Quiz - Equipment - NDL

[Pedro Burrito]

Spoiler: Answer

d. all the above are correct

This question, as well as many others in the Dive Theory Exam, is a reminder to the new divers that their equipment is fallible. It does not cover anything other than NDL for the newer diver. We see it here sometimes where someone will discuss an unexplained DCS hit. I liked @Rollin 's quote:

Measure with a micrometer, mark with chalk, cut with an axe.

That's the theory of decompression. Computer is the micrometer, diver is the chalk and the algoritm is the axe.
It is not about the micrometer being not precise enough.

 

[BlueTrin]

Aren’t timing devices pretty correct usually unless they break ?

If one tries to account for a potential break in a timing device by modifying NDL times, that seems a bit wrong.

 

[tursiops]

Looking at it a little differently.....the danger is not that you are exceeding NDL, rather the danger is going into mandatory deco, possibly not knowing what to do, possibly not having enough gas, and possibly freaking out. In general, exceeding the NDL by a tiny bit gives you only a tiny bit of deco (usually gone by the time you surface, or even if you just go shallower, and only a tiny bit of extra gas requirement. it is a shame tht the teaching and literature takes this tiny problem and turns it into the potential for a freak-out. I'd rather see divers just a tiny bit more knowledgable...

 

[dmaziuk]

I agree it is a bad question, but not for the reasons @Ukmc mention. Not pushing NDLs should be avoided because dive tables and algorithms used in dive computers are probabilistic models, the closer you get to NDLs the higher the probability of DCS; that does not mean that if you violate an NDL by 1 foot or 1 min you will take a hit, that could happen 1 foot or 1 min under the NDL limits. They build in all kinds of safety factors/conservatism levels but probabilities are just that.

Cue the "solid line drawn through gray area" quote. It is clear that over NDL the risk of clinical DCS is "unacceptably high" whereas under NDL it is "acceptably low", for some value of "acceptable". What isn't clear is that there is a linear dependency.

Fraedrich study shows that the probabilities between different models only agree in GF 70..85 range and Spisni study suggests, however indirectly, that "more conservative" is not "safer".

I.e. it may well be that you're better off "not pushing NDL" at GF85 than "not pushing NDL" at GF60, and that a blanket "avoid diving to no decompression limits" is overly simplistic.

 

[TMHeimer]

Looking at it a little differently.....the danger is not that you are exceeding NDL, rather the danger is going into mandatory deco, possibly not knowing what to do, possibly not having enough gas, and possibly freaking out. In general, exceeding the NDL by a tiny bit gives you only a tiny bit of deco (usually gone by the time you surface, or even if you just go shallower, and only a tiny bit of extra gas requirement. it is a shame tht the teaching and literature takes this tiny problem and turns it into the potential for a freak-out. I'd rather see divers just a tiny bit more knowledgable...

Agree.
Regarding BluTrin's question about timing devices usually being pretty correct unless they break--
In one of my first "deep" (shore) dives (to 75') my watch did break. Freak accident- a shred of string from my collection bag wrapped around the thing you pull out to set the time and opened it up, if you can believe that). I wasn't near the end of the dive and not near the NDL. But if I was, it may have cause a problem. Maybe even panic at my newbie stage. I signalled my buddy and think we turned around and headed back to shore.
So, for that reason too, all answers are correct. Don't get too close to the limits as anything can happen.

 

[elmo]

Agree.
Regarding BluTrin's question about timing devices usually being pretty correct unless they break--
In one of my first "deep" (shore) dives (to 75') my watch did break. Freak accident- a shred of string from my collection bag wrapped around the thing you pull out to set the time and opened it up, if you can believe that). I wasn't near the end of the dive and not near the NDL. But if I was, it may have cause a problem. Maybe even panic at my newbie stage. I signalled my buddy and think we turned around and headed back to shore.
So, for that reason too, all answers are correct. Don't get too close to the limits as anything can happen.

But the question doesn't say anything about reliability. It relates to 'precision'. A watch that device that reports time to 0.01 of a second is more precise than one that reports time to the minute, but it might just as well be wrong. Similarly, reporting depth to the millimetre or 16th of an inch isn't going to help anyone, especially if the sensor is faulty, despite being far more precise.

 

[broncobowsher]

I have 2 PAID certs. Not even sure what they are. I just remember that when I was done I was not impressed with the end result.

I tried to play this game. Dumb it down to the very basics, dumb it down even more, and a little more. Still a horribly flawed question/answer options. This is the stuff I elect to cast a "none" vote even if that means leaving it blank.

 

[scubadada]

So, remember, when you dive to the NDL, and not over, you can ascend directly to the surface at the appropriate rate. This is true for all of the available dive computers and the various decompression algorithms. The safety stop is recommended to further reduce the risk of DCS. Actually, I think it is recommended to slow the rate of ascent in many divers.

 

[EFX]

Good point. If NDL = 0 is a line through a gray area of an increase of getting a DCS hit then the incorporation of the safety stop (aka non-mandatory deco stop) takes us out of that gray area. The benefit/cost ratio is high.

 

[scubadada]

Good point. If NDL = 0 is a line through a gray area of an increase of getting a DCS hit then the incorporation of the safety stop (aka non-mandatory deco stop) takes us out of that gray area. The benefit/cost ratio is high.

I have thought long and hard about this. The longer at depth, the increased risk of DCS. I have 1850+ dives on DSAT, a very liberal algorithm. Many of the dives have not been close to NDL, many have. This works well for me, perhaps not for everyone.