PO2 Set-points (WHEN!)

samisus

I am presently in class for my Inspiration Vision. My instructor is challenged in the sense that he does not answer my questions well. If not for the $1500.00 already paid I might consider another. Ok here is my only issue:

My Vision has a Auto-setpoint for the high and low P02. What is a good depth to go to the high setpoint and on accent what is a good depth to go to the low setpoint? I had an issue where he had me change setpoints at 18 feet and the counterlungs filled with gas to the point I had a issue staying down. I love the unit and really want to be safe and efficient on it.

_R_

steveharriss

I was taught that a good rule of thumb was to change to the higher set-point at approximately 2/3 of the planned maximum depth. I was not taught to switch to a lower set-point on ascent as you can have issues with the PO2 dropping anyway if you're not careful.

In fairness to your Instructor though I'd go back to him and say you're unclear on what you should be doing. You've paid your money and are entitled to be clear on procedures that may affect your safety.

My Instructor gave me an open invitation to contact him any time in the future if I needed to check things out and most good Instructors will have a similar policy I'm sure.

NB You will find that this isn't an issue once you get to a reasonable dive depth anyway!

wedivebc

Thereare 2 stategies I use depending on the type of dive I am doing. For a dive with a gradual descent such as shore diving I change to high setpoint as soon as I reach 6m I also change back to low setpoint on ascent once I have cleared any deco I might have incurred.
For rapid descent such as boat diving on a wreck I descend as fast as my PO2 will allow me (it usually climbs as I descend) and switch to high setpoint when I reach max depth. I use the same ascent method as above.
I have had instructors who were evasive about answers and it turned out they wanted me to think for myself. Let's hope that is the case here.

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Dave....

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nigelh

That's called controlling your loop volume.
It puts it in, you let it out.

Most of us run what we call 'minimum loop' which means we are always edging it so the lungs bottom out when we inhale. This means the loop always contributes the same amount to your buoyancy. There are some added bonuses to this when things break but you have more pressing matters than them for now.

Good luck with the course. My instructor finished my last dive by making me hold a stop at 1 meter while he conducted a conversation with somebody above water and put his head under now and again to check on me. I thought it was the most stupid thing I had ever been asked to do but looking back it was spot on what I needed.

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nigelh

dave t

First I would learn to use your setpoint changing manually, that will get you used to check where you are and when it should be switched, and confirming that it does. When you are familiar with that you could use the auto function and as a rule of thumb and depending on the type of dive to be done, I would switch from low to high at or near reaching my target depth. The switch back to low can be just after leaving your 6m deco/safety stop, say 4m!
these points can be used in manual switch mode and auto switch mode
HTH

Dave

VIKTOR66

Hi,

I understand you about your instructor issues. I had a very experienced instructor but following his course recipe. You had to adapt to him not the other way around. So I checked with other expert people and my personal experience and I apply the following:

While in training changing to high set points too early and your limited buoyancy control at this level could provoke spikes in your PO2 due to very fast descents. I find change around 60 feet/18 mts comfortable enough. And avoids the continuous injection of O2 that adds difficulty to the already difficult buoyancy control for a newby.

As you are getting experience you have more control on descent and you can change earlier, but you have to consider that changing your set point has to do with your decompression requirements. As soon as you change to your high set point less deco requirement. But if you convert the PO2 to Open Circuit equivalent O2 percentage, you can figure out what is reasonable.

For example 1.3 at 60 feet/18 mt is equivalent to breath nitrox 46 and 0.7 at the same depth is equivalent to nitrox 25 so sound very reasonable this change at this depth. Earlier change don't have so much deco advantage (we are talking minutes here) and avoids the continuous injection of O2 and its effect in buoyancy.

For choosing the best high set point value you have to consider O2 toxicity versus deco requirements. You should have at least 1.0 (deco efficiency) and not more 1.3 (oxygen toxicity). If your dive is short with a mild deco you can go up to 1.3. If your dive is very long with a long deco you should go down to 1.1 probably. You can compare the OTU's and CNS toxicity with software like V-Planner to make your decision.

In my way up....I had more problems there. My first instructor force me to use the high set point during the ascent. The reason behind is that if you go to a low set point and you go up too fast your PO2 could drop down dangerously. The ascent in this case is probably the most difficult skill to master on a rebreather. I struggle with my ascents for many dives until I met my deep and cave rebreather instructor. He told me he changes to a low set point at the beginning of the ascent and keep the PO2 manually during the ascent. In case the PO2 drops bellow 0.7 then the injector will start working assuring a minimum life sustaining PO2.
I tried and it was magic... I add one or two pushes of O2 manually and I exhale by my nose a little bit at the same time. Buoyancy don't change and I can control much better my ascent rate so reducing the risk of a rapid ascent and subsequent drop in PO2. Once at the deco stop depth as you were keeping your PO2 manually you fine tune the PO2 manually and if the deco is long you can change to the high set point manually to relax a little bit.

I hope this help you, at least is what I do and works fine for me.

dave t

Did you also learn about minimum lung volume? using the low setpoint on ascent is not the best way to do it with an ECCR. There are many ccr diver who make very easy ascents using the high setpoint. but I guess it depends on what type of rebreather you dive. I dive an inspiration vision and always leave it on the high setpoint when ascending, in fact I dont change to the low setpoint until I have finished my 6m stop.

been doing it that way for about 15 years without problems

best

Dave

Batman

Totally Agree. I have been diving a Classic Inspiration for almost a year now and have no problem keeping it on the high set point when ascending, then I switch after my 6 or 4.5m stop.

Joe.

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mik

How far are you through your course? I think that if you are patient, everything will fall into place, there are many things about diving CCR that are not intuitive which you probably need to see for yourself. One of the things I noticed was just how slowly the PO2 drops at constant depth doing SCR. Maybe your instructor wants you to see for yourself that as you descend, the PO2 will increase and that switching to a high setpoint early on descent may cause a PO2 spike. This is not necessarily dangerous, just something to note. I am still a newbie and I always switch manually. For example, if I am doing a boat dive, I will descend so fast that my PO2 will usually spike to maybe 1.6 by the time I hit the target depth. I either inject a little dil at that depth if I am sick of the beeping or just breathe it down. Enjoy the rest of your course.

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Drmike

guess thats one way - doesnt make any sense at all to me though

If you are properly venting on ascent to maintain min lung volume you should have no issues ascending on high setpoint without any bouyancy issues at all. Shutting off the adv on ascent can help too

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