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

[Astran]

Suggesting that you were "correct" to ignore your computer is like someone playing Russian roulette, pulling the trigger and getting only a "click"... and then claiming there were "correct" to pull the trigger because the game is apparently safer than they were told.

Well no, not if you agree with the posters who state (and have gone through the time and effort to prove) that there is really no significant risk (to ignoring the 02 readings of the Oceanic dive computer in question), and that the algorithm and subsequent warnings are ultraconservative to the point of being impractical at best, and nonsensical at worst.

A better analogy than yours might be "I played Russian roulette with a gun that has only a blank cartridge and lived to tell the tale". In fact I don't even have as much as a tickle in my throat from excess 02 partial pressure exposure which is known to irritate the mucous linings of the pulmonary system.

The suggestion: why bother with a dive computer? Its only function is to give you advice (well, and to collect data that we'll be able to use to determine what happened to you), and you've already decided that if you don't like the advice you're going to ignore it

Asked and answered, for the 3rd time. I dive a computer because I use it for other stuff. It is reliable in regard to NDLs, gas time remaining, and ascent rate. The 02 graph and alarm is like the broken gas gauge on an airplane panel. It's there but it isn't really of any use.

 

[KenGordon]

Ken, if ignoring the computer's warning is the right thing, and there is a better way to do what the computer tries to do (i.e. calculate exposure to various risks to provide reasonable advice to the diver), or if the way it is done is too "simplistic"...

can you share with us the right and/or better way? I think the entire community would be grateful for this knowledge.

I ask only because it seems your post is agreeing with the OP that the computer was wrong, but the OP needs to know what is right.

Thanks!

The simplistic approach is to just add up the CNS exposures through the day and drop them after 24 hours. So if I do a 3 hour dive at a PPO2 of 1.3 at 9 am I am done for the day. (ok, I can do a half hour dive later but you see what I mean). Or If I do two 90 minute dives, four 45 minute dives etc. This is since the NOAA table has 100% CNS limit after 180 minutes at a ppo2 of 1.3.

But what we teach Sports Divers is that they can use a 2 hour half time for the CNS. So for that 3 hr dive resulting in 100% when they get out at 12, wait until 2pm and you are down to 50% and so can do another hour and a half etc. TDI etc use a 90 minute half time. BSAC is conservative in that regard, it also uses 80% of the full limit.

So for 4 45 minute dives:

Dive 9-9.45, get out at 25%
wait 90 minutes, now at 12.5%
Dive 11.15-12, get out at 37.5%
wait 90 minutes, now at 19%
Dive 1.30 to 2.15, get out at 44%
wait 90 minutes, now at 22%
Dive 3.45 to 4.30, get out at 47%

At this point the Oceanic is at 100%. Maybe it will let you have one more dive up to the full NOAA 210 minute daily limit. Who knows.

Or we can get in a cheeky night dive...
Wait 90 minutes, now at 24%
Dive 6 to 6.45, get out at 29%

etc etc

We don't teach this to Ocean Divers as they are limited to 20m, 36% so a ppo2 of 1.08 so they are even further away from an issue and don't get to calculate limits based on CNS etc. We do teach it at the next level up(Sports Diver) and all subsequent courses, especially ADP where rich mixes start being used for deco. I am surprised to hear it is not taught by all nitrox courses.

After 5 45 minute dives to 30m on 36% multideco gives:


DIVE PLAN #5
Surface interval = 0 day 1 hr 30 min.
Elevation = 0m
Conservatism = GF 100/100

Dec to 30m (1:30) Nitrox 36 20m/min descent.
Level 30m 43:30 (45:00) Nitrox 36 1.43 ppO2, 22m ead
Asc to 3m (47:42) Nitrox 36 -10m/min ascent.
Stop at 3m 6:16 (53:58) Nitrox 36 0.47 ppO2, 1m ead
Surface (54:58) Nitrox 36 -3m/min ascent.

OTU's this dive: 77
CNS Total: 61.4%
Gas density: 4.9g/l

2893.4 ltr Nitrox 36
2893.4 ltr TOTAL

 

[KenGordon]

Doing something similar with a Zoop in DM5 (but 2 hour SIs) it complains in the 5th dive after about 30 minutes that the OTUs exceed 300. If I set the mix to 39% I can get CNS warnings so I assume that there are half times applies. The manual is a bit vague:

The oxygen exposure calculations are based on currently accepted exposure time limit tables and principles. In addition to this the dive computer uses several methods to conservatively estimate the oxygen exposure. These include for example:
• the displayed oxygen exposure calculations are upped to the next higher percentage value
• for recreational scuba diving, the recommended upper limit of 1.4 bar PO2 is used as a default
• the CNS% limits up to 1.4 bar are based on 1991 NOAA Diving Manual limits, but the limits above 1.4bar are significantly shortened • the OTU monitoring is based on the long-term daily tolerance level and the recovery rate is reduced

These sequences of dives would be quite hard to do open circuit and essentially impossible without either stops or really rich mixes. I don't loose sleep over them. That is what dive planning software is for, and I have never had CNS be the limiting factor.

 

[JackD342]

Well no, not if you agree with the posters who state (and have gone through the time and effort to prove) that there is really no significant risk (to ignoring the 02 readings of the Oceanic dive computer in question), and that the algorithm and subsequent warnings are ultraconservative to the point of being impractical at best, and nonsensical at worst.

