Lessons Close call exiting Xunaan Ha Cenote 2021

[missionmtb]

I was recently reading the 2021 IUCRR incident report and 3D printing oxygen inline shutoff locks for a friend which got me to thinking about a near miss I had in Tulum in 2021.

I was diving a very shallow cave called Xunaan Ha with an average depth over our two dives of maybe 15 feet (5ish meters). This is a really fun, technical cave (for me) in places because the passages get tight and require a lot of very shallow depth changes. Because of this, PO2 varies much more widely than in a deeper cave on account of the shallowness.

My buddy and I finished our first dive without incident (besides me making a mess in an air chamber trying to get an over under shot) and went back in for a slightly different route. On both dives we went through a quite tight restriction, chest to bottle contact in back-mount CCR and a little positioning required to get thru. On the fourth pass going thru this restriction I unknowingly rolled my oxygen valve off.

As we continued our exit, nothing seemed to be amiss for about 5ish minutes, I dive on happy as a clam. However, nearing the exit with maybe 500 feet of line left to go, I see a red flashing warning on my HUD! I look at it and it says "warning - O1 pressure low" which means the pressure in my Oxygen bottle had dropped below my minimum setpoint (I think this is 300 psi?). My oxygen consumption was likely slightly higher than metabolic due to depth changes and flying manual for better buoyancy, but I shouldn't have gone thru it that fast! All the while my PO2 is still reading 1.2ish on the HUD.

My first thought is '****, I don't want to bailout! Maybe I can make it the rest of the way on this.' As I'm thinking this, I can see the pressure dropping quickly with oxygen injections. Too quickly. This can't be right.

Then it hits me, my bottle must be off! I check the hand wheel and sure enough it was lightly shut. I turn it back on, the warning clears and the pressure reading jumps back up to something over 1000 psi. Phew, and my buddy didn't notice either, how embarrassing that would have been

As time went on after this incident, the experience didn't feel like a near miss since it was over really quickly and only I knew it had happened. However, after considering different CCR configurations, I realized how much differently this could have gone.

I've been tempted to run SPGs on short hoses to have a clean chest like I see many do. I've thought about rebreathers with no HUD in the interest of cost, comfort and unobstructed view. I've run SPGs over my shoulders and seldom checked them out of complacency and resistance to the small effort to bend them into view. But all of these would have made detecting, making sense of and correcting my issue more difficult and delayed my corrective action.

Because I had my HUD programmed with this warning, I had a wireless transmitter and I had my pressure and PO2 right in my eyeball, I could see the issue and behavior right away. I was able to see the problem, diagnose it, and correct it in a matter of moments and continue my dive with little more than a reach for my bottle. Of course I could have just bailed out and never looked back, but this would have presented its own challenges.

Since this dive I've made it a habit to check my hand wheel position when my unit makes contact with aaanything. I've converted to hCCR as it is more efficient on oxygen consumption and easier on the buoyancy in caves like this in my experience. But beyond the improved behaviors I've implemented, I am thankful for the equipment that is available these days. I hope that others will consider their configuration and weigh cost, comfort and convenience against the risk of delay in diagnosis of a problem and possibly make an equipment change (or at least reinforce behaviors that accelerate problem resolution when they occur). I acknowledge that I am very fortunate to be able to afford the equipment I have and hope this doesn't come across as shaming to anyone who has to make that difficult cost decision.

Also, IMHO I think a rolled hand wheel and a slide shutoff on the oxygen hose are pretty similar, practically speaking. I think if I ever move to an mCCR I would likely also consider seriously using a needle valve as many have suggested here in place of an inline shutoff and CMF.

I hope you enjoyed this write up.

~M~

 

[kierentec]

Thanks for posting this. I don’t think people realize just how many near misses like this happen every day.

Out of curiosity, do you dive with your valves fully open, or just cracked open a turn?

 

[grantctobin]

Also, IMHO I think a rolled hand wheel and a slide shutoff on the oxygen hose are pretty similar, practically speaking. I think if I ever move to an mCCR I would likely also consider seriously using a needle valve as many have suggested here in place of an inline shutoff and CMF.

I hope you enjoyed this write up.

~M~

Thanks for sharing

 

[missionmtb]

Thanks for posting this. I don’t think people realize just how many near misses like this happen every day.

Out of curiosity, do you dive with your valves fully open, or just cracked open a turn?

Thanks for the question. I used to run them full open as I was trained in open circuit to make sure the valve was not a flow restriction. I had this corrected and explained to me that nothing attached to my onboard Dil and Oxygen needs that sort of flow rate. What's more troubling is having that full flow rate when an issue happens along with a delay in shutting the valve (especially at big depths). I now keep them open about 1-2 turns and I keep an eye/ear out for solenoid/MAV behavior to make sure they're getting what they need.

