gas switch: backgas? move bottles?

[HIGHwing]

Riddle me this. If you have a rebreather such as the Meg where the gas path goes from Diluent manual add in the exhale counter lung into the scrubber through to the cells then up to inhale counter lung, then to divers DSV how would this effect your plan?

My thought process give the situation above (I don't dive a meg) would be that it wouldn't work as you are manually adding diluent across the broken scrubber. Depending why you are bailing out open loop breathing may not work.

By the way I was not taught open loop breathing I don't think in my CCR Air Diluent Deco course nor was it discussed as a viable option for bailout in my CCR Cave course.

Garth -

If you think about open loop breathing very critically the gas path doesn't really matter. Even if the gas source is from the exhale CL the exhaled breath is going to exit the loop and not be returned, recycled, or reused. As long as you don't return the expired gas (CO2 source) into the exhalation CL you'll accomplish breathing gas directly from the dil cylinder (or wherever you're plumbed) which is the same intent of a BOV. The critical point here is YOU are creating the CO2 - not the CCR.

As an aside, depending on your rig, you actually don't have to use the MAV in favor of the ADV to get access to the onboard Dil. The O2ptima essentially has a 2nd stage right next to the DSV on the loop whereas the Prism2 & Meg have a bicycle tire valve encased in plastics which isn't as "elegant" in terms of breathing performance - but it will work when required. There has been some argument about the smaller hose/valve bore(s) not really being able to deliver the volume of gas required to pass CE at depth via some BOVs. Rarely do CCR divers consider the performance of valves and hoses until they bail to the equivalent of a lemon seed stuck in a drinking straw...

Regarding your inquiry about CCR Cave and Air-Dil Deco courses, SCR and open-loop breathing were featured in both of my courses utilizing different instructors & agencies.

Safe dives.

 

[kwinter]

Garth probably wouldn't know an Optima if it fell on him. Despite sharing the same electronics, Garth dives a Hammerhead rebreather that I'm sure he will be happy to tell you he won for free in a raffle.

As for open loop breathing by ADV instead of MAV, that really depends on the ADV and how you have it adjusted. If it breathes freely, then I agree that is easier than using the MAV on every breath. If it's like my rEvo, the ADV is a POS lung buster that would drastically increase WOB. BTW, I agree with you that the Optima ADV is among the best.

 

[Garth]

Garth probably wouldn't know an Optima if it fell on him. Despite sharing the same electronics, Garth dives a Hammerhead rebreather that I'm sure he will be happy to tell you he won for free in a raffle.

As for open loop breathing by ADV instead of MAV, that really depends on the ADV and how you have it adjusted. If it breathes freely, then I agree that is easier than using the MAV on every breath. If it's like my rEvo, the ADV is a POS lung buster that would drastically increase WOB. BTW, I agree with you that the Optima ADV is among the best.

I paid for the raffle tickets... hehe.

The AP adv is preferred by some divers as well. As for my buddy, he doesn't like it.. It might be an adjustment issue.

Garth


Sent from my iPhone using Tapatalk

 

[HIGHwing]

Garth probably wouldn't know an Optima if it fell on him. Despite sharing the same electronics, Garth dives a Hammerhead rebreather...

There's nothing Hammerhead about my O2ptima. I LEAPED at the opportunity to get an all Shearwater head and hud. How these very expensive machine seem to multiply, I have no idea, but it's nice having the DiveCan setup on my Prism 2 and the O2ptima. Nothing to remember in terms of differences that really matters in the displays, etc.

 

[kwinter]

Losing the vibrating DIVA is one disadvantage. The Rev D HUD has lots of advantages. And I'm finding a few things I miss about the HH as I learn my Shearwater.

As for rebreathers, yes they do multiply. Put two in a dark room with a lot of money and they breed like rabbits.

Please pardon any typos. Sent from my iPhone using Tapatalk

 

[Dr. Lecter]

You were not taught open loop breathing because it is pretty much restricted to SCRs. CCR divers learn SCR as a backup. SCR needs open loop as a backup or when shallow.

Wrong. Case in point being TDI's rEvo course, where hypercapnia drills involve 1 minute of open loop breathing followed by bailing.

