Deco cleared "on the go"?

[Zef]

There are a couple of different factors. Deco will begin to clear as you ascend from your max depth, but it will clear at a slower rate - that 1 min at 10ft might take 2 min to clear after you start your ascent, but before you reach the 10ft stop. Off-gassing occurs when you ascend, but the most efficient off gassing is at the designated stop depth (plus deco gas, but ignore that for now) where the deco time will clear 1 min in 1 min.

That is not totally accurate....our bodies have different types of tissues that nitrogen load at different rates. Depending on tissue saturation level, you can start ascending and some tissues will be off-gassing while others are still becoming nitrogen loaded during part of the ascent....one's deco obligation can increase even though they are slowly ascending. At a certain point in the ascent the rate of off-gassing off all tissue types (compartments) will be faster than the nitrogen loading and the deco obligation will start to reduce.

Edit: Forgot to add that the deco clears because it is a minor deco obligation on back gas. If you were to do a dive and accrue a 100% O2 deco obligation, it would take an extended time to off gassing on the ascent and would most likely not change your stop time--maybe a minute at most.

What is "a 100% O2 deco obligation"...I have never heard of this before. I know folks who are advanced nitrox certified and use 100% O2 at @ 6 meters to accelerate their decompression...but I have never heard of a 100% O2 deco obligation, but I am not a tech diver so despite what I do know, I readily admit there are plenty of things that I don't know.

-Z

 

[drbill]

Back in the days when I would repeatredly dive to 180-200 fsw, my deco obligation often was cleared by the time I was in the shallows due to the steep offshore slopes and gradual ascent in the upper water column.

 

[CptTightPants21]

That is not totally accurate....our bodies have different types of tissues that nitrogen load at different rates. Depending on tissue saturation level, you can start ascending and some tissues will be off-gassing while others are still becoming nitrogen loaded during part of the ascent....one's deco obligation can increase even though they are slowly ascending. At a certain point in the ascent the rate of off-gassing off all tissue types (compartments) will be faster than the nitrogen loading and the deco obligation will start to reduce.



What is "a 100% O2 deco obligation"...I have never heard of this before. I know folks who are advanced nitrox certified and use 100% O2 at @ 6 meters to accelerate their decompression...but I have never heard of a 100% O2 deco obligation, but I am not a tech diver so despite what I do know, I readily admit there are plenty of things that I don't know.

-Z

While technically correct, this is a newer diver who is asking about a single tank dive he did to 95ft- give him simple the base line answer. Also, I used ascend as in go up and end the dive; you are using it as decrease depth and continue diving on a multilevel dive profile. If you stay at a shallower depth, yes, some tissue compartments would be off-gassing and some would be on-gassing. Given the presented dive depths and time, if he ascends to end his dive he will not have any added deco obligation and the small amount he accrued will begin to clear.

A diver needs to set the gases he is diving into his computer and have them activated--back gas and deco gas. This dive profile had no deco gas set so it was 1 min of decompression on his back gas. A dive that doesn't have any deco gas set might show 10 min of decompression at 10ft, but that same exact dive would only have 3-4 min of deco if 100% O2 was programmed into the computer. Run a dive with GF 50/80 at 70ft for 70 min on 32% nitrox and set 20ft as the last stop. Then run the same dive, but program 100% O2 as a deco gas.

 

[doctormike]

While technically correct, this is a newer diver who is asking about a single tank dive he did to 95ft- give him simple the base line answer. Also, I used ascend as in go up and end the dive; you are using it as decrease depth and continue diving on a multilevel dive profile. If you stay at a shallower depth, yes, some tissue compartments would be off-gassing and some would be on-gassing. Given the presented dive depths and time, if he ascends to end his dive he will not have any added deco obligation and the small amount he accrued will begin to clear.

Actually, I think that what @Zef pointed out is more than a technical academic point, and one that's probably worth making to a new diver. People get the idea that if a slow ascent is good, a slower ascent is better - I have heard that here before. It's good to understand that even a single tank diver can be increasing their nitrogen load and deco obligation even though they are continually ascending, if you they it very slowly. One of the things that was stressed to me in tech training was that once you start to ascend, don't delay but ascend at a reasonably fast rate (within limits) to your first stop.

