View Full Version : Megladon Rebreather - Toxicity Query



Major Clanger
30-04-12, 09:05 PM
As all will be aware that own a meg, the O2 feed is to the inhale lung. There is talk about a possibility that a faulty valve could allow the injection of low levels of oxygen in to the inhale lung, the build up of which was undetected by the cells as breathed straight to lungs, casuing 02 toxicity. I emphasise that this may just be speculation. If so, what can be done to avoid this. I was thinking of feeding the O2 hose to the offboard valve on the exhaust lung or having a freeflow type switch and leaving it off unless needing manual inject. Thoughts?

NotDeadYet
30-04-12, 09:22 PM
Would it have been undetectable if it had been a leak? You wouldn't be burning enough O2 for the loop to appear normal by the time it reaches the cells surely? Admittedly you might not be getting particularly well mixed gas on each inhale so I can see why that would be bad. Genuine question, I've never been near a Meg.

mala
30-04-12, 09:30 PM
run the set point low and add manual to get what you want.
if you dont need to add you will know that there is extra o2 leaking in.

i cant see the 02 level being any different if you plug in to the exhale lung-just makes the hose run more complex.

Major Clanger
30-04-12, 09:32 PM
Third hand information. I couldn't say for certain. The guidance for manual injection advises short bursts to avoid this type of thing happening. As for the possibility of it occuring due to a faulty valve, I dunno. The person telling me seemed to think that the inhale lung basically was full of O2 that the diver was breathing straight off.

Major Clanger
30-04-12, 09:34 PM
run the set point low and add manual to get what you want.
if you dont need to add you will know that there is extra o2 leaking in.

i cant see the 02 level being any different if you plug in to the exhale lung-just makes the hose run more complex.

I think the point they were making was that the cells couldn't detect the high O2 as it was in the inhale lung or diver. Plugged in to the exhale lung, the gas would pass the cells before breathed in so changes more detectable in this scenario. Dunno, just mulling.

nigelH
30-04-12, 09:43 PM
I would say no.
A slow bleed would cause the loop ppO2 to rise slowly. Remember that you only take in the O2 you need to bring your body up to the loop level.
You exhale the rest so it goes round again and gets to the cells. If you were inhaling a high level you will exhale what your blood is at so the cells will be reading that within a breath or two.
Also the lag, even at quite high ppO2s, before symptoms hit is reportedly quite large so the situation must be sustained for quite a while.
Again big leak would be felt inflating things long before it hurt you - rebreather divers are very bitchy about buoyancy.
Something else is to blame so fixing that won't help.

Major Clanger
30-04-12, 09:50 PM
I would say no.
A slow bleed would cause the loop ppO2 to rise slowly. Remember that you only take in the O2 you need to bring your body up to the loop level.
You exhale the rest so it goes round again and gets to the cells. If you were inhaling a high level you will exhale what your blood is at so the cells will be reading that within a breath or two.
Also the lag, even at quite high ppO2s, before symptoms hit is reportedly quite large so the situation must be sustained for quite a while.
Again big leak would be felt inflating things long before it hurt you - rebreather divers are very bitchy about buoyancy.
Something else is to blame so fixing that won't help.

That helps put my mind at rest. I thought the leak would be detectable but couldn't quite see how.

philsiswick
30-04-12, 09:52 PM
I think the point they were making was that the cells couldn't detect the high O2 as it was in the inhale lung or diver. Plugged in to the exhale lung, the gas would pass the cells before breathed in so changes more detectable in this scenario. Dunno, just mulling.

NDY's point was that the high PO2 gas from the inhale lung would still be passed over the cells on the next breath, as you only metabolise some of the O2 you breathe in. Therefore, the higher than expected PO2 should have been obvious.

I do find myself wondering whether current limited cells or a very high set point that meant the diver was close to a tox anyway could have played a part. In the latter case, injecting into the inhale lung could have raised the PO2 to a suitable level, I guess.

I prefer the idea of injection on the exhale side, or into the head, as both the Inspo and JJ do, but I had understood that the difference was negligible if everything else was right (good cells and sensible set point).

Major Clanger
01-05-12, 07:15 AM
NDY's point was that the high PO2 gas from the inhale lung would still be passed over the cells on the next breath, as you only metabolise some of the O2 you breathe in. Therefore, the higher than expected PO2 should have been obvious.


