Nitrox max oxygen exposure question

[Jackie Treehorn]

I use a Teric these days, and a Perdix/Petrel before that. Even on a week-long liveaboard with 27 dives on 32% I've never come close to the limit, so I don''t worry about it. Using rich mixes for deco would be a different story completely.

This.

If you are single tank diving on Nitrox, your MOD (CNS Hit) is what you should be paying attention to. Here are a couple lines from PADI's Tec40 manual:

"In recreational enriched air diving, pulmonary toxicity is highly unlikely. Staying within no stop limits and using EANx blends with 40 percent oxygen and less, you have to dive your brains out to get anywhere near your exposure limits."

 

[NAM001]

I think the 2.5 hours is a otu thing. with a conservative OTU limit of 300 minutes and diving to mod means you are at 1.4 PPO2 then 300 divided by 1.4 is about 210-220 ,,,,,, that is same as 300 minutes at 100% on the surface or 1.4 PPO2 at mod. 2.5 hours is 150 mi so it sounds like they don't want you to exceed more than 70% of your daily OTU limit. Is it important to comply with that value. I dont know. a 30% safety factor is a real protection margin in any corporate lawyers mind. Quite often recommendations and science do not go hand in hand, when legalities and liability prevention comes into play. I did some reading on this and 300 otu a day is a conservative limit if not the most conservative limit. I seam to remember something about 360 OTU's a day.... If that were used then they are trying to keep your exposure under 50% of the daily limit. 50% os not a risk at all. If that is true then this is run amuk over protection. I would rather them tell me it is a recommendaiton for the over all population not to exceed XX% of daily allowable limit of xxx. with out the assumed daily limit you dont know anything other than a value not to excede. its like saying drive no faster that 90% of the posted speed limit of 80. You dont know if that is mph or km/h or feet per second. Wording like that starts ringing bells like those that say must be O2 clean at 23.5 percent and nothing about the basis for that statement. A sheeple mentality.

 

[GJC]

I think the 2.5 hours is a otu thing. with a conservative OTU limit of 300 minutes and diving to mod means you are at 1.4 PPO2 then 300 divided by 1.4 is about 210-220 ,,,,,, that is same as 300 minutes at 100% on the surface or 1.4 PPO2 at mod. 2.5 hours is 150 mi so it sounds like they don't want you to exceed more than 70% of your daily OTU limit. Is it important to comply with that value. I dont know. a 30% safety factor is a real protection margin in any corporate lawyers mind. Quite often recommendations and science do not go hand in hand, when legalities and liability prevention comes into play. I did some reading on this and 300 otu a day is a conservative limit if not the most conservative limit. I seam to remember something about 360 OTU's a day.... If that were used then they are trying to keep your exposure under 50% of the daily limit. 50% os not a risk at all. If that is true then this is run amuk over protection. I would rather them tell me it is a recommendaiton for the over all population not to exceed XX% of daily allowable limit of xxx. with out the assumed daily limit you dont know anything other than a value not to excede. its like saying drive no faster that 90% of the posted speed limit of 80. You dont know if that is mph or km/h or feet per second. Wording like that starts ringing bells like those that say must be O2 clean at 23.5 percent and nothing about the basis for that statement. A sheeple mentality.

It is known that if you are exposed to more than 850 OTUs in 24 hrs, you start to show symptoms of pulmonary O2 tox. Additionally, all the cells in your body are affected by free radicals and have biochemical disruptions yada yada yada (I'm not that interested in this level of detail). It takes some time for your body to recover from the exposure after you go back to air at sea level.

Every day you are exposed to 302 OTUs from air at sea level. That leaves 548 for diving.

But you might get bent diving and need a chamber treatment. A table 6 treatment exposes you to 268 OTUs. So if you want to be able to go into a chamber after diving, you only have about 300 OTUs for diving in 24 hrs.

But what if I'm too far away from a chamber, can I use those chamber OTUs for diving? Not if you want to breath 100% O2 on the boat if you get bent. 268 OTUs would be just over 4 hrs on 100% O2 at sea level.

That's why Oceanic computers track OTUs in the last 24 hrs and alarm at 300.

 

[NAM001]

It is known that if you are exposed to more than 850 OTUs in 24 hrs, you start to show symptoms of pulmonary O2 tox. Additionally, all the cells in your body are affected by free radicals and have biochemical disruptions yada yada yada (I'm not that interested in this level of detail). It takes some time for your body to recover from the exposure after you go back to air at sea level.

