First Dive Computer

[KenGordon]

Ken, if I could follow up on this again as I'm not sure I see why a Rec diver would need to take the computers to the (NDL) limits (or into deco) in order to assess their relative conservatism.

E.g. for a Rec diver, who happens to have two computers (A & B) on his/her wrist which run two different algos (let's call them algo A, and algo B). Let's say both those computers are programmed for a 3min SS and use usual ascent rates which the diver follows. That Rec diver dives to 25m, stays there for a while, and computer/algo A says 5min NDL, and computer B says 8min NDL. The diver then heads up, does a SS and surfaces. Therefore, for this particular dive, I could then state A is safer & more conservative than B. Correct?

Given that the rate of change of both of the NDL times should/must be the same in the above i.e. when A's NDL is saying zero, B's must say 3min when staying at 25m.

[edit start] And I assume that if the diver then went to 20m then the NDL differentials would also be approx the same [edit end]

And if the Rec diver did a whole variety of Rec dives over months with those two computers on his/her wrist and for each Rec dive A's NDL was less than B's, couldn't we then state A is generally safer & more conservative than B.

NDL is a nearly useless number to use to compare. As the nitrogen loading tends towards the limit the NDL is approximately (limit - current load)/loading rate So you see that heigh loading rates (being deep) or a smaller distance to the limit will make it shorter. As you come shallower though that loading rate tends to zero, and is eventually negative. Meanwhile the (limit - current load) is tending towards zero. So you are divinging one number which is hovering around zero by another number which is hovering around zero. We all know how that turns out, don’t we? (Anything can happen, resulting number wise.)

I did a chamber dive last week. The last stop was at 6m but fluctuated by a small fraction of a metre. The computer whose NDL display I could see was waving about between 120 minutes and 199 minutes and back again over a few 10s of seconds.

Planned NDL times or initial NDL at depth are more useful. If a dive profile is planned at 16 minutes at 30m rather than 18 minutes at 30m it is, everything else being equal, more conservative.

However, as pointed it in one of these threads in the past day or so by a proper expert, different compartments influence different dive profile/sequences. Thus your computers A and B may swap round which is more conservative depending on the particular dives done.

 

[dmaziuk]

I just happened to glance at my computer's screen when its readout went from 12 to 18 minutes yesterday -- I was actually checking the depth after coming up a metre or so. One second earlier and I'd've missed it and I'd've had 33% "less NDL". Or 50%, dep. on which way you count it.

<sarcasm>I felt half again safer right then.</sarcasm>