Total of 12 dives and already a few lessons and one almost "near miss"

[yle]

Be careful with that: changing one parameter to make things 'more conservative' does not necessarily make things 'more safe'.

Same here. Diving Nitrox is balancing two risks: DCS/Narcosis (caused by the Nitrogen) and OxTox (caused by the Oxygen). Reducing one necessarily increases the risk in the other.

This isn't exactly true, because the two risks aren't dependent on the gas mix in the same way. The risk of CNS oxtox is practically nonexistent for a recreational diver below a ppO2 of 1.4. The risk of DCS increases with time and exposure to ppN2.

Using 36% nitrox on a dive to 70 feet for a second dive of the day will greatly reduce the chance of DCS (compared to using air for the same dive), while exposing the diver to a ppO2 of 1.1... and virtually no risk of CNS oxtox (compared to the ppO2 of 0.65 were the diver to use air.)

 

[yle]

But remember: "a wider margin of where I am safe" from one risk (say, OxTox from PPo2 < 1.4) is also a 'narrower margin where I am at risk' (from DCS and narcosis from greater nitrogen). TNSTAAFL.

Stay safe -- but remember that always trying to be "MORE SAFE™" might actually be making you materially less.

Simply untrue in this situation. The diver might choose to dive a shallower depth... which would reduce both DCS and CNS oxtox risk. These two risks do not always move in opposite directions for all choices. The depth choice is a simple way to reduce both risks.

And for a new diver trying to be cautious and conservative as they build experience, choosing an appropriate depth limit is arguably the simplest way to do so. There's no reason to confuse the issue by suggesting that limiting depth in an effort to dive conservatively will necessarily increase risk in some other way.

 

[Manatee Diver]

And I was led to believe this is why the MOD of the nitrox mix is an important parameter typically included on the label or logbook when nitrox is used.

It still is, just like with DCS you don't know exactly how quickly O2 toxicity will affect your personally, as such going over a PPO2 of 1.4 and/or approaching the daily O2 exposure limits should only be done cautiously. The general recommendation I've heard is to take the NOAA's exposure numbers and subtract 33-50%.

But for recreational divers if they stay at or below 1.4, it isn't something they really need to be concerned about as long as they realize that going past 1.4 isn't a wall of death.

 

[Protondecay123]

It still is, just like with DCS you don't know exactly how quickly O2 toxicity will affect your personally, as such going over a PPO2 of 1.4 and/or approaching the daily O2 exposure limits should only be done cautiously. The general recommendation I've heard is to take the NOAA's exposure numbers and subtract 33-50%.

But for recreational divers if they stay at or below 1.4, it isn't something they really need to be concerned about as long as they realize that going past 1.4 isn't a wall of death.

NOAA still uses a pp02 of 1.6 for its divers with no reported deleterious effects according to the agency dive manual.

 

[Marie13]

Don't fight it, it is natural.

Hang on tight and tuck your head (so you don't lose your secondary) on the downstroke. Get your fins up one step at the stall point at the bottom of the downstroke. Stretch out a bit on the upstroke while remaining tucked. Ride it up. Grab another step at the top if you wish, but I don't. I just get comfortable for the next downstroke. No rush.

If I can manage a fin on ladder, anyone can. You just swing your fins out to the side to get on the next step. I refuse to dive on boats without a fin on ladder. Peeling off fins while in a drysuit - not fun

 

[lowviz]

Being blown off the ladder in sporty seas (lakes) without fins is even less fun.

 

[KevinNM]

Being blown off the ladder in sporty seas (lakes) without fins is even less fun.

Never give your fins to the crew until you are standing fully on the boat.

 

[KevinNM]

Ok thanks... I always thought the risk of CNS oxtox was primarily associated with a high ppO2, i.e. something higher than 1.4 or 1.6, which varies with an individual and the specific conditions of the dive. I was led to believe that CNS oxtox could occur in the first few minutes of a dive, given the necessary conditions, so that "time of exposure" was a minor contributor at best. And I was led to believe this is why the MOD of the nitrox mix is an important parameter typically included on the label or logbook when nitrox is used.

But thank you for correcting me and setting the record straight.

The entire CNS toxicity process is poorly understood last I knew. It is a combination of time and dosage, and likely a set of unknown factors. The original studies showed that some people could operate fine for hours at 2.1, and then the next run they would tox at 1.5 after 20 minutes.

But it's not a rapid process unless you do something insane like breath 100% oxygen at 100 feet. You can drop below the safe depth and it's fine if you don't stay there. It usually takes a while to mess you up. But you should still avoid doing this, as the penalty for being wrong is typically death.

 

[SapphireMind]

I appreciate all the discussion on the topic! In my field, we don't see oxygen toxicity the way it's viewed here (at least, we've never considered it, could be a fun research study maybe) Our issues/concerns are more with lung damage, creation of free radicals and retinal development.

 

[yle]

I appreciate all the discussion on the topic! In my field, we don't see oxygen toxicity the way it's viewed here (at least, we've never considered it, could be a fun research study maybe) Our issues/concerns are more with lung damage, creation of free radicals and retinal development.

Yep, two different types of oxygen toxicity: in a hospital environment you're more interested in pulmonary oxtox, which is due to lung tissues exposed to elevated oxygen levels over a long term (like having a patient on 100% O2 for hours.)

Divers are generally more concerned about CNS oxygen toxicity, which is primarily caused by breathing oxygen at a partial pressure of greater than 1.6 atm (of course this can only happen if you're breathing gas at an elevated pressure, i.e. as we do underwater... can't happen in a typical hospital environment.)