Exceeding the NDL during recreation diving

[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.

 

[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

 

[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.

 

[Miyaru]

Looks like the software forgot to recalculate density from metric grams per liter into imperial pounds per gallon

 

[MarkA]

Deleted

 

[scubadada]

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.)...

My post figure was straight off MultiDeco. I used my average RMV of 0.36 cu ft/min from 1,917 dives over the last 14 years under all kinds of conditions. The variation around this average is not very great, with a std dev of 0.04 cu ft/min. My avg RMV over the last several years is a bit lower than the overall average.

I was using @crofrog example of a planned rec deco dive of 15 min at 130 ft using air, GFs of 40/85, that ended up going over by 5 min. I would do this dive on 28% and do not use 40/85.

Deep air, oh, come on now.

.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.

I'm not sure why this planned dive, that ended up going over by 5 min, would result in an overall dive RMV of 1.5 times average gas consumption. If that were the case, I would probably just squeeze by with an AL80 with a gas requirement of just over 70 cu ft. Of course, I would still have my pony.

I encourage divers to know their gas consumption rate and to plan for the gas requirements for their dives. There has been a thread going on SB for over 8 years now discussing average gas consumption

Average Gas Consumption

There has been extensive discussion lately regarding average gas consumption. A poll may help give us a better idea of what the normal distribution of RMVs is. 1 cu ft = 28.32 liters 1 liter = 0.035 cu ft 28.3 l/min

scubaboard.com

 

[crofrog]

I'm not sure why this planned dive, that ended up going over by 5 min, would result in an overall dive RMV of 1.5 times average gas consumption. If that were the case, I would probably just squeeze by with an AL80 with a gas requirement of just over 70 cu ft. Of course, I would still have my pony.

You shouldn't plan on things going perfectly, you should plan on them going very poorly.

Then when they go well, like they do 99% of the time you've had a bit of extra conservatism and you can lament from the safety of the surface the fact you could have spent a few extra minutes down there, and when they go very poorly you get to still go home.

 

[scubadada]

You shouldn't plan on things going perfectly, you should plan on them going very poorly.

Then when they go well, like they do 99% of the time you've had a bit of extra conservatism and you can lament from the safety of the surface the fact you could have spent a few extra minutes down there, and when they go very poorly you get to still go home.

I did not say that I do not plan for contingencies, only that I did not understand why your example dive would require 1.5 times the average gas requirement.

In fact, I use 2 times my average gas consumption rate when planning my required pony volume (very close to your 0.75 cu ft/min).