02 Feed on exhale lung

trimixdiver

trimixdiver

Member
I have read story's about meg diver's changing 02 feed hose over to exhale Counter-lung with ADV, what's the pro's and con's for this change?
Thinking the 02 would be exposed to the cells faster....
 
trimixdiver said:
I have read story's about meg diver's changing 02 feed hose over to exhale Counter-lung with ADV, what's the pro's and con's for this change?
Thinking the 02 would be exposed to the cells faster....
Click to expand...

I had an instructor who was big on this. I started a thread on here and after the dust settled I have kept the O2 on the inhale counter lung.

Peter
 
I have the O2 feed on the exhale lung and have been diving it since training. My reasoning is that by adding on the exhale side the inhaled gas is truer to the readout on the cells. When adding on the inhale lung you are breathing in a much higher ppo than what the cells are showing.
Either is acceptable breathing in a high ppo for one breath surely is no great danger, so it is a personal choice as to which is within your comfort zone.
 
trimixdiver said:
What was your reason to switch sides Peter .
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I had about 150 hrs on my Meg with the standard set up of the O2 feed on the inhale lung when I undertook to do normoxic mix training.

Up here we have relatively cold water diving and most of us use some sort of dry gloves. When I switched the O2 feed to the exhale lung at the beginning of my course at my instructors insistence I found it very awkward. First off I was used to the feed on the right and would at first reach there. Next, the O2 feed was now in between the adv and the opv. With the clumsy gloves on I would feel my way down and around to find the correct button as opposed to reaching up to the only protrusion on the inhale lung.

The big reason my instructor gave for the switch to the exhale lung was the notion that a puff of O2 at depth was likely to cause O2 tox. As far as I can tell this is not true.

On bigger dives I like to use the MAV on the exhale lung for my redundant O2. I also use this O2 as a fall back suit gas. For me this is well set up as I unplug from the exhale MAV and into the suit nipple which is right there and pointed in the correct direction.

People have been doing big dives on the Meg for some time now and very few seem to feel the need to re engineer the rig.

For me, the stock set up is cleaner and has better options.

As for more accurate read outs, I don't turn myself inside out if the PO2 is not exactly on set point.

Peter
 
Exhale proponents: O2 mixes better to give a more accurate loop reading. And you don't want to take a big gulp of nearly pure O2 at depth if you manually inject O2 into the inhale lung.

Inhale proponents: There is no faster way of getting needed O2 in case of hypoxia than to put it right into the inhale lung. If pO2 drops, you want the shortest path between the O2 addition and the mouthpiece.

Bottom line: The Meg configuration has been used on many big, deep dives without any consequence of OxTox as far as I know. It probably doesn't make a bit of difference which CL gets the O2 MAV.
 
If the head was designed to have flow go the reverse way it would mimic many other Rebreathers and would eliminate the concern altogether about oxygen being injected prior to analysis and being consumed/breathed ultimately before it can be factored into deco etc. That being the other weird thing about the apeks not having deco.

I think it was interesting to hear Peter describe his instructor insisting on changing the intended location of oxygen add.


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Garth said:
If the head was designed to have flow go the reverse way it would mimic many other Rebreathers and would eliminate the concern altogether about oxygen being injected prior to analysis and being consumed/breathed ultimately before it can be factored into deco etc. That being the other weird thing about the apeks not having deco.

I think it was interesting to hear Peter describe his instructor insisting on changing the intended location of oxygen add.


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The logic on the meg is if you get hypoxic, you get instant o2. I can't see any ox tox concern unless you are hammering the o2 and often and for a long time. I went hypoxic once, it wasn't fun. It will hit you far far faster than an ox tox.

Having to wait for the o2 to get through the entire loop back to me wouldnt be very good.
 
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The gas mixes instantly. I can't recall the details of who, how and when but had this discussion years ago with Tom Mount who told me about some testing where someone put a sensor in the loop to confirm the gas mixed instantly upon addition.