A better analogy than yours might be "I played Russian roulette with a gun that has only a blank cartridge and lived to tell the tale". In fact I don't even have as much as a tickle in my throat from excess 02 partial pressure exposure which is known to irritate the mucous linings of the pulmonary system.

Okay, I think the reason I am still engaging with this at this point is because I am still having a hard time with reacting to OP's emphatically negative characterization of the O2 exposure algorithm that his computers were using. Thanks to all of the back-and-forth on this thread I now have been educated on at least a top line view of half-time calculations, and feel that they are in use by multiple providers and not just a one-off theory that needs more research. (although I have no idea yet if everyone is using the same or similar calculation, or if there is also a wide variance on implementing anything beyond the 24 hour clock version.) So I agree that there was very little risk of harm involved in that decision to ignore the computers, I strongly disagree that it was the "right" thing to do, or that ignoring potential O2 exposure in the future is a wise choice of action.

Let's try this metaphor instead:

• The PADI DSAT Recreational Dive Planner is a useful tool, and provides a relatively risk free way to plan dive NDLs. I doubt anyone would describe it as "liberal." Due to the assumption of square profiles, and "round-ups" on both time and depth, it has a baked-in conservative factor.
• Now, use the eRDPml to plan multi-level dives using a slightly different implementation of the same DSAT algorithm, and bottom times are now extended. Probably still relatively conservative, but longer total dive times are now "allowed" due to a slightly better precision versus a square profile plan, and 5 foot depth increments for pressure groups instead of 10 foot increments.
• Then you move that same DSAT algorithm into one of many Pelagic Pressure Systems dive computers, and you have a recreational dive computer algorithm that is widely known as one of the most liberal algorithms, if not THE most liberal. And an algorithm that I suspect is more likely to "allow" a bent diver pushing limits and having a bad day, than most other algorithms.

Sounds like O2 Exposure is very similar. First the single 24 hour clock "cliff" calculations, apparently adapted "as is" into the OP's 3 different computers. Then the slightly more liberal repetitive dive versus single extended dive limits. The some variation of half-life calculations.
Doesn't mean the starting place was wrong, any more than the RDP is wrong. It is just a different, simpler implementation of the theoretical limits. And anyone diving an aggressive series of dive profiles needs to start paying closer attention to BOTH NDLs and O2 exposure, rather than just being complacent with the thought that either limit "rarely comes into play."

 

[TrimixToo]

<snip>

A better analogy than yours might be "I played Russian roulette with a gun that has only a blank cartridge and lived to tell the tale".
<snip>

...or not. Firearms are another pursuit for which ignoring safety rules is perilous:

Blank Pistol Kills Actor, The Son of Bruce Lee

 

[martincohn]

Ok erroneous might be a bit of an exaggeration but certainly the data supplied by the Oceanic in regard to 02 exposure can be taken with the proverbial grain of sea salt, especially given that under the same diving conditions your less conservative computer has never issued an 02 warning.

I carry enough gear as it is, I don't need to be carrying around a paperweight.

OK so this has been a blast to read, but inquiring minds want to know...

What do you plan to do to monitor your O2 tox the next time you do "3 days of concentrated EAN diving"?

 

[KenGordon]

Okay, I think the reason I am still engaging with this at this point is because I am still having a hard time with reacting to OP's emphatically negative characterization of the O2 exposure algorithm that his

blah blah

limit "rarely comes into play."

The simple version is presumably designed to make it easier for the calculation to be done manually. Given the limit is unlikely to be reached that will usually be fine. It is quite painful doing all the maths for CNS limits so people generally don't bother. A bit different with a computer. It might as well do it properly.

 

[scubadada]

Oceanic computers calculate O2 exposure just like the NOAA tables do, but more precisely calculate depth. If one did not have a computer, or had a computer that did not calculate O2 exposure (long story, but Watoom Cyano does not), you would calculate O2 exposure this way. Nothing wrong with it. It is much more conservative than using a 90 min half life of elimination, like many computers do. It seems embarrassing how little data is actually available on this topic. When I have more time, I will do a more exhaustive search.

 

[JackD342]

Oceanic computers calculate O2 exposure just like the NOAA tables do, but more precisely calculate depth. If one did not have a computer, or had a computer that did not calculate O2 exposure (long story, but Watoom Cyano does not), you would calculate O2 exposure this way. Nothing wrong with it. It is much more conservative than using a 90 min half life of elimination, like many computers do. It seems embarrassing how little data is actually available on this topic. When I have more time, I will do a more exhaustive search.

And @KenGordon says he teaches using a 2 hour half life. So even the "more liberal" method does not have an agreed upon standard. (I think that is a special law applicable to dive computers, consensus must be avoided at all costs.)

 

[chillyinCanada]

I've learned a lot in this thread as well, though perhaps still not enough. Aston wants to continue diving aggressively (with which I have no particular problem) but I'm confused as to why he isn't going to change his PPO2 to 1.4. Why dive 1.6 anticipating alarms only to ignore them?

Have I missed a point that would be important to me personally? I dive Oceanics at 1.4 and have been known to do some aggressive diving though on a liveaboard.