Lesson for Life, what do new divers think about this?

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doctormike

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I generally enjoy Eric Douglas' column "Lessons for Life" in Scuba Diving magazine. But a recent one had me concerned. Maybe I am missing something...

This column is called "Pushing the Limits on a Liveaboard leads to DCI for this Scuba Diver", and it involves the risks of doing multiple dives on a vacation. Fair enough, good point. My concern is that the dive profiles that the author uses are so ridiculously OVER the "limits" that it actually affects the conclusion. The point should be that you can stay within your computer's NDLs and still get bent. But I can't imagine how he came up with these numbers...

The first day in the water, Heath made four multilevel dives while following his dive computer. It was set up to calculate his no-decompression limits based on the gas mix he was breathing. His first dive was to 106 feet with a total bottom time of 53 minutes. He had a surface interval of 1:49. Dive two was to 123 feet for 68 minutes, followed by 2:27 out of the water. Dive three was a dive to 91 feet for 75 minutes with a surface interval of 1:35. His fourth and final dive of the day was to 93 feet for 43 minutes. He finished up in time for dinner at 6:30 p.m. Heath didn’t experience any problems on the dives and performed a routine safety stop at the end of each one.

These profiles are SO far into deco, the diver didn't have a chance of doing even one of them without major risk, let alone four. Using MultiDeco and a very aggressive gradient factor of 100/100, I get 20 minutes of deco for the first dive. And even if he DID that (which he didn't), he would have had over an hour of deco on his second dive. The article states that he did no deco at all. I guess they do state that they were "multi level" dives, so maybe the diver spent a lot of time at shallower depths. But they don't say that, and the way it is written you would really need to assume a square profile, otherwise the whole point is lost.

The problem with this is that a new diver will think that this is sort of in range but "pushing it", and not realize how way out of line this dive plan is. We often talk about the "sanity check" needed for computer generated dive plans. A diver doesn't have the ability to do these ascent profiles in their head, but they should at least be able to look at something and decide that it sounds like it's out of the ballpark.
 
I’m not very new anymore, but I still think that’s ridiculous.
 
DoctorMike:

I self-identify as intermediate, not a newbie, so hope posting to your thread isn't inappropriate. I have no deco training, so I have to ask...are you assuming square profile dives where he spent most of the dive time at those depths, and assuming recreational practices where he avoided going past NDLs (as opposed to doing deco stops & clearing his deco obligations before surfacing)? And it says he planned to dive EAN32 all week, yet "Dive two was to 123 feet for 68 minutes, followed by 2:27 out of the water." Well past the max. operating depth for that mix (I don't recall EAN 32's contingency depth, but over an hour at contingency depths seems unlikely).

The article seems a little short on detail (like where was this live-aboard? Those are some deep dives!). To make sense of it, a newbie would need to know a bit about nitrox (if only to not be confused by the term), able to intuit those likely weren't square profile dives, aware that this guy was a dive instructor and tec. diver so he was likely quite advanced (not your 'average' diver), and as to whether 59's getting a little past the prime and time to be more conservative...well, somebody else can brave that discussion. There are plenty of folks in their 50's fitter than me (as I approach 49...).

Richard.
 
There have been several instances cited recently of this author presenting highly unlikely or impossible scenarios as fact in these stories. I don't understand why he can't just stick to reality and still make his points. As for repetitive dives on liveaboards, one of the things I like about the Shearwater computers is their detailed tissue loading graph, which allows you to closely monitor tissue loading over multiple days of multiple dives.
 
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I generally enjoy Eric Douglas' column "Lessons for Life" in Scuba Diving magazine. But a recent one had me concerned. Maybe I am missing something...

This column is called "Pushing the Limits on a Liveaboard leads to DCI for this Scuba Diver", and it involves the risks of doing multiple dives on a vacation. Fair enough, good point. My concern is that the dive profiles that the author uses are so ridiculously OVER the "limits" that it actually affects the conclusion. The point should be that you can stay within your computer's NDLs and still get bent. But I can't imagine how he came up with these numbers...



These profiles are SO far into deco, the diver didn't have a chance of doing even one of them without major risk, let alone four. Using MultiDeco and a very aggressive gradient factor of 100/100, I get 20 minutes of deco for the first dive. And even if he DID that (which he didn't), he would have had over an hour of deco on his second dive. The article states that he did no deco at all. I guess they do state that they were "multi level" dives, so maybe the diver spent a lot of time at shallower depths. But they don't say that, and the way it is written you would really need to assume a square profile, otherwise the whole point is lost.

The problem with this is that a new diver will think that this is sort of in range but "pushing it", and not realize how way out of line this dive plan is. We often talk about the "sanity check" needed for computer generated dive plans. A diver doesn't have the ability to do these ascent profiles in their head, but they should at least be able to look at something and decide that it sounds like it's out of the ballpark.

All I can imagine is that they were bounce dives and that the maximum depth may have been what is stated but the majority of the bottom time was done much shallower.
 
