JJ CCR newbie questions

[doctormike]

Thanks, guys... that's all very helpful! I think that I'm OK with the stock JJ approach at this point.

I got started thinking about it when I read that Dead in 7 minutes article in Advanced Diver magazine. They portray constant flow as a lifesaving backup in case (1) the solenoid fails, and (2) the diver doesn't check the PO2 for 7 minutes and (3) ignores the HUD hypoxia alerts. I guess those things are pretty unlikely to all happen, right?

 

[Effervescent]

Thanks! Yes, I should have posted my typical diving profile. I'm aware of the depth limitation, it just was never an issue with me because I have no interest in very deep dives. I currently dive mid range wrecks (up to 120-160 fsw), and I do a lot of photography (was interested in the sea life interaction issue of the CCR).

Hi Mike - Do you ever find yourself in a head-down position when diving or taking pictures? I noticed that this disclaimer is in the JJ manual:

"The JJ-CCR does not meet the requirements of EN 14143 section 5.6.1.4 (Hydrostatic imbalance test) with a pitch of -90 degrees (Vertical head down position). A vertical head down position during a dive is an extremely unusual position for a diver and normally never used when not absolutely necessary. Within a vertical head down position an unintentional activation or freeflow of the ADV can possibly occur. To avoid it when the diver is forced to a vertical head down position he can close the in line shut-off valve on the ADV and use the manual diluent adding valve to maintain an optimized loop."

I ask because I find myself in that position sometimes (like this past weekend when I was heads down in the Oregon with my rb) but I guess this wouldn't matter much if you are rarely in that position.

 

[doctormike]

Good question! Pretty much all of my photos are with the lens pointed up, I rarely am even slightly head down, let alone completely head down. I have always found that to be very uncomfortable even on OC, and I avoid it.

I remember someone else telling me that about the JJ, but I didn't get the ADV to free flow during class... Wayne's unit had the manual dil valve with the shutoff, I didn't have that.

 

[sea_ledford]

Shearwater got rid of the .19 setpoint option. Easier to do that and make divers replace batteries more frequently than deal with guys who can't be bothered to actually follow procedure. I know my Meg checklist has a setpoint check in it, so does my Pelagian. If I am willing to ignore those and jump in the water with a computer that isn't on, isn't ready to dive, valves off, I deserve whatever comes my way, and there's no piece of gear that is capable of saving me. "Functional Safety" won't turn on an O2 valve, no matter how angry the computer is. You cannot engineer out human stupidity, no matter how hard you try.

Did Shearwater get rid of the .19 setpoint? That would be a shame. My SF2 is less than a year old and it has it.

The point isn't to just save battery life, but to save O2 as well. If you don't have a .19 setpoint and your unit is on the surface, it'll keep adding O2 constantly because it won't reach .7 without intervention on the divers part. So if you either forget to turn it off, it gets splashed or is just wet enough for the wet contacts to fire up, it'll piss all your O2 out the OPV, open BOV, whatever. Unless you left it in a greater than .7 pO2 state and sealed up.

I would think the likely reaction to that after once or twice would be to shut down the O2 valve, even if that isn't part of a regular routine. If you don't do a pre-jump check and forget to turn your computer on, it'll most likely power up on it's own. If you jump in with out your O2 valve open, that is certainly not going to fix itself.

Of course a followed checklist will solve both those problems.

-Chris

 

[JohnnyC]

The WOB due to hydrostatic imbalance will be different in all positions. It's just the nature of the beast. realistically, there will be minimal difference in WOB whether you're head up or head down as the distance from counterlung centroid to your own lung centroid will be the same and under the same amount of pressure. However, swimming on your stomach vs swimming on your back will probably be more perceptible.

@sea_ledford, unless you're diving super deep with less than 100% O2, I'm not sure how your loop would be incapable of reaching a .7 setpoint, why wouldn't you be able to hit 1.0 at sea level? Realistically you may never be able to truly reach an FO2 of 100% due to other inert gas in the loop, but it shouldn't be enough that it would be pissing O2 out all over the place in an effort to reach 70% in the loop. Do you leave it at maximum loop volume at .19? I guess I never leave anything on my rebreather open to the environment once I pre-dive it, so it never crossed my mind that it would be anything other than low setpoint at a minimum. Once pos and neg are done, a setpoint check is done and it stays in that state until the dive starts. Making sure it maintains a stable setpoint should be part of your pre-dive anyway. What scenario would you do your predive and then leave the loop open? Hitting 1.6 for a 6m cell check is gonna be a bigger waste coming from .19 than .7. I guess I just don't understand why you'd have your loop at anything other than low setpoint or higher once you've predived the unit. I'm not saying it's incorrect, I just haven't seen a scenario in my rebreather diving where it would ever arise.

