Exceeded NDL by a bit, but computer cleared me. What should I do?

[arew+4]

Any time you have an accent ceiling, you are on a deco dive. If you or your buddy have an equipment failure you cannot go to surface without high probability of getting bent. If you find your self with an unplanned ceiling, and your gas supply is marginally sufficient to clear it you had better get to your stop. If you have excess gas by all means go up to 30-40 and let it clear, watch your N2 loading graph and try to wait it out till it's back in the yellow or better yet green. Then do a safety stop.

 

[boulderjohn]

I think that is a strange thing to say. Your computer does all the math and says you have entered into Deco, yet you don't consider that to be "real". I think that is a dangerous way to look at the situation and not the way it should be viewed by a person like the OP who is unfamiliar with the situation and is seeking guidance and perspective.

We have enough trouble with terminology where we already have "deco", "safety stop", "mandatory safety stop", "optional/recommended safety stop" and now you want to add to the mix a new term, the idea of "not real deco"?

I'm more simplistic in my thinking. Either you have a safety stop or a deco stop. Pick one, they are not the same, they may both be estimates based on theory, but the concepts ARE DIFFERENT.

Not that I am advocating that someone drastically modifies their subsequent diving that day, if they wander into deco for a few minutes (and clear it), but I would rather that the terminology be simpler and more easy to understand. Adding in all these "made up" adjectives doesn't help with the basic comprehension of the situation nor make things safer.

Let's try to get beyond the confusion of the words that are used and understand the reality of the situation. When you do this, you will see that vocabulary is the problem, because we keep trying to impose a binary way of thinking and speaking on a system that is not binary.

There is a big difference between what we typically call a decompression dive and what we typically call a NDL, no stop, or recreational dive. With a decompression dive, not only must you do a stop on the way up, your ascent rate matters. If you ascend too slowly or do stops that are too deep, you will pay a penalty by having to do an extended stop or stops in your later ascent. With a NDL (no stop, recreational, whatever) dive, once you begin your ascent, it doesn't seem to matter how slowly you ascend, as long as you don't take long enough to go into deco, and you don't have to do a final stop. The two are indeed very different.

The problem is that there is no bright line between the two. That means there is another type of dive, the fuzzy area in between the two, an area where the different physiology of different divers and the lack of full understanding of decompression science makes us unsure of what to say or do. We lack the vocabulary to describe such dives, and we lack the precision of procedures to deal with them. In that fuzzy realm we use terms like "mandatory optional safety stops," which means "OK, maybe you have strayed into decompression, so you really should do a decompression stop just in case."

We also have a situation where you have gone into decompression, but your decompression obligation is minimal. When a computer's algorithm decides you have gone into deco, it creates an ascent plan that should clear you to surface if you follow it. That plan includes two aspects: the rate at which you ascend and the stops you need to make. If you are in that fuzzy area, ascending at the computer's recommended rate may be all you need to do to clear your decompression obligation, and the computer will therefore no longer require a decompression stop. According to that algorithm, you were in decompression, and according to that algorithm, you completed your required decompression. You can then go on with life accordingly.

 

[mcohen1021]

I'm fairly certain that the computers are all programmed with a margin of error, not to the point where if you exceeded the limit by a little, you're bound for a chamber trip. Just keep a better eye on things in the future

 

[boulderjohn]

I think the point was that the deco stop cleared during ascent before reaching the stop depth. No deco done, not a deco dive... But of course, if they’d have felt an urgent need to do a CESA at the point they were in deco, then it would indeed have been a deco dive…

I tried to explain this in my previous post. The problem with this is thinking that dealing with a decompression dive only means doing a deco stop. There are actually two aspects of clearing a decompression obligation--decompression stops and ascent rate. If you have a minor decompression obligation, then it is possible your ascent rate alone can clear that obligation. If that happens, it does not mean you were no longer in a deco dive--it means you were in a deco dive and you cleared your obligation through your ascent rate.

You would be strongly advised to do a long safety stop at that point.

