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mempilot · 17th April 2008, 22:59

Here's my thought process on this as a Meg owner:

I always run a DIL FO2 that gives me approximately a 1.0 PPO2 flush at max depth. At the recognition of low PO2 during the dive, not deco, I have a high-flow access to breathable gas that will bring up a low PO2 and bring down a high PO2 to a breathable level, no matter how long I push on the firing mechanism. If using a hypoxic DIL for the deep portion of the dive, I would plug in an offboard breathable gas to the MGB.

Whether you have the O2 manual addition on the inhale or exhale side, in a panic, you may put too much O2 in, and all you are waiting for at that time is a validation from your HUD or handset to tell you if you put in enough, not enough, or too much. In a low low PO2 situation, you may hammer the O2 too much thinking more is better in the panic situation. You can never add too much DIL. I will agree that plugging the O2 into the exhale CL makes more sense for this improper reaction, but I believe the ADV is the proper response to low or high PO2 at depth.

For running my unit manually, I think about it this way. What would the solenoid and injection timing do? If I'm running a 1.2 setpoint, and the PO2 drops to 1.1, the solenoid injects a short burst of O2, and then the computer waits to analyse that mix to see if it needs to add more. So, when flying manually, I do the same. I give a short shot of O2 from the manual add and then I wait to see the outcome. Multiple short bursts of O2 are much safer than one long hammer to maintain setpoint when flying manually.

On deco, where the setpoint is high and the DIL may be hypoxic (shallow), I use this same method of maintaining setpoint. I could either 'block' in offboard breathable DIL from a deco bottle to the ADV, or just plug in to the MGB. Deco can be done via onboard O2 and if a flush is needed, just use the ADV or MGB. This way, the procedure is the same, no matter what the depth and DIL, since an ADV flush would always provide a breathable mix.

Flying a unit manually comes with obvious advantages and some additional things to mind. The beautiful thing about the eCCR Meg is that you can set an 'alpha floor' injection, say .7, to keep you from coming anywhere near a hypoxic mix in the loop. This alleviates the need to ever hammer the O2 manual add until the 20 fsw flush. The short burst, wait, analyze, short burst, wait, analyze method stays valid.

The only time that hammering the O2 manual add on the Meg becomes valid in this mindset is during a 20 fsw flush for deco or cell validation. My muscle memory reflex is always ADV first for flushes, and short bursts of O2 for 'minor' adjustments.

I think this is where there is a fundamental difference between the Megalodon and other CCRs. With this setup, there should never be a reason for a slug of O2 to hit the intake side of the loop. Using a moderate setpoint and regular checks of the monitors, a diver should be able to identify and easily manage a creep from a leaking manual addition valve (easily correctable by a DIL flush).

This is my .02, and not neccessarily the way others will view it.

17th April 2008, 22:59	#10 (permalink)
mempilot What is this..terrafirma? Current Rebreather/s: Megalodon Other Rebreather/s: Megalodon Join Date: Aug 2006 Location: Ft. Lauderdale Posts: 1,089	Re: O2 slug Here's my thought process on this as a Meg owner: I always run a DIL FO2 that gives me approximately a 1.0 PPO2 flush at max depth. At the recognition of low PO2 during the dive, not deco, I have a high-flow access to breathable gas that will bring up a low PO2 and bring down a high PO2 to a breathable level, no matter how long I push on the firing mechanism. If using a hypoxic DIL for the deep portion of the dive, I would plug in an offboard breathable gas to the MGB. Whether you have the O2 manual addition on the inhale or exhale side, in a panic, you may put too much O2 in, and all you are waiting for at that time is a validation from your HUD or handset to tell you if you put in enough, not enough, or too much. In a low low PO2 situation, you may hammer the O2 too much thinking more is better in the panic situation. You can never add too much DIL. I will agree that plugging the O2 into the exhale CL makes more sense for this improper reaction, but I believe the ADV is the proper response to low or high PO2 at depth. For running my unit manually, I think about it this way. What would the solenoid and injection timing do? If I'm running a 1.2 setpoint, and the PO2 drops to 1.1, the solenoid injects a short burst of O2, and then the computer waits to analyse that mix to see if it needs to add more. So, when flying manually, I do the same. I give a short shot of O2 from the manual add and then I wait to see the outcome. Multiple short bursts of O2 are much safer than one long hammer to maintain setpoint when flying manually. On deco, where the setpoint is high and the DIL may be hypoxic (shallow), I use this same method of maintaining setpoint. I could either 'block' in offboard breathable DIL from a deco bottle to the ADV, or just plug in to the MGB. Deco can be done via onboard O2 and if a flush is needed, just use the ADV or MGB. This way, the procedure is the same, no matter what the depth and DIL, since an ADV flush would always provide a breathable mix. Flying a unit manually comes with obvious advantages and some additional things to mind. The beautiful thing about the eCCR Meg is that you can set an 'alpha floor' injection, say .7, to keep you from coming anywhere near a hypoxic mix in the loop. This alleviates the need to ever hammer the O2 manual add until the 20 fsw flush. The short burst, wait, analyze, short burst, wait, analyze method stays valid. The only time that hammering the O2 manual add on the Meg becomes valid in this mindset is during a 20 fsw flush for deco or cell validation. My muscle memory reflex is always ADV first for flushes, and short bursts of O2 for 'minor' adjustments. I think this is where there is a fundamental difference between the Megalodon and other CCRs. With this setup, there should never be a reason for a slug of O2 to hit the intake side of the loop. Using a moderate setpoint and regular checks of the monitors, a diver should be able to identify and easily manage a creep from a leaking manual addition valve (easily correctable by a DIL flush). This is my .02, and not neccessarily the way others will view it. __________________ MEM "Da Pilot" Black holes are where God divided by zero. "If at first you don't succeed, don't dive silent." "Would you mind not shooting at the thermo-nuclear weapons." ~ Vic Deakins "Donkey's kill more people annually than plane crashes."
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