Accidental DECO and mild panic in a non tech certified diver.

[Gareth J]

What also needs to be considered when the topic of pushing ndl with a rich mix is: what is the benefit? How much less dive time would you have if you backed off the mix 1 or 2 percent? Those numbers seem to be relevant when trying to formulate a “reasonable” dive plan.

I would respectfully suggest that diving a mix close to 1.6 PO2 as an NDL dive is significantly more risky than doing the same dive on a leaner mix and doing the required decompression stops.

With the exception of Ocean Divers, most of my club do dives including decompression stops (they are trained and qualified to do this). Even those that choose not to do dives with compulsory decompression, will, if close to the NDL, automatically do a stop at 6m, prior to ascending for the safety stop. If they where to overrun their NDL, they are trained and capable of doing the required stops.
I struggle with the ethic's of not teaching decompression skills as part of general diving training. The OP is not the first time I have heard of divers charging towards the surface when the NDL time is close to zero, or when they have overshot the dive time and have moved into compulsory decompression. This is not helped by the attitude of some skippers who stop a diver doing subsequent if they move from NDL, to dives requiring decompression stops (not a policy practiced in the UK).

This thread has rather wondered away from the OP.

 

[uncfnp]

To the best of my knowledge- there has never, ever been a single documented case of a recreational single tank diver breathing <40% EAN getting an 02 hit at any depth. Not one.

I have hesitated to get involved since there truly are so many threads on oxtox risk and ppO2 ...

But for what its worth, I believe a lack of documentation of incidents is a very low level of credible evidence given that few “modern era” recreational divers dive to 1.6 for any significant length of time (hence the sample size is low) and all too often we never know the true cause in unwitnessed dive fatalities. So saying it is safe for recreational divers to intentionally plan dive to 1.6 based on a lack of supporting evidence to the contrary is not valid .

Personally I am known to push my ppO2 but for medical reasons, not just a desire to stay deeper longer. But still I would not plan a dive when a large portion of the dive would be at 1.6. As others have said, under those circumstances, the consequences of an oxtox event out weigh the limited benefit I would get pushing the ppO2 to 1.6.

 

[Bob DBF]

I would respectfully suggest that diving a mix close to 1.6 PO2 as an NDL dive is significantly more risky than doing the same dive on a leaner mix and doing the required decompression stops.

I would agree, however on this side of the pond deco is no longer looked at as just an advanced diver skill. It is only available in technical training, and any useful deco information need not be covered in Recreational training because you need only follow your computer, which got us here on this thread.


Bob

 

[Caveeagle]

Not sure I follow your reasoning. On your point about a buddy going into unplanned deco. Surely by having your DC matched to theirs, there is less chance of this happening as you both have eyes on roughly the same NDL (hopefully). Regarding lost buddy scenario, you would be in exactly the same NDL situation if you were also diving air, the only difference being you have some (albeit unknown) additional padding.

Don't get me wrong, I am not advocating air setting for nitrox as a standard practice, but in this unusual scenario it could be an option. Ideally both divers would be diving the same gas and one wouldn't be reliant on the other for situational awareness, but unfortunately we don't live in an ideal world.

I will try to explain my reasoning. If your air diving buddy gets lost, how long do you look for him before surfacing to call for help? If you are diving 32% you have a better profile to do a 5m search and then surface within NDL and seek help. Running the air profile, you will not really now your true TTS or NDL.

Similar for If your air buddy accidentally racks up unplanned deco and then runs low on gas before completing deco obligation. If you are on your 32% profile, you “could” surface within NDL and be in a position to deliver extra gas to the diver. Or even hang with them and share gas presuming you have enough to safely do so.

Diving the air profile with EAN In your tank just gives you unnecessary limitations. And I just don’t see the benifit.

I once went to 144’ with 32% Nitrox (PO2 of 1,72) for 2-3 minutes without any problem.

Yeah, and people survived air dives to 250’ as well. But that doesn’t mean it was safe.

 

[fsardone]

In the example cited, the diver HAD to be fairly deep and/or breathing a very rich EAN mix to get to, and maintain 1.6 for 40 minutes.

So I "might" need to clarify that there has "never been a documented case of an 02 hit on a single tank recreational (non tech) single tank diver breathing <40 percent 02 other than a test case where heavy exercise was done for 40 minutes and maintaining a 1.6 the entire time.

