The Low Setpoint myth (or misunderstanding)

[JVos]

I talked with another rebreather diver this weekend at a Christmas party and he said he is planning on diving a wreck next year that is around 180-190 ft deep. Knowing he is only air diluent trained, I asked him how he was going to do that with air, he said he would bump up his PO2 to 1.4-1.5 to reduce his nitrogen uptake since this is what his instructor taught him! Is this really what is being taught?

 

[wedivebc]

I talked with another rebreather diver this weekend at a Christmas party and he said he is planning on diving a wreck next year that is around 180-190 ft deep. Knowing he is only air diluent trained, I asked him how he was going to do that with air, he said he would bump up his PO2 to 1.4-1.5 to reduce his nitrogen uptake since this is what his instructor taught him! Is this really what is being taught?

I used to do air dives that deep. I didn't bump my setpoint figuring any N2 penalty I had for the short duration of the deep portion would be compensated from the higher PO2 going up. It's really dumb to do those dives nowadays where HE is readily available.

 

[JVos]

The problem I have with the idea is running that high of a setpoint at depth...

 

[doctormike]

The problem I have with the idea is running that high of a setpoint at depth...

Yes, and if your air dil is that hot at depth (over 1.4 at 6.7 ATA), you aren't going to have much ability to flush down a high PO2, right? Even if you aren't trained in hypoxic dil, using something like 18/35 would be a better choice, I think... both for PO2 issues and narcosis.

 

[Kay Dee]

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Now going back to the 20m Low Set Point 0.5. (Not switching to High Set Point) eventually your loop volume would decrease (due your body that is metabolising the O2 and CO2 being scrubbed) and the ADV would add Diluent (21%) so your controller would read somewhere in the region of 0.63 (0.21*3). Since its not dropping the solenoid wont fire O2 and you would burn Dil instead of O2. Although not an efficient way of diving this won't kill you right?

But it will slowly be dropping, even with the ADV.

After all, you are only 'burning' the O2 in your loop, not the Dil, and while that O2 may only be in / from the Dil, at some point you will metabolise that O2 (in the loop from your Dil), the PPO2 will drop below .5 and presto that heartwarming sound of your solenoid firing O2 into the loop will bring it back to .5, and the procces will start all over again, keeping your PPO2 at or about .5. So, at the risk of being sarcastic, sorry to dissapoint you but...............no, rest assured you wont die (from that occurrence anyway).

But as others have said, a fast ascent from that depth at that setpoint may result in a completely different outcome if your not manually injecting O2 to keep up with the rapidly falling PPO2.

Now onto another topic embedded in this thread and hotly debated it seems now the world over; and that is some / many folks seem deathly (pardon the pun) afraid of / or are opposed to using a high set point on / during the whole dive. Well at the risk of being accussed of giving 'advice' (which I am not, just stating my personal practice) I dived my whole CCR carrer with the highest set point I could run (1.4) and never had a problem I couldn't handle, as I was taught to monitor my gauges like a hawk and take appropriate action if / when I needed to, which I will admit I needed too, but on only a very few occaisons. It seems some folks think that accidentally crossing the red line barrier of 1.6 for even a few minutes is going to kill them. Hell, my Oz Navy instructor (no I was never in the navy) taught / tried to teach me to just breathe it back down (below 1.6) but in truth I liked to get it down a bit faster than that so I rode the manual add Dil button religiously till it got back to about 1.5, or at least below 1.6.

And yes, I just know the wolves are going to be baying and the duelers sharpening their sabres re "but what about your CNS Clock overunning (on deeper longer dives)", but I wont even get into that as, God forbid, I might be accussed of 'advising' somone on what may be consider by some to be improper practices.

But you know, I just gotta laugh at some of the folks I know / knew, some on this forum, that overun their CNS Clock on more than just 'some' dives (and some regularly) but would never admit it publicly. Guess some have a reputation to uphold and real jobs to worry about, so fair enough (for them).

