Question PO2 for bottom bailout when off-boarding diluent

[SavageSeaCow]

I’ve noticed that some divers for especially deep or technical diving are choosing to off board diluent in their bailout cylinders. This is more or less how the whole sidewinder/Gemini community does it in my understanding. My question for anyone who does this is, how do you choose the PO2 of your bottom gas? My training taught me to limit the PO2 of bottom dil to 1.0 and maximize PO2 for bottom bail out to 1.6 (OW) and 1.4 (cave). If you’re using one bottle for both are you choosing to meet in the middle, or do you sacrifice additional deco time in the unlikely event of bailout in the name of having an appropriately lean bottom gas? If you do that, won’t you end up pushing the limits of the necessary amount of gas you need to bring in the event of a deep bailout? This seems like it would be especially problematic in a cave if you had a prolonged deep exposure to a PO2 of 1.

 

[JonG1]

1.6 on bottom BO might be a bit rich on the basis that if you were bailing due to a CO2 issue the increased vasodilation can then trigger a tox event.

I think the concept of minimising deco by choosing a high PO2 has gone out of fashion in favour of a BO choice based on gas density and narcotic effect.

I generally dive an onboard 3 of dil, so usually match the dil and bottom BO, that way I can add the BO via BOV if I need to for any reason, plus I know the gas is breathable at any depth the dil is.

 

[tbone1004]

I’ve noticed that some divers for especially deep or technical diving are choosing to off board diluent in their bailout cylinders. This is more or less how the whole sidewinder/Gemini community does it in my understanding. My question for anyone who does this is, how do you choose the PO2 of your bottom gas? My training taught me to limit the PO2 of bottom dil to 1.0 and maximize PO2 for bottom bail out to 1.6 (OW) and 1.4 (cave). If you’re using one bottle for both are you choosing to meet in the middle, or do you sacrifice additional deco time in the unlikely event of bailout in the name of having an appropriately lean bottom gas? If you do that, won’t you end up pushing the limits of the necessary amount of gas you need to bring in the event of a deep bailout? This seems like it would be especially problematic in a cave if you had a prolonged deep exposure to a PO2 of 1.

Maximize as in that is what you are aiming for or it's an absolute max? I.e. 1.4 max with a target of 1.2 to give you a bit of flexibility or max/target of 1.4?

Two schools of thought that I feel are valid, both surrounded the concept of standard gases, i.e. EAN32+helium

First is on the CCR you are diving one gas deeper than what you would on OC to minimize WoB and to be able to bring the ppO2 down during a dil flush. Obviously this has negative implications if you have to make an SCR or bailout exit. At 100ffw, 21/35 with ppO2 of ~0.85
Second is to dive whatever gas you would be diving on OC. At 100ffw, EAN32 with ppO2 of ~1.3

Pros and cons to each, I much prefer to have a higher ppO2 for bailout but I am also really susceptible to hypercapnia so the lower gas density outweighs the ppO2 advantages. At 100ffw 30/30 would be a more ideal gas IMO but it's a nonstandard gas and the benefit does not outweigh the hassle.

In OW, I would definitely go for lower gas density as you are able to get to your deco gas a lot faster than you can in a cave so go for lower gas density when possible.

 

[rddvet]

Maximize as in that is what you are aiming for or it's an absolute max? I.e. 1.4 max with a target of 1.2 to give you a bit of flexibility or max/target of 1.4?

Two schools of thought that I feel are valid, both surrounded the concept of standard gases, i.e. EAN32+helium

First is on the CCR you are diving one gas deeper than what you would on OC to minimize WoB and to be able to bring the ppO2 down during a dil flush. Obviously this has negative implications if you have to make an SCR or bailout exit. At 100ffw, 21/35 with ppO2 of ~0.85
Second is to dive whatever gas you would be diving on OC. At 100ffw, EAN32 with ppO2 of ~1.3

Pros and cons to each, I much prefer to have a higher ppO2 for bailout but I am also really susceptible to hypercapnia so the lower gas density outweighs the ppO2 advantages. At 100ffw 30/30 would be a more ideal gas IMO but it's a nonstandard gas and the benefit does not outweigh the hassle.

