Ginnie Springs diver missing - Florida

[tbone1004]

I'd been curious about the effect of off gassed nitrogen remaining in a CCR loop used for deco. If it accumulated enough to make the deco less effective, or if occasional venting was needed to keep the ppN2 down. Thanks. That you off gas enough helium to affect the physical volume of the loop is fascinating.

Taking just the nitrox case, if you are at a constant depth and your ppO2 is going down faster due to N2 off-gassing and you do not try to keep it high at say 1.4 but let it drop to say 1.2 by definition you've increased the ppN2 of the closed loop of gas you are using for deco and so decreased its effectiveness as you off-gas more and more N2 into it. How much, I do not know.

ETA: I imagine it is less effective by the difference between 1.4 and 1.2 ppO2 on nitrox at that depth. And I mean less effective on time, not on the effective use of the O2 available in the cylinder. My original curiosity came from the case of grabbing an O2 rebreather because of omitted OC air/nitrox deco and hanging at 20' on it. Over time you fill the loop with more and more N2 if you never vent gas, making it less and less of a pure O2 deco and more breathing the same N2 you flushed from tissues.

The effectiveness of deco is a function of inspired inert gas pressure against the tissues inert gas pressure. As you offgas into the loop, the pressure of inspired inert gas increases. I.e. a true EAN50 flush at 70ft is ppO2 of 1.5 and a ppN2 of 1.5. If you are coming off a dive and the ppO2 drops to say 1.2, then you are now breathing what is effectively EAN40 which is less effective than EAN50 because there isn't as much of a pressure gradient. Running min loop volume and maintaining it while you are offgasing is critical for efficient and effective deco. If you're riding the computer it is constantly tracking all of that anyway, but if you're running tables it can become a problem. Same at the 20ft stops, you're supposed to be breathing pure O2 at 1.6, but if you drop to 1.3 you are now breathing 80% instead of 100%. It compounds rather quickly. I don't know anyone running tables on CCR's, but it can affect the preplanned runtimes that are assuming you are at a stable ppO2.

Effectiveness of the O2 in your cylinder is part of the equation though because the more time you are in the water the more O2 you use, but also that the smaller the gradient between inspired ppN2/He and the pressure in the tissues the less effective/efficient the decompression itself is so the O2 isn't working as well as it normally would be if it was at 100% instead of 80%.

A 100ft dive for 150mins on EAN32 is not an uncommon cave dive. On a GF of 50/80 that calls for 45mins of deco on 100%. On 80% that's 54mins. It may not be that significant, but it adds up pretty quickly

 

[MichaelMc]

Thanks for the clarity tbone.

 

[Kay Dee]

1.0-1.2 on the bottom/working phase of the dive has been normal for like 20 years.

The norm for like 20 years? Where? Certainly not in the groups / places we dove wrecks. I started diving CCR in 98 or 99, and was taught 1.4 for the bottom by a very experinced CCR instructor trainer along with an Aust. Navy / USN Mk16 instructor, and dived 1.4 my whole CCR diving career, and did some big enough dives during that time, and 1.4 served me and others well.

What people do today - and in caves - may be a different story. So I am not saying you or anyone should change your happiness zone, but what you suggest certainly hasn't been the 'norm' for 20 years by a long shot. Maybe where you dive, but not in the parts of the world I lived and travelled for diving and instructing during that time by any means.

EDIT: Oh, I meant to add I have never been a big believer in the cns clock, as have seen it overextended dramatically on too many deep wreck expeditions to place much faith in it. Of course in an instucting of a new tech / CCR diver situation I would have taught folks to take it into account. Which of course is the old dont do as I do, do as you were taught hypocrisy I suppose.

 

[PfcAJ]

The norm for like 20 years? Where? Certainly not in the groups / places we dove wrecks. I started diving CCR in 98 or 99, and was taught 1.4 for the bottom by a very experinced CCR instructor trainer along with an Aust. Navy / USN Mk16 instructor, and dived 1.4 my whole CCR diving career, and did some big enough dives during that time, and 1.4 served me and others well.

What people do today - and in caves - may be a different story. So I am not saying you or anyone should change your happiness zone, but what you suggest certainly hasn't been the 'norm' for 20 years by a long shot. Maybe where you dive, but not in the parts of the world I lived and travelled for diving and instructing during that time by any means.

EDIT: Oh, I meant to add I have never been a big believer in the cns clock, as have seen it overextended dramatically on too many deep wreck expeditions to place much faith in it. Of course in an instucting of a new tech / CCR diver situation I would have taught folks to take it into account. Which of course is the old dont do as I do, do as you were taught hypocrisy I suppose.

You do you bro. But it’s been the norm here and everywhere I’ve traveled to.

The CNS clock is clearly a poor measure. But look at the dudes doing absurd dives. What works? 1.2 (or less) on the bottom, 1.6 max on deco, gas breaks to a low ppo2 mix during the oxygen stop.

