How much wiggle room?

[kr2y5]

I have two closely related questions that have to do with the amount of wiggle room with respect to the pre-established plan, or within the boundaries agreed on beforehand, that one finds comfortable during a tech dive. I apologize in advance if this topic has been beaten to death, and/or if my post offends some people's sensibility.

1) Clearly, there are dives, e.g., recreational dives within NDL, on which many people will just set very rough boundaries (such as max bottom time, max depth, no deco, rock bottom gas, and so on), but where otherwise, they will not follow a rigid schedule. One can improvise, ascend and descend repeatedly, end the dive a bit sooner or later, depending on whether gas or NDL turns out to be a limiting factor. Let's call this an area with a great amount of wiggle room.

At the opposite end of the spectrum are tech dives, as taught in tech classes, where there is a fair amount of prior planning that culminates with a schedule that one follows to the letter, and where being as much as a minute late is unacceptable. Much has been written here and elsewhere about how planning, and following a plan, is the core of a technical diver's mindset. Some (perhaps many) will insist that on any tech dive, one has to follow a rigid schedule, and no compromise is possible. Let's call this an area with very little wiggle room.

In between these extremes lies a gray area that appears to be inhabited by a great many divers. Some will do tech dives with two or more redundant computers. They will establish some boundaries, much as on a rec dive, but otherwise allow for some changes on the fly, perhaps even "ride" their computers all the way. Others may resort to a method such as ratio deco to do real-time adjustments. Yet some others may prepare and follow a strict schedule during most of the ascent, but they will still get out of the water faster if their computer clears their deco sooner than what the ascent schedule requires.

To folks who live in the gray area: I am curious where you draw the line, on what kinds of dives you will follow a strict schedule, on which you will make things up on the fly, and on which you will do something in between. Max N depth? Max N minutes of deco? Max N of whatever? Something else? How much flexibility are you comfortable with vs. how much do you want to pre-determine, etc. Please feel free to interpret "where you draw the line" in whatever way makes some amount of sense to you.

2) Clearly, there are situations, in which all divers on the team follow precisely the same strategy, e.g., when there is a strict schedule established beforehand. There are others, in which this might not be the case. Maybe someone has 31% fill, and someone else has 32%. Someone might be diving algorithm X, or computer brand X, another might be diving Y (obviously, this does not apply to folks who reject computers). When folks take advantage of computers, they might remain at different depths, and incur different amount of deco obligation, and their computers may require stops of a different length. In those circumstances, there will inevitably be some variability.

Again, to folks, who live in the gray area: I am curious how much variability are you comfortable with. Would it be OK if one person ends up with N minute more deco than the other? What N is OK and what isn't? What, and how much, is OK to vary? Can the amount of deco vary? Can the algorithm/conservatism vary? Can gas, or something else vary? How much can it vary? Or does everything have to be precisely the same? Again, where will you draw the line? And, what do you do to make sure you won't cross it?

If you think those are dumb questions, that's OK... just do your best to help a brother in need, and share your thoughts, anyway... all feedback is welcome. Thanks!

 

[TSandM]

There is no dive in the world where I follow a strict schedule. Rick Murchison here on SB, a very old and wise diver, has opined that decompression is measuring with a micrometer, marking with chalk, and cutting with an axe -- and he is completely right. Run a deco schedule on 30/70, or V-planner; you will get slightly different profiles. Is one right and one wrong? Unlikely. Plus, no dive goes precisely as planned; in fact, it's been said that no plan survives contact with the water. You get to the wreck, but the interesting part is a little deeper or a little shallower than you thought. Or you get cold faster than you anticipated. Or somebody is having problems with leg cramps, or a piece of equipment starts malfunctioning, and your dive time is shorter or your average depth is different from what you planned.

One of the reasons I love RD is that it makes it extremely easy to deal with all of that. But I do set a max depth and max time, and I don't violate that, because the gas I'm diving is a gas for the depth I plan, and the gas supply and deco gas supply I have is planned for the max time.

I don't think you'll find very many people who dive a schedule to the minute.

