PPO2 maximum safe value: 1.4, 1.6

[uncfnp]

Max ppO2 for virtually every agency is 1.4ppO2. The only one I know that differs is ANDI, who use 1.45 ppO2....//...

I have seen this posted on SB before and its been a few years since my class so I got my TDI nitrox manual out and after a quick review I can't find that statement.

Instead I see a discussion on the NOAA limits and procedual information then a section on determining personal limits ending with "it is the responsibility of each individual diver to evaluate the circumstances for himself, and then to select the most appropriate nitrox mix to use on a particular dive."

 

[hroark2112]

If you're going to start doubting the accuracy of the analyzers used by your shop, then buy your own. You can be in control of calibration, sensor age, and storage of the unit.

Many shops use a hand held analyzer permanently installed in the system for filling banks. I guess I am not understanding your distrust of analyzers. Most of us calibrate the analyzer in atmospheric air before testing. Sure, it might be off by a few tenths, but unless you're diving serious depth, you're fine.

To be honest, the idea of pushing the recommended limits for the working part of your dives for the purpose of making it easy on you to deal with fills is dangerous and lazy. You're risking your own life and quite honestly the life of your future buddies.

If I remember correctly, you're a very new diver. I'm curious as to your dive experience, certifications and training, and whether you've actually been certified in nitrox. There are a great many divers, both on this board and otherwise, who have decades of experience and the limits currently in use have been well established and the reasons for them well documented.

Permanently marking a tank for anything other than 100% is useless. All shops have a roll of tape & a marker with their analyzer. You're going to analyze & mark them with the actual content anyway. As a rebreather diver, even my diluent tanks are only marked diluent, no content marking is permanent.

 

[NAM001]

I don't doubt what you're saying, but that's new information for me. The medically-based controlled studies I've seen haven't documented a case at below 1.7. What symptoms did your acquaintance have? Was hypercapnia a possible alternative explanation for what occurred?

One of my questions was whether all agencies use 1.4. I know PADI does. Do GUE, TDI, SSI, etc., also use 1.4?

I think there is someone that is talking 1.3

I was under the impression that the lack of studies, an abundance of caution, and wildly varying and poorly understood variation from one individual to the next (and one dive to the next) also played a role. I am unaware of any confirmed cases of ox tox at 1.6 in the literature.

I think that there was a reference to one somewhere but there were other factors involved.



If I were to always use 32% I would never have to dump the tanks, right?

Not necessarily .. You will burn O2 in the tank If there is rust and is sitting for a long time. I Just took a tank filled in 12 with air and 1000 psi in it and put 530Psi of O2 in it and got a 40% mix. figure that. The shop meter read 37%.

Im not sure quite how to respond to that. There are plenty of other ways to control and confirm gas mixing ratios. If I were doing PP filling I would be using as accurate a pressure gauge as I could reasonably use, and would also use an accurate scale to weigh all cylinders before and after. Mass is something that can be measured cheaply and with great precision. With knowledge of the molar mass and a little math the FO2 can be determined with great precision.

I think you are making this too complicated This is all covered in the 1.4-1.6 buffer. Another reason why not to dive to 1.6

 

[DevonDiver]

From what I can see, is a repeated use of the phrase "Nitrox is not appropriate for depths beyond traditional recreational limits".

That's from the TDI Nitrox Instructor Guide.

It also states "for simplicity, use 1.4 or lower".... but I do see that the emphasis is upon setting a personal limit (max 1.6) based on multiple considerations, including repetitive dives, dive duration, water conditions, exertion etc etc

From the TDI website, https://www.tdisdi.com/nitrox-selecting-your-best-mix/

"PO2 for Diving Nitrox
Now that we have determined some of the environmental factors, it’s time to look at PO2’s. For recreational diving the maximum PO2 is 1.6. For a diver to choose a 1.6 PO2, all environmental conditions must be perfect: good visibility, no current, warm water temperatures, a low workload for the diver and the planed bottom time of the dive cannot exceed 45 minutes. If any of these factors are not perfect, you must lower the PO2. As an example, if your dive involves current, low visibility, etc. it would be best to set your PO2 to a maximum of something less than 1.3."

What is important to note, is that selecting your best mix is an honest, prudent process. Often, it's about decreasing the PPo2, not using the process as a means to justifying diving the highest ppO2 for convenience...

