Nitrox - What do you dive PO2 - 1.4, 1.5 or 1.6?

[Steve_C]

Since dives get canceled and I do not always know ahead of time what depth I usually fill with 30%. Gives me max of 1.3 at likely sites and I am usually diving 1.2 - 1.3. On the rare deeper dive I will dive 28% to keep it at or below1.3.

 

[Colliam7]

This point stuck out at me - "Please note: There is no more danger . . . . Would I be allowed to dive 30% at 1.5 or is that pushing the limits? . . . . So I am wondering do most folks dive 1.4 for conservationism or is the article above misleading?

I am not sure I agree with the statement, as written. I do agree, that not only PPO2, but time as well factor into the overall risk of untoward events. But, if you hold other factors constant - e.g. time - then PPO2 becomes a controlling variable. If you are at a given depth, for a given time, your risk is greater with higher PPO2 (e.g. 1.6 vs 1.4). Of course, if the risk of an event, say a generalized tonic-clonic seizure, increases from only 0.01% to 0.02%, many would say the risk is trivial, even though it has doubled. And, that may be the basis for the statement the change im risk is so small as to be almost unmeasurable. But, as TSandM pointed out: 'You can survive the bends. You are highly unlikely to survive an oxygen seizure.' A generalized seizure at 130 ft, even if it was very unlikely to occur before it actually did, creates a scenario which some would describe as, 'It sucks to be you'.

As to what you might be allowed to do - what a given charter op might do - that is dependent on the OP, and all we on SB can say is what we have seen, which is quite variable in my case. I have seen, more often than not, ops not even asking what you are breathing on dives to 130-140ft. The behavior suggests the belief that a certified diver is expected to make their own decisions, on the basis of knowing the planned depths of the sites. But, I have seen other ops that are more OCD on the issue.

As for what I do, like several other posters, I use a limit of 1.4 for the 'working' portion of a dive, and 1.6 for the deco portion as a general rule. If I expect to be strenuously 'working' - dark, cold, current, etc. - I will drop my max to 1.2, 'just in case' - no hard data to guide such a decision. But, one of several points in the article that I do agree with is that there may be no premonitory signs before a seizure. Somehow, in our teaching of enriched air diving, a few students come away with the erroneous notion that VENTID(S) is sequential or some sort of cascade, and that you will experience minor signs / symptoms before the occurrence of a seizure - not the case at all. No premonitory signs, no 'aura'. But, the consequences of a generalized seizure occurring underwater are so substantial, it isn't worth the 0.02% (or whatever) risk, for me.

As with most things, in balancing PPO2, time, depth, mixture, etc. against risk, there are no solutions, only trade-offs. As an aside, the article refers to the proceedings of the DAN conference in 2000. It may also be helpful to reveiw more contemporary DAN publications, such as an article that Petar Denoble published in the December 2013 issue of Alert Diver): Alert Diver | Understanding Oxygen Toxicity.

 

[Basking Ridge Diver]

Have you heard the bit about better to be on the boat wishing you were in the water than in the water wishing you were on the boat?

I am starting to realize that knowing what the calculations are and what they mean versus using them in the regional setting are really two different things entirely.
I am realizing now that warm blue water vs cold dark water may warrant further caution and knowing local dive habits... That said I appreciate that I have more to learn about what I thought was relatively straight forward.
I appreciate the responses.

 

[dumpsterDiver]

I use 1.6 as a max and try to stay above that a little (maybe 10 feet). I am generally drift diving and "hang high" for most of the dive and then drop down to catch a lobster or fish and then try to get higher in the water column without delay.

I know some very experienced people who will push 34% to even 130 feet, but it is really stupid. You really need to balance the extra no deco time you get from a somewhat rich mix versus a lower mix.

Is it worth the risk of a deadly seizure to gain 4 extra minutes? If you must have the extra 4 minutes, it is probably safer to use a lighter mix and just do the 4 -6 minutes of deco it causes you. After all, if you are diving close to the limits of the no-deco constraints, you really can't be safely shooting for the surface if something goes wrong - so you should be carry some redundancy and that redundancy should be sufficient to handle a little deco, if need be.

I'm not saying recreational divers should be doing deco, but if you are going to be an idiot, at least be smart about it.

 

[tbone1004]

I'm with Dumpsterdiver surprisingly on this.
If I know I'm not planning on being on the sand for most of the dive, the sand is 1.6, max, if the wreck is only 10-15 ft off the bottom, then the planned depth is 1.4 and the sand is what it is. If I'm planing on being at or near the bottom for a while, 1.4 it is, or whatever standard ish mix is close enough.

