How much gas in case of accidental deco.

[Agility]

Yes, computers do provide different algorithms - but not with the sort of deviation I highlighted.

Some do
see
A sense of algorithm - Divernet
it is a comparison of typical rec dive computers. Not multiday but on two aggressive profiles on one day.

 

[ianr33]

Lowviz, you CAN'T get V-planner to do what Andy is describing, because V-planner will give you credit for the offgassing that occurs as soon as you ascend past the offgassing ceiling of the fastest compartment. The models used by recreational computers, and in particular Suunto, are very Buhlmann-based, and don't begin to consider you as decompressing until you get very shallow. Therefore, deco continues to accrue, sometimes rapidly, throughout what other models would consider part of your ascent and decompression.

That may well be true,but I think that's a defect of the computer. If you are at 80 feet and your computer indicates a 5 minute stop at 10 feet, what that SHOULD mean IMHO is that if you ascend immediately at the rate the computer expects (probably 30 feet/minute) then when you ascend to 10 feet you will need to do 5 minutes of deco. Obviously if you ascend at 5 feet/minute then it will start adding on deco.

This may be a problem with recreational computers pushed into deco dives,which is (yet another) reason why getting into deco without the appropriate training is not a good idea.

 

[DA Aquamaster]

Ok, here's a real life example of unintentional deco.

I got paired with a newly minted diver on an NC boat trip - an insta buddy. He was a nice guy with eventual technical diver aspirations, but very new, not overly familiar with his equipment, and not real aware of everything that was happening. We did a dive on the Spar and on ascent at the NDL from a planned no decompression dive he discovered he had 11 minutes of deco. The problem was that while our computers had been very compatible the day before, his computer had defaulted to air over night and he missed it both pre-dive and during the dive.

Now, it was obvious that it was not really a deco dive, but if we came up, his computer would get cranky, and I got the impression he was not really understanding the problem underwater anyway, so we did the time and discussed it later on the boat. Fortunately he had the gas to do it - and if not, I had doubles, a long hose and bags of reserve gas.

I suspect this kind of error combined with a lack of SA or understanding of how the computer is set and what the computer is telling them results in "deco" more often than the real kind of deco, but with the tendency of some computers to lock divers out, and with many divers not really knowing whether they have a deco obligation or not on the dive, doing the time is almost mandatory.

 

[tracydr]

Really? I use a typical recreational nitrox computer. I often do a 180 ft dive and get 8-15 minutes of deco on that first dive and then 90 minutes later do another pretty long dive in 80 or 100 feet and approach deco limits again with a rich nitrox mix and somethimes do another dive too, with a little deco.

This weekend (in a priod of 6 hrs or so) I did 4 dives with depths between 165 and 135 on light nitrox and went into deco on 2-3 drops, I forget actually. My Oceanic computer has little trouble with a first deep deco dive and then repet dives less than 100 afterwards. It only really gets mad at me when I do repetative dives past about 120 feet.

To make a general statement that mainstream recreational computers give unfair amounts of deco for small transgressions into deco, is not necessarily true in my experience. What people need to be aware of is that.... staying 2 minutes past the no-deco limit at 100 feet on the first dive will give you minimal deco.... doing the exact same 2 minutes past the no-deco limit on the 3-4-5th dive can casue the computer to really spank the diver with a lot of deco. This is an important distinction...

What happens if you do some agressive diving, on air, each day for a week or two? Not even 180 feet but say, 110-120 feet?

 

[dumpsterDiver]

What happens if you do some agressive diving, on air, each day for a week or two? Not even 180 feet but say, 110-120 feet?

I have never done that. I only gave examples from my personal experience.

 

[AfterDark]

Ok, here's a real life example of unintentional deco.

I got paired with a newly minted diver on an NC boat trip - an insta buddy. He was a nice guy with eventual technical diver aspirations, but very new, not overly familiar with his equipment, and not real aware of everything that was happening. We did a dive on the Spar and on ascent at the NDL from a planned no decompression dive he discovered he had 11 minutes of deco. The problem was that while our computers had been very compatible the day before, his computer had defaulted to air over night and he missed it both pre-dive and during the dive.

Now, it was obvious that it was not really a deco dive, but if we came up, his computer would get cranky, and I got the impression he was not really understanding the problem underwater anyway, so we did the time and discussed it later on the boat. Fortunately he had the gas to do it - and if not, I had doubles, a long hose and bags of reserve gas.