I would also expect this to be the case and also think that there may be other factors at play. The manual O2 inject is below the loop and the person telling me appeared to be saying that a low level weep had caused a pocket of O2 to collect in the bottom of the inhale lung but, as mentioned, even breathing this for a short time, I would expect O2 levels to still be sensed quickly. Have to keep a listen out to see if more details become available.

matthewoutram
01-05-12, 09:49 AM
This is one of the reasons I don't like the Meg - the O2 coming in on the inhale side is a hazard. However I had thought it was only a hazard for using the MAV (although I confess I didn't study the set-up in minute detail).

If you are not diving at MLV (and a leak would make it very difficult to do so anyway) then perhaps the oxygen can build-up. But why would it not pass the cells - surely after one breath the expired gas is en-route to the cells (NDY's point)? I have no clue how many breaths of high ppO2 gas are actually needed to cause a problem, I do know it is less in the presence of CO2, but I have no absolute numbers.

Matt.

Major Clanger
01-05-12, 10:25 AM
Found a bit more information elsewhere. Incident was specifics of incident removed. I think the cause may still be under investigation so probably best to draw a line under this thread to avoid speculation and unnecessary distress to anyone specifics of incident removed. Sorry, should have checked further before posting. Mods please remove if inappropriate.

Mal Bridgeman
01-05-12, 10:33 AM
Mods please remove if inappropriate.

So long as the thread concentrates on the mechanics of the rebreather and how best / safely to operate it, I see no issue. If the thread strays into a discussion of the specifics of the incident to which you refer, then I think it should move to the Incident forum.

Either way speculation about that particular incident is inappropriate.

HTH
Mal

EBT
01-05-12, 10:42 AM
To be honest, I dont see a leak into either lung causing an issue. Your next breath would cycle the loop gas past the sensors and make it clear what the problem was. For the O2 to get high enough in 1 breath would require a massive leak (ie you'd hear or feel it) and some very unlucky numbers on satans dice roll. I've had a reasonably fast injector leak on the kiss, felt the bouyancy go a bit wierd then listened and heard the injector noise was slightly different.

The whole 'inject in small bursts' line is about avoiding a massive slug of high PO2 gas. I seem to remember leon preferred injection in the inhale as a rapid recovery technique for a hypoxic loop.

anecdotally, when I do a big inject on the kiss (6m flush etc) I see the slug of gas pass the sensors and spike them wildly, it smooths out over a few breaths as the loop gas mixes. ignoring sensor placement for the moment, it'd be much the same on all units. You can have a lot of fun watching the 'experts' faces when they watch your displays as you do this (deco is boring, got to get amusement somehow).

whaam68
01-05-12, 07:51 PM
Although sadly I have been unable to dive for 2 years for health reasons I did several years on a classic Kiss and latterly a couple on a COPIS meg and the missus mentioned this thread. At first I was concerned about manual o2 injection into the inhale lung but in practice it wasn't a problem even on the deeper dives. As other posters have said the gas is constantly and fairly quickly mixing as you breathe, the adv fires, opv dumps etc and spiking quickly shows on the displays post inject......if it really worries you move the manual inject hose to the exhale lung as some meg divers do.

I would be more concerned about Checking the seal on the internal o ring in the cell carriage as if not maintained it poses a real risk of bypassing the stack....which is IMHO a bit of a strange design....or probably the quality of cells available these days....

Slightly OT but Im not a massive fan of the emphasis in the meg training which stresses locking the adv When no bov is fitted either....it's obvious when the adv is going to fire as you can feel it well before it cracks....Being used to the Kiss I forgot I had locked it closed on one of my early dives and stupidly nearly killed myself in 6m...Which was educational ;o) anyways I thought it safer after that to actually have access to dil when I needed it.....

Anyway I doubt the "pocket of O2" was the actual problem but obviously we will never fully know know. All units have their foibles but I would say when properly maintained A meg is as "safe" as any other unit.

Mike

Major Clanger
01-05-12, 08:00 PM
I would be more concerned about Checking the seal on the internal o ring in the cell carriage as if not maintained it poses a real risk of bypassing the stack....which is IMHO a bit of a strange design....or probably the quality of cells available these days....



It does seem to be a weakness that a fault in the seal or a cracked sensor case can create a path.

Major Clanger
02-05-12, 07:30 AM
Not sure what other rebreathers do but on the Meg does anyone see any problems if an inline shutoff is connected between the O2 MAV and hose, leaving it off unless needed? Would help reduce any problems if an inflator sticks open or weeps unknowingly.

matthewoutram
02-05-12, 08:07 AM
Not sure what other rebreathers do but on the Meg does anyone see any problems if an inline shutoff is connected between the O2 MAV and hose, leaving it off unless needed? Would help reduce any problems if an inflator sticks open or weeps unknowingly.