Normal dives are PO2 of 1.4 max, preferably 1.3, but I'm not too picky depending on standard mixes. 32% is normal in cave country where 100 is planned max depth but it can dip lower than that for a bit in most of the caves, but seeing 31% out of the compressors isn't abnormal unfortunately. Doesn't particularly matter though. 30% generally off the coast of NC, so 1.5 to the sand, but even if sand is 130, if you're 10ft off the bottom you're at 1.4.

Now, where this really is something I'm passionate about is if you're regularly diving below 100fsw, you should have some sort of decompression training, whether on O2 or back gas doesn't matter, but without it you are cutting everything far too close with NDL's, certainly on repetitive dives.

 

[RJP]

Now, where this really is something I'm passionate about is if you're regularly diving below 100fsw, you should have some sort of decompression training, whether on O2 or back gas doesn't matter, but without it you are cutting everything far too close with NDL's, certainly on repetitive dives.

Agreed, at a minimum some "contingency familiarity." How often do you see a recreational diver get back on the boat freaking out because their computer gave them a 4min stop instead of a 3min stop "I went into deco.. I went into deco... will I die?!?" Worse, of course would be the person who gets hit with more unexpected deco than they have gas to handle.

I find with many students that a simple example of what "just a few more minutes" on the bottom can mean in a deco scenario. I'd point out to the OP, for instance, that if you planned to spend 10min at 130ft you're still within NDL, and V-Planner says you'd need 58cf of gas. However, if you got distracted while trying to get that lobster out of a hole and spent 12min at 130ft you've now got 7min of deco and need 75cf of gas. How about another 2min? Now you're looking at 10min of deco and 85ft of gas. A real problem if you're diving an 80cf tank. Because they have zero familiarity with deco, most students are shocked to learn that 2 extra minutes on the bottom means 7 minutes of deco.

As my tech instructor (Wayne from DiveSeekers) drilled into us: "Schedule. Schedule. Schedule."

 

[Centrals]

Agreed, at a minimum some "contingency familiarity." How often do you see a recreational diver get back on the boat freaking out because their computer gave them a 4min stop instead of a 3min stop "I went into deco.. I went into deco... will I die?!?" Worse, of course would be the person who gets hit with more unexpected deco than they have gas to handle.

I find with many students that a simple example of what "just a few more minutes" on the bottom can mean in a deco scenario. I'd point out to the OP, for instance, that if you planned to spend 10min at 130ft you're still within NDL, and V-Planner says you'd need 58cf of gas. However, if you got distracted while trying to get that lobster out of a hole and spent 12min at 130ft you've now got 7min of deco and need 75cf of gas. How about another 2min? Now you're looking at 10min of deco and 85ft of gas. A real problem if you're diving an 80cf tank. Because they have zero familiarity with deco, most students are shocked to learn that 2 extra minutes on the bottom means 7 minutes of deco.

As my tech instructor (Wayne from DiveSeekers) drilled into us: "Schedule. Schedule. Schedule."

Perhaps all the diving agencies should include couple of pages on decompression theory in their OW manuals.

 

[RJP]

Perhaps all the diving agencies should include couple of pages on decompression theory in their OW manuals.

I'm guessing that's a can of worms legally. Suppose some OW student decides to push NDL beyond the limit into "light deco" on a dive. Unfortunately he's done this based on nothing more than "a couple of pages of theory" and he ends up dead. Wife sues claiming that the agency/instructor provided insufficient training for deco diving which, irrespective of any disclaimers, they tacitly endorsed by bringing the subject up in the first place.

Beyond the "you'll get sued" drama, in all practicality if someone were to introduce deco theory in an OW course... where would you STOP the discussion? Deco diving is definitely in the "just enough information to kill yourself" category.

 

[Peter Guy]

Perhaps all the diving agencies should include couple of pages on decompression theory in their OW manuals.

a. Do you know any agency that does NOT include a "couple of pages on decompression theory" in the OW class?

b. What does diving on enriched blends have at 100 - 130 feet have to do with any agency's OW class?

c. What all this discussion shows is that deep dives really need to be planned based on at least three limiting factors -- decompression status (Yes, ALL dives are decompression dives); O2 status; and consumption (including decompression regardless of it being a "no stop dive"). If one is going to dive deep, get used to the idea of "doing deco" and plan accordingly.

 

[tbone1004]

I'd like to see limited back gas decompression incorporated into nitrox diving, not OW, but I think it's something that can be discussed in a nitrox course, even if it is limited to 5 minutes of back gas decompression it is enough to fix most issues with diving deeper than 100ft.