Hypothetical question about decompression

[MXGratefulDiver]

Thakyou for the responses so far.

So i guess the quick answer is no?

Is it because there is a certain depth where u are still on gassing faster than off gassing? cause i did notice the part in nereas's Vplanner profile where it says ""offgassing starts at 97.1ft"" for that particular profile.

The quick answer is no.

The answer to your second question is "it depends". Nereas's V-planner profile is nothing more than the solution to a set of equations that may or may not have much to do with your particular decompression needs.

The thing you have to understand about decompression is that (a) it's a complex subject (which is why nobody's ever come up with an easily comprehensible treatise on the topic) and (b) an accurate explanation does not lend itself well to internet forums. But I will try to provide a simplified explanation that may answer your question.

First, let's start with some basic statments.

1. All liquids contain within them a certain amount of dissolved gases.
2. The amount of dissolved gases any given liquid will hold is impacted by pressure.
3. In this respect our body acts like a liquid.

So let's look at what pressure does to us when we dive.

At the surface, our body is effectively "saturated" with dissolved gases ... meaning that it contains all that it's designed to hold in solution. When we dive, increased pressure increases the amount of gas our body tissues will hold ... the deeper we dive, the greater the amount until those tissues once again become saturated and won't dissolve any more. And so our body takes in more gas from the air we breathe.

Furthermore, because we're breathing pressurized gas, the deeper we go the denser the gas we're breathing. This denser gas creates a "differential" between the pressure of the dissolved gas in our body and the pressure of the inhaled gas in our lungs. Think about our lungs and body as two "containers", separated by a permeable membrane that allows equalization to take place over time ... as illustrated in this diagram ...

When we begin our ascent, the reverse process takes place. Now, as you can envision from this analogy, coming up slowly creates a very tiny pressure differential, which slows the offgassing process. It is primarily for this reason that the answer to your initial question is "no". But that is not the only reason.

Decompression is further complicated by the fact that our body comprises literally hundreds of different "tissue" types ... blood, nerves, muscle, fat, bone, ligaments, etc. ... each of which ongasses and offgasses at different rates. At any given time or depth during the dive, some of these tissues may be saturated while others will not be. Those latter tissues will continue ongassing on a slow ascent, even while the saturated ones may begin offgassing.

In order to make the process of tracking decompression manageable, what a program like V-Planner does is to group certain types of tissues into what they call "compartments" and track what each compartment would theoretically be doing, based on a set of assumptions about ongassing and offgassing rates. Up to a certain point during the ascent, certain "compartments" will still be ongassing while others will be offgassing. Your ascent rate plays an important role in the equations that determine which "compartments" will be doing which.

In reality, what your body is actually doing will approximate those assumptions, but not precisely ... there's too many variables, based on your physical makeup, the condition of your circulatory system, and even such things as how rested you are or what you drank before the dive. The program knows nothing about those things. Therefore following a program without understanding the assumptions and how your particular risk factors might affect them is risky ... that's why training is so important. In decompression programs, you accommodate your particular risk factors by adding or subtracting "conservatism" levels, which add or reduce decompression stop times accordingly. In nereas's example, he has his conservatism level set to +3 ... but without explaining to you why he does that, the numbers won't really be useful, because your body may not be much like his.

Hope that helps a little ... those references rstofer provided are worth reading if you really want to understand the subject better.

... Bob (Grateful Diver)

 

[texdiveguy]

I don't know what you are reading but it should be "Deco For Divers" by Mark Powell.

Good book.....though I am still reading it at present, have been impressed so far.

 

[SteveAD]

Well said NWGD

 

[jaycanwk]

The quick answer is no.

The answer to your second question is "it depends". Nereas's V-planner profile is nothing more than the solution to a set of equations that may or may not have much to do with your particular decompression needs.

The thing you have to understand about decompression is that (a) it's a complex subject (which is why nobody's ever come up with an easily comprehensible treatise on the topic) and (b) an accurate explanation does not lend itself well to internet forums. But I will try to provide a simplified explanation that may answer your question.

First, let's start with some basic statments.

1. All liquids contain within them a certain amount of dissolved gases.
2. The amount of dissolved gases any given liquid will hold is impacted by pressure.
3. In this respect our body acts like a liquid.

So let's look at what pressure does to us when we dive.

At the surface, our body is effectively "saturated" with dissolved gases ... meaning that it contains all that it's designed to hold in solution. When we dive, increased pressure increases the amount of gas our body tissues will hold ... the deeper we dive, the greater the amount until those tissues once again become saturated and won't dissolve any more. And so our body takes in more gas from the air we breathe.

Furthermore, because we're breathing pressurized gas, the deeper we go the denser the gas we're breathing. This denser gas creates a "differential" between the pressure of the dissolved gas in our body and the pressure of the inhaled gas in our lungs. Think about our lungs and body as two "containers", separated by a permeable membrane that allows equalization to take place over time ... as illustrated in this diagram ...

When we begin our ascent, the reverse process takes place. Now, as you can envision from this analogy, coming up slowly creates a very tiny pressure differential, which slows the offgassing process. It is primarily for this reason that the answer to your initial question is "no". But that is not the only reason.

Decompression is further complicated by the fact that our body comprises literally hundreds of different "tissue" types ... blood, nerves, muscle, fat, bone, ligaments, etc. ... each of which ongasses and offgasses at different rates. At any given time or depth during the dive, some of these tissues may be saturated while others will not be. Those latter tissues will continue ongassing on a slow ascent, even while the saturated ones may begin offgassing.

