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

[Astran]

If I've set my PPO2 max at 1.4, then presumably I'll get 150mins that day. But from the Shearwater table I only get 45 mins at PPO2=1.6. So if I've set my PPO2max at 1.6, and dive to that limit, I only get 45 mins allowable exposure.
So it isn't that setting your PPO2 max changes the allowable clock exposure, but it DOES change what that number is that is the one you need for your diving.

This makes a ton of sense and is written in a way even I can understand what you mean.

My slight disagreement is to the posts that say computer settings do not matter. It does if the diver then dives to that limit. If the diver sets his computer lower, say 1.4 and then dives under that limit it will be harder for him to hit the 24 hour limit.

The reference to his liberal algorithm, if such it was (Oceanic), it will allow said recreational diver a longer dive time (longer ndl) letting the diver hit up against the 24 hour limit. Again, especially if said diver is diving to 1.6.

This one also helps to clarify the point, thank you for that.

That much being said- on no dive did we go any deeper than the MOD for PPO2 1.4 even though our dive computers were set for 1.6.

 

[BlueTrin]

I'll try and make my point again.
Look at the Shearwater link; it is as good as any. You get 150 mins (in a 24h period) at PPO2=1.4. If I've set my PPO2 max at 1.4, then presumably I'll get 150mins that day. But from the Shearwater table I only get 45 mins at PPO2=1.6. So if I've set my PPO2max at 1.6, and dive to that limit, I only get 45 mins allowable exposure.
So it isn't that setting your PPO2 max changes the allowable clock exposure, but it DOES change what that number is that is the one you need for your diving.

The derivative of time vs PPO2 is not linear at the edges on this table.

 

[tursiops]

This makes a ton of sense and is written in a way even I can understand what you mean.



This one also helps to clarify the point, thank you for that.

That much being said- on no dive did we go any deeper than the MOD for PPO2 1.4 even though our dive computers were set for 1.6.

Yes, you only nudged 1.4+ on that first dive Sunday. My point was more methodical.
In your dive sequence, I haven't worked out the details, but it is entirely possible that your last few dives Saturday plus your first two Sunday put you way over the allowable CNS clock....it is a 24h averaging window, not a calendar day, that counts. If your last 3 dives Saturday were before noon, and your first two Sunday were before noon, then your accumulated dive time was 226 mins, and the CNS clock limit is 150 mins. How close you were depends on your profiles, but apparently the computers all thought you exceeded the limits.
Best practice is to keep to an 80% exposure level, not exceed 100%!

 

[Astran]

it is entirely possible that your last few dives Saturday plus your first two Sunday put you way over the allowable CNS clock....it is a 24h averaging window, not a calendar day, that counts. If your last 3 dives Saturday were before noon, and your first two Sunday were before noon

The 3 Saturday morning dives were 8:15 am, 10:07 am, and 11:54 am.

The 3 Saturday afternoon dives were 5:00 pm, 6:30 pm, and 7:40 pm.

Sunday morning dives were 7:40am and 9:30 am.

 

[Manatee Diver]

Thanks for pointing that out- first post is updated to show dive times for the last 4 dives.

On Sunday you also accumulated about 100% O2 exposure. Since you computer is doing a rolling windows it is certainly possible to exceed the CNS clock.

 

[tursiops]

The derivative of time vs PPO2 is not linear at the edges on this table.

So? Why should it be? It is an empirical table.
https://www.scubaboard.com/community/threads/otus-where-do-the-limits-come-from.77221

 

[Manatee Diver]

So? Why should it be? It is an empirical table.
https://www.scubaboard.com/community/threads/otus-where-do-the-limits-come-from.77221

He probably figured it would be like the deco tables where they adjust an equation to fit the empirical results.

 

[scubadada]

Hi @Astran

The answer to your question is actually quite simple. Oceanic employs an O2 exposure algorithm that works the same way as the NOAA table. It calculates your O2 exposure for a 24 hour period and does not give you credit for O2 elimination until 24 hours after that exposure. So, if you do four dives in a day, it will count all 4 dives as exposure, If you do a dive the next morning, it will be a 5th dive in the 24 hour window. You get no credit for elimination until 24 hour after the end of the dive for the morning before. I have gone over the 24 hour O2 exposure on my Oceanic VT3 on many occasions. This is particularly true when I am doing long drift dives in SE Florida at 4 per day, diving 36% nitrox. My VT3 computer will continue to track nitrogen exposure, it will be on an alternate screen or the nitrogen exposure bar graph. I have a newer Geo 2 that I was using for a backup that I believe tracks O2 exposure like many other computers (see below)

Shearwater, like on my Teric, and Dive Rite, like on my Nitek Q track O2 exposure differently, by giving you a 90 minute O2 elimination half life. For every 90 min you are on the surface, you eliminate one half of your O2 exposure. This carries over less O2 from dive to dive for repetitive dives and essentially eliminates O2 exposure over night. I have never come close to the O2 exposure limit with this algorithm doing 4 or 5 rec or light deco dives per day. For this reason, I will disregard the O2 exposure information on my VT3 and continue diving, paying attention to my nitrogen exposure as always.

Good diving, Craig.

 

[Astran]

For this reason, I will disregard the O2 exposure information on my VT3 and continue diving, paying attention to my nitrogen exposure as always.

Ok, so you are saying that my decision to disregard the 02 warning mirrors your experience and strategy, you do it because you have determined that the Oceanic method of calculating 02 exposure is flawed?

 

[tursiops]

Ok, so you are saying that my decision to disregard the 02 warning mirrors your experience and strategy, you do it because you have determined that the Oceanic method of calculating 02 exposure is flawed?

Flawed? No. Different? Yes. Oceanic is more conservative? Yes. Remember the Shearwater is fundamentally a Tech computer that can be used recreationally. In the tech world, more risk is assumed by the diver.