General thoughts on the dive physics questions
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Not sure what you mean by "fair". I think a DM should understand ongassing, compartments, etc. whether this info. ever comes into play during his/her duties or not. A good backround is necessary and as well no knowledge is "bad" knowledge.
Having said that, in my limited time with OW courses no student ever asked me to explain this stuff (my rapport with students, I think, was very good--with my school teacher backround).
jcp2-- Right. You never caught me working for tips. The great majority who do are IMHO doing a big disservice to those who seek pay. A very old topic and pet peeve.
wstorms
Contributor
The concept of ascending "too slowly" is quite interesting.
Let's see at this problem historically.
Initially there were just the US Navy tables. They were assuming that the time was starting at surface, when starting descending, and was ending at maximum depth, when starting to ascend. And an ascend rate of 18 meters/minute was the standard one. If going slower than this, the additional ascend time had to be added to the bottom time. Hence, ascending slower than 18 m/min was not dangerous, was actually safer, if this additional time was taken into account as larger bottom time. The problem was if this additional ascend time was not added to the bottom time, as someone was wrongly doing (but the US Navy manual was very clear on this point).
Then, in the seventies, some study did demonstrate that an ascend speed of 18 m/min was too large, and a new ascend speed of 10 m/min MAXIMUM was recommended. Going even lower was safer, as now the whole ascend time had to be considered part of the bottom time. So the bottom time to be considered was starting at surface, and was ending after the ascend, when reached 9m, launched the SMB, the diver was looking at his tables for calculating the length of the deco stops (no NDL to be respected, in the seventies, deco was the standard).
Again, with this approach (which was the one I was trained for in 1975, and which I followed until one year ago) there was no risk slowing down the ascend, as this was simply increasing the bottom time, and of consequence the duration of the deco stops.
Then the computers arrived, and with them it appeared evident that if the ascend speed was very slow, the deco time was increasing, so instructors started recommending to avoid too slow ascend speed. In reality, for a dive with deco, this is simply a longer dive, as it was when using the tables, and means no additional risk: simply this will cause you making longer deco stops.
The problem exploded when training agencies started to consider it safer to dive WITHIN NDL limits, and to plan the dive accordingly. If you stay at depth for the maximum time allowed for NDL, and you ascend too slowly, you exceed the NDL, and this started to be seen as "dangerous", as the new safety rule was set to always stay within NDL.
So now, suddenly, ascending slowly became a problem, and it became very important to teach new divers to ascend at the "correct" speed, as now ascending too slowly or too quickly was equally wrong.
In my opinion, this evolution makes now diving more dangerous than in the past. Divers are taught to base their diving plan on their computer, and to "ride the NDL" in such a way that they stay down as long a possible, and then they ascend "at the proper speed", following the sharp edge of NDL. They end up at surface in conditions very, very close to initial DCS symptoms, despite the "safety stop", which is too short for providing a true deco effect.
The old deco dives with tables, for a number of reasons, resulted instead systematically in a gross overestimation of the Nitrogen absorbed, and in very slow and long deco profiles, ensuring ample margin below the critical edge of DCS.
But in the end I had to surrender, and one year ago I bought my first computer (very basic and cheap). In most cases, it causes me to make more aggressive dives than those I was doing previously following the tables.
Now I can stay down longer, and make no deco. How this can be safer is something I do not understand...
I still try to stay on the safe side of what the computer says, making a longer safety stop, and possibly going very slowly from 5m to surface at the end of it. And I still continue ascending from depth much slower than 10m/min, accepting that this will reduce the time I can stay down, or that I will be forced to some deco stop before emerging.
Now back to the topic: what is correct to explain to students at their first course, and what to explain to divemasters or instructors? Do they need to know all the history?
Probably not for a beginner's course, but they should also be warned to not "ride the NDL", always keeping a robust margin before the limit which will allow to ascend at a much slower speed, still arriving at 5m with the computer signalling a significant time before the NDL...
On the other side, a Divemaster should know the whole history and the reasons for which a slower ascend need to be considered as a longer bottom time. He should also understand that a dive planned with deco is actually SAFER than a dive within the NDL, of course requiring a longer training (and certification, possibly), proper equipment and logistics, and proper planning.
Let's see at this problem historically.
Initially there were just the US Navy tables. They were assuming that the time was starting at surface, when starting descending, and was ending at maximum depth, when starting to ascend. And an ascend rate of 18 meters/minute was the standard one. If going slower than this, the additional ascend time had to be added to the bottom time. Hence, ascending slower than 18 m/min was not dangerous, was actually safer, if this additional time was taken into account as larger bottom time. The problem was if this additional ascend time was not added to the bottom time, as someone was wrongly doing (but the US Navy manual was very clear on this point).
Then, in the seventies, some study did demonstrate that an ascend speed of 18 m/min was too large, and a new ascend speed of 10 m/min MAXIMUM was recommended. Going even lower was safer, as now the whole ascend time had to be considered part of the bottom time. So the bottom time to be considered was starting at surface, and was ending after the ascend, when reached 9m, launched the SMB, the diver was looking at his tables for calculating the length of the deco stops (no NDL to be respected, in the seventies, deco was the standard).