 

[KenGordon]

What I do for dives involving significant deco obligations:

Plan the dive for the weaker o2 mixture I or my buddy has. I use DM5 as we both dive suunto computers, otherwise I plan with both DM5 and multideco set appropriately. Do deeper, longer etc and find appropriate schedules that will not bend the computers. Put these on a slate.

Dive the plan as an envelope. Typically that means leaving the bottom no later than planned then following the planned stops to the shallowest stop unless a computer objects and holds us back. Once at the shallowest stop follow the computers, maybe stay longer if padding seems like a good idea.

Since the dive was planned with software matched to the computer the differences are about not actually being as deep as the planned depth for the entire bottom time.

What I am trying to do is keep with the planned ascent and be sure that there is plenty of gas for the ascent and any contingency that might arise on the ascent. Gas is usually the limiting factor although being prepared to do the deco is also a serious consideration.

I am happy to do simpler dives on a more relaxed basis, but I still want to know I will have the gas for whatever plan. What is enough also depends on who I am diving with, what contingency they have and so forth.

 

[LiteHedded]

on some dives, particularly on deep cave dives, it's pretty much impossible to plan it to the minute and not go under or over.

for a dive this weekend for example we're shooting for 40 minutes but bringing more than enough gas for 60 minutes on the bottom.
if i were to take a guess i'd say the runtime will end up being 45 or 50 minutes at 260 avg. there are just too many variables on a dive like this to say exactly how long it will be before we start our ascent

 

[tbone1004]

the hard line for tech for me is primarily working dives. If I am going down and I have a job to do and it is well defined. Recovery/salvage, etc.

Gray line is all "recreational" dives, i.e. it is for fun. I always have rules that won't be violated. It is usually gas and I'll ride the computer out for decompression, especially in caves, and loosely follow the Navy 120 Rule if the computer craps out for caves. I have most of the decompression schedules memorized for planned dives, so will have a planned duration and 10 minutes after just in case. These aren't technical trimix dives or anything, just ye old standard nitrox cave dives with a stage or two and O2 deco. This is all for fun, so if we turn at thirds and feel like exploring side passages on the way out, we'll recalculate and make sure the deco doesn't start getting too terribly bad. It is almost impossible to plan cave dives very strictly due to the flow, so in that case it is plan with a micrometer, mark with a paint roller, and cut with cannonball when you're trying to plan decompression with tables or plan based on dive time. Again, ocean diving is very different, but for cave diving it is all but impossible to accurately plan a dive without historical data.

That said, in low flow caves, like say Twin or P1 when it isn't flowing hard, I kick at 50fpm *much to the annoyance of my dive buddy, but it is like clockwork*, I know what my sac rate should be, and what the avg. depth should be, so I can plan fairly accurately, but as soon as the flow kicks up significantly it's game over. What is weird about that though is that I kick 50fpm into or out of the flow in say P1 at low-normal flow rates, so I think it is more my body saying I want to move at this speed than it is kicking at a steady thrust/rate.

I will also say that I am a HUGE proponent of taking AN/DP early because it removes the bindings of NDL diving. It may only be a 5 minute mandatory backgas decompression, but removing the "oh **** I have 1 minute left, time to go up before I hit NDL" removes a lot of stress from recreational divers because decompression is beat into their heads as "scary"

 

[kwinter]

Other than during training, time is not critical. Gas is the critical factor. Watch your TTS, or know how long it will take you to exit and surface including stops. Know your SAC rate. So when you plan your dive, you leave a comfortable margin. Then you know how much you can possibly dip into that margin and still leave at least a slim reserve. You also know that in an emergency, you can fast track your ascent. It's all a matter of balancing risk and reward. To stay a bit longer than planned in order to pull your buddy out of that restriction he is having difficulty with might be worth invading your margin. Staying too long to take a photo or grab a lobster might not be worth it. Watch your TTS and assess your risk.

 

[waterpirate]

For ocean wreck diving it is all the grey area. Each diver/team sets the parameters for margins of safety ect. My limits are based on MOD for back gas, and RT. Nothing gets you asked not to come back quicker than violating your estimated RT in the Mid Atlantic.
YMMV
Eric

 

[tbone1004]

oh, forgot to actually answer the original question.