 

[DA Aquamaster]

I talked with another rebreather diver this weekend at a Christmas party and he said he is planning on diving a wreck next year that is around 180-190 ft deep. Knowing he is only air diluent trained, I asked him how he was going to do that with air, he said he would bump up his PO2 to 1.4-1.5 to reduce his nitrogen uptake since this is what his instructor taught him! Is this really what is being taught?

I've noticed when checking cells in a pressure pot that the percentage of error increases the farther I get away from the 1.0 calibration point. The problem with running a set point higher than 1.2 or 1.3 is that with cells that have been in use a while, and have some moisture on the cell membrane, at a reading of 1.3 you're really running around 1.35, at 1.4 you're closer to 1.5 and at 1.5 you're probably running 1.6 or a bit more actual. If your run times are long you'll potentially run into CNS issues.

I agree that a higher PPO2, even in the 2.0 range isn't going to kill you dead right now, but how long it takes to actually tox depends in large part on workload, varies by diver, and for each diver will vary from day to day.

At 180-190 ft, I'd much rather be using 18/35 to maintain a reasonable END, and to be able to flush down to a1.2, rather than being stuck at 1.4.

---

Where a low "set point" is a problem is at shallow depths with an mCCR, where a sudden increase in workload and a little distraction can quickly result in the PPO2 falling to dangerously low levels. In that regard, I don't normally run mine at anything less than .7 and don't have an issue with 1.0, unless it's a hot drop where a dil flush creates more delay than I'd like.

 

[Kay Dee]

"I agree that a higher PPO2, even in the 2.0 range isn't going to kill you dead right now, but how long it takes to actually tox depends in large part on workload, varies by diver, and for each diver will vary from day to day. "

***Agreed

"At 180-190 ft, I'd much rather be using 18/35 to maintain a reasonable END, and to be able to flush down to a1.2, rather than being stuck at 1.4."

****Disagree. How can you be 'stuck' at 1.4 when you have a manual overide Dil button at hand to flush down to whatever the PPO2 of the dil you are running is. Sure it will work back up to 1.4 if your running it, but it aint 'stuck' going below there.

PS. Now see / realise you are referrng to another post with a diver on air, not a gas dive (or mine next above), so in that air situation totally agree. It was my own practice that I was erroniously referring to with my 'disagree' answer, cause with air at the depth in question you are 'stuck' at or about 1.4 as you say. Perdoneme senor!

I'll use the excuse that I havent had my morning coffee for that one for missing the 'fine print'!

PARDON NOT INSERTING YOUR QUOTES BUT THIS DAMN TABLET HAS A MIND OF IT'S OWN AT TIMES AND REFUSES TO CUT AND PASTE TODAY!

 

[billgraham]

The issue is that sensors will under report the PPO2 as the faces become coated with condensation. If you run the rig at 1.4 because you are too cheap to make up the appropriate dil, (what this is really about), you could actually be doing the working part of your dive at 1.5 or more.

 

[Kay Dee]

The issue is that sensors will under report the PPO2 as the faces become coated with condensation. If you run the rig at 1.4 because you are too cheap to make up the appropriate dil, (what this is really about), you could actually be doing the working part of your dive at 1.5 or more.

My cyber friend, I have been fortunate enough to never have been too cheap to buy or do anything in my diving career, so the dil mix had nothing whatsoever to do with it, it being running 1.4. Nor was replacing sensors regularly an issue. And I would still do so (run 1.4) given the chance.

I ran 1.4 religiously on dives up to 110m for 13 years, sometimes on extended expeditions for weeks at a time, two dives, or sometimes one very long one, depending on depth, a day during those expeditions. Am I recommending you or others do that, no, but then again from my own experience, I wouldn't recommend against it either. I am just telling it like it was for me.

But then again, I dived all those years using a MK15.5, not just any old CCR.

 

[JVos]

I know a guy that ran a high setpoint all the time. It worked for him up until the day it didn't. The end.