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)
Bob, to clarify the intent of my question, I am not looking for any magic black box formulas, and I do not expect there to be any "hard" line... I am simply interested in how different people approach the question, knowing that it lies in a gray zone. Of course, the judgment is very subjective, but still, in specific circumstances, somehow folks decide to do things one way or the other, and those decisions do effectively define a (very wiggly and irregular, but nevertheless a) "boundary", if you will. Certainly not a boundary that represents a universal law of diving, not necessarily one that is easy to capture by a simple prescriptive formula, but a fuzzy "boundary" that reflects actual decisions people have made in the past.
I realize this highly depends on circumstances, and different people will make different choices. I use terms like "boundary" or "draw the line" mostly because of the need to formulate that question somehow... the intent of my question is not even so much in precisely delineating any "universal" boundary (which probably isn't feasible, or doesn't even make sense whatsoever), and more about understanding what kind of decision process different people followed, based on what personal philosophy... and some real-life examples to support this.
Maybe a different way to phrase the same question would be: under what circumstances might you hesitate one way or the other (e.g., use fixed runtimes, or not), and what would be going on in your head at the time? What factors would you then be taking into account? Can you think of past examples when this was the case, and explain what you decided on (e.g., what limits you had set vs. what you had left for yourself to decide on the fly), and why? Were there any situations, in which you had to adjust your schedule on the fly, and afterwards you realized that you did not plan quite enough ahead of the time, and that you left too much wiggle room... and so on.
I know that some folks are very strict about planning, and look down on things like "riding a computer" in any circumstances, or at least that is the impression I got from some of the posters in the past (some would even refer to computers, condescendingly, as "magic bracelets", and insist that they develop dependence on technology). A number of books I've read convey some of such message as well. Even though nobody here expressed such sentiment yet, as some posted here, there are examples dives, on which they would stick with a fixed sequence of runtimes. I always imagined this is also what happens on all "big" dives, and it's interesting to find otherwise (assuming that LiteHedded was talking about 260ft average depth, to me that sounds firmly in the "bad ass" zone). All the responses so far were pretty interesting...
Nobody I met in real life, so far, was actually very strict, indeed.
Even the instructor in my AN/DP class, after first confirming that I can plan and follow runtimes to the minute, insisted that we apply common sense, and shorten some of the stops if the computer I carry cleared those sooner. I'm not saying that I find it surprising, or that I don't understand the reason... just that it represented a slightly different philosophy than what was communicated elsewhere, even if those differences were subtle. As was proposed here earlier, I was taught to mostly focus on ensuring that I have enough gas, and pay attention to things like TTS, but to otherwise be flexible. While I was sitting on a boat writing my deco schedules on duct tape to tape them to my fins (following examples from Gary Gentile's book), the rest of the boat was probably having good laughs
I have yet to meet someone who actually does that sort of thing for real.... meanwhile, my instructor was making a point of carrying 3 computers.
I guess in this thread, and pretty much all my other threads, I am not really looking for textbook or any other "official" knowledge, fixed rules, etc., I am more interested in reconciling things I've read here or there, with what other divers actually do in real life. I don't mean to sound defensive, but I hope you understand that "read Deco for Divers", or "it will all be explained in the class" is very far from the kinds of answers I'm looking for (I've read that book, BTW.) Not to dismiss textbook knowledge (and I try to read a lot), but I'm actually more interested at this point in the sort of open discussion and real-life stories that can't be found in any books, and I'm quite OK with getting a lot of fuzzy, unclear, and contradictory answers. I thought I should clarify this because it has been a source of misunderstanding in a number of past threads and conversations. I think there's a lot of merit to having an open discussion on an online forum.
I realize that much of the time, the answer is "it depends". Many folks are OK with that answer, I like to dig deeper... not necessarily expecting to find a golden rule of thumb. I think digging itself can be very educational, especially if I get exposed to a lot of different people's perspectives in the process. Knowing different perspectives, to me, carries a high value.
