Exceeding the NDL during recreation diving

[tursiops]

I'm not sure if this is the current version, but it does mention gas planning including deco stops: archives.cmas.org/technique/self-rescue-diver-training-programme - Self-Rescue Diver Training Programme

Those are both interesting documents, but are not really about planned solo diving but rather about inadvertent buddy separation. Decompression stops are only mentioned once, in the Training documents, not the Standards document.

 

[johndiver999]

So you're only planning to be able to safely handle one failure?



They've got redundant back gas and a separate decompression cylinder, and they do the planning so they can lose 2 of those and finish safely.



The fact that you don't know this shows your ignorance.


Entanglement, loss of visibility, navigational error, dealing with an unexpected equipment issue, unexpected workload, and task fixation are a few reasons that you could be delayed leaving the bottom.

Let's do some quick analysis; you're planning a
15 minute dive to 130ft on air using 40/85
View attachment 859341
That gives you 7 minutes of planned decompression which is inside that 5 to 10 minute range you talked about

If you're delayed in leaving the bottom for 5 minutes now your real close to or actually out of gas in your back gas
View attachment 859340
And you're looking at 17 minutes of decompression time. Which is going to outstrip the capacity of your pony pretty quickly specially if you had to use any on the bottom.

Sure you could blow 10 minutes of that off and pop out of the water at 100% gf. Doesn’t seem like a very intelligent plan in the middle of no where.


Doing that dive using GUE T1 protocols and procedures for 30 minutes at 130ft.

We’re leaving the bottom with 60cuft of back gas untouched in double 80s and 54cuft of deco gas left over if you’re diving an 80 with 50% in it.

We could be delayed around 10 minutes before we started outstripping out back gas reserves, and we’d still be no where close to the deco gas.

We could safely finish the dive without blowing off any deco with various combinations of significant delays, a complete loss of back gas for a team member, missing decompression cylinders, failed regulators etc

GUE dive to 130 on air? Is that what they teach? Or are you trying to compare one diver using air and another using mixed gas and three tanks to prove????

 

[crofrog]

GUE dive to 130 on air? Is that what they teach? Or are you trying to compare one diver using air and another using mixed gas and three tanks to prove????

No obviously not. It'd be 21/35 and 50%. In fact I mentioned that in the post, but you can't get over your trigger words and actually read.

The point is if you want to do decompression dives, you need to bring equipment, skills and knowledge appropriate for technical diving, to effectively manage the risk of decompression diving, and that technical diving equipment and procedures greatly reduce the risk. I can only speak to the GUE ones because I don't know what gas planning the other technical agencies do, but I'm pretty sure it's not sling a pony and hope.

 

[LI-er]

[*]Why do you write deco in capitals? (I really need that spelled out)

I don't know! I think maybe because that's how my dive computer shows it when I exceed my NDL.

 

[LI-er]

So you're only planning to be able to safely handle one failure
Entanglement, loss of visibility, navigational error, dealing with an unexpected equipment issue, unexpected workload, and task fixation are a few reasons that you could be delayed leaving the bottom.

Thank you for that detailed answer, unfortunately except for the extremely low possibility of a sudden, unexpected, immediate catastrophic loss of gas from both cylinders I don't see any of those other issues as being realistic enough to be concerned about causing a 10 minute delay on the bottom, except possibly if in the midst of light deco I swam through a mass of monofilament line while deep inside a shipwreck.

As part of my plan, I'm not spending those last few minutes deep inside a wreck or a cave, I'll be in the general vicinity of the exit point or line, and if I should experience a "navigational error" I still know which way is up. I can't begin to imagine some sort of equipment error that would delay me on the bottom for 10 minutes. Task fixation? Please. If I've got that much OCD that I completely abandon my dive plan and forget about my DECO obligation and sit on the bottom and drown I'd probably deserve it

 

[crofrog]

Thank you for that detailed answer, unfortunately except for the extremely low possibility of a sudden, unexpected, immediate catastrophic loss of gas from both cylinders I don't see any of those other issues as being realistic enough to be concerned about causing a 10 minute delay on the bottom, except possibly if in the midst of light deco I swam through a mass of monofilament line while deep inside a shipwreck.

As part of my plan, I'm not spending those last few minutes deep inside a wreck or a cave, I'll be in the general vicinity of the exit point or line, and if I should experience a "navigational error" I still know which way is up. I can't begin to imagine some sort of equipment error that would delay me on the bottom for 10 minutes. Task fixation? Please. If I've got that much OCD that I completely abandon my dive plan and forget about my DECO obligation and sit on the bottom and drown I'd probably deserve it

A 5 minute delay… that’s pretty easy to do. You have a 5 minute delay and you’re close to OOG or you’re blowing off deco obligations

 

[LI-er]

A 5 minute delay… that’s pretty easy to do. You have a 5 minute delay and you’re close to OOG or you’re blowing off deco obligations

All I can come up with that would cause a 5 minute delay would be swimming through a mass of monofilament line while inside a wreck during the last few minutes of a dive. I have already mitigated that risk by being near the exit point towards the end of the dive.

None of the other factors you mentioned as possible causes of a delay are any more realistic than an asteroid hitting the dive boat.

 

[scubadada]

...If you're delayed in leaving the bottom for 5 minutes now your real close to or actually out of gas in your back gas
View attachment 859340
And you're looking at 17 minutes of decompression time. Which is going to outstrip the capacity of your pony pretty quickly specially if you had to use any on the bottom.

Sure you could blow 10 minutes of that off and pop out of the water at 100% gf. Doesn’t seem like a very intelligent plan in the middle of no where...

What RMV and cylinder size did you use in your example?

Using my average RMV, I would have enough gas to do this dive with an AL80

 

[lairdb]

What RMV and cylinder size did you use in your example?

Using my average RMV, I would have enough gas to do this dive with an AL80
…

What RMV are you using? Setting aside deep air questions, I get a bigger obligation and need 78cf.

I have to reduce RMV to 0.30cfm to get your numbers, and that's... impressive. I'm jealous of that.

(Edit: on Nx32 with a plan RMV of 0.5cfm I get more reasonable stops and only 65cf needed... but that's still an ugly plan and pushing ppO2 and way under mingas.)

(Also, since I couldn't resist, here's @LI-er's asteroids:

)

 

[crofrog]

What RMV and cylinder size did you use in your example?

Using my average RMV, I would have enough gas to do this dive with an AL80
View attachment 859430

.75cuft/min

Which is a reasonable default to use to take into account the effects of stress. I'd recommend using at least a 1.5 multiplier or .75cuft/min whichever is more to build in a safety factor.