Planning multi-level deco dive?

[victorzamora]

Stuart: If you're using the same algorithm, the distribution should be incredibly similar. More than similar enough to get you out safely. I think this is one of those "measure with micrometer, mark with chalk, cut with an axe" traps.

Of course changing the profile dramatically will give you some slight variation in what the deco planner is telling you to do....I just can't imagine it being significant enough on a dive of that length/size being a real issue on your ascent.

 

[stuartv]

Stuart: If you're using the same algorithm, the distribution should be incredibly similar. More than similar enough to get you out safely. I think this is one of those "measure with micrometer, mark with chalk, cut with an axe" traps.

Of course changing the profile dramatically will give you some slight variation in what the deco planner is telling you to do....I just can't imagine it being significant enough on a dive of that length/size being a real issue on your ascent.

Understood. But, I'd like to get my procedures dialed in now in such a way that they are still sound even when (someday) I get to doing bigger dives where it does matter. In other words, I'm just trying to do it "right", even if the difference in real life (with today's parameters) is essentially inconsequential.

 

[rjack321]

Understood. But, I'd like to get my procedures dialed in now in such a way that they are still sound even when (someday) I get to doing bigger dives where it does matter. In other words, I'm just trying to do it "right", even if the difference in real life (with today's parameters) is essentially inconsequential.

Re: "Big"
You mean if you were to do an 75min dive over the same profile? To do it right in that sense where this plan scales up you would be bringing 2 deco gases (EAN50 and O2 most likely, not likely EAN70). Scaling it up again you bring the same 2 deco gases and a bottom stage. Scaling it up again you are into 2 bottom stages, and 2 deco gases.

 

[packrat12]

rjack321 - I think the mix tends to be somewhat regional. Here in South Florida, diving ocean with 100% is not all that common. I see more divers on 80%. I started deco'ing at 100% but later switched 80% due to CNS loading during deco. I find I have a deep mix in the 20's - 30's and 80 for deco for 2 gas deco or 80 for 1 gas deco. I also personally do not like rolling around at 20' with 100% deco (I do not do 10' stops). The idea of 50 and 100 is not set in any stone. Mix that will work for you is better. I would say that 70% is fine for deco if it works.

 

[KenGordon]

the thing about two deco gases s having a spare if one fails. Eventually, especially with much helium, you cannot deco out on back gas. You could use your buddy's gas but being self reliant having two is better. Do the numbers and you will find that the choices of size are not obvious. I never use 100% for deco. Stops in the sea at 6m are ok, but leaves little room to spare. Often your first stops will be sufficiently deeper that getting onto a weaker gas earlier will not cost extra time, but now your back gas doesn't need to get you through those deeper stops,

As for the dive itself, what counts is gas. You could take 5 computers and they will be no use if you have not planned the gas before you get in. So, figure out the likely levels and plan for those. Write that plan on a slate (and deeper, longer etc) and dive the plan. You are likely to use less gas and go shallower than the plan, but that is ok. The plan dictates that you must be at or above waypoint depths at particular times. So long as you are early or shallower you know you will be ok for gas. You can still follow the computer up, and probably get out earlier than the plan, but you cannot overstay your waypoints deep.

It is important to be realistic about all parts of planning. If you round in a conservative way at every step you end up with something which will not match reality. Do it realistically and and any extra conservatism once at the end. Know your actual SAC, your actual descent rate, ascent rate and actual profile. Do the dive and check you came out with the expected gas. If not why not? If more were you shallower or is your SAC unrealistic?

Also consider that the dive maybe too much. Have you done enough similar but slightly shallower dives to make this only a little step up? Or is this a big leap?

Btw an Ali80 is perfectly nice in the water, but beware its empty buoyancy. Do do a weight check.

It is possible to over think this too. Today I was at 40m and everyone else (diver soup) was on a single. My buddy still got out to plan with 70 bar.

 

[rjack321]

rjack321 - I think the mix tends to be somewhat regional. Here in South Florida, diving ocean with 100% is not all that common. I see more divers on 80%. I started deco'ing at 100% but later switched 80% due to CNS loading during deco. I find I have a deep mix in the 20's - 30's and 80 for deco for 2 gas deco or 80 for 1 gas deco. I also personally do not like rolling around at 20' with 100% deco (I do not do 10' stops). The idea of 50 and 100 is not set in any stone. Mix that will work for you is better. I would say that 70% is fine for deco if it works.