 

[boulderjohn]

I'm fairly certain that the computers are all programmed with a margin of error, not to the point where of you exceeded the limit by a little, you're bound for a chamber trip. Just keep a better eye on things in the future

The algorithms used by computers all have that safety margin to begin with. The research that developed those algorithms over more than 100 years led to the creation of plans that will almost certainly result in a safe dive if followed properly. There should be no need to add an additional safety factor, but most computer algorithms allow the user to do that anyway--such as GFs in Buhlmann.

 

[jborg]

Yeah… I was going for some subtle sarcasm in noting that the supposedly non-deco dive would indeed have become a deco dive if they’d had to surface during the time they were in deco… so how non-deco was it really

 

[boulderjohn]

If you have excess gas by all means go up to 30-40 and let it clear,

I must have missed the context for this.

Are you talking about adding an arbitrary decompression stop at 30-40 feet, a stop not recommended by the computer's algorithm, in the event the computer has indicated you are in a decompression dive?

If that is what was meant, I would recommend against that. If a diver has gone into deco during a dive and begun an ascent, it is possible that following the computer's recommended ascent rate will clear that decompression obligation along the way. Doing a stop that is not recommended along the way is imposing your own procedures to override the procedures developed in the algorithm. In decompression diving, doing stops deeper than the requirements of the algorithm generally add to the decompression obligation.

 

[arew+4]

I must have missed the context for this.

Are you talking about adding an arbitrary decompression stop at 30-40 feet, a stop not recommended by the computer's algorithm, in the event the computer has indicated you are in a decompression dive?

If that is what was meant, I would recommend against that. If a diver has gone into deco during a dive and begun an ascent, it is possible that following the computer's recommended ascent rate will clear that decompression obligation along the way. Doing a stop that is not recommended along the way is imposing your own procedures to override the procedures developed in the algorithm. In decompression diving, doing stops deeper than the requirements of the algorithm generally add to the decompression obligation.

No, I'm talking about some one doing a typical reef profile who stays down around 80' a little too long on second or third dive.

 

[boulderjohn]

No, I'm talking about some one doing a typical reef profile who stays down around 80' a little too long on second or third dive.

I still don't understand.

If a diver stays too long at 80 feet and goes into decompression, then that diver has a decompression obligation. It is the same as if the diver went to 150 feet and went into deco. Either way, that decompression obligation must be met through a combination of ascent rate and stops. Either way, a diver using a computer should follow the advice of the computer, advice based on a tested algorithm.

 

[scubadada]

I tried to explain this in my previous post. The problem with this is thinking that dealing with a decompression dive only means doing a deco stop. There are actually two aspects of clearing a decompression obligation--decompression stops and ascent rate. If you have a minor decompression obligation, then it is possible your ascent rate alone can clear that obligation. If that happens, it does not mean you were no longer in a deco dive--it means you were in a deco dive and you cleared your obligation through your ascent rate.

You would be strongly advised to do a long safety stop at that point.

I'm entirely with @boulderjohn If you do not enter deco, it was a no stop (NDL or recreational) dive. If you enter deco, regardless of the time of the decompression obligation, it was a decompression dive.

If you are very close to the NDL, or exceed it, and go into decompression, you can increase your safety margin by extending your safety stop or padding your deco stop. Better yet, you can follow your SurfGF and surface with the safety factor you choose.

The algorithms used by computers all have that safety margin to begin with. The research that developed those algorithms over more than 100 years led to the creation of plans that will almost certainly result in a safe dive if followed properly. There should be no need to add an additional safety factor, but most computer algorithms allow the user to do that anyway--such as GFs in Buhlmann.

I believe that all the available decompression algorithms are safe. However, the NDLs of the various algorithms vary, further complicating this discussion. For example, at 60 feet, on air, the NDL for DSAT is 57 min and for PZ+ it is 48 min. At 55 min, diving DSAT, I have a couple of minutes of NDL remaining while you, diving PZ+, have several minutes of obligatory decompression. You have to dive the computer you have decided to use. If you decide to skip the deco stop because my computer says there is NDL left, you will be violating the guidance given by your computer. In addition, with many/most dive computers, you will be locked out of using your computer for 24-48 hours.