O2 slug

NEDIVER

I was very torn in purchasing my first eCCR and as you can see I went with an evolution, which I am VERY happy with.

The Meg was my other consideration and still may be a second unit that I purchase in the future. I have a question about the addition of O2 with the Meg's, and who better to ask than current owners.

It is my understanding that the O2 addition is on the Inhalation counter-lung rather than the exhalation counter-lung. I was thinking about the thought process about this and was a little confused. Perhaps there is a benefit that I am not seeing.

If I manually add O2 (say, flying manually or maintaining high O2 during shallow deco) wouldn't I be inhaling almost pure O2 before it went around the loop and to the sensors? Would this cause problems at deep depths where O2 toxicity could occur with a slug of pure O2, should you have to fly it manually?

I am very interested in finding out what everyone thinks about my line of questioning. Believe me I am not defending another unit compared to the Meg, so I am hoping this will stay on the topic of my question.

Like I said, I am still thinking about a meg as a second unit, so all the help in getting me to understand the issue is greatly appreciated!

Thanks in advance for the responses.

__________________
-Tom-

"slow is smooth, and smooth is fast" TM
"Insanity is repeating the same thing over and over again, expecting different results" Albert Einstein

jkaterenchuk

I will sit on the side with interest to see if you get a more reasonable answer that seem to reflect the same detailed thought process that ISC put into the other parts of the unit. My posting was less than satisfying in this regard.

John

scuba_phil

Tom,
The standard for O2 input on the Meg is on the right hand side. On the left hand side, the Meg has a mixed gas bypass (MGB) valve. It is the same valve as the O2 inlet valve. The MGB is used to plug in off-board gas. Many Meg divers move the O2 hose to the MGB for the very reason you state. This then allows the old O2 input to be used as the MGB. The hose is long enough to reach. Point is that on the Meg, you have options - It is your choice - no extra cost!

Phil

jkaterenchuk

Direct from the source....

Hi John,

This is a fairly common question, at first glance injecting oxygen on the inhale side would seem counter intuitive. However, CNS is not a condition that "strikes" with a breath of pure O2, it is a combination of time and level of PO2. If your operating envelope is anywhere near normal PO2 range a CNS hit is not an issue from a an occasional injection of O2. On the other side of the envelope if your PO2 had dropped to dangerously low levels and you were on the edge of Hypoxia you may not have the 3 or 4 breaths left to pull O2 from the exhale side, through the loop to your lips before you pass out. The overall design of the Meg is based on crictical controls being assessable to either hand and the ability of a diver on the edge of unconsciousness from whatever cause to have the best chance of self rescue. As you mentioned it is easy to switch the O2 to the exhale side and when I dive a 4 cell lung that is what I do to maintain a stable reading on the 4th cell.

Best Regards,

Jerry Whatley
General Manager
InnerSpace Systems Corporation

UKSteve

As someone who always looks over the fence and likes to learn from others I found this a really interesting post. The subject has been debated before on here but I don't recall this rationale being mentioned before.. but I might of course simply be having a senior moment!!

It also made me think... what is the "right" response to looking at your handsets and seeing 0.20 0.18 and 0.19? Other than "Oh sh1t!!" I guess you have a choice - hit the manual O2 button or hit the manual Dil button (or suck hard and fire the ADV) .. but which?

I am trying to recall what (if anything) I was taught on my courses. Am away from home at moment - but my instinct is at depth, hit the dil button and dil flush and in the shallows - 6m or less, hit the O2 button. Based on that the rationale above only makes sense in the shallows .. but perhaps that is where you are most at risk of a sudden drop in ppO2?

Interested in others thoughts...

Steve

OceanOpportunity

This was one of the reasons that I chose against the Meg. I think the unit has tremendous build quality, adn a number of other nice features, but given the post, there is opportunity to look at both sides of the coin.

I am a strong proponent of ALL rigs following the 'rich on right, lean on left' mantra. Standardizing this, both OC, and CC supports our intuitive actions when switching from unit to unit, and mode to mode. That being said, the O2 button on the Meg is on the 'correct/right' side IMO. However, I dont like that it is on the inhale side of things. Moving it to the other lung, again IMO, complicates things as both manual adds are too close together. May not be an issue for the diver, but for a diver rendering assistance that isnt familiar with the unit, herein lies an issue.

My thought on adding O2 on the inhale side is of mixed opinions, but its mixed enough that I elected not to go with this route until I see some better data out there. My rationale, in general, which is only supported by hypothetical data, is that both gasses should be injected pre-scrubber (although controls should be on rt/left respectively) to aid in mixing/blending through this 'turbulence', and then pass over the cells. IMO, in ALL units, the cells should be the last thing gas passes before it enters YOU. This isnt the case with a number of units however (so I built my own ). Now of course there is a gap in this arguement as wel, that being that cells response time is relatively slow and they may not be caught up with each breath...but then this is an issue with all units anyway.

my $.02

__________________
Michael Lombardi
Oceans of Opportunity
www.oceanopportunity.com

Gear Reviewer, The Summit Journal

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Twain

I believe the standard response taught in Meg Mod1 for a low PO2 is a dil flush. The logic being that a dil flush with the high-flow ADV will get known and breathable gas to you before O2 injection would. And without the danger of spiking.

Sutty

I don't dive a Meg, or indeed an ECCR, but like to mentally kick around problem scenarios as I believe it helps develop a "mental toolbox" which can be useful in emergencies.
On an ECCR if you look at your handsets and they are "0.20 0.18 and 0.19" then several of things come to mind: If they are reading correctly then you are breathing a perfectly life-sustaining gas mix. If they are reading correctly your ECCRs O2 addition system has failed in some way. If they are all incorrect (some major problem with software, frozen display, etc) then you may have no idea what you are breathing - about to pass out from hypoxia, or have a hyperoxic convulsion.
Therefore my first response would probably be to bail out to BOV unless I was sure the displays were reading correctly to avoid hypoxia/hyperoxia, and then reassess. If no BOV then you are into a debate as to whether O/C bailout is quicker than breathing dil by triggering ADV. If firing ADV gets dil to you fast then that sounds like a good plan, progressing to dil flush to check cell function. I probably wouldn't immediately try to add O2 even if I believed displays were right as 1. I may not be judging it well if hypoxic, 2. If O2 delivery has failed it may not work! immediate realisation that O2 is turned off, and displays are right, would be the only circumstance I wouldn't go to breathing dil by whatever route - in this case no big rush as still on life-sustaining mix, turn on O2 and bring up PO2.

Just my thoughts - but then I'm an MCCR diver at present

Neil

__________________
Never forget that life is a finite resource.

jkaterenchuk

Regardless if we are talking about O2 injection as a reaction to Hypoxia or DIL Flush to combat Hyperoxia. I am wondering if anyone has done a mathematical reality check by looking at the loop volumes, lung volumes, gas absorption and blood circulation at minimum loop to determine the dwell time seen by the brain from point of recognition of the problem until physiological change would be evident?

My University Biomechanical Engineering classes were far to long ago for me to remember any of the specifics to take a cut at it myself. Just looking for a rough estimate. Are we talking <1 sec, a couple seconds or > 5 seconds?

Would be interested in how the time changes based upon inhale or exhale manual add locations?

John

mempilot

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.

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