EAN 70 is fine as a single deco gas for this particular dive. But its been chosen because its a "best mix" for this dive, it does not scale to bigger dives with 2 deco mixes very well.

80% then adding 32 or 36% is scaling up
50% then adding 100% is scaling up

If you start with 70% and switching at 40ft what is the 2nd gas that gives you a reasonable number of stops over which you're using it?

 

[packrat12]

rjack321 - Why be concerned about scaling up? Each dive is unique. I have no issue running non-optimum gasses if I already have them and they will meet my need. Also, 80% is a lot easier to obtain then 100% in many locations as you can fill it using a T bottle without a booster and some people do not like to handle 100% O2 @ 3000 PSI (Not that 80% is much better though). I see common use of even 50% single gas decos (much more common then the 50 and 100 two gas deco. Normally I see xx% and 80% when in SF). It is what ever approach works for the user.

I usually used either my rich mix (80%) or both gasses (30%ish and 80%ish) depending on my dive depth/time profile. I never felt the need to remix without reason. Also, the deco gas in this case is for accelerated decompression and not a long duration decompression dive. His profile is quite doable with just back gas. Remember that Stuartv is currently AN/DP so really should not need a 2nd bottle anyway. Even his planned profile is a light AN/DP dive. He is more interested in how to account for the widely variable depth in planning then how to plan the dive. His questions are very reasonable and honest. We all may have different approaches to handling that dive profile and use different gasses to meet our individual requirements.

Also, I have had long duration dives with a variety of gasses including near 70% on my rich deco. A lot of times, I would just use a dwindling mixture and refill cheaply with nitrox or air. Works well vs having it remixed. The actual time to get out of the water is not that significant and with modern technical computers, handling the odd mixes was not an issue. It is just a question of how much time in the water the diver is willing to have and how much CNS risk the diver wants to tolerate. The trade-off is a personal choice, unless he is diving GUI that is.....

As for the 70% switch point, it does not matter. Switch @ 40', and still be at a PPO2 of 1.12 @ 20'. On my CCR, if I am lazy, I will deco at 1.3 the whole way up. Besides, it is much better to deco at the PPO2 of 1.12 for 70% then it is for the PPO2 of .34 for air @ 20'.

Oh and I doubt he would have deeper than a 60' first deco stop for his profile.

 

[DevonDiver]

Two methods that I've used before:

1. Max TTS: Pre-dive planing based on a max depth (square profile). Note the total deco time (gas requirements). On the dive, you find that the bottom profile isn't square (you want to spend time shallower) so you can make use of TTS. Keep TTS within pre-planned total deco time. Needs a common-sense check, based on your understanding of the algorithm you use and the ascent curve you'll follow to ensure nothing deviates significantly in respect to time on individual deco gasses.

2. Average Depth: if you've done that dive route/profile before you can make note of the average depth of the bottom portion of the dive and plan based on that. I do this routinely for wreck penetrations that involve multiple levels, but aren't progressively shallower. Not had one that differed more than 1-2 minutes in total deco (laptop plan vs computer actual).

 

[RyanT]

I agree with packrat here. When using a single deco gas, if 70% is a better mix, there is no reason not to use it. Later you can train to use two gasses with a more standard 50 & 100 or 32 & 80. Stuart, I'm also planning to dive the Oriskany at the end of the year, please give us a report when you get back!

 

[tbone1004]

I thought about using an 80. But, I don't have rigging for an 80 and have never tried slinging one. But, you gave me an idea. Maybe I could sling 2 40s of the same gas? That would allow me to use 1 whole bottle for myself and still have a reserve for a lost gas scenario. And not have to worry about switching to the wrong gas and killing myself.

I ran a few numbers and it actually appears that using 70% for deco is the sweet spot. It gives me a shorter runtime than 50%, 80%, or 100%.

I'm looking forward to eventually being able to carry and use 2 different deco gases, but that time is still out in the future.

do you have 2 bolt snaps, a cam band, a slide lock, and a piece of rope? No need for worm gears unless you want them for a more permanent solution.