Descending/restarting ascent at end of dive

[Belzelbub]

In general, I don't see anything inherently problematic with your examples.

Your computer will be tracking your exposure throughout. I've certainly done similar with no ill effects, and my computer had no issues tracking as a single dive, or in a couple situations as a second dive after a short SI. That said, the computed results will vary based on the type of computer you are using. Some will penalize for short SI or sawtooth profiles. Overall risk will depend on a number of factors, but in general unless you are riding the NDL, this shouldn't be a big issue.

 

[dmaziuk]

Going down to 10 metres shouldn't be a problem: typically, the "no-limit depth" is somewhere in 7-8 msw range and as long as you stay there, it's all just one long safety stop. Between there and 10 msw it's a bit murky, but at/under 10 msw you start on-gassing again. But if you only descend there for a minute or two, you won't on-gas enough to make an appreciable difference.

I believe dive computers will want you to do a safety stop after that, even you've completed one already.

 

[mdwalter]

My questions are:
1 - am I wrong in my assessment? Are any of these examples considered risky?

No and No

2 - it is my understanding that the major issue with descending after an ascent, is the possibility of gas bubbles being recompressed and passing over into the arterial blood flow, and that this is an important reason for the mandatory surface interval - is this correct? Are there other considerations?*

No and yes but not in your examples

3 - Where do you draw the line?
A) In the first case: what are the limits of exposure and bounce you would be comfortable with?*

I would bounce down as far as your average depth from the whole dive without a thought. If the bounce were only 1 to 2 minutes I would just count it as additional bottom time to factor into any safety stop or other deco calculation, but give yourself credit for any deco you may have already done. This is what most good computers will do for you. If it were a longer bounce, add the time of the bounce to your total bottom time and start over with your ascent as though you had been down the whole bottom time. Some computers may also do this for you.

B) In the second case: how much of an exposure would you say warrants a mandatory surface interval?

There is no requirement for a surface interval, ever. A surface interval is really just a deco stop at 0 ffw (or less if at altitude). Remember, there is no such thing as "no decompression" diving. Your tissues will on-gas during any time spent at more than 1 ATA and will off-gas as you move back to lower ATAs, regardless of whether you make conscious stops or slower ascent or not. This is why tables have surface intervals for adjusting allowable time on repetitive dives, because you are doing your deco that did not happen in the water up in the air. So, as others have said, and as most tables will tell you, you're basically just counting the next dive as a continuation of the previous dive. Most good computers will do this for you.

4 - Or am I completely overthinking this out of proportion, surface intervals are not necessary when diving with a computer on NDL dives and bounces are not a problem?

Yes you are. Yes, they are not. And yes they are not.

*I expect that for deco dives this will be more problematic both in terms of available gas (if you were to redo any deco stops) and also in terms of higher load and more chance of bubbles - ie. dive your plan, if you drop something, let it go...

Every dive is a deco dive (see above), so I would apply the same criteria I listed above to any dive. In some cases this means restarting the deco, in others it means adding a bit to deco to compensate for the bounce, and in other cases it means just returning to where you left off and finishing the deco.

 

[DrWilliam]

I believe the major issue is the 2nd one you described. Possibility for re-compressed bubbles to cross into arterial circ thus setting you up for AGE.

A diver did a re-descent on a multi-day trip recently. Shark popped up down below when diver was at 10 feet so he dropped down to about 35 feet for a couple of minutes to check it out. His Cressi computer called for 18 minutes safety stop.

I would not do it, but especially not for a double ender.

 

[boulderjohn]

Every dive is a deco dive (see above),

Let's be careful with language.

If you google the phrase decompression dive and read 100 responses, almost all of them will tell you that a decompression dive is a dive with required deco stops, in contrast to NDL dives that do not require such stops. That is how the phrase has evolved among divers, and that is what they mean when they use that phrase.

What you mean to say is that every dive needs to have some decompression. That is true, but that is not how the phrase is commonly used. You are using language differently from the norm.

As it turns out, there is a significant difference between dives with required decompression and dives commonly called NDL dives. When you are within NDLs and begin an ascent, there does not seem to be any limit to how long it takes you to ascend, as long as you do not violate NDLs. You can hang out at different depths for different lengths of time and still end the dive with a direct ascent to the surface or a standard safety stop. On such a dive, descending a short distance to pick up a dropped item will generally not be any problem whatsoever.

With what we commonly call decompression dives, it is very different. Taking a longer than expected ascent time can add time to your required decompression stop times and depths. Descending to a deeper level to pick something up during decompression stops will also impact your decompression time and depth requirements.

 

[divezonescuba]

 

[DrWilliam]

I would like to clarify the issue however of ultra minimal surface intervals. I thought I saw this from DAN but can’t find it. The common scenario being ascending from a dive and while getting back on boat dropping something overboard (like your BFK). So someone volunteers to drop down to say 50 feet and retrieve it, knowing they had plenty of NDL left.

The issue isn’t about getting bent, the greater, though rare, risk is AGE (Arterial Gas Embolism).

The concern is that upon surfacing bubble nuclei have blossomed into micro-bubbles. Now you recompress them and somehow (PFO??) they cross into arterial circulation and when you surface you now have arterial bubbles which can cause Arterial Gas Embolism.

But there are also reportedly “undeserved” cases of bends when divers go back down quickly to free the anchor.

 

[steinbil]

Did you read the OP? Do you know what a double-ender is? Do you suggest I tether double-enders to me?