DoctorMike:

I self-identify as intermediate, not a newbie, so hope posting to your thread isn't inappropriate. I have no deco training, so I have to ask...are you assuming square profile dives where he spent most of the dive time at those depths, and assuming recreational practices where he avoided going past NDLs (as opposed to doing deco stops & clearing his deco obligations before surfacing)? And it says he planned to dive EAN32 all week, yet "Dive two was to 123 feet for 68 minutes, followed by 2:27 out of the water." Well past the max. operating depth for that mix (I don't recall EAN 32's contingency depth, but over an hour at contingency depths seems unlikely).

The article seems a little short on detail (like where was this live-aboard? Those are some deep dives!). To make sense of it, a newbie would need to know a bit about nitrox (if only to not be confused by the term), able to intuit those likely weren't square profile dives, aware that this guy was a dive instructor and tec. diver so he was likely quite advanced (not your 'average' diver), and as to whether 59's getting a little past the prime and time to be more conservative...well, somebody else can brave that discussion. There are plenty of folks in their 50's fitter than me (as I approach 49...).

Richard.

Hey, Richard...

Of course, always happy to hear your input! I just meant that newer divers might not intuitively see that something is way off in the numbers, and might interpret this as just "pushing it", which might be OK in some circumstances, as opposed to "ridiculous".

I am assuming square profiles because that's how the article is written. The point is that they specified those depths and times. It's describing a dive trip, it's not a deposition, so it wouldn't make sense for the author to justify the plan after the fact by saying "oh, yeah, I meant to say that on that 68 minute dive he just touched bottom and then spent the rest of the dive at 20 feet".

You are right about the MOD of EAN 32 (it's 111 for 1.4, 132 for 1.6 but that's just used for deco normally).

The location of the liveaboard, the age of the diver, and the certification level of the diver would be irrelevant in this situation - a 21 year old navy seal certified as an expedition trimix cave CCR whatever diver would still be just as bent (if not dead) doing those dives and blowing off such a huge amount of deco!
 
All I can imagine is that they were bounce dives and that the maximum depth may have been what is stated but the majority of the bottom time was done much shallower.


Sure (see my response to Richard), but my point was about the way the article was written. If you have to assume all sorts of things that aren't in the text to have it make sense, then it's not written well. I know that the whole point of a dive computer is to give you deco credit for time above the bottom.
 
Mike, let’s say you were on a wall, performing a NDL dive on 32%. Let’s also say you went to the Spree boat captain school of nitrox where we don’t believe 1.4 PPO2 is any sort of limit, and that 1.6 is perfectly acceptable limit. So you are on your OC dive, and you went to 123 feet to see the blinker fluid container, then came to the top of the wall in 60 feet and made the bulk of your dive, and then came across the sand to 15 feet to your safety stop.

This profile is made thousands of times a day in Cozumel.

So the dive wasn’t really 123 feet for 68 minutes, it was 123 feet for less than a minute, followed by 60 feet for 42 minutes, followed by a slow ascent to 15 feet for 13 minutes, followed by a safety stop, all without ever violating an NDL. But it’s way quicker and meets convention (a liveaboard records depth and bottom time, not profile). And, why pay for 37 words where 3 suffice?
 
One of the harder things to get a sense of early in diving is the common question, 'How long does/should a tank last?' The answer varies amongst individuals and over time, but judging not only from my own struggles with airhoggery but what I've seen posted by others, I think managing a 50-60 minute dive at modest depths (average over the dive maybe 30-something feet, including safety stop) on a typical AL80 is a bit of a milestone for some.

So a 68 minute dives to 123 feet, even if we knock it down a bit for descent, ascent and safety stop (let's call it 50 minutes at 123 feet for sake of argument) sounds pretty wild to me. Granted, this guy was seasoned and probably had significantly less gas usage than I do, but still, that's around (actually a little under) 5 atm pressure. Even if deco. weren't an issue with such a dive, that'd be something!

Most recreational live-aboards I read of mainly offer 80-cf tanks, sometimes 100-cf tanks. If you breathe a 100-cf tank empty at 5 atm over 50 minutes, 100 cf/5 atm/50 min. = SAC 0.4 cf/min? Hope I ran the numbers right. That leaves no gas for descent/ ascent or a safety stop, or a 'back on the boat with 500 PSI' style reserve. There are people with SAC rates around 0.4 cf/min. (I'm thinking seasoned dive guides, some petite females, some advanced divers with superb in water skills), but even so...

A relatively new diver isn't going to grasp that. Question: at what point in their diving (let's say, by dive count) do most divers gain a 'ball park' working knowledge of roughly how long an AL80 tank ought to last them at typical recreational depths? I'm not asking at what point they should know; I'm asking at what point they do. I learned 'experientially' (still not great at it!).

Richard.

P.S.: I wasn't precise on my atm pressure; I have a fondness for round numbers.
 
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