Quiz - Equipment - NDL

[Pedro Burrito]

From the PADI Dive Theory Exam:

Divers should avoid diving to the no decompression limits because _____ may not be precise - even a slight variation can put a diver at risk if the limits are pushed.

a. dive computers

b. depth gauges

c. timing devices

d. all the above are correct



I will post a daily question from my exams to help newer divers and to encourage more experienced divers to interact gracefully and helpfully with the newer divers.

Reminder - this is a post in the Basic Forum and it is a green zone. Please be nice and on topic.

Thank you for your patience while we try to give people something to discuss other than Covid-19 and/or Politics. I will post the answer covered by the spoiler tag later today.

 

[Angelo Farina]

Of course my favourite answer had been e) which is missing:
e) as diving just within the edge of NDL is unsafe, it is safer to program the dive slightly OUTSIDE NDL, training and equipping accordingly.

 

[Ukmc]

Bad question. Depth gauges are precise enough. Timing devices are very precise. Nowadays computers are very precise to do what you want them to do and many will allow to choose how conservative one can be. I wonder if authors of the question are looking for computers not being individualized enough and because of that want students not to ride an NDL... will see when the answer comes

 

[Pedro Burrito]

To quote Segal's Law: "A man with a watch knows what time it is. A man with two watches is never sure."

I usually dive with two dive computers, one on each arm. Side by side, they will differ by up to a meter in depth.

 

[loosenit2]

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.

 

[Angelo Farina]

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.

I agree entirely. Which is the reason for which no one is really thrusting NDL limits, hence the practice of always doing a deco stop (which is renamed as "safety stop", but actually does not differ significantly from a true deco stop).
And this in the end proofs my missing option e), which always mandates for planning for a deco stop.

 

[Edward3c]

Bad question. Depth gauges are precise enough. Timing devices are very precise. Nowadays computers are very precise to do what you want them to do and many will allow to choose how conservative one can be. I wonder if authors of the question are looking for computers not being individualized enough and because of that want students not to ride an NDL... will see when the answer comes

On many occasions I’ve brought my computer and depth gauge together and seen over a meter deference (not always the same way round), the same situation has existed with my buddy’s instruments. I now dive with two computers and consistently get different readings

 

[elmo]

I doubt I'll ever find myself sitting the PADI instructor exam, but I would struggle to bring myself to answer this question if I were to, even though I have no doubt which answer would be marked correct.
The question is terribly confused about the difference between precision and accuracy, not to mention all those instruments are precise enough given the accuracy of our decompression models, and of course the fact that many of us are capable of planning and executing dives to and well beyond the no decompression limits.

 

[rhwestfall]

another bad question....

 

[scubadada]

The article posted by @Dominik_E in discussion of another DCS quiz question is applicable to this question also Quiz - Physiology - DCS Factors The vast majority of episodes of DCS were "unexpected" and the divers did nothing outside generally recognized "safe" no stop diving.

Perhaps what this question is trying to get at is that there is a gray (or grey) area surrounding the NDL limit and that susceptibility to DCS consists of a variety of personal factors in addition to the depth/time limit. One could write a much better question if that is the case. The previous question tried to illustrate some of the personal risk factors.

And, of course, many of us our sensitized to risk of DCS, especially as we grow older. I pay considerable attention to my ascent rate and pad my SS if I have been close to NDL. Today, I follow my SurfGF to aid me in when I choose to surface.