gas switch: backgas? move bottles?

[PfcAJ]

Interesting.

I've also heard of divers incapacitated from hypoxia and it being corrected by a flip to OC. In my mind, co2 and hypoxia are 2 convincing reasons to have it.

 

[HIGHwing]

It's less complicated, has fewer failure points, and doesn't weigh a frigging ton hanging out of my mouth for hours on end. if I'm so out of control on my breathing that I know I can't transition from the DSV to my BO without involuntarily sucking in a lungful of water, all I have to do is go to open loop breathing.

At that point I've effectively turned the DSV into a diver controlled 2nd stage and will be reducing my CO2 levels regardless of whether the scrubbers are broken through. After I'm under control in terms of breathing, BO normally.

Exactly my plan as well. I already have access to gas, so why do I need a reg breaking my jaw for a three hour dive?

BTW - I've been watching videos on YouTube noting some individuals who are cruising around at 200 - 240 feet with an AL80 marked 220. I wonder what's really in those bottles? Is it air or 21/35 or similar mix? Imagine that bailout straight to air. Whoa! I'd be a drunk monkey at that point, but I admittedly narc easily. I'm also trying to imagine the CCR diver who is sanity breathing on air (it's safe because it comes out the BOV) and getting all good an narc'ed up while "fixing" the CCR at depth to get back to Trimix.

What scares me is CCR divers are cheapy cheap when it comes to bailout gas...

 

[ajduplessis]

??????????????????????????????????????????????????????????????????????????????????????????????

 

[jadairiii]

....I guess all my tables lied to me when they showed a whopping CNS difference for the same deco on 80 vs 100.

Let see the dive you are talking about? Details make this discussion so much easier.

 

[Dr. Lecter]

I think we may need to move part of this thread to the RB section

Let see the dive you are talking about? Details make this discussion so much easier.

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%.

But let's hear all about O2 windows and how 100% is so much more efficient at "proper" decompression. The only really great reasons I can think of for having 100% over 80% is that it requires no blending and you can plumb it into your CCR in a pinch. Since you DIR ninjas blend trimix for 90' dives, blending a proper deco gas should be childsplay. Likewise, you guys aren't known for your love of CCRs, so that's another reason for 100% out the door.

You can fall back on people supposedly choosing 80 over 100 because they're not good enough divers to safely dive O2 all you want. Doesn't change the fact that 100% is a suboptimal gas used by a group of people who whine all day long about the importance of always diving the best gas for the dive no matter the inconvenience or expense involved.

 

[PfcAJ]

Call me nuts, but a deco gas zero inert gas seems like a pretty clear advantage over one that has inerts.

 

[Dr. Lecter]

Call me nuts, but a deco gas zero inert gas seems like a pretty clear advantage over one that has inerts.

I'm sure you've seen the chart of relative deco efficiencies for various EAN blends and 100%. Why don't you pull it up and tell me how much more efficient 100% is versus 80%?

 

[PfcAJ]

Can't find it. Did a fair bit of google-fu, also.

Makes me wonder why the Navy, recompression chambers, the guys doing REALLY big dives, etc use 100%. Weird.

 

[LiteHedded]

Can't find it. Did a fair bit of google-fu, also.

Makes me wonder why the Navy, recompression chambers, the guys doing REALLY big dives, etc use 100%. Weird.

because they can control their position in the water column

 

[PfcAJ]

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.

---------- Post added April 30th, 2014 at 06:02 PM ----------

because they can control their position in the water column

That skill IS lacking a bit these days, I suppose.