Lots, and at the same time none is the answer. There are hard and fast rules that aren't to be violated. Those rules in caves are thirds, in ocean it's rock bottom. You turn the dive if you hit those. The other one which sort of follows that is how much decompression gas you actually have. I.e. if you have an AL40 as a deco bottle, assuming it is full, you have about 30 minutes of O2 deco, so you need to make sure you actually have enough gas to decompress fully. If you have time tables that are fixed for whatever reason, then you don't violate the time, the depth limits for MOD are fixed, those don't get violated, but if I have a light mixture and something is at 134ft and I only planned on 130, I'll go down there, but I won't violate PO2-1.6 for an emergency MOD or 1.4 for working portions. Some rules are hard and fast and can't be violated, others are more guidelines than rules, the diveplans are usually pretty loose, but the limits aren't.

 

[MXGratefulDiver]

I have two closely related questions that have to do with the amount of wiggle room with respect to the pre-established plan, or within the boundaries agreed on beforehand, that one finds comfortable during a tech dive. I apologize in advance if this topic has been beaten to death, and/or if my post offends some people's sensibility.

1) Clearly, there are dives, e.g., recreational dives within NDL, on which many people will just set very rough boundaries (such as max bottom time, max depth, no deco, rock bottom gas, and so on), but where otherwise, they will not follow a rigid schedule. One can improvise, ascend and descend repeatedly, end the dive a bit sooner or later, depending on whether gas or NDL turns out to be a limiting factor. Let's call this an area with a great amount of wiggle room.

At the opposite end of the spectrum are tech dives, as taught in tech classes, where there is a fair amount of prior planning that culminates with a schedule that one follows to the letter, and where being as much as a minute late is unacceptable. Much has been written here and elsewhere about how planning, and following a plan, is the core of a technical diver's mindset. Some (perhaps many) will insist that on any tech dive, one has to follow a rigid schedule, and no compromise is possible. Let's call this an area with very little wiggle room.

In between these extremes lies a gray area that appears to be inhabited by a great many divers. Some will do tech dives with two or more redundant computers. They will establish some boundaries, much as on a rec dive, but otherwise allow for some changes on the fly, perhaps even "ride" their computers all the way. Others may resort to a method such as ratio deco to do real-time adjustments. Yet some others may prepare and follow a strict schedule during most of the ascent, but they will still get out of the water faster if their computer clears their deco sooner than what the ascent schedule requires.

To folks who live in the gray area: I am curious where you draw the line, on what kinds of dives you will follow a strict schedule, on which you will make things up on the fly, and on which you will do something in between. Max N depth? Max N minutes of deco? Max N of whatever? Something else? How much flexibility are you comfortable with vs. how much do you want to pre-determine, etc. Please feel free to interpret "where you draw the line" in whatever way makes some amount of sense to you.

2) Clearly, there are situations, in which all divers on the team follow precisely the same strategy, e.g., when there is a strict schedule established beforehand. There are others, in which this might not be the case. Maybe someone has 31% fill, and someone else has 32%. Someone might be diving algorithm X, or computer brand X, another might be diving Y (obviously, this does not apply to folks who reject computers). When folks take advantage of computers, they might remain at different depths, and incur different amount of deco obligation, and their computers may require stops of a different length. In those circumstances, there will inevitably be some variability.

Again, to folks, who live in the gray area: I am curious how much variability are you comfortable with. Would it be OK if one person ends up with N minute more deco than the other? What N is OK and what isn't? What, and how much, is OK to vary? Can the amount of deco vary? Can the algorithm/conservatism vary? Can gas, or something else vary? How much can it vary? Or does everything have to be precisely the same? Again, where will you draw the line? And, what do you do to make sure you won't cross it?

If you think those are dumb questions, that's OK... just do your best to help a brother in need, and share your thoughts, anyway... all feedback is welcome. Thanks!

You post some interesting threads, my friend ... and for the sake of transparency, let's establish that these questions are the result of the dives you and I have been doing recently which, while remaining within recreational depths, have involved some mandatory deco and some differences in how you and I calculate our deco schedules. So ... given that I was there on those dives ... I'll add my perspective to your questions.

