Deep Dive Strategy

[Doc Intrepid]

Here are some things that I plan out for a 100 ft dive:

- I know my rock bottom at 100 ft on the tank (1000 psi)
- I know how much gas I expect to use in the first 10 mins (500 psi)
- I know how much gas I'll use at depth every 10 mins (800 psi)
- I know that my SAC is 200 psi / ATA every 10 mins and can adjust
my gas consumption expectations based on my depth.
- I check at 10 mins to make sure that I'm not consuming gas at an unusually high rate
- I know what my NDL is at 100 ft (20 mins)
- I know at what depth I'll be limited by NDL and at what depth I'll be limited by air.

Jim,

You've gotten good advice in this thread, including taking a class. So long as nothing goes south, dives to 100' require more planning but execution of the dive would be the same as you're doing now. One major difference between planning dives to 50-60 fsw and 90-120 fsw, however, is that if something DOES go south, you have significantly less time to futz around dealing with it. If it happens to involve a panicky buddy, there are other complications added to the task loading, simply as a consequence of the depth you're at.

In addition to an understanding of decompression fundamentals and calculation of personal variables, such as SAC/RMV or consumption rate at depth, rock bottom, etc., planning dives to 90-100 fsw and deeper involves a commitment to redundant equipment, redundant gas sources and planning, practicing with gas switches, closing off valves that may suddenly begin leaking, and other skills. The idea is to pre-plan a variety of response behaviors and equipment options to each of the number of things that could suddenly go wrong down there. By all means continue to research all the background information. But a class is the place where it all gets put together.

FWIW. YMMV.

Dive safe,

Doc

 

[BigJetDriver]

-hh, I think your discussion can be summarized in the bar graphs of the changing compartment saturations, no? That's why I personally think the simulation software is so much better to learn from than just the tables. You can get a better sense of some very complex interactions. But again I'm just guessing.

Anyway, I'm still not clear on the practicality of managing those dives where NDL, not SAC rate is a controlling factor.

Jim,

You are getting some very good advice in this discussion. If I may add some general points to consider:

(1) Remember that ALL deco programs are mathematical reprsentations of what is going on in our bodies. The compartments we all talk about are NOT actual tissues. They are mathematical pictures, if you will, of how certain types of tissues react to gas on and off-loading. There are different assumptions in each model about gas up-take and off-gassing, such as linear-linear (old) or exponential-linear (newer), to take two examples. Various models also take different approaches to the basic assumption about how to deal with the dissolved gas, such as gas only in solution (Haldane and Buhlmann), or gas in solution and gas in free-phase (RGBM and others).

(2) Since most humans learn a lot visually, you are quite right about graphs giving a good visual picture that enhances your understanding.

(3) Almost no real-world dives are actually square profile, so depth averaging is always part of the computation.

(4) ALL DIVES, WITHOUT EXCEPTION, ARE DECOMPRESSION DIVES. It's just that some dives take care of that decompression by controlling exposure time, and dealing with the off-gassing solely through ascent rate.

(5) Obviously, as you get deeper, more issues come into play, such as redundancy for safety, and adequate gas supply.

Enjoy your studies. I know I do. At present, my light bed-time reading is Bennett and Elliot (5th). :11:

Cheers!

Rob

 

[DepartureDiver]

The ability to think in terms of and mentally track compartment loadings may change your willingness to dive various profiles. For example, exceeding NDL at 100' by a couple minutes is of less concern to me than being at 60' with a couple minutes of NDL remaining.

Getting a good margin of safety before surfacing is lot easier coming from slightly over NDL at 100' than from near, but still within NDL, at 60'. This difference is that for an immediate descent to a flat 100' dive, the limiting compartments are the fast ones. OTOH, if I'm at 60' and my computer is saying 5 minutes NDL left, then the problem is that the medium speed (30 and 40 minute HT) compartments are near the limits. Obviously, those 30 and 40 minute compartments take a lot longer to offgas than the 10 minute compartment that controls the 100' dive.