Also, that's one of the theoretical criticisms of the deep stop algorithms - that what you gain by offgassing your fast tissues with a deep stop may be offset by extra loading of incompletely saturated slow tissues.

 

[Zef]

While technically correct, this is a newer diver who is asking about a single tank dive he did to 95ft- give him simple the base line answer. Also, I used ascend as in go up and end the dive; you are using it as decrease depth and continue diving on a multilevel dive profile. If you stay at a shallower depth, yes, some tissue compartments would be off-gassing and some would be on-gassing. Given the presented dive depths and time, if he ascends to end his dive he will not have any added deco obligation and the small amount he accrued will begin to clear.

A diver needs to set the gases he is diving into his computer and have them activated--back gas and deco gas. This dive profile had no deco gas set so it was 1 min of decompression on his back gas. A dive that doesn't have any deco gas set might show 10 min of decompression at 10ft, but that same exact dive would only have 3-4 min of deco if 100% O2 was programmed into the computer. Run a dive with GF 50/80 at 70ft for 70 min on 32% nitrox and set 20ft as the last stop. Then run the same dive, but program 100% O2 as a deco gas.

Again you are not totally accurate. First, if a newer diver, as you have assessed the OP is diving in such a way that they are incurring deco obligations then something is very wrong. Standard recreational scuba diving is defined as no-decompression diving, at least by NAUI, PADI, and SSI.

Second, it is just plain wrong to think that you can just go up and end the dive....depending on your deco obligation you may have a specific depth ceiling limit, discussing this in too basic a way may give someone enough info to make them dangerous to themself should they employ that info not understanding the bigger picture.

Third, for the OP to understand the "why" aspect of their question there needs to be a theoretical discussion to one degree or another. Dumbing things down does not necessarily make it better, in this case you are just leaving out vital information.

Fourth, a diver only needs to set the gasses in their computer if they are diving with different gas mixes. It is interesting that you are introducing this notion to the discussion while you want to dumb down the basics. One can go into deco on good old air and conduct their decompression stops breathing that good old normal mix of 21% O2 + 79% N2. Nothing to set in the computer. Your introduction of the use of 100% O2 edges into even more advanced discussion and is totally irrelevant to anything indicated in the OP's post. How do you arrive at discussing computer gas settings and accelerated decompression on 100% O2 when you have assessed the OP as a "newer diver"?

You are correct though, given the dive profile in the graphs the OP presented, as they ascended from the depth they were at they would not be incurring an increase in deco obligation. But it is important for the OP to know, from a basics standpoint that this will not always be the case.

My initial response to your post was not meant to be a "poke in the eye" or a "hey look how smart I am in comparison" type of thing. It was meant to present balanced info to the OP whom I think there are 3 pertinent questions that they should answer:

1. What computer are/were they using?
2. How much dive experience do they have?
3. If not specifically trained to do so, which based on the nature of the info they presented and the question(s) they asked, what are they doing going into deco that this is an issue/question?

other than the responses to those questions the OPs post is a great catalyst to discuss the basics of deco....and given the relative danger, especially if not properly planned, the basic discussion should not be overly simplified, at least not in my professional opinion.

-Z

 

[jgttrey]

Again you are not totally accurate. First, if a newer diver, as you have assessed the OP is diving in such a way that they are incurring deco obligations then something is very wrong. Standard recreational scuba diving is defined as no-decompression diving, at least by NAUI, PADI, and SSI.

I haven't attempted to deconstruct the profiles, particularly the reference to the 17 minute stop, which is quite odd, but I'll just say that you need to distinguish between "no deco" diving, which is the common but incorrect description of recreational profiles and "no stop" diving, which is the more accurate statement.

