Use a gag strap?

[JohnnyC]

The ADV on my Pelagian is just some second stage guts built into the top of the head. You can roll on your back and the thing will freeflow since it's hydrostatic, but when you're in trim, you do have to suck. Lots of guys will pull the guts out, blank the top, and run it with an aftermarket MAV.

The SF2 is second stage guts inside the top of the CL like the RB80's. You've gotta bottom out the CL to get it to fire.

Both can leak since they're both essentially just a second stage, but like anything else rebreather related, some judicious maintenance and proactive attention should prevent it. I trust them both enough that I find the benefits of a gag strap outweigh the rather small risk of hypoxic dil leakage. That being said, I have a shutoff on my SF2 and I use it.

 

[kensuf]

The big question mark in my mind is hypercapnia. I suspect that if you go hypercapnic to the point of LOC, the only way you are going to even have a chance of coming back from that is if you get much shallower (maybe) or to the surface and have a chance to start breathing surface air. Or if you have a BOV and someone switches it to OC for you.

All of these scenarios seem to make a case in favor of a gag strap. And, really, in favor of a gag strap and a BOV.

Insight from someone who has more experience (which ain't sayin' much!) would be very welcome!!

You can track the first two H's by having good cells and maintaining them properly, but CO2 is one you can't and candidly, it's the gas that gets my respect. CO2 is more narcotic than any other gas you're likely to be exposed to while diving (google meyer-overton hypothesis and lipid solubility coefficients) and excess CO2 can cause increased respiration, blood pressure, and increases your risk of heart attack.

Earlier this year I was diving with someone that had CO2 breakthrough. It started around 170' deep and he was so out of it that it took three of us to manage the situation. We had to hand him appropriate gases, strip him of unused stuff, and manage his deco the entire way to the surface. One minute he was fine and posing for photos on the top of the wall, the next he was bailed out and trying to get to the surface as quickly as possible (not a good thing coming up from a 300' dive).

I'm not sure how a gag strap would have helped the situation, and if anything, I could see how it could have hampered the situation.

 

[doctormike]

You can track the first two H's by having good cells and maintaining them properly, but CO2 is one you can't and candidly, it's the gas that gets my respect. CO2 is more narcotic than any other gas you're likely to be exposed to while diving (google meyer-overton hypothesis and lipid solubility coefficients) and excess CO2 can cause increased respiration, blood pressure, and increases your risk of heart attack.

Earlier this year I was diving with someone that had CO2 breakthrough. It started around 170' deep and he was so out of it that it took three of us to manage the situation. We had to hand him appropriate gases, strip him of unused stuff, and manage his deco the entire way to the surface. One minute he was fine and posing for photos on the top of the wall, the next he was bailed out and trying to get to the surface as quickly as possible (not a good thing coming up from a 300' dive).

I'm not sure how a gag strap would have helped the situation, and if anything, I could see how it could have hampered the situation.

Wow! Scary. Great save.

This situation is a bit different than the ones that we were discussing in the French military study. This is a situation where a gag strap would have been a liability instead of an asset, but not the situation that they are designed for.

Although your buddy was "out of it", he was conscious and functional enough to bail out and protect his airway, right? Since you said you found him already bailed out, and since you said you handed him appropriate gasses, I'm assuming that he had a DSV and that he was able to switch regulators during deco, as opposed to having a BOV with the ability to plug in your donated deco gasses, right?

 

[kensuf]

If your loop goes hypoxic because you were on the bottom and ran out of O2, or your solenoid crapped out, or your sensors somehow screwed up in a way that indicated you still had O2 in the loop, well, there's probably nothing but a very good buddy that's going to save you.

First rule of diving a CCR - always know your PO2.
Second rule of diving a CCR - when in doubt, bail out.

If your five sensors (on the revo) are throwing wildly divergent numbers, bail out. If they're all showing you're at a PO2 that's likely to sustain consciousness, but in reality you're hyperoxic or hypoxic but they're just all displaying the same numbers, you need to start playing the lottery. When sensors fail, they have fail in different ways. Some current limit, others shoot high, and some just stop reading. 5 sensors failing in exactly the same way at exactly the same time seems REALLY REALLY unlikely. Ditto for 4 or 3 sensors. And if you've got 3 saying one thing, and 2 saying something else, bail the F out.