When a diver such as myself sets their dive computer for 1.6, it is highly unlikely that they would be diving diving 1.6 the entire dive- if they even get that high at all. It's just the upper limit. I don't think I EVER hit 1.6 during a dive.

Interesting discussion. I have little concern for what the computer is set but what it is in the tank and what ATA you are breathing it. Setting a Open Circuit Nitrox computer ppO2 only means that given the configured fO2 and current depth, when you see the set ppO2 you will get a warning. So if diving Nx30 and 30 mt you actually are at ppO2 1.2 and the computer is set at Nx30 and ppO2 1.6 will not give you a warning and correctly track your OTU and CNS. But everybody knew this ... just not clear from the context I guess.

But you don't know that it was a 'recreational' diving example. To clarify- I use the term 'recreational' to mean single tank, no decompression diving, at or below 130' and I also add "breathing at or below P02 1.6". There is nothing in the study that states whether the one example fits this criteria.
[...]
Whatever those numbers work out to be, I'll bet that either the 02 level is higher than 40% and or the depth (real or simulated) is higher than my stated acceptable diving parameters.
[...]
The article makes it clear that exertion increases risk of an 02 hit. I'm not debating that in the least. My issue is with working hard AND maintaining P02 of 1.6 for 40 minutes. Most if not literally ALL single tank recreational divers breathing less than 40% 02 will never experience that. It's almost beyond the realm of possibility.

I will not debate the single tank issue or recreational dive definition .... they make little sense in term of CNS clock. What I am concerned is the high oxygen exposure and repetitive diving with short surface intervals. If we look at CNS clock at 1.6 is 45 minutes for a single exposure and 150 for 24 hours.
The NOAA standard and safety manual (linked below) establishes (page 119) a limit of 1.4 for normal exposure for Nitrox diving .... also

The maximum allowable exposure limit should be reduced in case swhere coldor strenuous diving conditions, or extended exposure times are expected.

https://www.omao.noaa.gov/sites/default/files/documents/NDSSM Final_041217.pdf
Point being keep max ppO2 low and reaching the 24 hours limits means you need to have a 24 hours break in diving otherwise the risk of CNS Ox Tox increases.

For clarity, the diver must have been breathing a 40% mix at 30m.
po2 (1.6) / absolute pressure at depth (4) = 40% mix

If you choose to dive the bottom phase with a planed PO2 of 1.6, that is your choice. There are no dive police.
But please don't say it is a safe option, or encourage others. Even in the early 90's when I did my Advanced Nitrox we where told not to do it, and we have learnt a lot of lessons since then, all of them say high PO2's during the bottom phase are not a good idea. Remember 1.6 - 0.1 for any adverse factor (depth, temperature, work, fatigue, stress, etc).

I will and have planned decompression schedules with a PO2 of 1.6 - but that is at rest, on the shallow stops. I have had off board bailout schedules with a PO2 of 1.6, but that is an immediately falling PO2, the bailout directly precedes an immediate ascent.

I am a CCR diver, I can set my ppO2 to my wish and I need to plan for OC bailout as above. I dive 1.3 ppO2 for short dives and 1.2 or even 1.1 for longer in the bottom segment. Then coming up I will bring it up to 1.3 and then 1.4 at 6 meters. But my bailout (the OC gas I need to switch to if the reb fails) is more geared toward keeping no big jumps in ppHE o ppN2 so is very close to the mix I am breathing at dept (so low ppO2), my gas changes will bring the ppO2 to 1.6 during an OC bailout ascent, again briefly and for each change gas at the initial stop) the it will fall, 1.6 ppO2 is not a safe place to be.

What I am saying and many have said before me, deco is safer (if done properly) than high oxygen exposure (which cannot be done properly). All it takes is a little proficiency in buoyancy and some gas redundancy. I believe anybody going past the open water (PADI)/ ocean diver (BSAC) should be able to manage staged decompression and have the minimum redundancy needed to handle it (which includes a buddy with proper reserves close by).

Cheers

 

[fsardone]

Here's the dive profile. We ascended at about 24 minutes. I caught up to her on the line at 15' and we briefly ascended to 10' after I was able to convince her to ascend although the profile doesn't indicate we were at 10' - but that could be a sampling rate error as I said it was a very brief 10' stop.

