PPO2 maximum safe value: 1.4, 1.6

[2airishuman]

I don't ever intend to do deco dives, trimix dives, dives with gas switches, or dives deeper than 130 feet. The interesting local dives are cold water dives most often in roughly 60-100 feet of water, sometimes deeper. When I have the training and experience to do these dives, I anticipate diving 100cf doubles. NDLs will be the governing factor in dive planning. I will be diving over a hard bottom that will rarely if ever be deeper than 130 feet. My dive computer calculates CNS O2 loading.

Given all that, there's simplicity to be gained by using EAN32 for all dives. Banked EAN32 is widely available here and doesn't require the lead time of a custom blend. I could permanently mark the MOD of each tank. I could refill tanks after each dive rather than waiting to decide the location and depth of the next dive. I could be flexible enough to change dive sites when the weather requires, a common problem here. There would be no reason to keep the tanks O2 clean since there would never be a reason to use partial pressure blending. Using the same gas all the time would reduce the risk of ending up with the wrong gas. I'd still get an analyzer and check every tank after it is filled, and again at the dive site.

The question I have is the handling of dives to 120-130 feet. With EAN32, PPO2 is between 1.4 and 1.6 at these depths. NDLs on EAN32 at these depths are 18-20 minutes, give or take a minute or two depending on which agency's table you use.

Is there anything more than a hypothetical risk from the higher PPO2 of 1.6 under these circumstances? Given that NDLs limit the dive duration, it's hard for me to believe that there is. Do agencies vary in their teachings on this subject? Is the added CNS safety margin of a custom EAN28 mix meaningful enough to warrant the risks inherent in keeping track of a tank inventory with multiple mixes? Have there been accidents or incidents involving tox hits at 1.6 for dive durations this short?

 

[tr3a]

Where in MN do you expect to be diving that deep? I ask because I am also from MN. At those depths the cold will be more of a concern than getting more bottom time

 

[bamafan]

It is my understanding the Navy dives a 2.0. I have done it spearfishing in the Middle Grounds off Florida doing 5 dives a day to that depth for 25 minutes or so per dive. I wouldn't do a 1.6 on a dive with a long bottom time as your CNS would be through the roof. If you aren't working hard it is common practice for tech divers to do it on deco for periods longer than what you are talking about. It is a decision you will have to make about what amount of risk you are willing to accept. There are lots of people who will say you are going to die but it is done daily and I have never heard of an oxtox event from a 1.6.

 

[Jared0425]

Tr3A is right, nitrox is not really all that advantageous in cold water. For our region, nitrox is primarily used for deco gases due to the fact that the water below 50ft is a few degrees above freezing in the high to mid 30s.

To answer your question nobody has ever toxed at 1.4 PPA very very very few people have ever taken a tox hit on 1.6 PPA, but it has happned it all depends on your body and cells. Some divers will amp up to 1.8-2 in some cases but they do it at a much more elevated risk for a tox hit. Other people who know the science will chime in with better answers than me.

 

[KevinNM]

People have died from oxygen toxicity at 1.3. But it's your life.

http://www.scubaboard.com/community...ty-limits-symptoms.440726/page-2#post-7067933

 

[PfcAJ]

32% isn't an appropriate gas for dives deeper than 100ft. Both oxygen and nitrogen contents are too high.

 

[lerpy]

I wouldn't use 32% beyond 110, keep your PPO at 1.4 no more for the working part of your dive. As well when it comes to PPO and toxing, both the PPO and the time are the factor. 1.4 for 2 hours at 140ft depth, may have you at a higher risk to tox than 1.6 for 30 min at a 20ft stop. Its not just the PPO that is the factor. Vacation divers have toxed due to the use of 32% or 34% over multiple dives over multiple days. Start looking at the math of 34% for 5 dives a day, 7 day of the week. You would find you can actually be pushing the envelope.

The other factor is contingency. If you are at the max PPO for the mix then you have no room for error, what if something happens that might have to take you deeper? Are you prepared to leave your buddy if he goes deeper, now increasing your PPO into dangerous areas? Just because it can be done doesn't me it should be.

If your truly interested in getting depth and time, then you should really take the course, learn the science and the safety factors. Trying to figure out how far you can push a 32% could end up being the first domino in the string that will lead to a very bad situation.

 

[Storker]

At 1.4 bar max pPO2, MOD for EAN32 is 33.8m (110'). That's deep enough for my narked skull.

Maximum pPO2 is, like NDLs, a black line drawn through a grey area. You may tox on 1.3, you may be fine on 2.0. The major agencies have concluded that 1.4 planned, 1.6 contingency is a decent compromise. It's your life, there's no scuba police underwater. Just make sure your prospective buddy is competent to make an informed decision about joining you or not.

 

[stuartv]

Banked 32 isn't always exactly 32. I would never mark my tanks "permanently" (except for dedicated EAN80 and 100% O2 bottles, and even then it would just be "permanent" stickers). You should relabel them each time you get a fill, with your name or initials and the date. And you're going to analyze them anyway. How big a deal is it to put the FO2 and MOD on the label while you're doing it?

If the max depth to a hard bottom is 130, then you're probably not going to be actually swimming at 130. Unless you like crawling in mud...? If I were diving EAN32 and the bottom was 130, I would just swim up a bit off the bottom. Maybe just a little bit higher than I normally would.

If you have a fill of 32% and you are going someplace where Best Mix would be 28, you can drain your tank off to 2200 psi (assuming an HP100) and then top it up (to 3500psi) with air to end with 28%. Topping with air should be cheap and quick.

Risks from oxygen exposure don't go up linearly with partial pressure. The increase in risk going from 1.5 to 1.6 is higher than when you go from 1.4 to 1.5.

The risks also may increase with being in cold water and with doing anything (versus hovering motionless during deco).

A number of people told me that partial pressure blending is going away and pretty rare these days and that I shouldn't worry about having my tanks O2 Clean, as long they're VIP'ed for Nitrox (to 40%). In the 7 months or so since I got my own tanks, I've had fills from at least 3 different places that did partial pressure blending. O2 cleaning isn't THAT much extra when you're having your tanks VIP'ed, so I will always do it.

I usually get my tanks topped up with banked Nitrox (30 or 32, depending on where I am) and then get them topped up with air for subsequent fills (1, 2, or 3, depending). The EAN percent drops each time. When the percent gets down to 28 or so, then I get them topped with banked Nitrox again. That means they usually are never as high as 32% anyway, since I'm always just getting them topped up on top of some lower percent blend. Does my NDL end up a little bit shorter? Sure, but only by a few minutes, generally.

 

[Storker]

Tr3A is right, nitrox is not really all that advantageous in cold water. For our region, nitrox is primarily used for deco gases due to the fact that the water below 50ft is a few degrees above freezing in the high to mid 30s.

Huh? Offgassing is a lot less effective if you're cold. So, getting that extra margin to your NDL could be very nice, even if your dive is limited by gas or water temperature