Obviously if the valve were full open it would take a lot of cave dragging to close it, but if I busted a hose or fitting going thru it would take a long time to shut it. Rather just check it after contact.

Answering your question I also now realize I should be checking my Dil bottle since it would roll too far ON and take longer to shut in the event I needed to...

 

[kierentec]

Thanks for the question. I used to run them full open as I was trained in open circuit to make sure the valve was not a flow restriction. I had this corrected and explained to me that nothing attached to my onboard Dil and Oxygen needs that sort of flow rate. What's more troubling is having that full flow rate when an issue happens along with a delay in shutting the valve (especially at big depths). I now keep them open about 1-2 turns and I keep an eye/ear out for solenoid/MAV behavior to make sure they're getting what they need.

Obviously if the valve were full open it would take a lot of cave dragging to close it, but if I busted a hose or fitting going thru it would take a long time to shut it. Rather just check it after contact.

I understand where that logic comes from, but it is kind of flawed, unfortunately.

A busted hose or fitting wouldn’t really matter if it takes a while to shut down. It’s not likely you’ll be able to access that gas anyways. A stuck solenoid is pretty rare, you’ll notice it immediately due to the PO2 spike and buoyancy shift, we can handle an high PO2 for a few moments in most circumstances, and the volume of gas lost is actually minimal (I actually can’t remember reading a fatality report involving a stuck solenoid, but I’ve seen a lot of them and I’m sure I’m forgetting one). A stuck adv or mav should be pretty easy to identify and shutoff. You can vent out your mouth in either of those situations to manage the buoyancy shift. To top it off, it really shouldn’t take much longer to turn the valves a couple more turns.

And most importantly, you are MUCH more likely to have a rolloff, which can have far more dramatic consequences if task loaded and you don’t notice the PO2 drop.

Valves should be either open or closed.

 

[missionmtb]

I understand where that logic comes from, but it is kind of flawed, unfortunately.

A busted hose or fitting wouldn’t really matter if it takes a while to shut down. It’s not likely you’ll be able to access that gas anyways. A stuck solenoid is pretty rare, you’ll notice it immediately due to the PO2 spike and buoyancy shift, we can handle an high PO2 for a few moments in most circumstances, and the volume of gas lost is actually minimal (I actually can’t remember reading a fatality report involving a stuck solenoid, but I’ve seen a lot of them and I’m sure I’m forgetting one). A stuck adv or mav should be pretty easy to identify and shutoff. You can vent out your mouth in either of those situations to manage the buoyancy shift. To top it off, it really shouldn’t take much longer to turn the valves a couple more turns.

And most importantly, you are MUCH more likely to have a rolloff, which can have far more dramatic consequences if task loaded and you don’t notice the PO2 drop.

Valves should be either open or closed.

I see your points above. I still think that, particularly for deep diving and deep penetration, being able to save your oxygen supply is a higher priority than preventing the small risk of a roll off (of which PO2 monitoring, buoyancy monitoring, loop volume awareness and tank pressure monitoring are critical as you note). I could also see that lots of grinding and contact between the unit and the cave would change the risk of a roll off. However practically speaking, rolling 1 turn vs. 4 turns is a roll difference of less than a foot...

In my later courses, getting off the rebreather as a result of gas loss gets riskier and riskier with jacked SAC rate from stress, possible gas switch mistakes and all the resource issues with OC. Treating that O2 like a precious resource by making sure the genie doesn't get out keeps your CCR options open.

At the end of the day though, I believe we're all thinking divers with different environments, profiles and risks to consider.

 

[rjack321]

Thank you for sharing. Which unit was this incident on? Does it have an ADV and was it shut off?

One of the most interesting combinations of factors here is that you probably would have run out of volume before your 1.0+ starting ppO2 would have dropped to hypoxic levels at 15ft. Depending on the ADV situation you would fortunately had other warning signals of the issue.

 

[missionmtb]

Thank you for sharing. Which unit was this incident on? Does it have an ADV and was it shut off?

One of the most interesting combinations of factors here is that you probably would have run out of volume before your 1.0+ starting ppO2 would have dropped to hypoxic levels at 15ft. Depending on the ADV situation you would fortunately had other warning signals of the issue.

It's a shearwater meg with an ADV that doubles as a DIL MAV, it has a shutoff (and was).

Agree, with a constant depth or maintaining minimum loop I should have felt a decline in loop volume. In this cave though there are a lot of depth changes so maintaining minimum loop was not really practical. The first indication of an issue was low O2 tank pressure. Fortunately there was still enough pressure in the line to feed the MAV (and eventually solenoid) during this warning. If this had not been the case, I might have either noticed the MAV/solenoid wasn't behaving normally or eventually had PO2 rash on my HUD as well. This behavior makes sense since the low O2 pressure warning happened before the pressure dropped below interstage.