gas switch: backgas? move bottles?

[Dr. Lecter]

Wait, are you talking about this chart?
Decompression Gas | Theoretical Efficiency Ratios

The baseline is air. I don't do a lot of air diving, and even less air deco (never), but just futzing with numbers, it seems that o2 is a bit more efficient than the above chart makes out.

For sport, I put in air (eww, gross) for 120min at 80ft. I set last stop to 20ft, and o2 gives a final stop time of 29mins with VPM+2.

Do the same dive with no deco gas (so air deco), and the last stop (20ft) jumps to 130min.

I don't think 130:29 = 1:1.4, but I'm certainly no math wizard and open to suggestion. You also might want to double check my numbers. I did it with a phone app.

Let's control for whatever the gap is between your software and the math that generated the relative efficiency table: all else being equal, what's the last stop on your software using 80%? That'll give you an efficiency ratio for 80 vs 100 according to your software. Then let us know the CNS loading difference between 80 and 100 for that dive...with the entire last stop at 20', it's going to be bigger than the gap I got when using 10' as the final stop.

Nobody's going to argue that the 20% inert gas content doesn't make O2 more efficient. The question is whether the DIR voodoo bull in the oft-cited Baker's Dozen about 100% O2 has any validity, or, whether there's a tiny sliver of efficiency difference that seems to be trumped by the CNS loading differences for many diving applications.

 

[rongoodman]

You and me both, but open loop breathing seems like a better solution to me.

I don't understand what you mean by this.

 

[Dr. Lecter]

I don't understand what you mean by this.

Breathe in from DSV, manual add dil while exhaling into the water through nose/lips, breathe in from DSV...repeat until you're happy enough with your respiratory control that you can safely switch from the DSV to your BO reg.

 

[rongoodman]

OK, I'd never heard the term before.

 

[PfcAJ]

Cns% is bull****. You can tox less than 100% and not tox well in excess of 100%.

 

[jadairiii]

....Just doing a quick phone V-Planner +2 run for a dive I've picked 80 for before: 40 min BT at 270' on 15/30, with 21, 50, and 80 for travel/deco gas gives a 85% CNS and a run time of 161 min. Swapping the 80 for 100 raises the CNS to 110% while only dropping run time to 159 min. Note I'm too lazy to plug in the travel gas leg on the phone, so actual CNS will be slightly higher for a properly planned dive. This also assumes your last stop is 10' - stay at 15' or 20' for the shallow stops and the CNS divergence for the same dive looks more like 50-60%.

Before I even try to respond I want to make sure I understand this. For a :40BT in 270' you are breathing all your gases down as travel gases then back up for deco? So on the way down you're on 80 then 50 then 21 (air??) then your back gas of 15/30?

And while you're at it 15/30?? why so little HE? Although your back gas choice is explaining a lot about your choice of 80%.......

 

[Dr. Lecter]

Cns% is bull****. You can tox less than 100% and not tox well in excess of 100%.

Hey, I feel the same way about the MOD for air, but rules is rules

Before I even try to respond I want to make sure I understand this. For a :40BT in 270' you are breathing all your gases down as travel gases then back up for deco? So on the way down you're on 80 then 50 then 21 (air??) then your back gas of 15/30?

And while you're at it 15/30?? why so little HE? Although your back gas choice is explaining a lot about your choice of 80%.......

Gods no, just lazy writing on my part; 21% down to planned switch depth, usually at 200'. Prior to switching to CCR, I was not a huge fan of HE, and for the dive 30% gave me the END with which I was comfortable. Your narcosis management may vary

 

[Garth]

Breathe in from DSV, manual add dil while exhaling into the water through nose/lips, breathe in from DSV...repeat until you're happy enough with your respiratory control that you can safely switch from the DSV to your BO reg.

Riddle me this. If you have a rebreather such as the Meg where the gas path goes from Diluent manual add in the exhale counter lung into the scrubber through to the cells then up to inhale counter lung, then to divers DSV how would this effect your plan?

My thought process give the situation above (I don't dive a meg) would be that it wouldn't work as you are manually adding diluent across the broken scrubber. Depending why you are bailing out open loop breathing may not work.

By the way I was not taught open loop breathing I don't think in my CCR Air Diluent Deco course nor was it discussed as a viable option for bailout in my CCR Cave course.

What agency endorses such a practice?

Thanks,
Garth

 

[PfcAJ]

We discussed something similar (inhale, then vent gas out the nose), but the purpose certainly wasn't to try and stay on a compromised loop.

Using that technique seems like a whole bucket of 'no thanks' compared to flipping a lever.

 

[kwinter]

You were not taught open loop breathing because it is pretty much restricted to SCRs. CCR divers learn SCR as a backup. SCR needs open loop as a backup or when shallow.

No friggin way you will be able to breathe open loop in the case of a full on CO2 hit. Respiratory rate can skyrocket and far exceed the ability to either SCR or open loop or remove a DSV to go OC. I have said it a thousand times: there is no faster way to get bailout gas delivered than through a BOV. Say whatever you will about the disadvantages. In a pinch, a BOV can and will save your arse. How you choose to plumb it is a matter of personal preference.


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