I suspect this kind of error combined with a lack of SA or understanding of how the computer is set and what the computer is telling them results in "deco" more often than the real kind of deco, but with the tendency of some computers to lock divers out, and with many divers not really knowing whether they have a deco obligation or not on the dive, doing the time is almost mandatory.

In this case if the gas wasn't available the diver could just leave the PDC on the line and retreive it after its deco is done.

 

[Paulmal]

None! I have done NDL and Deco dives but every time I went into deco I planned to do it. If you think you are going to do a dive that requires deco grab an Al40 of O2 on the way in the water and once you start the dive it disappears and you will clear any obligation fast.

 

[DevonDiver]

I read his bio. He is a local diver with a straight-forward, pertinent, and quite reasonable question.

Really, does anyone think that he is diving in five day stretches to the limits of NDL?

Probably not... until the day he does something 'new' and decides to take his annual vacation on a liveaboard trip, or otherwise dive the heck out of a 1 week vacation in diving paradise...

My issue was with attempting to provide categorical advice ("15mins gas is sufficient"), when there are easily attainable scenarios where that advice is incorrect...and the consequences of that incorrect advice could be serious injury.

To make a general statement that mainstream recreational computers give unfair amounts of deco for small transgressions into deco, is not necessarily true in my experience.

How many times does it need to be true?

On the basis of giving advice that has a serious safety implication for the recipient, the salient point is that the relationship between depth, bottom time and decompression is complicated.

There are numerous factors which contribute towards the on-gassing / off-gassing process... and unless an individual is fully conversant with the nature of those factors, they should not make vague assumptions on the basis of past experiences or 'internet advice'.

What people need to be aware of is that.... staying 2 minutes past the no-deco limit at 100 feet on the first dive will give you minimal deco.... doing the exact same 2 minutes past the no-deco limit on the 3-4-5th dive can casue the computer to really spank the diver with a lot of deco. This is an important distinction...

Absolutely... and even more so with multi-day diving. Before my hard-drive died, I had a whole bunch of dive graphs that showed I was getting into the water in the morning with 80%+ saturation in my slowest tissues... from a 2 week holiday of 'recreational' dives. That's a situation that can really spank you... not just with rapid accrual of deco obligation... but with the consequences of a fast ascent, missed stops... or just bad luck.

What happens if you do some agressive diving, on air, each day for a week or two? Not even 180 feet but say, 110-120 feet?

See above. SDM was putting me in the water, first dive of the day, with heavy slow tissue saturation.

Hitting deco in those circumstances can mean that slow tissues are the controllers. That means more deco than you'd otherwise expect from 'normal' recreational dive profiles (where fast tissues control...and clear.... fast).

This sort of diving is common for me... wreck dives, square profile, in the 100-130ft range, repetitive (2-3) daily dives over multiple days. I wouldn't choose to do that - it's work.

Some do...see A sense of algorithm - Divernet

What I read is:

"CONCLUSION
Most of these computers give a sufficiently similar result for us to have confidence in them.....It's complex! If you dive with a buddy who uses a different computer, or one with a different setting for caution, always come up together, using the more conservative deco requirements."

The point I've been trying to make is.... It's complex!

Thus... categorical, universal, advice on 'necessary air for accidental deco' isn't possible or prudent.

If you think you are going to do a dive that requires deco grab an Al40 of O2 on the way in the water and once you start the dive it disappears and you will clear any obligation fast.

I think that a few minutes of missed deco are inconsequential compared to the potential repercussions misusing O2 in the water. Don't you?

Let's add the caveat "...with appropriate Advanced Nitrox training..." to your advice.

 

[freeclimbmtb]

The variance in deco accumulation is dictated by the controlling tissue compartment. Most of the time, for recreational divers, that compartment will be 'quick' - thus deco clears quickly/on ascent. However, after multi-day and/or repetitive deep dives, slower tissue compartments become the primary controllers. They won't clear quickly...and can continue to calculate on-gassing even on ascent.

Just had a HUGE "ah-ha!" moment.

This is just an example of my learning style. I can read a book, memorize it, and puke it out all over the test...but that doesnt mean I can apply it to a situation. EXPERIENCE is how I learn, and as so many of you preach, thats exactly what I need. Now that I have a few dives under my belt, and have been faced with some non-routine situations, I can revisit the course material and actually have it sink in.