No friggin way you will be able to breathe open loop in the case of a full on CO2 hit. Respiratory rate can skyrocket and far exceed the ability to either SCR or open loop or remove a DSV to go OC.

Let me get this straight...I'm going to outbreathe the total gas volume of my unit plus the dil reg's LP output through the MAV, but won't outbreathe the shoddy 2nd stage built into a BOV and plumbed through a bunch of suboptimal restrictions? Whatever you say.

I've tried to hyperventilate fast enough to outbreathe the combination of the rEvo's volume and gas addition via the MAV, and I can't do it. I can make myself dizzy as Hell trying, but between the size of my rEvo mini's lungs and the addition rate of the MAV, I'm not only not overbreathing the unit (i.e., bottoming out the CLs) I'm having the OPV fire in short order if I don't lay off the MAV.

Riddle me this. If you have a rebreather such as the Meg where the gas path goes from Diluent manual add in the exhale counter lung into the scrubber through to the cells then up to inhale counter lung, then to divers DSV how would this effect your plan?

My thought process give the situation above (I don't dive a meg) would be that it wouldn't work as you are manually adding diluent across the broken scrubber. Depending why you are bailing out open loop breathing may not work.

By the way I was not taught open loop breathing I don't think in my CCR Air Diluent Deco course nor was it discussed as a viable option for bailout in my CCR Cave course.

What agency endorses such a practice?

TDI and rEvo both apparently endorse it, as noted above.

I'll agree that a scrubber compromised by flooding could preclude OLB as an effective hypercapnia response under limited circumstances, but I think it very unlikely you'd find yourself in the position of simultaneously having an unexpected and severe CO2 hit and the kind of loop flood that makes dual axial scrubbers too soggy to breathe through effectively.

You're much more likely to either (1) have an insidious CO2 problem that results in severe hypercapnia from an uncompromised scrubber that passes gas normally (breakthrough, poor packing, expended sorb), or (2) have the kind of scrubber flood that would prevent OLB from working and notice it way before CO2 becomes a problem.

 

[kwinter]

Ok, Doc, you are correct that I might have done some open loop breathing in my rEvo crossover. I do remember having to contort myself into going head up position in order to make the ADV breathable. But open loop is just not something that should really be considered a good option. It isn't the inhale part, but the exhale part. The last thing you want to do in case of a CO2 hit is to increase any WOB including exhale. By having to exhale through the nose or around loose lips, you are increasing the exhale WOB. Open loop might be a handy skill to have, but there are much better ways to deal with the problem.

As for overbreathing the loop, I would have to see you do this for myself. The OPV also is restrictive, so while you might be able to inhale enough at 2-3 breaths per second, you are going to have a hard time exhaling. And buoyancy could also be an issue with max lung volume. Again, there are better ways. You are not going to be able to remove the DSV from your mouth and go to a bailout second. Your onboard dil supply could be depleted in seconds by trying to stay on the loop in open loop breathing, and the entire loop could be compromised as the cause of the CO2 hit. There will always be those who oppose the use of a BOV. I say live & let live. I'm keeping my Shrimps and my Divematics.

 

[Dr. Lecter]

It isn't the inhale part, but the exhale part. The last thing you want to do in case of a CO2 hit is to increase any WOB including exhale. By having to exhale through the nose or around loose lips, you are increasing the exhale WOB.

Again, you're not making any sense unless your head is constructed rather differently from mine. Exhaling directly out my nose and mouth is going to be harder than panting through the exhaust port of a bastardized 2nd stage?

Not sure what you mean about the OPV being restrictive in this context. I merely brought up the OPV to indicate that not only could I not inhale enough gas fast enough to overbreathe the gas supply provided by a rEvo being filled by its MAV, I had to be rather judicious in using the MAV or I'd fire the OPV no matter how fast I was removing gas from the loop via hyperventilation.

Also not sure who let you go through a crossover course flopping heads up to fire a rEvo's ADV...or why that would help do so. But rest assured it's possible to OLB like a firehose without coming out of horizontal trim, and there's no ADV involved. For me, the buoyancy thing isn't an issue: the mini's lungs are pretty well counteracted by the 3L steels and SS case plus my 2lbs of trim weight.

