How close to no decompression limit

[Hatul]

Rule #1. Don't believe everything you're told or read.

 

[bleeb]

The levels in medium density tissue did exceed the saturation limit toward the end of one of my dives but fell to just below the limit after doing the safety stop.

If you really did exceed a saturation limit and the software isn't just being confusing, then you were decompression diving. In itself, this is not a bad thing provided you did it intentionally, knew what you were doing, and were adequately equipped for it. Many people would say this means knowledge of gas usage and adequate gas reserves, and redundant breathing apparatus (dual isolateable or isolated tanks, and dual first and second stage regulators). If you did it unintentionally or didn't realise it until after being back on the surface, this may indicate bad planning or poor situational awareness. The issue isn't that it could be cleared with a safety stop. The issue is that if anything happened while you were saturated, you would not be able to directly ascend to the surface without significantly higher risk. Many (but admittedly not all) people regard regular OW/AOW recreational non-deco diving as never exceeding NDL (i.e. No Decompression Limit) at any time, because this always allows you to ascend directly to the surface.

 

[DevonDiver]

In itself, this is not a bad thing provided you did it intentionally, knew what you were doing, and were adequately equipped for it.

Calling it a 'safety stop' doesn't provide a warm glow of reassurance that he was

If you did it unintentionally or didn't realise it until after being back on the surface, this may indicate bad planning or poor situational awareness.

It certainly doesn't indicate good planning and awareness

Many (but admittedly not all) people regard regular OW/AOW recreational non-deco diving as never exceeding NDL (i.e. No Decompression Limit) at any time, because this always allows you to ascend directly to the surface.

Absolutely - because the training given at those levels provides absolutely zero knowledge or skills for the conduct of dives where immediate surfacing isn't always available as a contingency resort. The training agencies are clear on that. Instructors are clear on that.

Some agencies do allow various degrees of deco to be conducted. They also provide various elements of deco training - appropriate to the dives permitted. PADI, SSI etc don't.

Some experienced divers chose to do deco diving. Most have specific deco training (i.e. 'Technical Divers). A small few have vast cumulative experience that essentially replicates whatever they would have been taught on a tech course.

Some inexperienced divers also chose to do deco diving. For the most part, that's through sheer ignorance of the risks and the requirements needed to do it safely. They roll the dice of fate and come up winners of a few occasions. To them, that's a justification for doing it - they're 'safe'. The activity is 'safe'. They never got hurt did they? So they keep rolling the dice....

 

[waterpirate]

I rreally hate blanket statements about anything. I think that people who issue these statements without their listners fully understanding what was meant should re evalute their role as instructors or mentors. The answer you are seeking will not come from anyone here, it will come from within yourself. You need to read and understand:
1. what is a ndl?
2. how did we arrive at them?
3. advantages and disadvantages of a puter
4. understanding all the recent developments in modeling and algorythms

I know I am asking alot of you, but when you understand all this you will be able to make your own informed decisions about how you want to treat your body and why you made the decision you did for yourself.
Eric

 

[Jax]

The diving is done from a charter boat and comprises two dives. Details of dive depths are known a few minutes before jumping in the water. First dive to a maximum depth of over 65 ft. Second dive the same but within no decompression limits. Dive plan (second dive) is - jump in, swim around at some depth until 10 minute NDL is reached, reduce depth as required to increase NDL until air gets low, rise to level for safety stop and then get out. Dive charters often avoid a deep second dive so this is not a problem.

Fox, you are at a point where -- In my opinion, and you know "Opinions are like buttholes, we all have one." - YOU should be making your own decision on how close or not, or how over or not, to the NDLs you go. With over two hundred dives, you should learn the theory yourself and make decisions for yourself.

Find Mark Powell's "Deco for Divers". Read it, study it, then read it again. Learn for yourself what all this deco stuff means.

I am fairly sure you do not understand decompression, because you would not be asking the question you asked if you did understand.

 

[DiveNav]

Rule #1. Don't believe everything you're told or read.

Rule #2. Don't believe in Rule #1.

