depth 5cf
Gas needed for bottom time 99cf
First Stop 110ft / 33.3m (4.3ATA)
Time from 150ft to the First Deep Stop (110ft) @30ft/min 2 min
Gas needed to get from depth to first deep stop 10cf
Time to Solve an Emergency at Depth 2 min
Gas needed for Emergency 11cf
Average ATA traveling from depth to first deep stop 4.9
Average ATA for deep stops and gas switch 3.7
Time to Complete Deep Stops and do Gas Switch 5 min
Gas needed for deep stops and gas switch 19cf
Minimum Gas Reserve (MGR) 80cf
Total back gas needed 184cf
Back gas used if all goes perfectly 133cf
Deco Time on 50% 30 min
Deco SAC Rate 0.5
Average ATA of Deco based on profile below 2.2
Total Deco gas (50%) needed 33cf
Total run time of dive 70 min

 

[limeyx]

You really can't have "contingency" deco gas because you only have one regulator (one person breathing at a time). So, you use Limeyx's lost gas plan.

Agreed. As you can see from your gas plan, LP80's filled to 2640 are marginal for a 30 min @ 150 dive, so if you plan to do a ton of those, then 100's or 85s/95s might be a better choice. However, a lot of the T1 sites here (and even the deeper stuff) is pretty small, and you dont need 30 mins at depth, especially for a second dive.

I'd rather lump around 80's or 100's and save 20 pounds of dry weight if they fit the dive than break my back on the 130s (which i do use for deeper dives, but you can even do deeper dives on the 80s/100s because you can plan for a smaller increase in stressed SAC rate once you get to that level of experience and so use smaller reserves)

For me, 30 mins of mandatory deco is about the max I want to do on a single deco bottle. Above that, I think you get much better deco from adding an O2 bottle.

Not that I *wouldn't* do a few 1-off 40 min decos but at that point, I'd be pursuing (and did pursue) the training for 2 deco bottles. It is amazing (to me) how much better I feel when I have even 10 mins on O2 compared to just 50%

You also get better contingency plans with two smaller bottles over one larger.

Imagine the backgas reserves needed to handle a lost 50% AL80 with 50+ mins of deco to do, since we usually plan enough for each person to deco out on backgas and not share a deco bottle. Now you are looking at 100 mins of exponential shaped deco which seems like a lot. Admittedly you would of course share the AL80 (and in a team of 3 you have another AL80 to share).

 

[jjones9527]

You really can't have "contingency" deco gas because you only have one regulator (one person breathing at a time). So, you use Limeyx's lost gas plan.

Well, not exactly. You do 1.5X the stop time, and switch the reg between the team mates at each stop, (ie one is on deco gas at 40, the other gets it at 30 and so on). So you still need to carry at minimum 1.5X your deco gas, making a 40 too small for both dives you planned out. (Thanks for doing that by the way, I've just got home from a month of training and am catching up on all my sleep).

Just looked back at Limey's plan from the previous page. He's using 2200psi normally, and doubles the deeper stops for contingency. But double 2200=4400, still not enough reserve.

 

[rjack321]

The confusion is between those of us trained via the "old" tech1 vs. the "new". In my course (3 years ago) there was very limited 50% gas sharing going on for a lost deco gas. Maybe a little when it worked. The lost gas plan was double the time, do that on backgas with an exponential shape, period. That was your primary deco strategy, sharing when easy/plausible but not overstaying at some stops just to attempt to share gas when things have already gone sideways.

The current "carry 1.5x the deco gas needed" bit was mentioned in the pdf I have, but not time.

But from a practical persective its not easy to do and it begins to "misshape" the deco. Hence the recommendation I was given to 2x the time and reshape. Then share when you can. Still ugly deco but at least you are offgassing whether you are on the bottle at that time or not. This strategy requires you to keep the deco short, the recommendation I was given was, if you couldn't do it on an AL40 it was too much deco for Tech1.

And 30 min dive to 170ft was specifically called out (to me) as: 1) too much exposure for a Tech1 grad, 2) not having the right tools for the longer dive/deco with more slow tissue accumulation.

Now the current course appears to demand sharing the 50% and doing 75% of the stop time on 50% and 75% on backgas. Now I don't have a clue how this deco is shaped or if this even works. I have never done it and I'm not about to go making 3 min stops 4.5 mins and then doing 2.25 mins each for how long??

And as Limeyx can attest my old instructor is now advising to share for the full time each and then go to backgas when the bottle is exhausted. So that's a 3rd strategy.

Anyway the lost deco gas plans seem to have changed over the past few years. And probably will change again. I wish the grand poo bahs would try to keep us consistent with the "global" training. Probably more important to at least understand the current thinking - way more so than silly pocket contents standards.

