Accumulated 02 following a large number of repetitive Nitrox dives over 3 days.

[Astran]

I thought I understood Nitrox but following 3 days of concentrated EAN diving my computer indicated a dive time remaining of zero following a relatively short bottom time (about 19 minutes at about 60' depth) due to 02. Plenty of gas and no deco time left but the 02 went into the red, the up arrow was flashing and DTR went to zero and stayed there for the remainder of the dive.

I checked my backup computer figuring there was a malfunction but the numbers were the same. A few minutes later my dive buddy swam over and showed me her computer (same Oceanic algorithm although she dives a Veo and I have a VT Pro), which said the same thing.

I gave her the "it's ok, disregard sign" and we both continued our dives for another 25 minutes or so, followed by a second and final dive which was not limited by 02, and neither one of us had convulsions and spit our our regs.

I checked my P02 screen and at the time of the warnings I was at 1.03 and during the remainder of the rest of the dive the number dropped to about .95. Our computers P02 level is set to 1.6

We did the following dives:

Friday: 2 dives to 109' for 32 minutes each on EAN 32
Friday night 1 dive to 80 feet on EAN 34 for 57'
Saturday 5 dives to depths of about 90' for about 40 minutes each, on EAN 34, and 1 dive to 70' for 57' on EAN 34. (6 dives total on Saturday).
Sunday 4 dives to depths of 96' 33 minutes, then 67' for 56 min, 82' for48 min, then 69' for 50 min on EAN 36.

It was the 2nd morning dive on Sunday that gave the 02 warning.

The computers read close to the 02 limits for the last 2 dives on Sunday but never reached them.

Can someone please clarify exactly what happened during these dives and exactly what risk we were exposed to by simply disregarding the computer's indications.

Thanks

 

[tbone1004]

@Astran
Start here and let us know if you need any further clarification. Basically your body can only handle so much O2 at higher partial pressures. Lots of repetitive dives on nitrox with high ppO2's will run up your CNS clock fairly quickly
Shearwater and the CNS Oxygen Clock - Shearwater Research

 

[Subcooled]

Oxygen causes two kinds of problems.

1) If the partial pressure of oxygen is too high, it can lead to colvulsions and consciouslessness.
These typically lead to drowning.
1.4 ATM of oxygen is considered to be reasonably safe for ordinary diving (and 1.6 ATM at rest during decompression).

2) Oxygen irritates lungs. It is oxidicing, which means it burns things. In the long run it also damages and scars your lungs. This is called "pulmonary oxygen toxicity". Too much oxygen over a prolonged period of time damages lungs. Permanently. I do know a person who had to quit diving as he had been breathing high concentrations of oxygen for prolonged amounts of time (albeit not enought to cause convulsions and death).

I would assume that your computer warned of this irritation/damage to the lungs.

---------------
Oxygen enriched air (often called as nitrox) is beneficial as it contains less nitrogen. This mix enters your body during the dive, and tries to escape during ascent, and forms bubbles. It does contain less nitrogen than usual, which is good. This means that the risk of decompression sickness is less.
Oxygen can be consumed, nitrogen canot

 

[JackD342]

Well, you seem to be one of the rare divers that actually hits this limit. There is a limit to the amount of elevated partial pressure O2 you should expose yourself to during a 24 hour period, and you hit and exceeded that limit.

To give you a rough idea, if you manage to dive profiles that are consistently right at the 1.4 O2 pp ata, then you are limited to 150 minutes during a 24 hour period. If you push up to the contingency planning limit of 1.6, then you are limited to only 45 minutes of bottom time in 24 hours.
There are tables for calculating this, but your computers do a much more efficient job of it.

 

[kmarks]

What certification agency did you use for your EANx training? I'm only familiar with SSI because that's what I do, but I'm pretty sure that the CNS clock time is pretty similarly taught across all agencies. It's in Section 4 of the SSI EANx digital manual.

SSI even has a CNS Clock Exposure Time Table that is a little different from what JackD342 said above. At ppO2 of 1.4, you are limited to 180 minutes of total time in 24 hours and 150 minutes on a single dive. At 1.6 like you were, you are limited to 150 minutes in 24 hours and 45 minutes in a single dive.

What happened is probably that after your long day on Saturday, with an especially heavy morning load, you were saturated on your clock time during your Sunday morning dives, but by the time you did the third and fourth dives, a sufficient amount of time had passed to regain some clock time.

Whatever happened, you were already pushing limits by using 1.6 as your acceptable ppO2 level. I wouldn't have pushed the clock time limit and ignored my computer's warnings. But, that's my decision.

 

[Astran]

What certification agency did you use for your EANx training? .

Don't really need these sorts of comments thank you.

 

[Astran]

At ppO2 of 1.4, you are limited to 180 minutes of total time in 24 hours and 150 minutes on a single dive. At 1.6 like you were, you are limited to 150 minutes in 24 hours and 45 minutes in a single dive.

I'm not understanding the 2 posts above that seem to indicate simply setting the computer to 1.6 vs 1.4 limits the bottom time even more.

Well, you seem to be one of the rare divers that actually hits this limit.

Almost exactly what I was thinking when I was looking at my computer telling me I was out of time due to 02 limits. In almost 500 dives I've never seen anything even close to that and I've done at least one liveaboard with 5 dives per day for a week on Nitrox.

 

[grantctobin]

I'm not understanding the 2 posts above that seem to indicate simply setting the computer to 1.6 vs 1.4 limits the bottom time even more.

In light of this question, I don’t think that @kmarks question is irrelevant at all. Setting the computer max PO2 to 1.4 or 1.6 does not impact the running of the CNS clock. Perhaps a refresher is in order?

 

[tbone1004]

Don't really need these sorts of comments thank you.

it is a rather valid question as it shows that there was a failure to emphasize CNS loading during your class. It is a failure of most agencies and instructors. You can see in the link that I gave that there is a 150min limit in a 24hr period at a ppO2 of 1.4. Sounds like a lot, especially for recreational diving, but if you're doing liveaboard diving, then that may well happen quickly if you are using "best mix".

 

[JackD342]

I'm not understanding the 2 posts above that seem to indicate simply setting the computer to 1.6 vs 1.4 limits the bottom time even more.

It is not the computer setting that determines the 24 hour O2 exposure limit, it is your dive profiles.
Now, if you set to 1.6 and dive to the single dive profile limit, then you will hit the 24 hour limit sooner than if you had it set for 1.4 and did a series of shorter (time and/or depth) dives.