Planned deco on a recreational dive?

[Stoo]

Like I said before, people would be going hog wild if someone advocated for "lite" cave diving. There is no lite deco.

Respectfully, I don't agree. Blowing off an actual decompression stop of a minute or two (as indicated by most PDCs) will likely not result in a hit in the vast majority of people. Even of the person is bent, it will almost certainly come in the form of a skin bend, which may well be treated by sucking some O2 on the boat and/or a wee chamber ride. This is "deco-lite" and it's ENTIRELY different than that required on a "serious" dive... multiple stops at various depths, using different gasses and totalling many minutes. Blow those stops and you are going to get seriously messed up.

By contrast, once someone is inside a cave and well away from the entrance, they can no longer ascend directly. Period. It's dark, and getting lost is an obvious risk and I would suggest that a cave diver can get "just as lost" 150' in as they can 1500' in. Lost is lost. (Been there, when I was much younger, and much stupider.)

I am not advocating "sliding" in to decompression diving lightly, but the reality is, that we are all doing decompression dives anyway... ascending at a prescribed rate is a form of decompression. A safety stop is decompression stop. It's just one that if the diver blows it entirely, it's very forgiving.

Moving from a "required safety stop" (which, by definition, isn't really a thing) of 3 minutes to a 2 minute decompression stop AND a safety stop really isn't a big deal IMHO, but I agree that it shouldn't be undertaken without at least some basic knowledge of what could happen if you blow it.

Through all of this I will add that I am assuming the divers in question are "competent", can hold a stop, and won't run out of air.

 

[boulderjohn]

I trained in Los Angeles, there was certification available, perhaps not required, since 1954, the year I was born

I went to a conference a couple of years ago, and the speaker at a special breakout session told this story. In the late 1960s (1967, IIRC), he went on a very special dive trip in Australia, a full week of intense diving. The captain asked to see everyone's certification cards, and he did not have one. He explained that his father had first taught him how to dive when he was 7 years old, and since that day he had done thousands of dives, but he had never gotten certified. The captain was adamant--no card, no dives. He begged. No dice. Eventually, the crew prevailed upon the captain to make an exception in this one case, and he was allowed to dive. When he returned home, he went to the local PADI instructor and got a card so he would never have to go through that again. He opened his wallet to show us he still carried that card.

His name was Jean-Michel Cousteau, the second human being to breathe from a Gagnon-Cousteau regulator.

 

[scubadada]

I went to a conference a couple of years ago, and the speaker at a special breakout session told this story. In the late 1960s (1967, IIRC), he went on a very special dive trip in Australia, a full week of intense diving. The captain asked to see everyone's certification cards, and he did not have one. He explained that his father had first taught him how to dive when he was 7 years old, and since that day he had done thousands of dives, but he had never gotten certified. The captain was adamant--no card, no dives. He begged. No dice. Eventually, the crew prevailed upon the captain to make an exception in this one case, and he was allowed to dive. When he returned home, he went to the local PADI instructor and got a card so he would never have to go through that again. He opened his wallet to show us he still carried that card.

His name was Jean-Michel Cousteau, the second human being to breathe from a Gagnon-Cousteau regulator.

Cute story

 

[boulderjohn]

Respectfully, I don't agree. Blowing off an actual decompression stop of a minute or two (as indicated by most PDCs) will likely not result in a hit in the vast majority of people. Even of the person is bent, it will almost certainly come in the form of a skin bend, which may well be treated by sucking some O2 on the boat and/or a wee chamber ride. This is "deco-lite" and it's ENTIRELY different than that required on a "serious" dive... multiple stops at various depths, using different gasses and totally many minutes. Blow those stops and you are going to get seriously messed up.

By contrast, once someone is inside a cave and well away from the entrance, they can no longer ascend directly. Period. It's dark, and getting lost is an obvious risk and I would suggest that a cave diver can get "just as lost" 150' in as they can 1500' in. Lost is lost. (Been there, when I was much younger, and much stupider.)

I am not advocating "sliding" in to decompression diving lightly, but the reality is, that we are all doing decompression dives anyway... ascending at a prescribed rate is a form of decompression. A safety stop is decompression stop. It's just one that if the diver blows it entirely, it's very forgiving.

Moving from a "required safety stop" (which, by definition, isn't really a thing) of 3 minutes to a 2 minute decompression stop AND a safety stop really isn't a big deal IMHO, but I agree that it shouldn't be undertaken without at least some basic knowledge of what could happen if you blow it.

Through all of this I will add that I am assuming the divers in question are "competent", can hold a stop, and won't run out of air.

I'm all the way with Stoo on this one.

When you are diving, there is a wide spectrum of possibilities in regard to decompression. At one extreme, if you are in a swimming pool with people feeding you new tanks for as long as you can stay awake, you can still go to the surface with no problems. Go a lttle deeper, and you will still be able to go to the surface at any times with no danger of DCS. Get into the normal dive range we think of as OW diving, and there is a wide range of dive profiles and NDLs we think are OK. If you go into deco for a minute or two with one of them, you might be well within the NDLs of another. If you are into deco for a few minutes with a very conservative program and surface, the odds are very much that nothing will happen. Blow off an hour or two of deco, though, and you might not live to get on the boat.

I mentioned this earlier, but I will repeat it to show the difference:

1. Spend 25 minutes at 100 feet on air, and you are within NDLs on the old Navy tables. No stops required.
2. Spend 20 minutes at 100 feet on air, and you are within NDLs on the PADI tables (3 minute safety stop required), but if you spend 25 minutes (as allowed by the old Navy tables), you are in serious emergency decompression procedures.
3. Spend 9 minutes on air at 100 feet on air with the Buhlmann ZHL-16 C algorithm with GFs of 50/80, and you will be in deco with a 1 minute decompression stop required.