First off, I've rarely ever done a tech dive that involved sticking to the schedule by the minute. In some cases I've gone a bit deeper, some shallower, some longer, some shorter. In some I've adjusted my ascent profile based on circumstances. In some I've calculated the deco schedule ahead of time, and in others I've "rode" my computer ... with the caveat that I already had a pretty good idea what the deco schedule should look like and was using the computer more for verification than to determine my ascent profile. The only time I've ever stuck strictly to a predetermined schedule was during a class ... when it was mandatory to demonstrate that I could. Other than that, I establish max limits (depth, time, deco) and maintain my profile within those limits. As an example, on our last dive we established that our dive would max at around 110 feet and that we'd leave the bottom at or before one of us reached a max deco obligation of 10 minutes. We did, in fact, hit 110 feet and when you signaled to me you were at 10 minutes deco we were already on our way upslope.

On gas differences ... it's always best to match gases between two divers ... it just makes dive planning and execution so much easier. Minor differences can be accommodated easily enough (a 1% difference doesn't really amount to much) but should always favor the person who's likely to run into deco first. Likewise differences in dive computer algorithms, if that's how you're planning your ascent strategy ... set your limits based on the person with the most conservative settings. But don't base your decisions strictly on the information your computer is giving you. For example, you mention divers with different computers remaining at different depths ... bad idea. Stay together. If two divers computers are telling them that they need to stop at different depths, stop at the deeper depth and honor that obligation. What it's going to cost the other diver is usually more deco time than their (more liberal) computer is telling them they need. As long as you have the gas for it, and there's no other mitigating circumstances, then it's a no-brainer ... nobody ever got hurt doing more deco than they think they're required to do. This is why we remained at 20 feet the other night, even though my computer said I should be at 10 ... and why we remained there even after my computer said I had cleared deco ... we honor the obligation your computer calculated based on the conservatism settings you chose. Always "err" on the side of conservatism, assuming adequate gas supplies and lacking some more serious reason why you should get out of the water sooner.

As for "what N is OK" ... it needs to make sense. In our dive of three weeks ago, it was clear to me ... and I suspect it was clear to you ... that your computer was set at a conservatism level that was way out of whack. In my case, I was basing on previous experience ... the amount of deco you indicated to me clearly was out of whack (by a factor of about 3X) with what I would expect for the dive profile. In that case, I went to about 2X what I thought appropriate, and then indicated to you that we should just "bend" your computer and go in because I saw no downside to doing so, and a significant downside to remaining in the water. We discussed the reasons for that after the dive, and I expect you understand why ... there should always be a "why" to that type of decision, and it will rarely be based on some hard and fast formula.

The answer to most of these questions would come from a better understanding of what deco is, and why we do it ... at least at the level that most tech divers comprehend such things. Mark Powell's excellent book "Deco for Divers" is a great place to start developing that understanding. If you haven't read it, I'll be happy to loan you my copy (although to my concern this book should be in the hands and mind of anyone who's contemplating getting into planned decompression diving). This isn't something you can rely on hard answers for ... it's going to be a judgment call, based on the circumstances of the dive. And even dive profiles won't give you all the factors ... as there are several important ones you need to consider based on such things as how you're feeling on a given day (that affect your ability to offgas properly) and the conditions of the dive (how hard you're working, how cold you're getting, etc).

Deco is a lot like the weather ... it's based on a multitude of factors that interact with each other dynamically. You have to factor in as many of them as you can, and adjust your deco schedule accordingly. Algorithms anticipate some, idealize others, and usually factor in some level of conservatism. The conservatism settings on your computer can add even more "fudge factor" ... but those always come with trade-offs. You can't really put all of them into context by talking about it on the internet ... it's one of the reasons we're doing dives at that level right now. Practical application will help you develop a better understanding of the information your computer is providing you with, and how to make the best judgment calls based on that information. In effect, it's a bit of an artform ... and as a result there are few hard and fast rules that will apply in all cases.

I'll leave you with a famous quote from Richard Pyle's famous treatise "Fizzyology" ...

"If you ask a random, non-diving person on the street to explain what's really going on inside a diver's body that leads to decompression sickness, the answer is likely to be "I don't know".
If you ask the same question of a typical scuba diving instructor, the answer will likely be that nitrogen is absorbed by body under pressure (a result of Henry's Law); and that if a diver ascends too quickly, the excess dissolved nitrogen in the blood will "come out of solution" in the blood to form tiny bubbles; and that these bubbles will block blood flow to certain tissues, wreaking all sorts of havoc.

