Quote: (Originally Posted by
Dave Sutton)
I respectfully disagree. Pick a depth: I'll pick 250 feet. Deep enough to be deep but not scary deep:
21% diluent = 1.8 PP02
US Navy LAR-V 02 limits for 1.8 PP02 = *essentially infinite duration* (IE: 240 minutes, or *4 hours*)
If I flush my loop with diluent at 250 using that mix it'll be *leaned out* to 1.4 in about *4 minutes*..... (and note that the time base for this "burn off" changes with loop volume, if you purge to CLVmin it'll slide down faster than if you inject more gas into the counterlung)
Let's see, that's an hour of time available against each minute of time used... I'll take that bet!
Bottom line is that there is *utterly no reason* to EVER dive a mix leaner that 21% shallower than 250 feet.
Let's take it to the boolean logic string:
(1): There is *no way in hell* I'll get hurt with that mix *ever* as the time spent on that mix at depth is *either open circuit in which case I'm blowing thru it fast* or closed circuit *where I am decreasing the PP02 fraction thru metabolism* is *FAR shorter* than the time it would take to even come close to acute 02 tox time. And by meaning FAR SHORTER I'm talking *hours* at 250 feet.
AND
(2): I can use it as a VERY EFFICIENT SCC mix for bailout modes.
AND
(3): It is FAR BETTER as a basic deco mix either SCC or Open Circuit than a 10% mix.
AND
(4): The alternatively proposed 10% mix is LETHAL near the surface in SECONDS.
AND
(5): 10% makes a poor choice for a SCC gas for "on loop" bailout in case ya'll loose your 02 for some reason.
AND
(6): 10% dil makes a *piss poor* deco gas open circuit.
THUS
(7): Thus there is *no rationale* for using a 10% mix for this dive. None. No how. No way. Zero. Nada. Zippo.
END
That's not opinion, it's fact.... makes no difference if you carry bailout or not. 10% dil selection for this sort of dive is *just plain wrong* and I don't care who teaches that it's OK. They are wrong.
The only operational issue with this mix is that you need to purge the loop once on descent (or after reaching bottom), assuming that you allowed the loop to go to higher than 0.21 on the surface. I'll take on that workload in return for the advantages. With my Mark-15 it's easy as I jump in with the rig turned off and it's not trying to inject 02 until I turn it on. I don't turn it on until I hit the bottom anyhow. Makes my purge depth irrelevant. If you dive a rig with "auto-on" power, purge after your depth = your depth for setpoint = diluent PP02 at that depth, assuming that you are diving deeper than where PP02 (setpoint) = PP02 (diluent).
Educated Opinion: Where the dive training industry has *gone wrong* is that it has TOTALLY failed to distinguish in training between the HUGE time difference (MANY orders of magnitude) between the time it takes to have a problem due to high PP02 versus low PP02.
This is what I teach:
(1): *Respect* high PPO2 and do something about it *In good time*.
(2): FEAR low PP02 'cause it's gonna kill you *RIGHT FRIGGIN' NOW*.
Oxygen is life. Fear the gas that carries little.
Dave
I am a little confused at your reasoning. I was taught (and in all honesty it does make sense) that a rebreather is nothing more than a mixing station on your back. You sorta have an open circuit best mix mentality.
What about your OTU's. Doing multi day diving between 1.4 and 1.8 does have disadvantages too. While we probably will never come close to OTU limits; do say 3 hrs a day from 1.4 to 1.8 and need a chamber ride on the 3rd day there is a good chance they wont admit you. Right wrong or indifferent that is the way it is.
There is always the whole picture and not just a small piece of the puzzle when considering a dive plan, and your prior posts suggested that when it came to buddy diving /solo diving cave diving techniques... different circumstances depend on procedure.
I feel 10% is a good mix for dives deeper than 150' for a few reasons I can keep a gas at 1 to 1.3, I can easily do a flush and bring down an elevated PPO2, and I can do multi day diving and still keep my OTU's at a manageable level if god forbid I need a chamber ride. And you said it best in another post "Dr. Hamilton is gonna cry when he reads this" More like scream and cuss is a better analogy