What is your selection process for rich versus lean Dil mixes?

[Heat Miser]

Incidentally I think that argues for using pretty rich dil mixes that are in line with what you are using for a setpoint. I.e. EAN32 at 30m/100ft instead of air where your dil flush validates function at 1.3 instead of 0.8.

@tbone1004 I read on you other posts that you tend to run a lower set point @ 1.2 on your CCR. Are you proposing for a 30 m dive to run a rich dil higher than your high setpoint and breathe it down? I was taught to target a dil mix with 1.1 PpO2 at max depth, when I asked why not 1.0 or 1.2 I never really got an answer? Feel free to move to new post if not relevant to O2 cell longevity strategies of the OP.

 

[kensuf]

You want your DIL to be at a lower PO2 @Max Depth than your planned setpoint. That way if you wind up with a PO2 spike, you can bring your PO2 back down quickly via a DIL flush.

How much lower is what is up for debate, I know some that basically choose a DIL that's right at their SP. As mentioned, 32% at 30m would give a PO2 of ~1.3, which is higher than I personally run my setpoint (unless I'm on deco). I personally think that choosing a DIL that's higher than your planned SP is looney toons.

The PO2's I normally run:

1.2 "working portion" for any dive where the planned RT < 180 minutes
1.0 "working portion" for any dive where the planned RT > 180 minutes
1.3 for decompression, except I will do an O2 flush at 6m for a cell linearity check at the tail end of a dive, but then I let it decay back down to 1.3.

 

[tbone1004]

It's an unpopular opinion, but I believe in diving standard gases and I don't believe in using a dedicated diluent bottle. This applies mainly to what I do with cave diving, but I dive like that in the ocean. With caves I am rarely at the actual MOD of the gases, and admittedly at 30m in a cave I'm typically on 25/25 because I am very susceptible to CO2 which has a ppO2 a tick under where I normally run at 1.1. That said I have run "trials" where I've run the ppO2 up to 1.6 at depth and then dil flushed and it comes right back to 1.2-1.3 if I'm at the MOD of the mix. It then takes a hot minute with an mCCR to get back down to 1.1, but it gets there.
For me it's all about balances. My sidemount unit really isn't compatible with a standalone diluent bottle that will last for long/deep cave dives which means I have to run my unit off of the diluent bottle/s. On my rack mounted Meg I use the 3l bottle for suit/wing inflation and run the unit off of the LP50's in the rack. If I look at a 30m cave dive with a planned 60minute penetration and have to bailout to EAN32 I am looking at roughly an hour of deco. If I had air in those tanks I am looking at ~90mins of deco. My logic is that if I had to bailout to exit, I really don't want to increase my deco obligation 50%. That comes at the cost of not being able to crash a ppO2 back down in the event of leaking O2, but I can always bailout if the O2 is spiking and then deal with the O2 leaking into the unit while in OC and get back on the loop if I can/go to SCR etc. but if I'm diving a lean dil mix then the SCR deco is even worse than the OC bailout.

For deco, I plug in the offboard deco gases to get the helium out of the loop and it also allows me to do 1.6 ppO2 checks as soon as I get on a deep deco bottle and then let it decay down to 1.1/1.2 then go back up to 1.5-1.6 and let it fall back off. My body feels better with the dynamic ppO2's on deco for whatever reason.

 

[Tracy]

I use dilout or bailuent on both of my units. I aim to run my dil po2 at my setpoint for the bottom section. This minimizes bailout deco and minimizes the po2 crash on dives riddled with ups and downs.
I can still crash a high setpoint with a dil flush. Not to below my setpoint obviously, but down to it, which is where I want it anyway.
I am a tourist cave diver at best, I rarely do runtimes long enough to necessitate running my setpoint below 1.2. With my normal diving in the Great Lakes, thermal considerations come in to play long long before OTU/CNS considerations.

 

[rjack321]

Wreck dives using an onboard 2/3L dil I aim for about 1.0 but sometimes its way way lower. I mean I've done 40ft dives on 12/65 dil. It's a pita but you have a back mounted gas mixer - use it.

Cave dives using combined dil/bo I typically run a standard gas with minimum 1.0 and up to 1.2 at max depth - but 90% of the cave I'm rarely at max depth. In MX caves it's just 32% even though that's only like a ppO2 of 0.7 at typical depths.

 

[lermontov]

Wreck dives using an onboard 2/3L dil I aim for about 1.0 but sometimes its way way lower. I mean I've done 40ft dives on 12/65 dil. It's a pita but you have a back mounted gas mixer - use it.

Cave dives using combined dil/bo I typically run a standard gas with minimum 1.0 and up to 1.2 at max depth - but 90% of the cave I'm rarely at max depth. In MX caves it's just 32% even though that's only like a ppO2 of 0.7 at typical depths.

12/65 on a 40ft dive is a strange mix whats the logic behind that?

 

[Whitrzac]

12/65 on a 40ft dive is a strange mix whats the logic behind that?

Probobly what was in the RB at the time.

 

[rjack321]

12/65 on a 40ft dive is a strange mix whats the logic behind that?

On a liveaboard trip with some dives to 265ft and weather pushing you to B-list sites on some days you dive what you got.

 

[lermontov]

On a liveaboard trip with some dives to 265ft and weather pushing you to B-list sites on some days you dive what you got.

I know that feeling, and crying with every breath

 

[mdwalter]

I know that feeling, and crying with every breath

Yeah, but not crying too much on the RB, since those few cf of expensive mixed gas are only costing you a few bucks. I'd cry my eyes out using (whole tanks of) OC mix at 40' though.