In order to make the process of tracking decompression manageable, what a program like V-Planner does is to group certain types of tissues into what they call "compartments" and track what each compartment would theoretically be doing, based on a set of assumptions about ongassing and offgassing rates. Up to a certain point during the ascent, certain "compartments" will still be ongassing while others will be offgassing. Your ascent rate plays an important role in the equations that determine which "compartments" will be doing which.

In reality, what your body is actually doing will approximate those assumptions, but not precisely ... there's too many variables, based on your physical makeup, the condition of your circulatory system, and even such things as how rested you are or what you drank before the dive. The program knows nothing about those things. Therefore following a program without understanding the assumptions and how your particular risk factors might affect them is risky ... that's why training is so important. In decompression programs, you accommodate your particular risk factors by adding or subtracting "conservatism" levels, which add or reduce decompression stop times accordingly. In nereas's example, he has his conservatism level set to +3 ... but without explaining to you why he does that, the numbers won't really be useful, because your body may not be much like his.

Hope that helps a little ... those references rstofer provided are worth reading if you really want to understand the subject better.

... Bob (Grateful Diver)

Thank you Bob for taking the time to give your response. That did indeed aswer my question. In fact after I hit the "post" button on my origional post it ocurred to me, after thinking about it some more, that becuase of the difference in halftimes of the different tissue compartments, that a very very slow direct acent would not work as u clearly pointed out in you expalination as to why. I guess I hit the post button to quickly LoL

.

 

[dschonbrun]

Agree with Bob. The simple answer is no. You can have multiple dive profiles with a TAT of 60 minutes from a depth of 150 ft. The off-gassing characteristics of each one can vary dramatically.

 

[DwayneJ]

Well the poor guy has never seen a deco plan, and you are too much in a hurry to show him one, whereas I happen to have one ready to go, so I have done the honors.

Although I suspect that you don't know either.

Nereas - You did not answer the original question.

 

[eponym]

Great stuff here from both Dave and Bob. Kudos, guys!

The first chapter of Jolie Bookspan's "Diving Physiology in Plain English" provides an introduction to these concepts behind the tables, and to some of the non-Haldane models as well, for those just starting to explore the topic.

-Bryan

 

[wve]

I'm going to stick my neck out here.

While I agree with everyone that the practical answer to the OP is "No", I believe that a better answer to the hypothetical presented is: No one knows for sure.

Dr Deco (Dr Mike Powell) has reminded me that Decompression Theory is just that. Although we are learning more all the time, there is still much that we do not know about how the body actually deals with nitrogen.

Current dive practices have evolved from mathematical formulations of our best guess about what might be happening -- combined with lots and lots of experience. Together, these form the dogma that we adhere to.

But to address the hypothetical, might a tediously slow ascent work just as well? Maybe. And from a medical standpoint, I do not see why not. In fact, a very slow ascent is more in keeping with most normal physiologic processes -- which are continuous in nature -- rather than arbitrary stops along the path.

So, hypothetically speaking, I think it may be possible.

Nevertheless, I recommend sticking with what has been shown to work. Let someone else be the guinea pig... (or goat).

 

[eponym]

I'm going to stick my neck out here. While I agree with everyone that the practical answer to the OP is "No", I believe that a better answer to the hypothetical presented is: No one knows for sure.

No question that deco is theory, but please consider the following, from the DAN website (citation below):

British physiologist Sir Leonard Hill theorized that decompression should be by linear ascent to the surface; he strongly disagreed with Haldane's approach. However, in the end Haldane was able to prove, using goats, that a slow linear ascent was not only ineffective, but unsafe; too much nitrogen remained on surfacing resulting in frequent DCS.​

Full article is available at DAN.

-Bryan

 

[ianr33]

I
But to address the hypothetical, might a tediously slow ascent work just as well?

No.

A very slow ascent e.g. 1 foot/min will increase your bottom time. More N2 will be absorbed requiring more deco to be done.

If you dive to 150 feet for 10 minutes then take 10 minutes to ascend to 140 thats pretty much the same as doing 145 for 20 minutes.

A very slow ascent once shallow is great,but near the bottom its just adding to your gas loading.

Doing 150 for 10 and ascending at 30 feet/min V-planner gives this:
Dec to 150ft (2) Air 60ft/min descent.
Level 150ft 7:30 (10) Air 1.16 ppO2, 150ft ead
Asc to 40ft (13) Air -30ft/min ascent.
Stop at 40ft 1:20 (15) Air 0.46 ppO2, 40ft ead
Stop at 30ft 2:00 (17) Air 0.40 ppO2, 30ft ead
Stop at 20ft 2:00 (19) Air 0.34 ppO2, 20ft ead
Stop at 10ft 5:00 (24) Air 0.27 ppO2, 10ft ead
Surface (24) Air -30ft/min ascent.

Off gassing starts at 81.1ft

OTU's this dive: 14
CNS Total: 4.9%

48.5 cu ft Air
48.5 cu ft TOTAL

But ascending at 1 foot/min this profile results:

Dec to 150ft (2) Air 60ft/min descent.
Level 150ft 7:30 (10) Air 1.16 ppO2, 150ft ead
Asc to 60ft (100) Air -1ft/min ascent.
Stop at 30ft 13:00 (133) Air 0.40 ppO2, 30ft ead
Stop at 20ft 30:00 (163) Air 0.34 ppO2, 20ft ead
Stop at 10ft 52:00 (215) Air 0.27 ppO2, 10ft ead
Surface (225) Air -1ft/min ascent.

Off gassing starts at 63.6ft

OTU's this dive: 83
CNS Total: 31.2%

379.5 cu ft Air
379.5 cu ft TOTAL

The second dive would require a huge amount of deco as well as a huge amount of gas.