Air or Nitrox as a diluent?

[Capt Jim Wyatt]

More and more I am seeing/hearing about CCR divers using nitrox 30, 31, 32 etc. as a diluent in their CCR's for dives at and around 100 feet. I fail to see the advantage of this.

Anyone care to explain an advantage?

 

[Wstern5]

I’m not CCR trained but I know a lot of cave divers use the same bottles for dil and bailout, either manifold on the unit or sidemounted and plugged in. If they need to bailout in the cave, nitrox would better manage the deco. My guess.

 

[grantctobin]

More and more I am seeing/hearing about CCR divers using nitrox 30, 31, 32 etc. as a diluent in their CCR's for dives at and around 100 feet. I fail to see the advantage of this.

Anyone care to explain an advantage?

If you’re running your bailout as diluent, it often makes sense. E.g sidemounted bailout, onboard O2 and wing (or wing+suit) gas. With sidemount CCRs, you’re very likely using dilout (other than the Liberty or one-off sidekick or flex configuration). As such, a “rich” dil is an optimization of gas carried vs accrued deco obligation in the event of a bailout.

If on a BM unit running 3L dil onboard, I’m much more inclined to run 21/35, 18/45 etc and have closer to a 1.0 bottom PO2 for cell verification and even lower END. However, in a sawtooth profile cave, a 3L thats split dil and wing doesn’t last very long.

 

[rddvet]

I use 30/30 and 21/35. I have used 32% but prefer 30 or 30/30. My dil is also my bailout so I was taught to plan your bailout so that you can get a decent dil flush but also aren’t going to rack up a ton of deco if you need to bail out at max penetration. It’s about trade offs. I’ve never used straight air as dil. Not once. Closest is 21/35. It’s nice to get a big drop on p02 with a lower oxygen dil flush, but I’ve got to have helium in my mix if I’m at 100ft or deeper.

 

[rjack321]

For BM cave dives, I'm using dil/bo like described above. If its a cave I know and backmountable I bring an OC gas in the left side offboard 50, 80, 85, 108 or 130 of dil/bo and the onboard "dil" tank is my suit gas. Bringing a suitable OC gas makes my BO ascent plan much more comparable to my CCR plan. I can get out of mainland or from the Champagne bottle in hole-in-the-wall with 2x108s of 32%, or 2x85s and a stage of 32%. If I put air in that dil/out bottle the deco penalty is horrendous. The 32% is also nicer in that every dil injection isn't followed by the solenoid chasing the ppO2 around and adding even more gas that I will probably have to vent eventually.

Target ppO2 for my 3 to 5 hr vacation dives like this on 32% dil/bo (75 to 105ft range) is 1.1 or 1.2. Usually 1.2 as more than 4.5 hours is pretty rare.

If its an exploration cave in my area I am always sidemounting and using the deepest dil/bo I possibly can. I did over 3 hours in a max 132ft deep system a couple years ago on 18/45 because I had no idea how deep it might get. When you only get a rare shot at good conditions and the nearest helium is potentially an island away, I never want to be MOD limited when I could carry on exploration if I had just brought a deeper gas.

 

[JahJahwarrior]

There are a lot of important considerations for gas planning, but reducing deco obligation in an emergency situation is probably not the number one priority. After all, you won't survive to get to your deco stop if you tox at depth or can't validate cells.

Diluent po2 needs to be safe to breathe and also allow you to get a good verification of cells with a flush, reducing deco obligation is a smaller concern that can also be met with a safety bottle.

I'm in a similar camp as Rddvet of using a lower o2 for dilout, higher o2 for bailout. Assuming the issue is with your dil and the rebreather is still functional, having the higher o2 gas is nice for SCR mode to extend your gas supply. Having a low o2 gas for deeper dives gives you much more flexibility.

In a pinch, I don't think a 100 foot dive with 30% dil is the biggest risk, but if the cave drops down to 120 and suddenly I want to drop down there, then it starts to get dicey. If I'm going in the back of Ginnie to the point where minimizing deco obligation in an emergency is that big of a concern, then it shouldn't be a big deal to bring another safety bottle so I can stick with my lower o2 dilout. Of course, this is also why I have aluminum bailouts and steel bailouts and LP45 bailouts...get your gas well suited for your dive, including a buffer to go deeper than expected and to spend more time on deco than expected.

 

[rjack321]

In a pinch, I don't think a 100 foot dive with 30% dil is the biggest risk, but if the cave drops down to 120 and suddenly I want to drop down there, then it starts to get dicey.

I don't think anyone is using 32% dil when there's the possibility of dipping below a MOD. I never breathe down my dil ppo2 but I know @tbone1004 has said he does. Swimming to the stratosphere or the traverse or well at peacock there's not really any plausible plan that brings me over 1.2 on the 32% and it is nice to basically not have a giant deco plan change for a BO exit or the solenoid shooting off at every little dil add. If I ever get back to MX i will for sure be on 32% dil/bo there.

I've not used 30/30 in a decade but I have used 25/25 as dil/bo for long 90 to 115ft dives (scootering a wreck from shore). I had o2 for that as well since there was ~25mins of deco time.

If I had a "rule" for dives where I'm using offboard dil/bo it would be that I use the standard gas with the highest O2 content that doesnt send me over my target ppO2 (~1.1 or 1.2) at max depth. If I don't know the max depth ahead of time that calls for a different plan.

 

[Tracy]

I don't know that I see an advantage, but that is typically what I do in Florida. I am set up for offboard dilout. My bailouts are 32% there because it is easy, cheap, and effective as a bailout. I don't see a downside to it in the cave systems I typically dive.

 

[tbone1004]

More and more I am seeing/hearing about CCR divers using nitrox 30, 31, 32 etc. as a diluent in their CCR's for dives at and around 100 feet. I fail to see the advantage of this.

Anyone care to explain an advantage?

what do you see as the advantage to using air at 100ft instead of nitrox?

I firmly believe in diving the same dil gas as you'd use if doing that dive on open circuit.

 

[Wibble]

what do you see as the advantage to using air at 100ft instead of nitrox?

I firmly believe in diving the same dil gas as you'd use if doing that dive on open circuit.

Just to expand this a little...

Dil should have a lower PPO2 so that you can flush down the loop. Thus a dil should be around 1.0 PPO2 at the bottom.

A bailout would, if using OC principles, be around 1.3 at the bottom.

So for deeper dives, say 60m/200', you'd be looking at a dil of 15% but a bailout of 18%. Or would you simply match your bailout to the dil - say 15/55 instead of 18/45?