 

[limeyx]

Well, not exactly. You do 1.5X the stop time, and switch the reg between the team mates at each stop, (ie one is on deco gas at 40, the other gets it at 30 and so on). So you still need to carry at minimum 1.5X your deco gas, making a 40 too small for both dives you planned out. (Thanks for doing that by the way, I've just got home from a month of training and am catching up on all my sleep).

Just looked back at Limey's plan from the previous page. He's using 2200psi normally, and doubles the deeper stops for contingency. But double 2200=4400, still not enough reserve.

But you still even in this case have your rock-bottom gas (at least 75% of it) to "aid" in the shallower deco.

Once you hit 30 or so feet, the high PPo2 of the 50% does not do much for you.
If you have worst case 64 cft Reserve gas.
Say you thumb the dive when you have exactly 64cft left.
By the time you go to the 50% you should have more than half of that left (for a new T1 grad) as that 64 cft is enough to get 2 stressed divers up to 70 feet, so you should use way less than 1/2 of it for 1 non-stressed diver.

So you arrive at 70 feet with say 48 cft of gas.
Turn deco bottle on poof no gas.
So you share with your buddy until the deco bottle runs out -- either of the many many strategies, and then back that up with back gas and double-time for the shallow stops (you should have 48 cft * 2 (bg) + 40 cft of deco gas to deco out on with a team of 2) maybe not "ideal" but doable (and yes, you now cannot have OOA and a lost deco but that's always the assumption)

And I definitely like AG's take on lost deco gas more than any other as it's simple to calculate, uses up the (non-sharable) deco bottle first so you dont need to share anything on the shallower stops, and emphasizes the 70,60 foot stops where it does you the most good.

This is my "lost deco plan of the day"

 

[rjack321]

And I definitely like AG's take on lost deco gas more than any other as it's simple to calculate, uses up the (non-sharable) deco bottle first so you dont need to share anything on the shallower stops, and emphasizes the 70,60 foot stops where it does you the most good.

While it is simple I'm not so sure its a great strategy when you don't have the O2. Cause let's say you are doing 3 mins each on the bottle at each stop. That's somewhere around 10-12 mins of time below ~30ft (about where the AL40 will probably be exhausted) and you're probably on-gassing some slower tissues during that time.

Now whether the double stops shallow negate that I dunno. Certainly having the O2 will be able to address this minor amount of on-gassing.

Buhlmann Decoplanner doesn't have anything like either one of these strategies and there's almost no way to "force it" to do something like these switches (either 75% of planned time or 100% of planned time at each stop with backgas on the remaining time).

Hence my preference at the T1 level to reshape first, share second. I know that the exponential decoplanner shape will get me out, probably pretty wasted but out. All other strategies are damned hard to validate. And I'm not going to try them so I can develop a baseline experience of working/not working approaches for lost deco gas.

At the T2 level you have other gasses at your disposal to manage your total deco.

(BTW I know from experience that screwing up the shape and overemphasizing the mythical O2 window can definately bend you. Time at ppO2 1.5-1.6 is not the "end all" of deco. You still need time shallow, with a lost 50% bottle - lots of time shallow)

 

[limeyx]

While it is simple I'm not so sure its a great strategy when you don't have the O2. Cause let's say you are doing 3 mins each on the bottle at each stop. That's somewhere around 10-12 mins of time below ~30ft (about where the AL40 will probably be exhausted) and you're probably on-gassing some slower tissues during that time.

Now whether the double stops shallow negate that I dunno. Certainly having the O2 will be able to address this minor amount of on-gassing.

Hmmm, I dunno either but I am willing to give either way a go I guess. Andrew seemed to be advocating it in either case but it's possible I mis-understood.

I think either way, worse case is you will have some aches & pains but walk away with that much "deco" want to try it sometime ?

 

[jjones9527]

I'd say as long as you and your team mates are on the same page, it doesn't matter what method you use.

 

[limeyx]

I'd say as long as you and your team mates are on the same page, it doesn't matter what method you use.

For dives in the T1 range, I would agree.

For deeper stuff I am betting it starts to matter more, but also that you have more choices in how to "fix" it (maybe a 120 bottle or doing more O2 time as long as you didnt let the bubbles grow too much etc)

 

[rjack321]

For dives in the T1 range, I would agree.

For deeper stuff I am betting it starts to matter more, but also that you have more choices in how to "fix" it (maybe a 120 bottle or doing more O2 time as long as you didnt let the bubbles grow too much etc)

Agree, share:
75% of planned time on 75% off
100% on, 100% off (each)
Reshape and share when you easy/plausible

Will all probably work just as well if disaster strikes and the bottle is totally lost/empty. Its odd that there are so many different approaches in class over just the past 3 years though. Maybe not, all this stuff is still evolving.