PO2 Set-points (WHEN!)

silent running

Yes Mike, proper venting happens easily with a nice, loose OPV, aided by pushing down on the CLs with one's hands and the occasional nose vent. My AP buddies all complain about the super tight stock OPVs and how they only activate by pulling on the dump cord, a design feature which may help facilitate pressure tests on the surface, but which certainly isn't convenient in the water, either during a slow ascent or even potentially injurious during a fast, emergency ascent. And why is the OPV nearer to the bottom of the CL? A great feature, if you dive upside down and like pulling on a string.

None of my 5 AP buddies used the OPV and vented through their noses or around their mouth seals, which I try to minimize as I don't like messing with my mask seal too much or letting any water into the loop through the DSV.

Never had a problem keeping min loop volume and high SP on ascent.

What exactly is the point of having an ECCR, if you're not going to let it help with solenoid injects on ascent, when O2 addition happens most frequently and when you need the most help keeping up a falling PO2?


In answer to the original question: IMHO, SP change to low at 5M, above your last stop and change to high at 5M, as during a descent, the dil gas coming from the ADV filling the loop should slow the injects as you descend and minimize overshoot. The only times I've had any notable overshoot was when I have paused for a while during a descent. If one were to use a rich dil targeted to the max depth, overshoot will be even less, even if you pause during descent, which is another argument for richer dils...

dave t

Whats wrong with venting through the mouth on ascent? I just relax my lips and it vents. infact if you are sensitive to the loop it will tell you when to vent as you can feel an increased pressure in your mouth. The thing is with nose venting is you have to breathe it first and you can only let go a lung tidal volume at a time (more or less) and it will upset your mask, well it does mine.

I think the AP dump valve is positiond so its easy to get to as an emergency dump. I think mouth/nose dumping is meant to be primary.

Where you switch setpoint on desent depends on the style of your inject pattern, as far as inspo's are concerned its best to switch at or near target depth.

best

Dave

Drmike

I never let the CL fill so much on ascent that the opv has to open - no need, and no need to crush lungs manually. On ascent I simply shut the adv off and nose vent to keep min loop and that also helps keep setpoint too at same time (high) People not maintaining min loop and letting OPVs control lung vol an ascent is probably one reason you seldom see ECCR divers with horizontal trim during ascents I suspect if people find they cant keep up with expanding CL volume on ascent by nose/mouth venting they are probably ascending too fast.

If mouth venting you only break lip seal when you sense a pressure differential. No water should enter the dsv then provided you maintain depth or are ascending. Unlike nose venting, the trouble with mouth venting is you cant maintain min loop vol this may effect trim adversley during ascents


why dont they lower the opv opening pressure when diving by unscrewing it? (thats how its designed to be dived and how I dived my inspiration) Locking it down is done for over pressure testing only.


Dont you mean the other way around?. Rich dils aid overshoot

What do people think will happen if they spike their ppo2 to say 2bar on descent for a few seconds before further descent causes the adv to fire in dil that will lower it? - sweet FA I should think

personallly I always spike my ppo2 momentarily well over setpoint on descent as I want to test for current limitation before the dive not after

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Mark Chase

Personally on a shallow >40m dive I switch streight after the bubble check at 6m. On a bigger dive like say 70m+ I my leave it till 30m.

On ascent i liked to drop to low set point at 6m and run it manually. That way if i suddenly go up I don't have issues with gas injecting making things worse.

Running the unit manually gives you something to do on deco and i found on my ECCR it was the time when i focused on my displays the most and spotted anomalies.

ATB

Mark

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bambang

I switch to high PPo whenever the loop is contain Nitrox 28 with 0,7 PPO. I think you can calculate yourself what depth the loop will hit 28% O2

That is how I do it..

Mixaddict

I am assuming that your "AP buddies" are actually opening their OPV after they have done their pressure test? There have been cases of lung over expansion injuries due to people either forgetting to do so or not being aware that this step is necessary. The AP OPV works just fine, it just shouldn't be necessary for it to be triggered. A diver should EASILY be able to recognize when there is a pressure differential and vent gas out their lips or nose or both without the OPV ever releasing. The OPV is there for those "once in a blue moon" situations where due to task loading or some emergency the diver is not paying attention to things and the OPV releases gas to prevent injury.