As for a buddy doing a dil flush, the rEvo does not have an OPV on the loop that can be manually actuated. I.e. my buddy cannot vent my loop using any OPV. If the loop is full, it would push out past my mouth, but still be full - unless my buddy were to turn me so my mouth/DSV were at the highest point. Then it might vent without help. But, then it could vent to the point that the loop has too little gas in it to breathe. And trust me when I say that the rEvo ADV is unlikely to fire based on the breathing of an unconscious person.

I must be misreading something, but you'd want your buddy to bring you up and hope the gas would go around your lips on ascent? Huh, risk of lung barotrauma seems high here, it only takes 1.5psi difference to put a hole in your lung. You might consider rethinking this strategy.

 

[kensuf]

This is all actually mostly likely (and has happened) on the surface.
Splashing with O2 off or O2 drained
Leaking ADV or ADV firing when its not supposed to
Distracted by surface currents, swimming to the shot line etc etc.
Sure you can get a hypoxic loops at depth too but its a whole lot more to go wrong. ADV firing hypoxic dil fairly shallow is the most likely way.

Breathing all the O2 out of your loop at 200ft? That takes a crazy long time compared to going hypoxic due to an ADV leaking 10/50 dil on or near the surface.

Keeping the DSV/BOV in your mouth with a gag strap can even allow the boat crew to jump in and save you in surface situations. Compared to having it fall out and flood making you sink like a stone.

I had an ADV stick open on my SF2 while I was at 20' with hypoxic dil once. PO2 dropped FAST!

 

[kensuf]

The SF2 is second stage guts inside the top of the CL like the RB80's. You've gotta bottom out the CL to get it to fire.

Both can leak since they're both essentially just a second stage, but like anything else rebreather related, some judicious maintenance and proactive attention should prevent it. I trust them both enough that I find the benefits of a gag strap outweigh the rather small risk of hypoxic dil leakage. That being said, I have a shutoff on my SF2 and I use it.

I had a small piece of sorb that got past the scrim plate and forced the adv on my SF2 open. The shutoff was the trick to solve it.

 

[kensuf]

Wow! Scary. Great save.

This situation is a bit different than the ones that we were discussing in the French military study. This is a situation where a gag strap would have been a liability instead of an asset, but not the situation that they are designed for.

Although your buddy was "out of it", he was conscious and functional enough to bail out and protect his airway, right? Since you said you found him already bailed out, and since you said you handed him appropriate gasses, I'm assuming that he had a DSV and that he was able to switch regulators during deco, as opposed to having a BOV with the ability to plug in your donated deco gasses, right?

He bailed out at the first sign of anything "unusual" as he had been taught by his instructor. From there, we collectively managed the situation.

Here's a screen grab of the start of the whole thing when he bailed out while I was trying to figure out what gas he was breathing (bottle marking, another lengthy discussion).

 

[doctormike]

He bailed out at the first sign of anything "unusual" as he had been taught by his instructor. From there, we collectively managed the situation.

Here's a screen grab of the start of the whole thing when he bailed out while I was trying to figure out what gas he was breathing (bottle marking, another lengthy discussion).

View attachment 541348

Yeah, that makes a huge difference, right? I think that the pro-gagstrap scenarios basically refer to an unconscious diver being rescued by a buddy.

Did you figure out the source of the hypercapnea later on? Scrubber failure? Mushroom valve? Overexertion?

 

[kensuf]

Yeah, that makes a huge difference, right? I think that the pro-gagstrap scenarios basically refer to an unconscious diver being rescued by a buddy.

Did you figure out the source of the hypercapnea later on? Scrubber failure? Mushroom valve? Overexertion?

I'm reasonably sure we know the cause.

 

[stuartv]

I must be misreading something, but you'd want your buddy to bring you up and hope the gas would go around your lips on ascent? Huh, risk of lung barotrauma seems high here, it only takes 1.5psi difference to put a hole in your lung. You might consider rethinking this strategy.

The rEvo loop does have an OPV - it's just not accessible to manually open it from the outside. I.e. you cannot vent the loop other than via the DSV, one way or another.

But, part of the checklist is to verify that you can blow into the loop until it's full and then have the OPV open to let gas out as you blow in. In other words, even if the gag strap maintained its seal to the diver's mouth and the diver was unresponsive and being surfaced, I think the OPV would vent the loop before the diver would experience lung overexpansion. After all, that is the point of the OPV, right?