In OW, I would definitely go for lower gas density as you are able to get to your deco gas a lot faster than you can in a cave so go for lower gas density when possible.

Everything he said is spot on. I actually prefer 30/30 for 100ft dives. It’s my go to gas for most dives I do. Good balance of pO2 and deco if you bailout.

 

[Heat Miser]

Using a bail-out for dil, which we call a dil-out solution, I would target a max pp02 of 1.2 (assuming a setpoint of 1.3) however using onboard dil I would target max pp02 of 1.1 (assuming a setpoint of 1.3) and then pair that with a bail-out having a max target of 1.4 pp02 at max depth.

Whilst adjusting dil-out and onboard dil for any change in setpoint - some people prefer 1.2 - and with the decision on TX versus NX subject to/based on gas density calcs and nitrogen narcosis calcs i.e. keeping max PPN2 < 2.765

 

[rjack321]

In a cave on OC, I plan a max ppO2 of about 1.2 Any higher and after a couple hours of dive time you start running very low on pulmonary OTUs and even if you're willing to exceed 100% CNS limits it starts to feel pretty dicey doing a long time on O2 even with gas breaks. You won't offgas well with lungs full of phlegm.

What does this mean on CCR when dive times can be even longer?

For me on the sidewinder I use standard gas (32%+helium) that keeps me under 1.2 dil-bo if I am planning a total dive time of 4hrs or less (basically 4hrs x 1.2 = ~100% CNS). If I am looking at a longer dive than that or the possibility of lots of O2 based OC deco in a BO situation I lower my dil-bo ppO2 further because I am planning on diving a much lower ppO2 of e.g 1.0

 

[kensuf]

I think it depends.

What does it depend on? Duration of the dive and your target setpoint.

The grey area comes when you start thinking about using SCR mode as one of your recovery options. One school of thought says that you should target a DIL with a PO2 as high as possible, while still being under your preferred setpoint; this is mostly to maximize PO2 after a DIL flush in SCR mode and reduce decompression obligations.

However, remember, one of the purposes of your DIL is to be able to blow your PO2 down in the event that you have an inadvertent O2 spike. This means your DIL PO2 needs to be lower than your target setpoint.

Generally speaking, for any dive less than 3 hours I run a 1.2 working PO2 and 1.3 deco. For something like that, I would want a DIL that has a PO2 of no greater than 1.1 at max depth and 1.0 might be better.

For any dive that's more than 3 hours I normally run a 1.0 working and 1.3 deco. For something like that, I want a DIL that has a PO2 no greater than 0.9 at max depth.

The reality is, when going into SCR mode, a 1.2 PO2 DIL vs a 1.0 or 0.9 isn't really going to make that much of a hill of beans difference in your total decompression obligation. But being able to blow your PO2 down after an O2 spike is really useful.

IMHO 30/30 or 32% for a 100' dive is a bit too hot. I view the utility of DIL with 30% or more having a max depth of around 80'. If I were going "custom mix" I'd probably target something with 25% oxygen for 100' dives; 25/25 is for some people a "standard" gas and actually does a decent job of covering PO2 and gas density. Personally, I just suck it up and use 18/45 for anything deeper than 80' and shallower than 160' when using a DIL-OUT; I really like having the helium on the rebreather and I don't mind the decompression.

 

[justinthedeeps]

There are also descents and depth changes near the MOD to consider. A diluent ppO2 of 1.2 can quite easily cause loop ppO2 to exceed that.

 

[rjack321]

There are also descents and depth changes near the MOD to consider. A diluent ppO2 of 1.2 can quite easily cause loop ppO2 to exceed that.

Depends how you run your unit. If you let your ppO2 drop when you come up on the tops of sawtooths then it won't spike when you re-descend. This happens more or less naturally on an mCCR, just don't add o2 for the small ascents, the orifice or needle valve will maintain your new 1.0 ppo2 and it'll rise again to 1.2 or whatever when you drop back down on the next saw drop.

eCCR obviously you need to have your setpoint adjusted lower to avoid all the O2 injections and resulting spikes on the next descent.

"Constant ppO2" doesn't really have to be as constant as people make it out to be. Your deco ends up being based on averages anyway and a little shallower with lower ppO2 ends up being pretty similar deco wise as a little deeper with a slightly higher ppo2