Once you get a feel for what is going on in the various speed compartments, you will also see the problem with "riding the NDL" back up. What happens as you slowly ascend while keeping the computer a few minutes inside NDL is that the controlling compartment shifts to the slower ones and you end up at 40' or 50' feet with a large nitrogen load that will be slow to offgas.

When talking about a single dive, or repetitive dives with nice surface intervals, then Charlie's point is a good one. When making a nice ascent with safety stops, you can lose more gas and get a better safety margin when surfacing. The other comment I would add is that many of the gas loads allowed in the slow compartments in many tables/computers/software, etc. seem to be greater than my liking. So not pushing the gas load of the slower compartments is always a good idea ... especially since it takes these compartments longer to off gas during safety stops. While the math can always be played with, reality and common sense should also be taken into consideration ... which should discourage big sawtooth profiles that Charlie has mentioned. Just because a computer may allow it does not mean that it is really a good idea.

 

[lostinspace]

xman & scottfiji have me right

 

[StSomewhere]

A little bit of knowledge can be a dangerous thing? Outstanding if you are using such information to expand your knowledge, but this board is no subsitute for training if you want to start pursuing more advanced techniques..which will ensure you have all the information required to adequately plan such dives...

Again, compliments on your search and thirst for knowledge...just a caution.

You make a great point that decompression diving shouldn't be self-taught, but I don't think that fits the case here. In my neck of the woods, if you want an OW certification then your LDS choices are affiliated with either PADI or SDI. Period. At the OW level, PADI teaches diving within the NDL using tables/RDP and SDI teaches diving within the NDL using only dive computers. I would guess that either approach (and for the record I went with PADI) probably works out in the end for most average, ordinary, vacationing recreational divers.

But I'm not sure that either of these "trust me" methods (and that's exactly what they amount to in my mind) really gives the average recreational diver enough understanding to actually take ownership of their own dive management. Read DandyDon's "Do you understand your dive computer?" thread if you don't believe me. The question is, how does the safety-conscious recreational diver ever develop this understanding if its only available in some advanced tech diving course? Especially when that diver has no current interest in tech diving.

I feel like I'm asking to see the secret handshake or something...

 

[mccabejc]

I guess the concept I was addressing in my original question is that it seems to me that shallow dives (say 30-50ft.) are pretty linear. In other words, planning your dive time can be not much more than monitoring your fairly constant and linear SAC rate, and when air is getting low you ascend. Pretty simple and predictable. NDL time usually isn't a factor.

However, in deep dives (90ft. or so), there is introduced another, much more nonlinear factor that you have to contend with, and that is NDL time. Assuming you want to limit your dive to non-decompression, you now have to keep close track of remaining NDL time, which may be dropping very quickly. In addition, your SAC rate varies much more with depth, so that is an additional variation you have to contend with. Plus, you don't know if your NDL will keep dropping even as you ascend, which makes things even more complicated.

Bottom line, I guess you can probably get by with not much planning of dive time for shallow dives, but I guess that's not the case with deep dives. And my original question was intended to say something like "in practice, do folks usually preplan dive times for deep dives, or are you familiar enough with the limitations so that you can plan-as-you-go while underwater?"

Yeah, I know. Take a class.

 

[Doc Intrepid]

...my original question was intended to say something like "in practice, do folks usually preplan dive times for deep dives, or are you familiar enough with the limitations so that you can plan-as-you-go while underwater?"

Put that way, you don't need to take a class. Its pretty straightforward.

Folks usually preplan dive times (and expected gas consumption/ gas reserves) for deep dives. At least folks who know what they're doing. Do it often enough to become extremely familiar with it, and you'll likely find yourself using the same plan over and over - but there will still be a plan in place (for duration, expected consumption, contingencies, etc.).

People who adopt a 'don't-worry-be-happy' attitude for dives to 100'-130' are often those who either have interesting stories to tell about incidents at depth, or else you read about them on the boards.

 

[DepartureDiver]

Yeah, I know. Take a class.