Now if the diver is consciously venting gas out the OPV by pulling on the string, that's a different story. It works great, and the diver may choose that method instead of venting out his lips, or he may use both, but it works great. As long as they have the OPV open and not shut down, I really can't understand what their complaint about the AP OPV would be. I have never experienced a problem with it, nor have I ever heard anyone complain about it.

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silent running

Hi Dave, when I have vented mostly through my mouth I find that I get some water in the DSV and I hate hearing any gurgling as it makes me worry that there's a leak somewhere.

Yes, I agree and I try to keep it to a minimum for the same reason. But my mask doesn't ride up much if I only use my nose occasionally as a fine tuner.

OK, but I wonder whether someone who is in a panic will be able to vent through the m/n fast enough and will have the presence of mind to pull the cord. It is my understanding that the aveoli are made a very thin tissue and that it doesn't take much to rupture them and that having a loose OPV may be more likely to prevent an over-expansion injury.

Yes, I had forgot that the AP inject pattern is a bit different and I'm sure you're right, thanks for the correction, -Andy

silent running

Yes, I'm sure you're right that if somebody were to let their CLs fill up to the point that the OPV opens on it's own, their trim will be lousy and they're going up too fast. In my case, the OPV is high up on the lung, where the most of the volume is and only takes me lightly putting my hands on the lung or OPV to get the excess gas out.

OK, maybe I get water in the DSV through mouth venting for some other reason, maybe it was because I was using it for the fine tune at the stop and not so much during the ascent itself.

I don't know, maybe they just didn't want to bother with it. But I remember at least one of them saying that even when it's opened all the way, it's still too stiff.

Maybe, I've only used a 1.8 MOD dil a few times and I remember the solenoid firing less frequently during the descent. My ascents tend to be a bit fast though.

Yes, I have never had a problem doing this and yes the dil does in fact, dilute the O2 when the ADV fires, who knew?

silent running

Hi Randy, yes as above, I'm sure they open the valve up after the pressure test, but at least one has told me they thought it was still too tight and didn't like bothering with the pull dump. I didn't mean to give the impression that they were a bunch of dummies who all forgot to open the OPV after the test.

IMHO, while the closing off of the OPV is helpful for the pressure test, it also means it's possible to forget to open it again. And having a strong spring which requires a pull dump style actuation means that it will probably always be tighter than a dry-suit style OPV, which IMHO may be too much. And I don't doubt that any CCR diver worth their salt will know when the lungs are too full, but will they remember to reach down and pull the cord in an emergency ascent? -Andy

tekdiver170

Samisus, to help answer your question about set points. I like to switch to my high set point (HSP) approx 15-30' above my max depth. If you switch to HSP at the bottom then you will have to vent excess gas out of the loop to keep min loop volume, (the O2 being injected will help to offset the compression of gas in your loop and the pressure increase will help raise your PO2 to the HSP), you should find it aids with learning buoyancy control.

There is no reason why you can't use the auto set point switch, when you're learning this is a great tool so you don't forget to make the switch during the decent, especially starting out when you're mind is thinking about 100 things at the same time. Just make sure you don't get complacent and forget the most important rule of CCR diving, make sure you know your PO2. So on decent make sure your watching that handset especially when you hear that solenoid fire.

As for switching to your Low Set Point (LSP), when training you can progressively shallow up your switch. Buoyancy control of course is one of the biggest changes from OC to CCR and learning to maintain min loop volume makes the biggest difference on your buoyancy and it just takes practice. Do what your instructor briefs you on. But it's often a basic teaching technique to make the LSP switch the first couple of days at around 26 feet, then shallow it up to 20' the next day or 2 then on the final day maintain HSP all the way up to 16 feet.

There have been lots of suggestions for venting excess gas out of the loop when first starting out venting through the nose if often the best way to stay in control of how much gas to "burp" out. As you learn and dive more on the unit you may start learning to vent out from around the mouthpiece. Just remember too loose of lips will allow water into the loop, it's all a matter of practice and finding what your comfortable with.

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