Recreational dives are "no stop," not "no deco." It is often the case that a recreational profile will generate a theoretical decompression ceiling. However, if one follows the prescribed ascent procedure for the computer or tables, typically 30'/min, the ceiling clears before you reach it -- thus, no stop.

Decompression stops would be generated when that 30'/min ascent is insufficient to allow the necessary offgassing and an actual stop is required.

Put another way, just because you don't have any "stops" showing doesn't mean that you have no ceiling. It just means that at a 30' ascent, you're ceiling will clear before you get there. That's why if you bolt for the surface you can, in fact, bust a decompression ceiling even on a recreational dive and why dive computers have ascent rate indicators that need to be followed to remain in compliance with their calculated obligations.

 

[Zef]

I haven't attempted to deconstruct the profiles, particularly the reference to the 17 minute stop, which is quite odd, but I'll just say that you need to distinguish between "no deco" diving, which is the common but incorrect description of recreational profiles and "no stop" diving, which is the more accurate statement.

Recreational dives are "no stop," not "no deco." It is often the case that a recreational profile will generate a theoretical decompression ceiling. However, if one follows the prescribed ascent procedure for the computer or tables, typically 30'/min, the ceiling clears before you reach it -- thus, no stop.

Decompression stops would be generated when that 30'/min ascent is insufficient to allow the necessary offgassing and an actual stop is required.

Put another way, just because you don't have any "stops" showing doesn't mean that you have no ceiling. It just means that at a 30' ascent, you're ceiling will clear before you get there. That's why if you bolt for the surface you can, in fact, bust a decompression ceiling even on a recreational dive and why dive computers have ascent rate indicators that need to be followed to remain in compliance with their calculated obligations.

Great clarification of a commonly misused term.

Thanks!

-Z

 

[doctormike]

Put another way, just because you don't have any "stops" showing doesn't mean that you have no ceiling. It just means that at a 30' ascent, you're ceiling will clear before you get there. That's why if you bolt for the surface you can, in fact, bust a decompression ceiling even on a recreational dive and why dive computers have ascent rate indicators that need to be followed to remain in compliance with their calculated obligations.

An excellent point!

 

[scubadada]

Hi @leslietaur

If the timestamp is correct, you had 1 min of deco at 3m while you were at 19.2m. Five minutes later, at 17.8m, you had 17 min of deco at 3m. One minute later at 12.9m, you had 1 min of deco at 3m. Less than a minute later at 13m, you had no deco. This simply does not make sense to me, or you, apparently. I assume the 17 min of deco point is incorrect. What do you think? It might still help us if you would supply the computer used, algorithm run, and settings chosen. Is this behavior something you have seen previously with your computer, or was it a surprise? I look forward to your additional comments. Edit: Sorry, did not see that you were diving a Garmin Descent at 30/75, all the more strange.

I fully believe that very short deco obligations can clear on ascent. In my experience that would be 2 or 3 minutes of obligation. Here is a light deco dive I did two days ago. At the end of the dive at 93 ft, I had 5 min of deco at 10 ft. By the time I reached my stop, 3 1/2 minutes later, I had only 2 min of deco at 10 ft, 3 min of deco had been cleared on the ascent. Two min later, I had cleared deco and spent an additional 3 min at the stop before surfacing.

 

[jgttrey]

If the timestamp is correct, you had 1 min of deco at 3m while you were at 19.2m. Five minutes later, at 17.8m, you had 17 min of deco at 3m. One minute later at 12.9m, you had 1 min of deco at 3m. Less than a minute later at 13m, you had no deco. This simply does not make sense to me, or you, apparently. I assume the 17 min of deco point is incorrect. What do you think? It might still help us if you would supply the computer used, algorithm run, and settings chosen. Is this behavior something you have seen previously with your computer, or was it a surprise? I look forward to you additional comments.

Totally agree. There's no way your 3m stop obligation went from 1 minute to 17 minutes by staying 5 minutes longer at 17 or 18m.

It's fuzzy on my screen. Are we sure it isn't 1.7 minutes rather than 17? Just guessing here but its all I can think of. It would sure make more sense.