A guide about setpoint selection for deep dives

dan@scuba-training.net

Nitrogen (and/or Helium) will hurt you, but Oxygen will kill you.

mikedive

Hey Cedric,

Very good article and make clear for new CCR diver with limited knowledge about the danger of use higher SP on the bottom fase of longer dives.
I'm and Matt always keep the SP around 1.0 to the bottom time and boost to 1.2 or 1.3 on the deco. The increase on the deco obligation time, in my point of view, doesn't justify boost the SP to 1.3/1.4.
If I need, for any reason, use higher SP I'd do that just using a FFM.
Cheers,

__________________
Michael Netto - www.rebreatherclubedobrasil.com.br
ISC Megalodon CCR instructor and dealer
IANTD CCR Trimix and CCR Cave instructor
IANTD Dive Rite O2ptima instructor

Mixaddict

Cedric,

Interesting article. It has caused me to give your conclusions some additional consideration.

I am curious about one thing: In the past, when have done OC deco, what flavor of gas did you use at 6 meters? (Just wondering about your pO2 choices on OC as compared to CCR)

Regards,
MixAddict

AD_ward9

http://www.nwdesigns.com/rebreathers/Nuggets/BodyCount.htm and look for Ghassem Gheissary.


To get an idea of how accurate the reports are, search for "convulsion" on this forum. It is implicated in quite a few deaths.

Alex

Genesis

I have to agree with AD and a few others here - including Cedric.

I think we've got a major problem here with "agency blindness", and this is a prime example of the kind of tomfoolery that I often rant and rave about with regards to being forced into an agency mold - no matter why and for how long.

We've got quite a few "unexplained" deaths recently. A couple of them, however, have eyewitnesses to what sure as hell look like O2 hits immediately precedent to the victim going deceased!

Then we have a handful of others where nobody lived that saw the original event - but these were very experienced people, who it can be presumed would not ignore a solenoid not firing and a HUD (and handset) alarming for the several minutes it would take for a loop to go hypoxic. Further, we have nothing to suggest that these folks screwed the pooch in regards to their scrubbers, and to the extent it can be determined we've yet to identify a "fatal flaw" in the hardware itself.

That leaves either a medical incident (which is not a diving accident) or an O2 hit. While SOME of these may be medical incidents, my money is on a sizeable number of O2 hits being involved here.

Extrapolating OC practices to CC is IMHO quite dangerous. As I pointed out in the other thread on this matter the patterns with regards to actual exposure are very different. When I plan a dive for a 1.4 PO2 at the MOD of OC gas, I'm only there for 10, maybe 20 minutes out of the dive. Then on deco I spike at my gas switches to 1.6, but quickly (as I move off that stop) the PO2 falls dramatically. On a 50% bottle I'm only at 1.6 at the 70' stop, which is a shortie - 2-3 minutes most of the time - then I'm up to the next stop, and the PO2 starts coming down.

When I get to my 20' stop I do a cleanup break first (on backgas) which by that time has a VERY low (well under 1.0) PO2, then go on 100%, which again spikes. But I usually do either 8/4 or 12/4 - which you don't normally do on a rebreather.

When you do a dive on a rebreather set to 1.3 you're there for basically the entire dive. This is a VERY different pattern than you get on OC. There's no "cleanup" time. It would appears from the evidence that the CNS clock theory may be dangerously wrong in two ways:

1. The accumulation time is overstated. That is, maximum continuous allowed exposure is OVERSTATED. This is probably ok on OC because you plan for the entire bottom time at the MOD - even though you RARELY dive there. But if this is true, that is a fatal error on a CCR!

2. The decay time is overstated. If the CNS decay time is much shorter than postulated, that explains why "air breaks" actually work and people don't tox right and left on big OC dives. The breaks are effective in cutting the CNS risk.

We also know that succeptability is highly variable, even from day to day or hour to hour.

These errors, if they exist, may not be a big deal for OC dives. But for a CCR diver they could be very, very big deals indeed.

IMHO the agencies (and manufacturers) are pushing unsubstantiated theory here and there's plenty of evidence that they're wrong! Now a bunch of us could be all wet behind the ears here, but if the reason to dive a CCR is shorter total dive time, but the trade-off is a MUCH higher risk of a tox hit while performing the dive, do you still want to dive the CCR? Or would you prefer to dive with a more reasonable profile, more runtime, and dive tomorrow too?

I'm still building here, but from what I've seen there ain't no way I'm diving at 1.3, and I don't think I'll even allow the K1 controller to go there, other than on deco when shallow. If you want to dive at that setpoint at the bottom, you have to pop it up manually and keep it there with your finger using the manual add.

