Anyone doing breathing down diluent diving? What is your maximum diluent po2?
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I'm willing to go as high as 1.2 (max) but I plan for 1.0
Sometimes you dont bring the right dil for the dive and have to make do. Cave diving it can be helpful to have a relatively hot diluent to avoid wild swings in loop ppO2 with a dil injection. 32% when diving a max of 100ft is actually fairly common cave diving dil/bo.
And when using BO as dil cave diving you dont want too low of a ppo2 in your BO because your bailed out exit will rack up crazy deco time should that happen.
Breathing down the O2 fraction to save helium makes absolutely no sense at all. You dont "save" or conserve helium by metabolizing the other gas.
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what @rjack321 said
I breathe down my diluent in a lot of dives because I usually dive the same gas for dil as I would if diving OC. I don't breathe it down to "save" helium, I breathe it down because I prefer to keep my loop ppO2 at 1.0-1.1, and since I use the "dilout" concept where diluent and bailout are on the same tank, that may well be at 1.4 if I'm close to the MOD of that mix.
I breathe down my diluent in a lot of dives because I usually dive the same gas for dil as I would if diving OC. I don't breathe it down to "save" helium, I breathe it down because I prefer to keep my loop ppO2 at 1.0-1.1, and since I use the "dilout" concept where diluent and bailout are on the same tank, that may well be at 1.4 if I'm close to the MOD of that mix.
Interesting. I was warned at course not to do this, because in case of high po2 diluent flush would be hard and slower.
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it's not "harder" or "slower", it's the exact same procedure. The ppO2 doesn't drop as fast because it isn't diluting as drastically, but you can dil flush most CCR's very quickly and I don't personally see any real benefit of "lean" diluents. Sure 1.0 is probably more convenient than 1.4 for things like dil flushes, in cave country we are not at the MOD of our gas mixes for the majority of the dive.
I think he meant that if using a DIL flush to manage a PO2 spike, the PO2 drop would be slower.
@Valyngar there's a difference in operational planning on a cave dive where you may have an extended exit with deco versus operational planning in OW. For OW CCR diving, I shoot for a 1.0 PO2 in my DIL and maybe bailout (bailout may be higher), for cave diving, I'm looking at a higher PO2 partially to add flexibility with SCR mode.
@Valyngar there's a difference in operational planning on a cave dive where you may have an extended exit with deco versus operational planning in OW. For OW CCR diving, I shoot for a 1.0 PO2 in my DIL and maybe bailout (bailout may be higher), for cave diving, I'm looking at a higher PO2 partially to add flexibility with SCR mode.
JonG1
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I tend to aim for 1.2 as a max on dil at MOD but I dive the in-board 3L as a current default, my bailout is then either matched to the dil or more commonly 1.3 at the MOD of the dil.
It's a balancing act though and TBF I am more concerned about straying into a hypoxic incident due to a low oxygen content at the surface, but still would have to think twice about a dil PPO2 over 1.2 personally at MOD.
There is a reasonable amount of evidence that a high PPO2 during a CO2 event can make a bad situation worse, which is why the 1.6 bailout mixes disappeared from trend, even though they minimised deco obligation.
It's a balancing act though and TBF I am more concerned about straying into a hypoxic incident due to a low oxygen content at the surface, but still would have to think twice about a dil PPO2 over 1.2 personally at MOD.
There is a reasonable amount of evidence that a high PPO2 during a CO2 event can make a bad situation worse, which is why the 1.6 bailout mixes disappeared from trend, even though they minimised deco obligation.
In most tourist caves the floor is a pretty known depth, and its not a sudden straight down drop. So you can fairly easily run a warm (o2 content wise) offboard diluent in the 1.2 range that also serves as your BO. Basically diving the diluent close to what you would do on OC.
Wreck diving with a straight down drop (or at least usually faster than a cave) has much more potential to spike O2 and you don't necessarily have huge volumes of onboard dil. So being able to flush down your ppo2 as you drop (since you want it high of the surface to avoid hypoxia) usually requires a much lower fO2 diluent which ends up being 1.0 or less by the time you get to the bottom. 10/50 is a very common wreck dil for instance (but makes for a terrible BO gas)
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