You are 100% correct that OLB does not scale well to depth, however. I have a relatively large onboard dil source and my wing/suit are driven off a separate bottle, but I'd still have a very limited amount of OLB time at 300'. I very well may go to a shrimp when I trade up my rEvo II for an Expedition that's plumbed for the kind of offboard gas supplies necessary for dives well past 100m, but for now 300' is about my floor and I prefer the simplicity of a DSV.

 

[kwinter]

I had a few minutes to run some schedules using 80% vs. 100% for deco. I haven't run OC schedules in a while, but I think that was the area we were discussing.

80 meters, 40 minute BT, VPM +4, Bottom mix of 14/45, Deco gasses of 21/25 and 50%, along with a final deco gas of either 80% from 9m or 100% from 6m.

Total run time with 100% is 198 minutes. Total run time with 80% is 203 minutes. Using 80% adds a whopping 5 minutes, or 2.5% to the deco time.
CNS total using 100% is 123.4% while CNS using 80% is 96.6%. Using 80% instead of pure O2 reduces CNS total by over 20%. That's a significant risk reduction in my book. Even if there is nothing magical about CNS% and you could tox below 100% or not tox at 200%, it is still reducing the risk of oxtox. And if you tox and seize while still underwater, even if it is only at 3m/10 feet, you are very likely to die. Much better to have slightly less efficient deco and suffer DCS an hour later when you're back on dry land.

There is more art than science to all this voodoo. Every diver should do what he or she feels is in their best interest.

---------- Post added May 1st, 2014 at 06:05 PM ----------

Again, you're not making any sense unless your head is constructed rather differently from mine. Exhaling directly out my nose and mouth is going to be harder than panting through the exhaust port of a bastardized 2nd stage?

Not sure what you mean by bastardized. My Divematics BOV uses a standard Poseidon Xstream second that breathes better than any other second stage I have. The Shrimp uses an Apeks second that is only "bastardized" to the extent that it is on a switch lever, but functions exactly like a standard Apeks. Any BOV will have dead space, and therefore equate to some WOB increase due to retention, but breathing in and out of the BOV will absolutely, positively be easier than exhaling against the seal of my mask.

Not sure what you mean about the OPV being restrictive in this context. I merely brought up the OPV to indicate that not only could I not inhale enough gas fast enough to overbreathe the gas supply provided by a rEvo being filled by its MAV, I had to be rather judicious in using the MAV or I'd fire the OPV no matter how fast I was removing gas from the loop via hyperventilation.

What I meant was the same as exhaling against the OPV would be as hard or even harder than exhaling against the seal of my mask. The gas in the loop has to go somewhere other than back into the loop.

Also not sure who let you go through a crossover course flopping heads up to fire a rEvo's ADV...or why that would help do so. But rest assured it's possible to OLB like a firehose without coming out of horizontal trim, and there's no ADV involved. For me, the buoyancy thing isn't an issue: the mini's lungs are pretty well counteracted by the 3L steels and SS case plus my 2lbs of trim weight.

Instructor is the US Distributor for rEvo. I think he knows what he's doing. And if you ask around or check the rEvo forum on the RB sites, you will find tons of posts about the difficulty of breathing through the ADV, especially in horizontal trim. I know you haven't been RB diving all that long, but walk a mile in someone else's shoes. My ADV does NOT breathe easily except in a head up position. The position is a function of the hydrostatic pressures.

 

[Dr. Lecter]

Instructor is the US Distributor for rEvo. I think he knows what he's doing. And if you ask around or check the rEvo forum on the RB sites, you will find tons of posts about the difficulty of breathing through the ADV, especially in horizontal trim. I know you haven't been RB diving all that long, but walk a mile in someone else's shoes. My ADV does NOT breathe easily except in a head up position. The position is a function of the hydrostatic pressures.

Jason? I have a hard time seeing him advocating head up for anything but flushes, but I guess if that's what it takes for your unit to pull dil, that's what it takes. The reason I said I'm not sure why going heads up would help has to do with the hydrostatic differences: the ADV fires based on reduced volume in the upper CL, and going heads up seems to me like it would expand the gas in there relative to the same CL being in the lower horizontal position.

Personally, I like that it takes real intention to fire the rEvo's ADV; I've backed the nut on mine off until it's just barely activatable.