 

[DiveNav]

How close to the deco limits?
It depends.

As waterpirate said, first we need to clarify what is the NDL limit.
Or, even better, is there a thing such as "the" decompression limit? Or is it more of a "range" of different values based on who/what defines them?

Let me try to explain (or confuse even further) this subject with few images.

Let's start from a profile of a real dive.
This is a multi-level dive at the Devil's throath in Cozumel. Max depth is 136ft and bottom time is 40 minutes.

​

Clearly, if this was a dive with a square profile .... it would have been a big NO-NO on a single AL80 as it is way outside any table ....

Being a multilevel dive, it was not a deco dive. But, how far was I from deco? And according to whose definition?
There are multiple answers to these 2 simple questions.
Let's see 4 of them

1) Tables.
If we average depth this profile we get a depth of 59ft for a bottom time of 41 minutes. That means group Q on PADI tables and "safety stop required"

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2) Conservative dive computer
If we use a "conservative" dive computer .... or a "normal" dive computer set in conservative mode, we might get an indication of NDL load very close to the "deco limit".
In the image below NDL load is 99%. Too close for my tastes.
But ... is it really that close?

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3) Moderate dive computer
If we use a "moderate" dive computer instead (or a conservative computer set in liberal mode ..... or a liberal computer set in conservative mode), we might get an indication of NDL load a bit more distant from the "deco limit" than before ...... But, it is still the same dive profile. How is that possible?
In the image below NDL load is 93%.

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2) Liberal dive computer
And lastly, ff we use a "liberal" dive computer instead (or a conservative computer set in very liberal mode ..... or a moderate computer set in liberal mode), we might get an indication of NDL load that is even more distant from the "deco limit" than before ...... But, it is still the same dive profile. How is that possible?
In the image below NDL load is 88%.

​

So, why is deco limit moving around?
Maybe is just a matter of perspectives?

Alberto (aka eDiver)

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[Thalassamania]

Back when computers first came out and we were diving in the Arctic, 4 days from helo evac, we instituted a rule that when using a computer you needed to surface with 5 minutes of no-D time left on it. What that meant operationally, was that when you hit 5 minutes of no-D time at, say, 60 feet, you started slowly up and after ascending ten or twenty feet you could stop to continue observations. Keep in mind that as you ascend your available no-D time increases. So you'd work at the shallower depth until you hit the 5 min. mark again, and you'd ascend a little more. By the time you had thus ascended to 20 feet (where a decompression limit did not exist, even fully saturated) you were, by then, out of air and would make a normal ascent to the surface. This procedure assured that no one was ever forced, by an OOA situation or a sudden problem, to ascend with too much nitrogen in their system.

If you understand that, then you will understand what I perceive the SSI recommendation to be, though I see as too conservative. It is not a matter of hitting a 10 min. to decompression situation and having to ascend to the surface, it is a matter of hitting 10 min. to decompression situation and ascending ten or twenty feet to increase your available no-D time.

 

[scubadada]

Why is this simple topic being made so complicated? You've chosen your algorithm based on the computer you purchased, I assume you trust it, I do. You don't exceed NDL and you perform an elective safety stop before you ascend to the surface. You exceed your NDL, you fulfill your deco obligaton with or without an additional elective safety stop and you ascend to the surface. The key, of course, is gas managment. You need enough gas to finish off the plan. Every diver should have this important skill, it's not difficult. I always run a simulation of my dive so that I know the depth and time limits. A backup timer could save your dive, I dive a backup computer.

I've followed this stategy for 589 dives now and have not had a problem to date. Best of luck in your diving future.

Good diving, Craig

 

[BDSC]

I recently did the advanced adventurer course through SSI and was advised in very strong terms that you should never get less than ten minutes to the no decompression limit on your computer. What do you think?

Seems way to conservative to me. I do uw video so when I dive in the Caribbean I seldom go deep enough that any of this matters but when I'm off the coast of NC where the diving is generally deeper, I usually go within a minute or two of my NDL according to my computer which is a Sherwood Wisdom. I'll sometimes even go into deco if the shot is worth it.