Yes, but there is a wealth of info here too that you may not be exposed to in a class. What I would add to your question is that gas consumption is always a factor that must be considered. For example, if you dive to 100' (4 ata) for 20 minutes and your SAC/RMV is .8 cubic foot per minute and your diving with an aluminum 80 (really a 76), then do you really have enough gas? The gas used on the bottom would be 64 cubic feet. Since you would need to plan to bring both you and your buddy back on one tank in case things went wrong while still doing your nice slow ascent with proper safety stops, I would say that you must still plan this dive around your gas consumption and not the NDL. So while many people may just go and do the dive, it really should be planned with many details being considered.

 

[ianr33]

I guess the concept I was addressing in my original question is that it seems to me that shallow dives (say 30-50ft.) are pretty linear. In other words, planning your dive time can be not much more than monitoring your fairly constant and linear SAC rate, and when air is getting low you ascend. Pretty simple and predictable. NDL time usually isn't a factor.

However, in deep dives (90ft. or so), there is introduced another, much more nonlinear factor that you have to contend with, and that is NDL time. Assuming you want to limit your dive to non-decompression, you now have to keep close track of remaining NDL time, which may be dropping very quickly. In addition, your SAC rate varies much more with depth, so that is an additional variation you have to contend with. Plus, you don't know if your NDL will keep dropping even as you ascend, which makes things even more complicated.

Bottom line, I guess you can probably get by with not much planning of dive time for shallow dives, but I guess that's not the case with deep dives. And my original question was intended to say something like "in practice, do folks usually preplan dive times for deep dives, or are you familiar enough with the limitations so that you can plan-as-you-go while underwater?"

Yeah, I know. Take a class.

I think the above is a pretty fair summary. A plan for a 30 foot dive can be "lets swim around looking at the pretty fishes for a while and when one of us gets low on air (500 psi) we will surface. " that will work fine

Lets put some numbers in for a 100 foot dive: I know my computer has an NDL at 100 feet of 19 minutes on air. I also know that that is pretty aggressive so I decide to back it off a bit and decide that we will start to ascend at 15 minutes.I also know that the Rock Bottom pressure to get 2 divers back from 100 feet is around 1000psi (Al 80, stressed divers,SAC rate of 1) I will pad that a bit as my gauge may be innacurate,and my reg may not work too well with only 100psi in the tank,so lets say our Rock Bottom is 1250 psi

We start to ascend at either 15 minutes OR 1250 psi,whichever comes first. I know that I normally use a bit less than 100 psi/minute at 100 feet so can reasonably expect to have at least 1500 psi remaining after 15 min utes,unless we are working hard swimming into a strong current or something.

If you have no idea what your SAC rate is then that is an excellent excuse to go dive and measure it !!

What you absolutely DO NOT want to do is get to 15 minutes at 100 feet and go chase a Whale Shark for 5 minutes. Suddenly you have a deco obligation and only 500 psi left in your tanks which is when Murphy raises his head and ruptures one of your low pressure hoses. That would be the start of a real bad day

 

[Dive-aholic]

The question is, how does the safety-conscious recreational diver ever develop this understanding if its only available in some advanced tech diving course? Especially when that diver has no current interest in tech diving.

I feel like I'm asking to see the secret handshake or something...

One thing you can do without going through the expense or time commitment of taking a tech class or DM class is to buy PADI's Encyclopedia of Recreational Diving and their DM book. The Encyclopedia provides some pretty in depth information on physics and physiology. The DM book has a chapter on NDL and M-values (although the M-values info is kind of confusing).

Do we plan all dives according to the tables? Not the shallow ones if they're the first dive of the day, but we've done enough diving that we know we'll run out of air long before we hit our NDL. However, just to keep up on using the wheel and tables (if you don't use them, you can't explain them), we do figure out our pressure groups in between dives. We also try to hold off diving again until we're As or Bs (sometimes we'll push a C). We do plan all deep dives, though, because our air can outlast our NDL time. And what if your computer malfunctions on a dive? Do you want to just quit diving for the day? Not us. When we dive deep, we dive with ponies and redundant gauges. That way, if our computer malfunctions (hasn't happened yet, fortunately), we can still continue to dive off the tables.