__________________
"A venturesome minority will always be eager to get off on their own, and no obstacles should be placed in their path; let them take risks for Godsake, let them get lost, sunburnt, stranded, drowned, eaten by bears, buried alive under avalanches - that is the right and privilege of any free American."
http://www.denninger.net
http://www.diversunion.org/liability.htm - Fix the Diving Cert racket

dave t

I couldnt agree with you more.....until you said this

I, other than on deco when shallow. If you want to dive at that setpoint at the bottom, you have to pop it up manually and keep it there with your finger using the manual add.[/quote]

As you quite rightly stated we are at a constant po2 so what difference does depth make? if you dont like 1.3 at depth why do you allow it at shallow depths?

Dave

Genesis

Because the history suggests that for decompression, when one is resting, it is safer to run at a higher PO2.

The evidence seems to suggest that maintaining a constant higher PO2 during the working part of the dive is NOT safe.

If the PO2 is limited during the working portion of the dive to, say, 1.0, it may be entirely reasonable to allow a higher setpoint (e.g. 1.3) for decompression.

I can think of two ways off the top of my head to "know" if you are decoing or "diving":

1. Depth. That one's obvious. If you allow a 1.3 setpoint at, say, < 70', that seems reasonable. It does leave a window of risk open though for a shallow dive run at a relatively high setpoint, where you'd have an immense NDL and thus an immense time you could stay down. That might not be such a good idea.... If you limit it to 40 or 30', however, then that precludes most of that sort of foolery (but again, not entirely.)

2. Add rate. Simply don't allow a setpoint over 1.0 to "stick" if the add rate suggests that the diver is working. If your metabolic rate is too high, as evidenced by your O2 consumption, then the controller goes to "low" setpoint mode. It will only allow you to select "high" when your metabolic work rate is low. This can be "reasonably" measured through your oxygen demand. Of course this would allow you to pop it up if you just went down and SAT, which might be bad.....

The "setpoint over 1.0 is only valid at depth under 30'" option seems to be fairly bomb-proof. Yes, someone could do a dive to 30' and try to tox themselves at 1.3, but there's really no point to doing that, since at 30' you have a near-indefinite NDL anyway.

__________________
"A venturesome minority will always be eager to get off on their own, and no obstacles should be placed in their path; let them take risks for Godsake, let them get lost, sunburnt, stranded, drowned, eaten by bears, buried alive under avalanches - that is the right and privilege of any free American."
http://www.denninger.net
http://www.diversunion.org/liability.htm - Fix the Diving Cert racket

dave t

Ok, point taken but as we were considering one of the reasons to run a lower setpoint was to offset somewhat potential errors of po2 which could lead to toxicity then switching the setpoint higher at the shallow stops could potentially push you over the toxicity edge so to speak.
Foe me personally I prefer the "middle road" 1.15-1.2 and thats where it stays for the whole dive unless its very deep and/or very long then its 1-1.1 and I run the whole dive like that. I am in the water longer, yes but less stressed (from an o2 point) From experience I know my body does not like high po2's if I deco on anything above 1.3-1.35 I get out of the water feeling shitty. On OC I can run higher po2's because of the drop back between gas switches, but not on CC

Dave

Genesis

That may indeed be the kicker.... there's no "drop back".

This may suggest that you'd feel better (and have less risk) if you ran a 1.0 and then decoed at a 1.3 than the "common" way of running a 1.2 or 1.3 setpoint through the dive.

What appears to be fairly clear though is that there are quite a few hits being taken with what should be - according to the mantras of those who claim to "know" - safe exposures.

Some people appear to advocate going towards a FFM to mitigate the risk. But an FFM is not a guarantee of survival - if you take a "for real" hit and your buddy is unable to correct the problem and surface you all evidence suggests that you'll be "non-functional" for a relatively extended period of time - long enough that the FFM probably won't matter.

That would seem to suggest that the only "fix" is to avoid the hit in the first place.

The sort of gross errors that are likely to burn you due to cal problems are unlikely to be addressed by this, other than, perhaps, the 1.0 linearity issue.

If you force a cal on 0.209 and then force a validate on 1.0, and restrict the setpoint to 1.0, provided the calibration is only valid for a short period of time (e.g. 1 day) you significantly reduce the risk of not catching a cell that is going non-linear. If you then also force a manual validation of hyperbaric PO2 before you permit one to be selected for deco, you also place some controls on that risk. The risk of a pressurized head can be controlled if the unit has an ambient pressure sensor in the loop (the K1 does)

With all this taken together you can reduce some of the risk factors. Eliminate, no, but reduce, yes.

__________________
"A venturesome minority will always be eager to get off on their own, and no obstacles should be placed in their path; let them take risks for Godsake, let them get lost, sunburnt, stranded, drowned, eaten by bears, buried alive under avalanches - that is the right and privilege of any free American."
http://www.denninger.net
http://www.diversunion.org/liability.htm - Fix the Diving Cert racket

dave t

yes indeed, I have had the misfortune to rescue a convulsing rebreather diver from 44m and its a VERY scary affair, a FFM would have been a hinderence in this instance not a help, but thats another story

Dave