I'd never do this. The last thing you want at the point of hypoxia is no access to O2. I agree with Mike above, it sounds very unlikely that this fault was the culprit.

Matt.

daviem
02-05-12, 08:15 AM
Not sure what other rebreathers do but on the Meg does anyone see any problems if an inline shutoff is connected between the O2 MAV and hose, leaving it off unless needed? Would help reduce any problems if an inflator sticks open or weeps unknowingly.

Although you should never be in the situation of being near hypoxic, as we are trained to monitor the PO2, should you find yourself in that situation, a couple of seconds to open the isolator on the O2 MAV could mean the diference between life and death.

If you are worried about the MAV leaking, make sure you do your high pressure test before every dive, and service the MAVs regularly. This should prevent any issues occuring.

matthewoutram
02-05-12, 08:18 AM
Being used to the Kiss I forgot I had locked it closed on one of my early dives and stupidly nearly killed myself in 6m...

I agree this is a potential killer. I always open mine and check it (breath, hear, watch gauge does not fall, OC style). I do the same on the O2 side with the MAV too. Pre-dive checks are uber critical, much more so than OC. I'd never get in the water if without doing them and I'll never kit up if stressed.

Matt.

EBT
02-05-12, 08:20 AM
You already have 2 shut offs....disconnect the inflator hose, or close the tank valve. Adding more kit just adds to the stuff you need to test on each dive and adds to the list of things to service.

Timw
02-05-12, 08:22 AM
I wouldn't add a shutoff there either. Keep the MAV clean and lubricated and it should behave. If you think it is passing gas, switch the hose to the MGB on the other lung. Unless you put a great long squirt of O2 in at depth, I can't see having the O2 on the inhale lung being a problem - and why would you do that? If you watch the PO2 after injecting, it takes a few breaths to settle so I wouldn't think a single pass through the scrubber would make a significant difference if the O2 was on the exhale side.

matthewoutram
02-05-12, 08:52 AM
You already have 2 shut offs....disconnect the inflator hose,

I wouldn't do that as the first port of call - I guess it depends on the MAV, but the Inspo one is open to the water if you disconnect due to free-flow (are they all the same?). I'd always switch off the cylinder and then figure out the problem.

matthewoutram
02-05-12, 08:54 AM
Unless you put a great long squirt of O2 in at depth, I can't see having the O2 on the inhale lung being a problem - and why would you do that? If you watch the PO2 after injecting, it takes a few breaths to settle so I wouldn't think a single pass through the scrubber would make a significant difference if the O2 was on the exhale side.

The amount is quite small at larger depths - try it at 80m if you want to frighten yourself.

Major Clanger
02-05-12, 08:59 AM
I definately wouldn't disconnect the inflator hose either unless absolutely necessary. It's difficult enough to connect on land let alone with the contortions it'd require at depth.

So the consensus is not to have a shutoff incase the loop was going hypoxic. Playing devil's advocate, surely monitoring would pick this up before then, also, if it went hypoxic and needed to be responded to hyper-quickly with no time to switch the cutoff on, what about a dil flush or BOV bailout and then sort it?

EBT
02-05-12, 09:06 AM
I didnt specify an order, just listed some options that already exist. It might just be me, but i think bolting extra kit on is an obvious answer and the less obvious downsides often get missed.

Major Clanger
02-05-12, 09:08 AM
For Meg owners, my query about this, and countermeasures, stems from a rebreather facebook thread on the meg that's running about the possibility of excessive ppo2 in the loop, due to a leaking O2 hose, before the cells detect it. Still speculative, but interesting nevertheless.

Major Clanger
02-05-12, 09:10 AM
I didnt specify an order, just listed some options that already exist. It might just be me, but i think bolting extra kit on is an obvious answer and the less obvious downsides often get missed.

I see that. Going through my mind isn't so much what it'll achieve but what knock-ons would it introduce that's less than immediately obvious.

matthewoutram
02-05-12, 09:10 AM
I definately wouldn't disconnect the inflator hose either unless absolutely necessary. It's difficult enough to connect on land let alone with the contortions it'd require at depth.

So the consensus is not to have a shutoff incase the loop was going hypoxic. Playing devil's advocate, surely monitoring would pick this up before then, also, if it went hypoxic and needed to be responded to hyper-quickly with no time to switch the cutoff on, what about a dil flush or BOV bailout and then sort it?

