Diving 32% Nitrox with "Air" Algorithms

[Joneill]

The OTU tracking he is referring to isn't the same as the oxygen toxicity you are referring to.

Oxygen toxicity can happen in different ways. You have a high level exposure that can cause rapid toxicity, this is what most think of in terms of ox tox scenarios. You switch onto an oxygen bottle at 70' instead of a 50% bottle and go into a seizure. Or more likely in the recreational realm, you are breathing 40% and diving at 120' for a few minutes.

It can also accumulate over time, you have a daily exposure limit of what they call OTUs or oxygen toxicity units. This is limited to 300 per day. While a huge dose at once can cause major issues, so can a healthy dose prolonged over a period of time. Your computer tracks these OTUs for you if you give it the correct gas information.
For recreational dives on recreational nitrox, I suppose it could be possible, but you would have to try very hard to ever exceed this. It comes heavily into play in technical diving.

I don’t recall the term OTU being used in my PADI Nitrox class nor do my Shearwater manuals appear to mention OTU? PADI did go over CNS oxygen toxicity as a % and this can be displayed on my SW DCs. How does CNS % relate to OTU?

 

[formernuke]

I don’t recall the term OTU being used in my PADI Nitrox

OTU usually doesn't show up in recreational nitrox diving. I don't remember if my nitrox class covered it or not. It definitely shows up in advanced nitrox and CCR training. But that is tech so longer and deeper dives.

 

[GiraffeMarineSalvage]

I know people who set their clocks 10 minutes ahead of the actual time to avoid being late. This is just regressive laziness and a sad attempt to fool yourself out of bad habits. Doing the equivalent on your dive computer is just as backwards and ridiculous. Sorry!

If you want to have a safety buffer then increase the conservatism of you computers algorithm. If you have GF then use something like 30/70. You will have a bigger buffer than if it you tracked to air, and your stats will accurate and actionable.

I fully agree with you on setting dive computers to the accurate gas mix being far preferable, but setting a clock ahead is totally reasonable for the OCD among us (but in my defense I only set the time 7 minutes early, 10 minutes does seem rather excessive...).

 

[inquis]

How does CNS % relate to OTU?

Different things. One is tracking toxicity to the Central Nervous System (CNS), the other to the pulmonary system (lungs).

The latter accumulates over the course of consecutive days, with an ever decreasing daily limit, eventually reaching a limit of 300 OTUs per day. (Some will use a daily limit of 300 OTUs from Day 1.) OTUs (like the CNS%) accumulate at a rate dependent on the PO2. Unlike the CNS%, OTUs don't have a 2 hr half-life and therefore do not reduce during the surface interval or effectively "reset" after a 12 hr surface interval.

At a PO2 of 1.3 atm (e.g., EAN32 at 100 fsw), OTUs accumulate at 1.48 OTUs per minute, so about 200 minutes per day at that PO2 and assuming the 300 OTUs per day limit. (The rate is lower for lower PO2s).

In contrast, CNS% accumulates at 0.56% per minute (at PO2 of 1.3 atm). A single dive exposure (from 0%) therefore is limited to 180 mins (= 100 / 0.56), but it's common to use a limit of 80% (144 min single dive). Due to the half-life of 2 hrs, after a 2 hr SI, you would go from 80% to 40%. This leaves room for another 40% for dive #2 (temporarily putting you back at 80%) which is 72 mins. However, that would also put you over the pulmonary limit.

In summary, CNS% drops during surface intervals, so multiple dives can usually stay away from the CNS% limit. However, the pulmonary limits may be reached over multiple days of diving (especially if the conservative daily limit of 300 OTUs is adopted from Day 1). Most modern computers track CNS%. Few computers (if any) track OTUs; Shearwaters, for example, do not.

 

[Redtwo]

I agree with everybody about putting in the right numbers and, if you want, change your conservatism. I did my nitrox certificate last summer with SSI and in their e-learning setting your computer to air is repeatedly suggested to add more safety. I dont agree, but was surprised they advocated it that much.

 

[Searcaigh]

I think both of these outfits bank their Nitrox and have found that the O2 percentages are usually within .5 to 1 % of 32. And when it's not 32, it's usually a bit under rather than over. We always analyse to confirm. My question is, with these types of small percentage variances, do most folks just leave their computers set for 32........or do you re-adjust for the exact O2 percentage for each tank after analysing.

In these situations I generally set my computer to the lower number e.g. set at 31% for both dives and use the 31% tank for the first dive and 32% tank for the second dive.

 

[Angelo Farina]

The idea that short-term hyperoxia may be good for you seems to be gaining traction: https://www.researchgate.net/public...roxia_on_Aging_Biomarkers_A_Systematic_Review

This is exactly the concept I was referring to: under medical control, high oxygen pressure is a great and powerful tool, exactly as a drug.
But I do not like the idea of exposing my body to a powerful medical treatment without medical control nor prescription: this is exactly the reason for which I do not assume any drug without previous suggestion by a medical doctor.
Think about radiotherapy: a powerful medical treatment killing cancerous cells. But do you expose yourself to ionizing radiations for no reason? Certainly not!
See also the recent posts by @tursiops and others regarding oxygen toxicity for lungs (OTU).
I understand that with mild recreational profiles using Nitrox for just a pair of short dives per day is usually not a concern.
But my previous health problems made me very anxious about lung inflammatory processes.
So why getting exposure to high oxygen pressure, when it gives no advantage in terms of prevention of DCS, as the dive profile is still far away from NDL even if breathing air?
In such cases, I prefer air to Nitrox, as there is a small, but not negligible, cumulative effect on lung inflammatory processes, and no gain at all in preventing DCS.
And in any case my computer will be set to the exact mixture I am breathing.

 

[Divin'Papaw]

I don’t recall the term OTU being used in my PADI Nitrox class nor do my Shearwater manuals appear to mention OTU? PADI did go over CNS oxygen toxicity as a % and this can be displayed on my SW DCs. How does CNS % relate to OTU?

View attachment 769463

It is covered in Advanced Nitrox courses. It only comes into play in technical diving when breathing mixes well above the recreational 40% limit.

 

[Belzelbub]

LOL.
For what reason to compare individual dive profile?

You claimed that your NDLs were the same as another diver using a Shearwater. The only way to know for sure is to compare the profiles. Small differences in depth over the time will lead to differing NDLs.

No DC can promise 100% safety no matter how much you paid for it.

I never said that they could.

ZLH16 was developed yrs ago.

Agreed. Doesn't necessarily mean that your 1995 DC has the hardware to run the algorithm to its potential. I don't know if that's the case, but it's possible.

I do not need bell and whistle for scuba diving and I won't pay for it!!

Clearly. You are definitely unique in your love for your 1995 DC. Others, however do see the value in some bells and whistles. Not just for dive computers, but bells and whistles are in just about everything today.

My dad has two cars currently. Both are licensed, insured, and in good working order to drive on the roads. One of them is a 2016 Chevrolet Camaro. The other is a 1930 Ford Model A. Both are certainly capable of getting from point A to point B. Which one do you think he uses more often?

I'm curious. If your Uwatec were to need to be replaced, what would you replace it with?

 

[kammel78]

I agree with everybody about putting in the right numbers and, if you want, change your conservatism. I did my nitrox certificate last summer with SSI and in their e-learning setting your computer to air is repeatedly suggested to add more safety. I dont agree, but was surprised they advocated it that much.

This is exactly why I stopped teaching SSI Nitrox (okay there were more but this was the final straw).

How lying to your computer as a best practice made it past the legal review, I have no idea.