Every day you are exposed to 302 OTUs from air at sea level. That leaves 548 for diving.

But you might get bent diving and need a chamber treatment. A table 6 treatment exposes you to 268 OTUs. So if you want to be able to go into a chamber after diving, you only have 300 OTUs for diving in 24 hrs.

But what if I'm too far away from a chamber, can I use those chamber OTUs for diving? Not if you want to breath 100% O2 on the boat if you get bent. 268 OTUs would be just over 4 hrs on 100% O2 at sea level.

That's why Oceanic computers track OTUs in the last 24 hrs and alarm at 300.

thanks a very informative explanation. Until you posted this I thought that 350-400 was a daily limit adn not an unclaimed limit with reserve quantities for a chamber.

 

[rongoodman]

How many dives a day were you doing? I feel like it might get close if you are diving EAN32, 5 dives a day at reasonably deep depths (90-100')

I looked at at few dives from my last trip aboard the T&C Aggressor, 27 dives over the 5 1/2 days on 32%. The CNS % at the end of each dive was in the 12-17% range.

 

[Boston Breakwater]

Oxtox comes in two flavors: CNS and pulmonary. In principle, the two are independent. The former is acute, the latter is chronic. At depth, what you should worry about is CNS. Through the whole dive and the subsequent dives, what you should worry about is pulmonary.

Don't mix them up.

Hello. You've actually omitted one. "Ocular Toxicity." I would "Expect." someone with your certification level to know this?
OXTOX: If You Dive Nitrox You Should Know About OXTOX — DAN | Divers Alert Network — Medical Dive Article
http://tecvault.t101.ro/Diving and Hyperbaric Opthalmology.pdf
Alert Diver | Understanding Oxygen Toxicity.
Oxygen Toxicity and Special Operations Forces Diving: Hidden and Dangerous
The quotes are designed to show "Emphasis." they are not "Scare Quotes." Don't mix them up.
Cheers.

 

[scubadada]

NOAA tables use exposure in a 24 hr rolling period. It is not difficult to exceed this limit with recreational diving with 4-5 dives in a 24 hour window. My Oceanic computer uses this method. Other computers give you O2 elimination with a 90 minute half life. It is essentially impossible to exceed this limit in rec diving. My Teric uses this method. I choose to follow the method with the elimination half life.

I have dived an Oceanic computer for 18 years. I have been diving a second computer using the 90 min half life for oxygen for a little over 5 years

I very recently did a week of diving in Florida. I ended the week with 4 dives on Saturday, 4 dives on Sunday, and 2 dives on Monday.. All the dives were on 36%, except one deeper wreck dive on 32%.. Average depth was just under 60 feet, average dive time was a little over 65 min. My Oceanic VT3 computer displayed O2 warning (80-100%) or O2 alarm (>100%) all day Sunday and Monday. The 24 hour window always contained 4 or 5 dives during this period. My Teric displayed a maximum O2 exposure of 44% during this same period. The biggest difference is starting each day at at an exposure of 0%, rather than having a carryover from the previous day.

This is a good example of monitoring O2 exposure by the two different methods. Fortunately, when my Oceanic computer is displaying O2 warning or alarm, all other dive information is still available on alternate screens

 

[ls1dreams]

thanks a very informative explanation. Until you posted this I thought that 350-400 was a daily limit adn not an unclaimed limit with reserve quantities for a chamber.

Never thought about reserve in case of a chamber ride... that is interesting.

 

[NAM001]

Never thought about reserve in case of a chamber ride... that is interesting.

Yes this has always been one thing I have not been abel to reconcile using a 300 min a day limit. especially or hospital stays on older days when you were is a O2 tent with pure O2 in it and having to come off at some time to vent. I dont know if,, like myself,, you were operating on the 300 +/- limit not knowing that it really was not a limit but more an allocation of a higher limit. If that is the case I see how,,, like this,, aspects of diving are considered factual when they are not and with out explanation they become accepted gospel. I take it from this that If I could get 450 of an 850 limit min in a day it would not be a health concern like it would be with a 300 limit. In a few years the O2 clean at 23.5% will be like this. not science based at all.

 

[Storker]

You've actually omitted one. "Ocular Toxicity." I would "Expect." someone with your certification level to know this?

PADI rec nitrox cert? Can't remember they covered that. Is it part of that curriculum?

We were taught about CNS and pulmonary and got to play with the oxygen clock table. That was when I found out that for my kind of diving, pulmonary isn't an issue, so I basically watch my MOD.