How close to no decompression limit

[dumpsterDiver]

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

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.

Ok that makes sense to me, but what do the guys graphs mean? What does 100% mean?

 

[Thalassamania]

Don't know, I can't see them. I'd assume that 100% means that the tissue has all the nitrogen that it's going to absorb, e.g., 6 half times.

 

[lowviz]

fjpatrum,

Your instructor may be more clever than overly safe. Taking a ten minute buffer really does limit you to recreational depths for a diver who claims 0-24 dives. -maybe just trying to cool your jets?

 

[Bombay High]

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.

It is not limiting for a diver with a lot of experience. You do the dive you have to, and then you do the deco you are obliged to do.

It is not complicated.

 

[redacted]

The reality is that if you had enough air and trusted your algorithm, you could wait until a ceiling showed up and then begin your normal ascent. Odds are, by the time you reached the indicated ceiling depth, the ceiling would be gone as a result of out-gassing during your ascent.

May work OK for the dive at hand, but if there is a next dive and a next dive, you may want to keep it on the conservative side. You have to decide where in the knee of the DCS risk curve you and your computer are. I am old and dive a fairly liberal computer, so I usually try to back off a bit when doing multiple dives over multiple days, That usually means extended safety stops rather than shortened dives.

 

[Dive-aholic]

I'm diving in the ocean within limits for advanced recreational diving ie. < 30 m. The precise profile of the dive is largely dependent on the profile of the reef structures below which are unknown before the dive. I could refer to my computer dive planner which would give me a maximum time at a single depth but that is largely irrelevant because there is usually a large range in depth around the reef and the depths that I'll dive depend on what is interesting. If I need to go deeper, that just means less time at that depth. Providing you are within the NDL, why is that a problem? If I start getting low on air I head to the surface. The question here is how close is it safe to be to the NDL.

So what you're saying is the boat captain is bringing you to dive locations that he's never been to before? That's quite unusual. Every charter I've been on goes to sites they are familiar with and the divemaster can brief on. I've never heard of a charter going to an unknown location. Part of the plan is to establish a maximum depth. If you "need to go deeper" you do so on a subsequent dive with that as part of the plan.

 

[fjpatrum]

fjpatrum,

Your instructor may be more clever than overly safe. Taking a ten minute buffer really does limit you to recreational depths for a diver who claims 0-24 dives. -maybe just trying to cool your jets?

For reference, I'm not the OP and no one has ever told me anything except not to exceed my NDLs. My jets are probably somewhere between cold and full afterburner, so no biggee there. Like I said, I'm not diving deep. I tend to be limited more by my gas consumption than NDLs anyway, except occasionally.

It is not limiting for a diver with a lot of experience. You do the dive you have to, and then you do the deco you are obliged to do.

It is not complicated.

It's very limiting if you're trying to stay in no-deco diving and someone tells you to not get within 10 minutes of NDL (on the short side, not beyond NDL).

If you assume that the NDL is just a line that you should be aware of and then plan to dive beyond it anyway, then yes, it isn't very limiting.

 

[MXGratefulDiver]

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?

I think you have found an instructor who doesn't truly understand what he's teaching.

There is nothing magical about no-decompression limits. Depending on the dive profile you can reach zero minutes on NDL quite safely and begin an ascent at that point. The important thing is to have a basic understanding of what NDL means to your body, and craft an ascent profile that allows your body to offgas safely as you ascend.

Someone earlier recommended Mark Powell's book "Deco for Divers" ... I heartily second that recommendation. It will provide far more useful information that you'll get by using rules of thumb.

As noted about, the graphs of tissue saturation indicated at some stages of the dive some tissue exceeded no decompression limits even with the ten minute buffer. That was a concern. There is no provision on the dive charter boats for dealing with a diver who goes into decompression (ie. spare tanks in the water) and we surface with 50 bar (725 psi) in the tank.

How do you go about ascertaining that, on a given dive, you will surface with a specific amount of gas reserve? How do you determine what amount is adequate to deal with a worst-case emergency at the deepest part of your dive? This information is far more important to someone pushing NDL's than anything your dive computer can tell you.

I don't have a good operational understanding of what to do if you do exceed no decompression limits. This is not taught or discussed at this level of diving. I'm not even sure if my computer would provide guidance on decompression stops needed if I did exceed the no decompression limits. The rule is plainly - don't exceed no decompression limits.

It SHOULD be taught and discussed in any class that covers dive planning ... particularly deep diving, where exceeding NDL's becomes rather easy to do. "Just say no" is not a good instructional method.

There's a couple articles on my website that may be helpful to you ...

NWGratefulDiver.com

NWGratefulDiver.com

And not to be harsh about it, but do yourself a favor and seek out an instructor who actually understands what they're teaching. Handing out rules of thumb without explaining why they're appropriate isn't my idea of proper dive instruction ...

... Bob (Grateful Diver)

 

[Bombay High]

It's very limiting if you're trying to stay in no-deco diving and someone tells you to not get within 10 minutes of NDL (on the short side, not beyond NDL).

If you assume that the NDL is just a line that you should be aware of and then plan to dive beyond it anyway, then yes, it isn't very limiting.

You referred to very experienced divers ... I do not think very experienced divers would be frustrated by staying within NDL's.
They would either stay within them or go over and do the deco.

 

[lowviz]

...//...My jets are probably somewhere between cold and full afterburner, so no biggee there. Like I said, I'm not diving deep. I tend to be limited more by my gas consumption than NDLs anyway, except occasionally....//....

Bolding mine. Love that reply, go for it.

My bad.