How close to no decompression limit

[Peter Guy]

The question here is how close is it safe to be to the NDL.

To quote the Master

You've got to ask yourself one question: 'Do I feel lucky?' Well, do ya punk?”

Or, as said by my favorite Diving Instructor when asked "How are you doing on your deco limits?" replied:

"He just asked me the wrong question. The right question is, How much gas do I have left?"

 

[Foxfish]

I don't understand your graphs and your quuestions... How can you reach 100% of allowable no deco nitrogen absorption and simultaneously contiue to have a 10 minutes of remaining no-deco time?

At 100%, you should have zero time right?

That was the next question I was just about to ask. For those who know a bit about the various algorithms used to determine NDL's, in this case the RGBM for Suunto, what triggers the NDL. Clearly it is not simply one part of the tissue saturation limit exceeding 100% since as stated previously I avoided getting closer than 10 minutes to the NDL. Obviously I'm asking the question because I dont know much about the models.

 

[Thalassamania]

That was the next question I was just about to ask. For those who know a bit about the various algorithms used to determine NDL's, in this case the RGBM for Suunto, what triggers the NDL. Clearly it is not simply one part of the tissue saturation limit exceeding 100% since as stated previously I avoided getting closer than 10 minutes to the NDL.

No, it depends on the "speed" of the tissue. The faster the tissue (shorter the half-time) the deeper that it can be completely filled and then directly surfaced without a problem. The slower the tissue (longer half-time) the less "stretchy" the balloon is and the more likely it is to cause problems at 100% saturation if directly surfaced at the normal ascent rate. So as time and depth exposure adds and/or subtracts to all the compartments (tissues) the "controlling tissue" (that is to say the one most like to cause you problems) changes. By taking a step back from the NDL (which you will see shift as you change depth) you will lessen the odds of being bent. How much, exactly, do you need to shift it? Who knows? On what day? There are way too many variables and unknowns. From my perspective, all the computers that are out today are, in reality, somewhat more conservative than the U.S. Navy Tables that I dove for years without incident ... so I use them the same way, I crank back when there is a reason (e.g., cold, arduous, remote to help).

 

[dumpsterDiver]

No, it depends on the "speed" of the tissue. The faster the tissue (shorter the half-time) the deeper that it can be completely filled and then directly surfaced without a problem. The slower the tissue (longer half-time) the less "stretchy" the balloon is and the more likely it is to cause problems at 100% saturation if directly surfaced at the normal ascent rate. So as time and depth exposure adds and/or subtracts to all the compartments (tissues) the "controlling tissue" (that is to say the one most like to cause you problems) changes. By taking a step back from the NDL (which you will see shift as you change depth) you will lessen the odds of being bent. How much, exactly, do you need to shift it? Who knows? On what day? There are way too many variables and unknowns. From my perspective, all the computers that are out today are, in reality, somewhat more conservative than the U.S. Navy Tables that I dove for years without incident ... so I use them the same way, I crank back when there is a reason (e.g., cold, arduous, remote to help).

I have absolutely no idea how that response answers Fox. (or my question)? If a tissue reaches 100%, then that should be the controlling tissue and any more nitrogen absorption puts you into deco (for whatever critical tissue) is involved.

 

[lowviz]

...//.... somewhat more conservative than the U.S. Navy Tables that I dove for years without incident ... so I use them the same way, I crank back when there is a reason (e.g., cold, arduous, remote to help).

NOAA for me, nowhere near the same history, but same mindset...

 

[lowviz]

I have absolutely no idea how that response answers Fox. (or my question)? If a tissue reaches 100%, then that should be the controlling tissue and any more nitrogen absorption puts you into deco (for whatever critical tissue) is involved.

By the time you surface, the fastest tissue may no longer be the determining tissue.

 

[Thalassamania]

I have absolutely no idea how that response answers Fox. (or my question)?
If a tissue reaches 100%, then that should be the controlling tissue and any more nitrogen absorption puts you into deco (for whatever critical tissue) is involved.

Let me first, by way of background, share this wonderful post of Mike Powell's:

Dear Readers:

This question arises anew from time to time.

M-values

They are variously referred to as ”matrix values” or “maximum values.” The method was instigated by Dr Robert Workman of the US Navy to assist in the calculation of decompression tables. This was in the 1950s.

The initial concept of John Scot Haldane was to allow the pressure to be reduced by ½ (2:1 ratio) in all of the “tissue compartments.” [This was actually 1.58:1 if allowance was made for the fact that much of the air was actually oxygen and not pure nitrogen.] It was found that this small ratio was only necessary for saturation diving. This ratio for pressure reduction was modified when it was noted from experiments that “fast” compartments (those with short halftimes) could tolerate a much higher supersaturation than “slow” compartments (large halftimes).

