Would YOU thumb this dive?

[TrimixToo]

Forgive my ignorance and the probably dumb question, as I have no experience with technical diving or doubles, but could someone please enlighten me about something? If SPG is run from left 1st stage and left post shut down, how would one know how much gas they have in the right cylinder they are now exclusively using? How would having a slung stage with bottom gas help with this loss of critical data and allow one to safely continue the dive? Thanks

These sorts of questions make gas planning a critical part of technical diving.

If one dives thirds, meaning that upon surfacing you plan to have 1/3 or more of your gas remaining, then at the worst possible time shutting down the post should leave you exactly enough gas to return. Assuming you turn the dive instantly, pressure information is not actually necessary. You either have enough gas, or you don't. At worst, you might need to share gas or skip a deep stop to get on a deco gas early, but those should be very rare events on a well-planned dive with a single failure. Adding an SPG to the right post reg adds points of failure to the system, as well.

A slung bottle of bottom gas might or might not increase safety. It adds complexity and decreases mobility in a current. If you need one to maintain the margins for your gas plan, that's one things, but otherwise it's best left behind in my view.

 

[SJT1961]

Thanks for helping me understand this.

 

[DanielRJones]

Thanks for helping me understand this.

Say i start with 2x 12l tanks with manifold i work out my rock bottom which is the pressure needed to get 2x divers from the bottom to the surface or 1st gas switch. I then split the rest of the tabk into 1/3rd if i have to get back to the line which is 1/3rd for me going out and 1/3rd for me to get back and 1/3 for my buddy who is air sharing.
If i can ascend at any time and it is not needed for me to get back i split into 1/2 for me to swim out and 1/2 for me to swim back if a buddy ooa can just ascend knowing i have my rock bottom to get us to gas switch or safety.

If i have a freeflow i instanly isolate the manifold which turns my linked 12l tanks into 2x 12l tanks i can then turn off the offending post with the freeflow and then unisolate the manifold. This means i still have access to my 24l of air even though the left or right post is turned off. If it is a problem with the valve and i cant turn the manifold back on i have my 1x12l tank stoll working for air. In this case i cant donate so the dive is over but even if right at the end of dive will have enough gas to get me to first gas switch or surface

 

[kensuf]

Two years ago I was part of the first group to work through the logistics of getting CCR's, helium, oxygen, and sorb to Cayman Brac. The trip easily cost me a bucket load.

One morning I was part of a 3 man team that was going to hit a 110m dive on a virgin reef, during the first two minutes of the dive I had a problem with my CCR. While I could have potentially managed the dive by manually adjusting a few things, I decided the wiser choice was to send my buddies off and head for the surface rather than putting myself in the situation where I was on a compromised unit on a fairly advanced dive.

Don't take stupid risks, even on bucket list dives.

 

[jerrwhy01]

Generally speaking if you're in situation and your asking yourself if I should thumb the dive then you already should have.

You suffered a malfunction to a piece of life support equipment. Like it or not, understand it or not the potential for the chain of failure and mistakes to build off of this one and continue until you reach a point of no return is now very much on the table. Whether or not it occurs is squarely in the hand of the statistics gods.

If it's the trip of a life time you're equipment should have been prepared accordingly. I've made that mistake of ignoring a minor gear problem and what my mind was telling me and it almost bit me in the butt: On the value of just stopping, or how I almost didn't swim out today..

 

[boulderjohn]

Team of 3 diving on the wreck they have traveled half way across the world to see and last chance to get on the wreck.

This part of the equation is the most dangerous, for it can drive you to do a dive you should not do for reasons that have nothing to do with dive safety.

You decend down and 5 mins into the dive you get a freeflow on your necklaced secondary nice easy fix you isolate left post having lost very little air.

Five minutes? You go to the surface, fix the problem, and start over.

 

[DanielRJones]

This part of the equation is the most dangerous, for it can drive you to do a dive you should not do for reasons that have nothing to do with dive safety.
Five minutes? You go to the surface, fix the problem, and start over.

Hell if it ever did to me i would be making sure i got the rest of my team back on that wreck even if i had to pay for a new trip for them.

 

[TrimixToo]

Generally speaking if you're in situation and your asking yourself if I should thumb the dive then you already should have.

^^^ This.

If you need to ask yourself the question, you already know the answer. Act appropriately and control the risk, *right now*. This annoys dive partners from time to time. I do not care. You should not care, either.

That said, the need to ask yourself certain questions will evolve over time. I have generally become more conservative even though my capabilities have, I'd hope, improved. But for some things, I'll just fix it (whatever it is) and go on when I would have thumbed the dive at some point in the past.

 

[EireDiver606]

Generally speaking if you're in situation and your asking yourself if I should thumb the dive then you already should have.

You’re right but he wasn’t asking himself. He was trying to gauge what the more experienced divers would do.

 

[rjack321]

The 5 mins into the dive and I don't even have any deco yet. Just go up and fix it.

I have done dives with broken stuff before. I did a 150ft dive on a backup light once cause my primary failed after I got in the water and it was slack "now" and there was neither time nor resources to fix it. I probably should have thumbed it even though I had a second backup on me - cause having a buddy separation at 150ft in dark swirly high current water is bad news.