Overshooting NDL and mandatory deco stops

[Mike1967]

Just did a dive on 32% and my buddy on air, we both got our NDL down to 3mins. After the dive I questioned what conservative setting he was using and he actually forgot to reset his DC to air, hence both of us having the same NDL. My question is, how much difference would that make to his deco obligations? i.e how far over his NDL would he have gone if it was set to air?

TIA

 

[dewdropsonrosa]

Just did a dive on 32% and my buddy on air, we both got our NDL down to 3mins. After the dive I questioned what conservative setting he was using and he actually forgot to reset his DC to air, hence both of us having the same NDL. My question is, how much difference would that make to his deco obligations? i.e how far over his NDL would he have gone if it was set to air?

TIA

Need more info:
-Depth
-Time
-Dive computer models (for algorithm)

 

[Mike1967]

Need more info:
-Depth
-Time
-Dive computer models (for algorithm)

Max was 33m average 21
37min
Not sure what his is but we both hit the 3 min NDL at the same time so heres mine. Perdix set to 40/85. Buhlmann GF ZHL-16C

 

[KenGordon]

Just did a dive on 32% and my buddy on air, we both got our NDL down to 3mins. After the dive I questioned what conservative setting he was using and he actually forgot to reset his DC to air, hence both of us having the same NDL. My question is, how much difference would that make to his deco obligations? i.e how far over his NDL would he have gone if it was set to air?

TIA

Compare the no stop times of air vs 32%. At 30m it is of the order of 20 minutes vs 28 minutes (BSAC88). So he was probably well into deco if you were down to 3 minutes. However, the actual ascent and profile makes a difference. With your computer set to /85 your air no stop time for 30m is probably more like 14 minutes, so your 32% no stop time might be 21 (guessing, my Perdix is downstairs) so your buddy might not have been so terribly into deco by the tables or more aggressive computer settings. From your max vs average depth I guess you were doing some multilevel dive with a shallower second half. If that is the case then any ceiling as a result of the deep initial section would likely be gone by the time you surfaced.

You may be able to load the dive into subsurface and change the gas to air to see the actual air ceiling. It would be interesting the rest of us to hear about that.

There are various strategies for diving with different gases/CCR vs OC/different computes, however everyone involved needs to be on the same page. Especially people have to not forget anything! Did you know your buddy was on air?

So, probably some poor practice, at least one untaught mistake, got away with it, don’t make a habit of it...

 

[Mike1967]

I guess you were doing some multilevel dive with a shallower second half

Boat dropped us off at the wrong spot and it took us about 17mins at 20-25m to get to the drop off, then 10 mins between 25-33, then another 10min to ascend and do safety stop.

You may be able to load the dive into subsurface and change the gas to air to see the actual air ceiling.

I tried that on DiveLog but it doesn't change anything else.
Edit: I've got subsurface but never used it,I'm downloading all my dives(can't pick one) then I'll have a play.

Did you know your buddy was on air?

Yes but we don't buddy check DCs are set right, does anyone?

 

[RainPilot]

Yes but we don't buddy check DCs are set right, does anyone?

Its part of my buddy checks, gas quantity and set on DC. I say aloud, check mine and buddy acknowledges and check theirs.

I dont see a lot of thorough buddy checks out in the wild though.

 

[Mike1967]

I dont see a lot of thorough buddy checks out in the wild though

We've been diving with each other for about a year, 50ish dives. I must admit we've become pretty slack, we check gas is on and quantity and that's about it. We've never checked DCs though.

 

[Mike1967]

You may be able to load the dive into subsurface and change the gas to air to see the actual air ceiling.

Did that and like DiveLog nothing else changes.

 

[KenGordon]

Yes but we don't buddy check DCs are set right, does anyone?

Given he is on air you, as a pair, need some sort of plan about keeping to an air profile. That might be sticking to the air no stop times, doing the air stops, abandoning your buddy to do his air stops while you get on the boat to eat his cake, or just ignoring his computer altogether and hoping to eat his cake while he is on the O2/been taken away by the helicopter.

I am loath to suggest setting your computer to air to match your buddy’s gas, but it is a plan. In these circumstances I tend to have one computer set to the truth for me and another set to air so I have a clue about what to expect, but I often dive with extreme novices. For a proper dive where I care about the backup being a backup I set it to the truth too, if my buddy makes a mistake like yours did that is their problem.

This starts to lead into a soft/people problem of how far to go in checking your buddy.

The ‘by your training’ answer would be to have had a dive plan for depth and time based on air and to have stuck to that.

The ‘tables tables tables’ and ‘rule of 120’ people might say you should have been able to guess his NDL based on yours and known not to get so close to your limit.

 

[KenGordon]

Did that and like DiveLog nothing else changes.


View attachment 562410

I am no subsurface expert, I have run it a handful of times, but I would guess that if no ceiling appears when you tell it it was an air dive then there was no ceiling. You could also try a more conservative GF to see if my guess is correct.

I think DiveLog uses the ceiling reported by the computer, whereas subsurface might be able to use either that or one it calculates itself. Maybe @atdotde can help.

The profile shows what looks like about a 4m/minute ascend and a reasonable stop, so maybe he was out of deco, but clearly my earlier guess was completely backwards compared to the actual profile.