A drivers license doesnt mean you know how to drive, it means your allowed to start learning. IMHO, a c-card lets you rent a tank...it doesnt mean you know how to dive.

OMG freeclimbmtb, I think that the answer is: "screw you, you shouldn't have done that."

That is 100% truely and absolutely the answer. But thats an easy statement to make from a keyboard (yours OR mine...and as stated numerous times before, fortunately we can both still make that statement from the comfort of a desk!). There are certain questions you just don't ask until after the fact.

In this case if the gas wasn't available the diver could just leave the PDC on the line and retreive it after its deco is done.

I have heard this line of thinking before...and in the interests of not attracting the angry mob...I'm going to go ahead and NOT prescribe to this line of thinking...

None! I have done NDL and Deco dives but every time I went into deco I planned to do it. If you think you are going to do a dive that requires deco grab an Al40 of O2 on the way in the water and once you start the dive it disappears and you will clear any obligation fast.

My Vyper does support gas switching...but in the interest of the KISS mentality...I think (for the purpose of this thread/topic) I am going to maintain I would cary a sling of air not O2. As stated, my intentions for a sling are multiple...having a bottle I can hand off, having an EMERGENCY gas reserve etc...the ABSOLUTE LAST thing I want is an instabuddy getting a hold of a sling of 50% O2 at 30' and then dragging it down to 80'. Why this would happen isnt the point. And yes I understand that a stranger somehow getting 2 bolt snaps undone and getting my sling away from me without me noticing is basically impossible. But recomending bringing a bottle of O2 in the context of this question I think is very much a complication not a solution.

As a response to the nay-sayers that believe accidental deco is impossible and bringing extra gear is not the answer, I have mixed feelings. I ride a sport bike and I wear gear. On the same thought process, a new rider says "do I need a full leather jacket with armour and a back protector?" to which an experienced rider says "no, you only need leathers and armour if you are doing a track day" EXCUSE ME??

Related: I put frame sliders on my bike. So clearly I have the intention of laying it down right? Because you will never "accidentally" crash. Yes, accidental deco is more a result of diver error than outside factors, and a motorcycle accident can be a perfect rider hit by another motorist. But you get my point...


As for my nitrox training, the question was brought up in the class about using higher blends for decompression...but this was given minimal attention and snuffed out rather quickly in the interest of maintaining a "recreational level course" but it was clear that myself and 1 other student of the 10 were "tech curious" I'll say. Did you know, in the computer oriented class they dont teach the "T equation" either? That class makes note of nitrox tables, but teaches you to fly your computer.

A month after I had my "issue" I overheard a girl on the dive boat I was on mention her computer said "Er" to which I thought to myself "Uh oh...I know what that means" so I went over to talk to her. Similarly to when it happened to me, she had no idea WHAT it was, or WHY it happened. I went on to explain to her what I knew about it. After I was done she looked at me and said "so can I still do the second dive?" I replied "I wont tell you what you can and cant do. But since you didnt know you had an obligation, you dont know how large it was, so you could have already gotten bent. Theres no way I would go back in."

THIS IS WHY I ASKED THIS QUESTION. Because of the people that don't ask! I would like to believe that there are people reading this thread and learning the answers to questions they didnt know they had, or were afraid to ask. Is it too advanced and overcomplicated for a large number of recreational divers? Of course it is, but I still think its important, especially for those that are thinking about this stuff.

(As an aside, if my tone seems defenseive, its not my intent...I greatly value all the opinions in this thread. Its just early here.)

 

[DevonDiver]

THIS IS WHY I ASKED THIS QUESTION. Because of the people that don't ask! I would like to believe that there are people reading this thread and learning the answers to questions they didnt know they had, or were afraid to ask. Is it too advanced and overcomplicated for a large number of recreational divers? Of course it is, but I still think its important, especially for those that are thinking about this stuff.

Personally, I feel that if threads like this dispel fear of deco...and instill respect of what it entails, then that's a good result.

There's nothing to fear about the 'magic number' on a dive computer, but a reasonable person should pale a little when they think of the potential medical repercussions of not being able to effectively resolve all that saturated gas in their bodies.

The 'problem' with DCS is that it means very little to us. For most divers it is just another 'abstract concept'. Over time, IMHO it becomes a bit of a fairy story or urban myth. It's easy to dismiss, or even forget.

Those divers who do have experience with DCS, either as victims or witnesses, rapidly develop a profound respect for decompression.