Start with NDLs

If one started with the no-decompression limits (NDLs) determined experimentally for a group of test subjects (age dependent, lean/fat dependent, physical fitness dependent, work load dependent, hydration dependent, etc., etc), it could be found that deep dives are short and are “limited” by small halftimes (5-10 minutes). Intermediate depths (60 fsw) are limited by medium halftimes, and shallow dives (35-40 fsw) are long half time dependent. The pressure in the limiting tissue is the “surfacing pressure” and its ratio to absolute pressure is the “limiting ratio.” This value (in fsw) is the pressure in a given half time compartment for ascent to the surface and is the Mo-value.

Deeper depths

As one moves from the surface to the ten-foot depth, the allowed tissue gas ratio becomes a bit smaller. The ATTACHMENT (from my Decompression Physiology class) shows the allow tissue pressure (M-value) for ascents to the ten-foot stop and deeper stops. There are ten-foot increments in the Mo values that allow the calculation of tables down to hundreds of feet.

Problems

These M-values do not always work as the tables go deeper and decompression becomes extended. This deco is conducted in chambers for commercial and military divers. I believe that the major failure in decompression is the result of tissue nucleation as the divers walk about in the chambers over the hours to days of decompression.

Safe decompression

It is clear for recreational SCUBA that you can add to the safety of decompression by

• simply reducing bottom time,
• slowing ascent, and waiting a bit at the surface (the “hidden stop&#8221 before boarding the boat, and
• reducing strenuous physical activity on the surface.

You can always reduce your strenuous surface activities. This increases safety and costs noting in terms of the need for new equipment.

One advantage of tables is that it is easy to conservatively read and apply the tables. Computers always give numbers that are the “table limits.”

Dr Deco :doctor:

See where he says, "This ratio for pressure reduction was modified when it was noted from experiments that “fast” compartments (those with short halftimes) could tolerate a much higher supersaturation than “slow” compartments (large halftimes)."?

This tolerance (think of it as stretchiness in a balloon) means that even though a fast tissue is at 100%, it will not cause DCS if you are shallower (well, if it is shallower) than it's supersaturation ratio would permit.

Mike goes on to say: "deep dives are short and are “limited” by small halftimes (5-10 minutes). Intermediate depths (60 fsw) are limited by medium halftimes, and shallow dives (35-40 fsw) are long half time dependent. The pressure in the limiting tissue is the “surfacing pressure” and its ratio to absolute pressure is the “limiting ratio.” This value (in fsw) is the pressure in a given half time compartment for ascent to the surface and is the Mo-value."

For example, the safe surfacing supersaturation ratio of the 5 min. tissue is 3.5, while the 120 minute half time tissue can safely oversaturate to 1.6. This means that you can be supersaturated to about 3.5 times surface capacity without concern for bubble formation and decompression sickness resulting from the faster tissue. But, even if it is at 100%, there is no way that it can be the controlling tissue as long as you are shallow enough that this 3.5 ratio is not exceeded. However, look at the other extreme, the 120 min. tissue can give you trouble at about half the depth that the 5 min. tissue can, but you have to have been there longer.

 

[fjpatrum]

Seems fairly ridiculous to me. The NDL for 100 feet is only 20 minutes, which would mean by the time you got to depth (let's say 2 minutes to the bottom), you're already within 8 minutes of that limit. Sure, there are some rounding errors there because the NDL wouldn't be 20 minutes for the whole descent, but even so, that's an unreasonable limitation on your dive time.

The NDL is the limit you don't want to exceed. Giving it a little leeway maybe a minute or two is being reasonably conservative and probably a decent idea, especially if you're doing multiple dives but 10 minutes is just unnecessary, in my opinion.

 

[lowviz]

Seems fairly ridiculous to me. ...//....

Assuming that you mean the 10 min buffer, I would have to agree. Massively limiting on deeper dives...

 

[fjpatrum]

Assuming that you mean the 10 min buffer, I would have to agree. Massively limiting on deeper dives...

Yes, I did mean the 10 min buffer. I don't dive deep but, even so, I've been within 5 minutes of my NDL on several different occasions at 40 feet (using an Oceanic computer, which I've been told means I probably would have overshot my NDL using a Suunto.) At that point I scooted up a bit in the column and swam around some more. 10 minutes would severely limit some of my dives even at 40 feet and I'm a newb. I can't imagine how limiting it would be to those who have a lot of experience and/or a low SAC.