Oxygen toxicity seizures, while rare, aren’t survivable. To me, marginally more deco associated with a 1.2 is cheap insurance.

 

[KevinNM]

What works? 1.2 (or less) on the bottom, 1.6 max on deco, gas breaks to a low ppo2 mix during the oxygen stop.

Oxygen toxicity seizures, while rare, aren’t survivable. To me, marginally more deco associated with a 1.2 is cheap insurance.

I know of people who have toxed and drowned at 1.3, I’ve never heard of one at 1.2 or less. Does that happen?

 

[nquinn]

@rddvet @TBone @_Slacker_ Thank you.

I also agree the PO2's should be 1.2 max vs 1.4. We have a history of a couple of females toxing at 1.4. on O/C cave dives.

As a recreational (no-planned deco) nitrox diver, would you ever consider reduce your MOD using 1.2 instead of 1.4?

 

[PfcAJ]

I know of people who have toxed and drowned at 1.3, I’ve never heard of one at 1.2 or less. Does that happen?

Idk. I’m sure it could given a big enough sample size. It seems supremely unlikely though.

 

[UCFKnightDiver]

As a recreational (no-planned deco) nitrox diver, would you ever consider reduce your MOD using 1.2 instead of 1.4?

Are you going to be at 1.4 for your entire dive open circuit?

 

[helodriver87]

As a recreational (no-planned deco) nitrox diver, would you ever consider reduce your MOD using 1.2 instead of 1.4?

Recreational, no deco diving is totally fine at 1.4. You might be able to find some cases of people toxing on rec profiles at 1.4, but you can probably also find cases of people spontaneously combusting with no clear cause. In either case, it's vanishingly rare. An average PO2 exposure of 1.2-1.3, especially with minimal workload, seems to be sustainable for extremely long periods, despite what the CNS clock suggests. 1.4 seems to be totally safe for recreational timeframes. It only starts to be a meaningful concern for the vast majority of people when you exceed 1.4.

 

[mderrick]

The discussion of PO2 exposures seems to be drifting off topic, but for those open-circuit readers I will share what I teach new rebreather divers...

Experienced open-circuit divers, who typically plan exposures of 1.4 for the bottom and 1.6 for deco may not realize their knowledge cannot be applied to CCR. Common target PO2s referenced in open-circuit do not take in to account that for most rebreather designs a set-point represents an average. That means (for example) while a PO2 of 1.4 on open-circuit is generally considered reasonable, on CCR that's considered excessive in part due to the fact that sometimes the loop would be at 1.3 and sometimes it will be at 1.5 (perhaps even higher, depending on some issues related to accuracy of the loop PO2 measurement.) Some years ago, to demonstrate this I installed an independent sensor in a rebreather and used it to indicate the instantaneous inspired PO2 without any buffering. I discovered that for the diver, who was a physically small woman diving a setpoint of 1.3, the inspired PO2 for single breaths would occasionally exceed 1.6 while the handset and even data download from the electronics maintaining setpoint indicated the unit was perfectly maintaining the configured 1.3 setpoint.

For a variety of reasons, nearly every rebreather diver has experienced circumstances where their loop PO2 exceeds the planned setpoint and this is common enough that their training includes appropriate responses to the situation. There is some evidence that even brief exposures to PO2 spikes may have greater effects on CNS oxygen toxicity than what the "oxygen clock" calculations would suggest. Further, large periods of time during a typical open-circuit dive will be spent at values significantly lower than the planned maximum but CCR divers will maintain the PO2 setpoint over virtually the entire dive and as mentioned previously sometimes the PO2 can be higher than setpoint. There are additional considerations related to the setpoint is almost always maintained using oxygen sensors at pressures well above those used in calibration, and whose non-linear sensor accuracy can vary due to temperature, age and particularly the quality of their most recent calibration. Thus, open-circuit planning values for PO2 have substantially more "room for error" than the PO2 setpoints used in rebreathers.

Looking back, diving best practice has for decades been gradually reducing what is considered acceptable risk oxygen exposures and that continues today. While it's true that 1.3 is generally accepted as having acceptable risk as a CCR setpoint, and DAN accident database has very few cases of CNS oxygen toxicity events for 1.3, there does seem to still be some risk associated with 1.3 exposures (although these mostly seem to be "edge" cases.) For exposures of 1.2 there does not seem to be any risk of toxicity for CCR dives within the 3.5 hour single exposure limit. There are also some operational benefits to using a setpoint the same as calibration value; as a practical matter virtually all rebreathers are calibrated using 1.0 PO2 (i.e. 100% O2 at 1 ATA.) Thus I teach that for a typical recreational dive, IMO the appropriate PO2 setpoint selection for new CCR divers is 1.0 and with little bit of experience (i.e. 25 to 50 hours) they might select 1.2 max as appropriate where the TTS or NDL becomes a concern.