 

[KenGordon]

Good question! Pretty much all of my photos are with the lens pointed up, I rarely am even slightly head down, let alone completely head down. I have always found that to be very uncomfortable even on OC, and I avoid it.

I remember someone else telling me that about the JJ, but I didn't get the ADV to free flow during class... Wayne's unit had the manual dil valve with the shutoff, I didn't have that.

If the ADV firing when you roll annoys you then you can close the shutoff. I make do, it annoys me but I like the hands off ADV so leave it open mostly. Some claim it is a feature as a dil flush is a roll left and pull the dump.

I believe a few people do run them as hybrids.

A lot of the database you refer to is complete fiction. I found a local death there which was actually an OW diver under training. There was probably no rebreather within 5 miles. It is an attempt to scare people into thinking the regular rebreathers are unsafe. The fact that they never got their own (proper ccr rather than a mccr or o2 ccr) to market is a little ironic.

I like my JJ. So far it has all just worked. I had one minor failure of the ADV shutoff (which the factory replaced without question).

You do need to pay attention of course.

My petrel does .19 set point, although there is newer firmware I have not got round to installing it. If not broken etc...

 

[Seya]

My JJ still has .19 and you can only have it set to that when the unit is out of the water. As soon as it hits water it changes to .7

 

[rjack321]

I'm not sure the fascination with hCCRs honestly. Seems like the revo people are the biggest fans and that it functions (for them) as a means to save solenoid batteries. Basically every other uniit you have to work to make it into a hCCR, custom modifications or whatever.

You should be aware that solenoids can fail open or closed. The usual failure mode for an eCCR is current limited cells opening the soelnoid over and over trying to bring the setpoint up. Having the actual solenoid stick open or close seems to be rarer

On a mCCR orifices aren't completely hazard free either. They can't get any bigger but the HP seat on the reg can fail driving the IP higher than its supposed to be which creates more O2 flow into the loop. And a tiny bit of water/corrosion can grow green crap in the orifice filter so it clogs and fails that way (for example)

The general idea behind the mCCR is that you know the flow right is a little below your metabolic rate and then have to look and add O2 to bring the setpoint up periodically. Basically you're "forced" to monitor and add the whole dive (which might only be 2 or 3 times on the bottom, but it'll be often on ascent)

Flying a eCCR manually has alot of adherents. BUT if you fail to monitor the ppO2 for whatever reason you have alot less time to fix the declining ppO2 - especially if you are working hard. Yes the solenoid is there as a parachute in theory. But you're still relying on it to catch you. Not much different than if you just let it control the ppO2 in the first place.

Personally, despite being trained to run my unit manually, I just let the solenoid do what it was designed to do. It does a way better job maintaining set point than I can. And I know its actually on and cause I can hear it every minute or two. I have all my ppO2, depth, time and deco info on my right wrist's shearwater so checking my precise ppO2 is natural as I check gauges. I use my HUD as an alarm. The two things I catch most on the HUD are not bumping up the setpoint once I arrive at the bottom. And if I drop too fast having the ppO2 spike and needing to do more of a flush than just what the ADV has added.

Every CCR has negatives, the biggest nits IMO on the JJ are the lack of flood tolerance and difficulty breathing with a full wing on the surface. Its also really hard to remove the lid if you get sand or debris down the top part of the canister tube. I would not try to add hybrid capabilities to it.

Enjoy your unit, get lots of hours, have fun. Cheers

 

[doctormike]

Personally, despite being trained to run my unit manually, I just let the solenoid do what it was designed to do. It does a way better job maintaining set point than I can. And I know its actually on and cause I can hear it every minute or two. I have all my ppO2, depth, time and deco info on my right wrist's shearwater so checking my precise ppO2 is natural as I check gauges. I use my HUD as an alarm. The two things I catch most on the HUD are not bumping up the setpoint once I arrive at the bottom. And if I drop too fast having the ppO2 spike and needing to do more of a flush than just what the ADV has added.

Thanks so much! This is pretty much what my instructor had recommended as well...

 

[KenGordon]

Some people swear by running them manually, leaving the so at 0.7. This forces you to pay attention to the ppo2. For me, I think the biggest risk is failing to pay attention and missing a failure which results in low ppo2, but I prefer to try to pay attention than to run manually the whole time. I tend to intervene more for accents as I know what is happening next so have an advantage over the computer.