 

[Stoo]

It's probably worth tossing this little bit of information into the mix, lest some aren't aware of it.

DECOMPRESSION IS ALL JUST A THEORY ANYWAY
To unequivocally state that "this or that" will happen following a missed two minute stop, or a blown "safety stop" is just plain wrong.

I have personally known people who followed a decompression plan to the letter and took a hit. I have also known people (well, one anyway) who blew off a whack of deco (20 minutes plus I think) following a 250' tri-mix dive when he was involved in the attempted rescue of a buddy who was in trouble... and sat outside the chamber while Doctors waited for something to happened, and never did.

 

[scubadada]

I'm all the way with Stoo on this one.

When you are diving, there is a wide spectrum of possibilities in regard to decompression. At one extreme, if you are in a swimming pool with people feeding you new tanks for as long as you can stay awake, you can still go to the surface with no problems. Go a lttle deeper, and you will still be able to go to the surface at any times with no danger of DCS. Get into the normal dive range we think of as OW diving, and there is a wide range of dive profiles and NDLs we think are OK. If you go into deco for a minute or two with one of them, you might be well within the NDLs of another. If you are into deco for a few minutes with a very conservative program and surface, the odds are very much that nothing will happen. Blow off an hour or two of deco, though, and you might not live to get on the boat.

I mentioned this earlier, but I will repeat it to show the difference:

1. Spend 25 minutes at 100 feet on air, and you are within NDLs on the old Navy tables. No stops required.
2. Spend 20 minutes at 100 feet on air, and you are within NDLs on the PADI tables (3 minute safety stop required), but if you spend 25 minutes (as allowed by the old Navy tables), you are in serious emergency decompression procedures.
3. Spend 9 minutes on air at 100 feet on air with the Buhlmann ZHL-16 C algorithm with GFs of 50/80, and you will be in deco with a 1 minute decompression stop required.

But at 100/100 25 minutes gives you just 3:15 at 10 ft. 20 minutes gives you 0:15 at 10 feet. DSAT gives you an NDL of 19 min at 100 feet.

 

[tursiops]

DECOMPRESSION IS ALL JUST A THEORY ANYWAY

I gotta say, this is a really unconvincing argument.
From the Merriam-Webster Dictionary:
However, there are two senses of theory which are sometimes troublesome. These are the senses which are defined as “a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena” and “an unproven assumption; conjecture.” The second of these is occasionally misapplied in cases where the former is meant, as when a particular scientific theory is derided as "just a theory," implying that it is no more than speculation or conjecture. One may certainly disagree with scientists regarding their theories, but it is an inaccurate interpretation of language to regard their use of the word as implying a tentative hypothesis; the scientific use of theory is quite different than the speculative use of the word.

 

[Beau640]

Well I have to respectfully disagree as well.

I agree there is a wide range of decompression obligations. I agree that tables differ. However it seems that the large proportion of underwater divers are diving computers, primarily computers with RBGM which is proprietary and we don't necessarily know how great it is as calculating deco obligations. I don't know of any technical divers using RBGM to plan dives/calculate dive profiles for decompression diving. Are you saying you potentially trust your life on your computer to accurately calculate your decompression obligation when you have not figured it out yourself pre-dive?

What we are talking about is deco diving being conducted by an average OW diver. That would mean people that most likely don't know the difference between agency tables, or maybe don't know tables at all. These are people just diving with their computer. They don't know about diffusion or bubble models. They haven't been taught about the different models or decided on which model gives them a personal acceptable level of risk for dcs. We are talking about blindly and without planning trusting RBGM for a deco dive which in my mind is inappropriate.

I agree that some people have blown off deco stops and are fine. But also some people have done extremely conservative profiles correctly, and still got DCS. People in general don't know how conservative their computer is compared to another (or they just believe random word of mouth they heard). They don't know how much risk they are actually taking by going into deco.

Also I really do disagree with your statement that deco does not create a ceiling. Deco does create a virtual ceiling that in almost no circumstance should be violated. If you are conducting a true deco dive, even if your obligation is only 2-5 minutes (Deco "lite" by people's terms), violating the ceiling or blowing through a stop creates an unacceptably high risk of DCS. You may get away with it one time. Or you may get a type 2 hit and be paralyzed even after hyperbaric treatment. This virtual ceiling is not the same thing as a safety stop at all.

It just comes down to people not knowing what they don't know. And like you said, we don't know a lot about deco and it's all theory. So if anything we should be conservative as hell and not be advocating non trained people to be doing "only a couple minutes" of deco.

 

[Stoo]

I gotta say, this is a really unconvincing argument.

Well I wasn't trying to "convince" anyone of anything... Only pointing out that there are so many variables involved in this study of decompression "theory", that nobody can say much of anything with certainty. For every "perfect" dive plan, someone will get an unearned hit. Similarly, someone will blow off some huge decompression "obligation" and walk away unscathed...

As a number of folks have pointed out in this tread, that a mere adjustment of the settings on a dive computer can make deco go away. I use a Petral... I adjust gradient factors a dozen different ways and completely change my deco requirements.... Or go on a bender the night before. Or not drink enough water. Or drink too much. Or get cold. Or overheat...

You get my point I'm sure.

 

[boulderjohn]

The issue was your comparison of going into deco and going into a cave. You said there was no such thing as "lite" Deco, just as there is no such thing as lite cave diving. Going a minute or two into deco is not a very serious problem. If you do get decompression sickness, it is a minor issue issue. Go an hour into deco, and there is a huge difference. In contrast, Go two minutes (100 feet) into a cave, turn around, and can't find the exit, you will die.