IMO, the only time hyperoxia would be an issue is if you intentionally held the MAV down for a long time. A normal (short) burst of gas is of no consequence in that regard.
 
Joe Citelli said:
The gas mixes instantly. I can't recall the details of who, how and when but had this discussion years ago with Tom Mount who told me about some testing where someone put a sensor in the loop to confirm the gas mixed instantly upon addition.

IMO, the only time hyperoxia would be an issue is if you intentionally held the MAV down for a long time. A normal (short) burst of gas is of no consequence in that regard.
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This sound logical to me the mav does what it suppose to do .
 
Joe Citelli said:
The gas mixes instantly. I can't recall the details of who, how and when but had this discussion years ago with Tom Mount who told me about some testing where someone put a sensor in the loop to confirm the gas mixed instantly upon addition.

IMO, the only time hyperoxia would be an issue is if you intentionally held the MAV down for a long time. A normal (short) burst of gas is of no consequence in that regard.
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I'm not sure about this. What I'm basing my comment on is when I use to do partial pressure blending once I get through topping the tank off with air and check the mix I would get one reading. Roll the tank on the floor for a few minutes and take a reading I would be closer to my intended mix.


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I suspect the turbulence in the loop would make a big difference and that is why it mixes instantly.

From a scientific perspective gases do not "layer" as we like to believe but homogenize immediately when mixed. That said, I too have experienced the variation in readings when partial pressure blending. It probably has to do with the high pressure and the lower level of turbulence in the tank. It has been my experience that if you "slam" the gas in you get correct readings immediately.

SlowDiver said:
I'm not sure about this. What I'm basing my comment on is when I use to do partial pressure blending once I get through topping the tank off with air and check the mix I would get one reading. Roll the tank on the floor for a few minutes and take a reading I would be closer to my intended mix.


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I agree with Fred. When partial pressure blending, there is even a big difference in doing it with tanks vertical vs horizontal. The gasses do layer to some extent. Entropy will eventually cause it to become homogenized.
 
Interesting subject on gas homogenizing and maybe someone who has studied this will chime in. Yes Ken the tanks that I partial pressure blended were vertical, now use a stick so don't have that problem anymore.
Here's something else to think about concerning gas homogenizing. When a person enters a underground cable vault they immediately start checking for explosive gas pockets which can be located in any part of the vault. Now I know the air movement is low but the fact that same gas molecules like to accumulate together (gas pockets) is a real problem at lease for the men and women that have to work below the ground.


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The central issue in all of this seems to me to be the risk of an O2 tox event due to the brief exposure to an elevated PO2 in the loop.

The accounts I have read of folks toxing out seem to indicate that some time is required for a toxic reaction to happen. Perhaps at great depths the time required is much less than in the relative shallows?

As I see it the O2 injected into whichever lung is relatively quickly a part of the mix in the loop. If one blows it and the PO2 is out of the comfort range, we all know how to quickly fix it.

What matters most is having a simple as possible a rig that is robust and that you understand and can manage through issues.

I am a great fiddler and have tried all sorts of different things on my rig. Most often I have found that the addition of gizmos or alternative arrangements for routing of hoses etc. have not worked out for me. Th ergonomics of all of the modern rigs seem very well worked out to me and I have grown to have more respect for those who engineer these rigs and less time for we armchair experts.

Peter
 
My experience with partial pressure "layering" is similar to others, it does appear to happen and if you fill very slowly its actually worse. I partial pressured some 250cf storage bottles with my 3cfm compressor once. After filling they analyzed at 24%, 3 days later (they were vertical) they were 32%. I suspect its mostly due to differences in temperature, not inherent density differences between O2 and air.

As far as the O2 MAV on the inhale lung, I have not found a compelling reason to change this. Not hammering on the O2 MAV at depth is prudent of course. I've never heard of or seen it causing a problem.
 
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