In diving, we often see 'maximum' limits taught in training; max ascent speeds etc... over time, these maximums seem to be translated as 'norms'. This is not the intended outcome.... because we are also taught to apply prudence and conservatism.... and to take other variable factors into account.

In ANDI, we set the 1.45ppO2 as maximum... but we also teach to reduce that ppO2 incrementally with each listed factor that applies. That's a formal process... and gives a recommended max ppO2 for given circumstances.

PADI and SSI set 1.4ppO2 as the maximum for recreational diving. They are very clear that 1.6ppO2 is for emergencies/contingencies only. Even in technical diving, 1.6ppO2 is used for resting decompression (i.e. if decompression isn't resting/low exertion, then 1.4 or less, should be used).

In short, it isn't wise to get complacent about O2 toxicity issues. Personally, I think there is absolutely zero justification for exceeding the industry norm-recommendations for the sake of convenience or cost.

O2 toxicity isn't a hypothetical problem (as, I am sure, it's easy for recreational divers to believe), it's a known killer and it can be unpredictable.

 

[uncfnp]

.... but I do see that the emphasis is upon setting a personal limit (max 1.6) based on multiple considerations, including repetitive dives, dive duration, water conditions, exertion etc etc...\\...

That is essential my understanding and what I got from my course. And I appreciate this approach. Instead of a "black line in a grey area" with a hard MOD, the instructor attempted to give me the information to make an informed decision and skills to modify it as personal and dive site conditions changed.

 

[KevinNM]

One of my questions was whether all agencies use 1.4. I know PADI does. Do GUE, TDI, SSI, etc., also use 1.4?

GUE teaches (at the Fundamentals level anyhow) that the limits are 1.2 for tech diving (active phase- not deco), 1.4 for rec diving and 1.6 for deco (resting). No idea what any of the other organizations teach.

 

[Tracy]

A responsible shop dispensing banked nitrox should be producing an accurate mix. +-2% is an industry standard, but it's easy to do better than that, and should be measuring by pressure or volume in such a way that the analyzer is just a process check. Right? I'm not getting breathing gas from some guy with a beat up Bauer in his garage.

You seem new, let me help you a little. In my experience, 9 times out of 10, the guy with the garage compressor will have a more accurate mix.

 

[NAM001]

Officially if you have nitrox you are suposed to be O2 cleaned if the mix is greater than 23.5 and 50 psi I think . Dumb YES but they figure that you can get 23.5 via PP blending.

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.

 

[hypnodiver]

I am skeptical of handheld analyzers and see them as a safety item rather than as a primary means of identifying the gas in a tank. Maybe the calibration gas was rich. Maybe the sensors were approaching end of life. Maybe humidity was a factor during calibration. Maybe the gas from the tank was diluted before it reached the sensor.

A responsible shop dispensing banked nitrox should be producing an accurate mix. +-2% is an industry standard, but it's easy to do better than that, and should be measuring by pressure or volume in such a way that the analyzer is just a process check. Right? I'm not getting breathing gas from some guy with a beat up Bauer in his garage.

Bear in mind that just because you've asked for 32%, doesn't mean you're going to get 32%. Sometimes dive shops make mistakes and put the wrong mix in a cylinder. Sometimes the banked "32%" isn't actually 32% at all (one of my local shops advertises and charges for 32%, but their banks usually contain something closer to 28%. Personally, I think this is unacceptable, but the dive shop concerned have a captive market who are happy to accept a +-4% error in their gas mixes.

Even if someone's partial pressure blending, they could calculate the required pressures completely accurately, but depending on the cylinders, the weather, and how much of a hurry the gas blender's in, the heating effect as the cylinders are filled can render the mix less accurate that you might have liked.

 

[tursiops]

I don't ever intend to do deco dives, trimix dives, dives with gas switches, or dives deeper than 130 feet.

LOL. I said that once, too. Now I'm Advanced trimix trained and very comfortable with multiple-bottle deco dives.

I think you are maybe confusing NDL with some other things. All NDL means is you have not taken on so much N2 that you can't come to the surface. But the big-deal Nitrox limit is not about N2, it is about O2. And your chances of ox tox GREATLY increase if you are narced and if you have a CO2 buildup. Sure, you don't hear about ox tox while people are at 20 ft on deco; yes, they are at 1.6, but they are also not narced and they are not building up CO2.

Don't push it. There is nothing to gain by living on the edge, or worse yet, dying on the edge.