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

[scubadada]

Ok, so you plan to do a dive with requiring 20 minutes of deco. Then, the dive ends up being slightly shallow and shorter than planned. Do you still do the 20 minutes of unnecessary deco? Of course not.

Some folks still do not seem to understand. No, you do the deco required by your computer at the end of your dive

 

[johndiver999]

Yeah I expressed my post really badly. I started by "explaining" the supposed point (no deco actually done => not a deco dive), but highlighting the fact that had they needed to surface it would have been obvious it was in fact a deco dive. So I agree fully.

There are lots of ways to express the ideas that a computer only generates a prediction of a physiological status, that people can get bent even when they follow the computer, people can get bent when they do no deco dives, that a minimal deco obligation will often clear deep if you ascend slowly etc. etc.

But if the captain of the boat says "no deco" and you tell him you didn't do a deco dive because (all 5 minutes) of it all cleared at 30 feet (below a real deco stop depth) , somebody may not be happy.

 

[scubadada]

There are lots of ways to express the ideas that a computer only generates a prediction of a physiological status, that people can get bent even when they follow the computer, people can get bent when they do no deco dives, that a minimal deco obligation will often clear deep if you ascend slowly etc. etc.

But if the captain of the boat says "no deco" and you tell him you didn't do a deco dive because (all 5 minutes) of it all cleared at 30 feet (below a real deco stop depth) , somebody may not be happy.

That can be covered by a dive time limit. If you have deco, you are still under the dive time limit.

 

[arew+4]

What's the point, you were in deco and cleared it at a shallower depth, allowing a longer dive time? That's one of the advantages of a dive computer, right?

I totally agree with you, Boulder thought I was advocating for deep deco stops or something.

 

[jborg]

As a possibly interesting aside, I was playing around in the Subsurface planner and noticed that in "recreational mode" it plans dives like the one in this thread. That is, it's happy as long as the normal, planned ascent with safety stop just avoids the deco ceiling.

(For those unfamiliar with this planning mode in Subsurface, it is intended for planning dives within NDL, according to the manual it "computes the maximum time a diver can stay at the current depth without needing mandatory decompression stops and without using more than the existing gas".)

For example, here I say that I want to dive to 50m on air with a bottom time of five minutes. The planner suggests extending that to eight minutes before heading up, at which point the SurfGF is predicted to be 107% (ceiling @8m). During the ascent that increases to a maximum of 112% (ceiling @10m). The generated profile avoids the deco ceiling, so it's all good.

 

[inquis]

That is, it's happy as long as the normal, planned ascent with safety stop just avoids the deco ceiling.

Minor correction, the calculated ascent *without* safety stop will just avoid the ceiling.

 

[L13]

Minor correction, the calculated ascent *without* safety stop will just avoid the ceiling.

Minor correction to your minor correction :
You are correct for most (all?) dive computers. However, SubSurface planning mode is a little different.

1) If you check the "Safety Stop" button in the planner, it uses the SS as part of the planned ascent for NDL calculation. Just as @jborg stated.

2) You can set your own ascent rates for each of 4 segments during the ascent, those rates are used for the NDL calculation. This can have a similar effect, and greatly change the results as well.

 

[jborg]

the calculated ascent *without* safety stop will just avoid the ceiling.

Well it sort of looks like that in the screenshot, and I also tweaked the ascent rates a little in that case, but it can also end up like below where you'd hit the ceiling if you skipped the safety stop.

Of course, you can uncheck the safety stop checkbox, then Subsurface won't plan a safety stop and will start the ascent earlier to avoid the deco ceiling.

 

[inquis]

Sorry, still incorrect. I'm using Subsurface 5.0.8 (EDIT: I don't recall any past version that acted differently), which definitely calculates bottom time to just clear without a safety stop. Enabling the optional stop setting does nothing to the bottom time, and exits 3 mins after clearing. In this screenshot, you can see the extension of the ascent would miss the ceiling:

@jborg, you've forced a longer than NDL dive through your choice of duration at depth in your second screenshot. You'll note that your initial screenshot (with variable ascent rate) misses the deco ceiling if you extend the ascent rate established just prior to the safety stop.

 

[inquis]

If you check the "Safety Stop" button in the planner, it uses the SS as part of the planned ascent for NDL calculation.

If NDL was calculated with that stop in mind, the bottom time would increase when enabled. When I check it, bottom time in the table on the right is unchanged.