Again, with this approach (which was the one I was trained for in 1975, and which I followed until one year ago) there was no risk slowing down the ascend, as this was simply increasing the bottom time, and of consequence the duration of the deco stops.
Then the computers arrived, and with them it appeared evident that if the ascend speed was very slow, the deco time was increasing, so instructors started recommending to avoid too slow ascend speed. In reality, for a dive with deco, this is simply a longer dive, as it was when using the tables, and means no additional risk: simply this will cause you making longer deco stops.
The problem exploded when training agencies started to consider it safer to dive WITHIN NDL limits, and to plan the dive accordingly. If you stay at depth for the maximum time allowed for NDL, and you ascend too slowly, you exceed the NDL, and this started to be seen as "dangerous", as the new safety rule was set to always stay within NDL.
So now, suddenly, ascending slowly became a problem, and it became very important to teach new divers to ascend at the "correct" speed, as now ascending too slowly or too quickly was equally wrong.
In my opinion, this evolution makes now diving more dangerous than in the past. Divers are taught to base their diving plan on their computer, and to "ride the NDL" in such a way that they stay down as long a possible, and then they ascend "at the proper speed", following the sharp edge of NDL. They end up at surface in conditions very, very close to initial DCS symptoms, despite the "safety stop", which is too short for providing a true deco effect.
The old deco dives with tables, for a number of reasons, resulted instead systematically in a gross overestimation of the Nitrogen absorbed, and in very slow and long deco profiles, ensuring ample margin below the critical edge of DCS.
But in the end I had to surrender, and one year ago I bought my first computer (very basic and cheap). In most cases, it causes me to make more aggressive dives than those I was doing previously following the tables.
Now I can stay down longer, and make no deco. How this can be safer is something I do not understand...
I still try to stay on the safe side of what the computer says, making a longer safety stop, and possibly going very slowly from 5m to surface at the end of it. And I still continue ascending from depth much slower than 10m/min, accepting that this will reduce the time I can stay down, or that I will be forced to some deco stop before emerging.
Now back to the topic: what is correct to explain to students at their first course, and what to explain to divemasters or instructors? Do they need to know all the history?
Probably not for a beginner's course, but they should also be warned to not "ride the NDL", always keeping a robust margin before the limit which will allow to ascend at a much slower speed, still arriving at 5m with the computer signalling a significant time before the NDL...
On the other side, a Divemaster should know the whole history and the reasons for which a slower ascend need to be considered as a longer bottom time. He should also understand that a dive planned with deco is actually SAFER than a dive within the NDL, of course requiring a longer training (and certification, possibly), proper equipment and logistics, and proper planning.
This isn't surprising. Many resorts/operators are much more concerned about day-to-day operations rather than continuing education for the staff. If one trains the staff, then one of them will leave taking that knowledge with them; at least that is the comments I've heard many times by owners who don't want to invest, whether time or money, in continuing education, not just in the dive industry.
a) It's not saf-er but it does not noticeably change your risk as far as the model is concerned.
b) A computer would typically factor in off-gassing during ascent in its NDL calculation. I don't know if you'd normally do that with tables. This would add to the difference, esp. on deeper dives.
It is also why it's dangerous to exceed the safe ascent rate when close to NDL: the calculation assumed that you won't. In this particular scenario tables give you a safety margin. (See also the difference between Buhlmann's ZH-L16 "B" and "C".)
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Point taken. I assume you have witnessed DMs being asked advanced questions on advanced courses.
While taking advanced courses myself I don't recall asking an instructor about anything regarding advanced theory. But that proves nothing, except that it never entered my mind, being a "memorizer" type learner ("Tell me what I need to know to be safe, you can include the Whys about it if you like").
tarponchik
Contributor
This is because helium was discovered originally in 1868 as a bright yellow line in the spectrum of the Sun. Of course, a bright yellow line was of little practical use. Only in 1895 was helium isolated on Earth in trace quantities, and in 1903 a drilling operation in Texas gave a fountain of gas that contained about 12% helium. Thus, the gap between the discovery of an industrial scale source of helium and its practical use in barrage balloons in WW1 was only about a dozen years. Compare this with uranium which waited to be used for about 70 years and found use only accidentally, because Germany ran out of molybdenum.
tarponchik
Contributor
Thanks! Now I know why young American scientists do not know chemistry. They teach them a lot of fascinating new stuff but for everyday work in a chemical lab silly trivia is way more important than quantum physics. I'll give you an example from my experience.
One late Monday morning I was approached by a scientist and a tech who asked me to figure out what had happened. Turns out, the tech was running SOP for a large scale DNA isolation. This was not a new task for her, she did this several times already. According to the protocol, the DNA was denatured by alkali, the solution was then put on ice, neutralized with acetic acid, and the DNA was precipitated by methanol. On Friday, the tech performed the denaturation step, then she realized she was running late and she won't be able to do the rest. So she made a sensible (or so it seemed to her) decision to put the stuff (a 2 L conical flask with about 0.5 L solution in it) into the fridge and get back to it on Monday.
On Monday, however, adding acetic acid resulted in foaming and the content of the flask ran out into the ice bucket and all over the bench. Whoa, what happened?! The guy has a PhD, BTW, and he could not figure this out. Guess, he did not memorize the silly trivia.
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