Somebody else explained the calculated ceiling concept.
I have uploaded a little more complex dive below to show that Subsurface has a lot of options and can calculate the ceiling in Buhlmann GF (in this case below GF is 45/85). Also if the dive computer reports the ceiling subsurface will display it too. In this case the red shaded area shows the computer displayed deco stops (and ceiling) while the green shows the subsurface calculated ceiling. You can see I was following the computer ....
If you computer reports the ceiling (look in the downloaded data) it will show in red. Missing red means it was not diplaying deco.
Also means that the configured GF (or algorithm) in Subsurface did not match what you have in you dive computer.


If you look closely, the last bit (from 7 mt to surface) took me 5 minutes. I used the ruler function of subsurface to measure: time difference 5.24 minutes dept difference 7.4 meters. I would recommend being very slow in the last bit (as pointed out from others) since this is where the maximum relative pressure change happens.
This is a relatively short dive (135) and as pointed out above I used 0.7 at start of dive, 1.3 in the bottom portion 1.4 in deco up to minute 126 and then switched down to 1.3 again, finished deco switched to 0.7 but in the climbout I flushed pure O2 to improve my flush out of residual inert from fast tissues and to delay bubbling upon surfacing.

All of this to say that:
-Oxygen is a dangerous beast and needs to be managed in term of ppO2 and exposure times
-Oxygen can be very beneficial in managing the decompression risk
-Oxygen exposure needs to be carefully managed (did I say that before? )
-Being a recreational diver doing 2 or 3 dives on Nitrox 40 @ 30 mt will expose you to a greater dose than the one in the dive below and on the second day you would be out of limits

So please be wary: too much or too little oxygen will kill you, silently and without prior notice. This is a lesson that all rebreather divers have hardwired in their brain.
Nitrox allows you to push the limits: learn about oxygen and decompression, what your instruments are telling you and enjoy being underwater.

I really appreciate this discussion (and I thank the OP for putting it in the open) because is highlighting a lot of great points!

 

[atdotde]

Also means that the configured GF (or algorithm) in Subsurface did not match what you have in you dive computer.
View attachment 520978

I am not sure the difference between the OSTC ceiling and the Subsurface ceiling is so big: The OSTC is always rounding to the next deeper multiple of 3m whereas Subsurface also shows intermediate values. There is a button that makes subsurface round up to multiples of 3m as well and I bet if you turn that on the difference will be minimal. Still, personally, I like the continuous display better.

 

[fsardone]

I am not sure the difference between the OSTC ceiling and the Subsurface ceiling is so big: The OSTC is always rounding to the next deeper multiple of 3m whereas Subsurface also shows intermediate values. There is a button that makes subsurface round up to multiples of 3m as well and I bet if you turn that on the difference will be minimal. Still, personally, I like the continuous display better.

I believe In my graph is quite close.
I was referring to @caruso having some ceiling from subsurface but not recalling any deco displayed by his computer ...

 

[Johnoly]

.... the topic of pushing ndl with a rich mix is: what is the benefit? ...

All of us divers dive differently and just like equipment, there is no perfect configuration nor situation.

In our case, we are spearos on the deep ledge in Jupiter. It goes from 118 on the top rim down with a steep dropoff to 145 at the bottom and run times are typically 35 minutes. We'll run a 32 mix typically on that site(s). If I shoot a grouper/hog/cobe, he's gonna try to run to 145 and try to rock up. I gotta go down there and lay in the sand digging under that rock to put a hand in his gills and pull him up. Might take 30 seconds. Might be a 3-5 min task.

Everyone when they are diving dives differently and at some point you need to make that decision when to LEAVE. That includes leaving gear or leaving your buddy on the bottom.

 

[Peter69_56]

I once went to 144’ with 32% Nitrox (PO2 of 1,72) for 2-3 minutes without any problem.

I have been to 2.2 during an incident, but wouldnt suggest for a second that its ok. We dealt with it and got sorted and went back to back gas. All good on THAT occassion. It shows no one figure is right or wrong, just a guideline on what we think is ok.

I try and maintain 1.6 or less always during deco and 1.4 or less on the bottom.