Pose the question to an experienced hyperbaric medical expert, and you will probably get an explanation of how "microbubbles" already exist in our blood before we even go underwater; and that ratios of gas partial pressures within these bubbles compared with dissolved partial pressures in the surrounding blood (in conjunction with a wide variety of other factors) determine whether or not these microbubbles will grow and by how much they will grow; and that if they grow large enough, they may damage the walls of blood vessels, which in turn invokes a complex cascade of biochemical processes called the "complement system" that leads to blood clotting around the bubbles and at sites of damaged blood vessels; and that this clotting will block blood flow to certain tissues, wreaking all sorts of havoc.

You will likely be further lectured that decompression sickness is an unpredictable phenomenon; and that a "perfect model" for calculating decompression schedules will never exist; and that the best way to calculate the best decompression schedules is by examining probabilistic patterns generated from reams of diving statistics.

If, however, you seek out the world's most learned scholars on the subject of decompression and decompression sickness, the top 5 or 6 most knowledgeable and experienced individuals on the subject, the ones who really know what they are talking about; the answer to the question of what causes decompression sickness will invariably be: "I don't know". As it turns out, the random non-diving person on the street apparently had the best answer all along."

To put that in the context of your final two questions ... there is no line. There's a continuum of different shades of gray ... and the thing we do when we decide our deco schedule is decide which shade represents the most acceptable level of risk we're willing to take. Occasionally we're wrong. The best way to reduce the risk of being wrong is to choose a shade of gray, then slide a bit to the conservative side of it and let that represent your profile choices. And always remember that there are pros and cons to any choice you make ... consider them all and prioritize accordingly. For example, never allow your deco schedule to take precedence over your available gas supply. Factor in things like being cold (which can become debilitating) or other physical factors. If you can anticipate those things, adjust your dive profile to reduce your deco ahead of the ascent. If not, choose according to what can most likely go wrong and what the potential consequences could be. In most cases, a few minutes of extra deco won't hurt you, so just stay down and do them.

... Bob (Grateful Diver)

 

[bradells]

While I don't tech dive, I do dive with a lot of Tech Divers while on a rec profile.

As eluded to above, most of the planning is the envelope. Max depth (or avg depth), planned run time, and ascent strategy (in the context of this thread), and knowing how the envelope can change on a dive and the results.

Heres an example:
Last dives, 17 of 18 divers hadn't been to this brand new wreck. All we knew is you couldn't go deeper than 110ft without some dynamite, and it's sitting upright.

Since the goal was to get 2 dives in a ~3hr window (or 1 long decompression dive), it makes planning pretty simple if you wanted 2 dives. 1hr dive window, 1hr SI, 1hr dive window.

My team was diving similar tanks (2 sets of 130's, 1 set of 100), and our first hard wall was our MOD of 30m wrt our gas, and the second hard plan for the first dive was to have 2000psi left in the 100's for the second dive. The third was again, the dive operating windows.

First dive, the plan was for an average depth of 24m, bottom time of 50 minutes.
We hit 28m for our max (avg 22m) with a 40min bottom time, which left us tonnes of gas for our second dive.

With a 1hr SI, our MDL were cut in half, so our plan was to average out around 21m with an estimated runtime of 30 minutes.
On that second dive, given our parameters, we hit a max of 22m, averaged at 18m, and a 35minute bottom time.

Again, surfaced with lots of gas, and had 2 full dives within our operating windows.


WRT tech diving, its pretty similar, as in, if you have the proper training, you know what your profiles should be (at least for simpler dives) for given depth/time, so you can plan for a shorter deco if you leave the bottom faster/stayed shallower, or stayed longer/went deeper.


Now another point. I wouldn't do those kind of dives with other divers outside of our core group or as a mixed team. I do some rec dives with other people from other backgrounds, but there is so much discrepancy between the organizations at that level (from - just follow your computer to deep air to a little deco is ok and everywhere in between to I'm in BM with a deco bottle, rebreather, etc on a rec profile).

BRad