Hypoxia doesn't have a lot of symptoms, and the ones it does have are not so useful. Try the video below from, 6m46s to see what I mean.

The scenario I'm thinking off where you survive hypoxia is the one where your buddy pushed the O2 button.

Matt.


http://www.youtube.com/watch?feature=player_detailpage&v=JqoansxbLT4

EBT
02-05-12, 09:11 AM
So the consensus is not to have a shutoff incase the loop was going hypoxic. Playing devil's advocate, surely monitoring would pick this up before then, also, if it went hypoxic and needed to be responded to hyper-quickly with no time to switch the cutoff on, what about a dil flush or BOV bailout and then sort it?

Wouldnt the same devils advocate argument apply to your high O2 situation also? Maybe its a personal thing but I find hypoxia more worrying than hyperoxia.

Its a good debate, but I think this 'undetectable o2 leak' facebook thread is a fairy tale. if you're breathing the loop the gas is moving past the sensors. The only way you'd get a loop buildup of o2 is if the loop isnt circulating (you can see this on bailout drills with leaky orifice ccrs when they bail back on).

As an idea of some of the risks;

- Shutoff valve fails open, because you're lazy and dont test it
- Shutoff valve slides shut. You dont notice it until you're in the shit with hypoxia and are using the last of your wits to hit the inject button..... are you sure you could diagnose it? (for that reason I quite like the swaglock quarter turn valves which are obvious to all as to their position)

I have a very jaded view of kit/divers. I tend to assume kit will break and divers (me included) are stupid underwater. Of course that influences my kit choices, but in this case it seems like you're adding complexity to solve a problem that doesnt exist. You've got multiple other options already, depending on how serious the leak is and how committing the dive is.

Amidst all this clever stuff, theres still the option to swear your arse off, shutdown the O2 and bail.

Major Clanger
02-05-12, 09:16 AM
There's an old adage that if it ain't broke, don't fix it. The potential problem here is not being able to detect something's broke in the first place. On the face of it it a leaking O2 hose should be detectable but...

Major Clanger
02-05-12, 09:19 AM
Wouldnt the same devils advocate argument apply to your high O2 situation also? Maybe its a personal thing but I find hypoxia more worrying that hyperoxia.

I agree that hypoxia is more worrying but this is an issue about euqipment failure that may have caused undetected hyperoxia.

matthewoutram
02-05-12, 09:19 AM
I see that. Going through my mind isn't so much what it'll achieve but what knock-ons would it introduce that's less than immediately obvious.

This is a good way to think, IMHO. And I also agree adding kit to solve problems is the wrong thing to do.

The only extra I've added is a bottle of O2 and a whip (no reg). That's the only thing I cannot manage without. My plan (such as it is) is that I dive the same config every dive, all depths. I'm totally familiar with everything and I always do the same things every time, without hesitation. As I said, its is not much of a plan as there is not much to it. I think by people like you asking these questions it gets others thinking about how they would manage it.

High PPO2: stop, check cells. O2 off at cylinder. Dump o2 from MAV to OPV. Cross-armed DIL flush. Check cells - do we trust the cells? (Let's assume we do as this check is a continuous check throughout the dive). Open and close O2. Check cells. OK? Continue. Not OK (still high) then O2 stays off. Upright-position. Unplug on-board O2, plug in off-board O2. Confirm off board is on. MAV to SP. Continue manual.

Matt.

matthewoutram
02-05-12, 09:20 AM
I agree that hypoxia is more worrying but this is an issue about euqipment failure that may have caused undetected hyperoxia.

Nevertheless the point is still interesting as by fixing one problem you could indeed create another, perhaps one that is worse...

Major Clanger
02-05-12, 09:26 AM
Leave as is then and extra vigilance on the pre-checks, especially when monitoring the spg for any drop in pressure...

Major Clanger
02-05-12, 09:28 AM
Thanks, so far, for no-one jumping down my throat :)

EBT
02-05-12, 09:41 AM
Leave as is then and extra vigilance on the pre-checks, especially when monitoring the spg for any drop in pressure...

On the occassion I had an O2 leak, it wasnt the SPG that gave the game away. If Im honest, the diagnosis went like;

"hmmm, bouyancy is wierd... what the f*ck is going on", that triggered me to stop and do a quick self check, which is when I noticed the injector noise and at about the same time saw 1.5ish on my PO2 displays. I did the quick nose dump and sucked to trigger the adv then carried on swimming in (it was a cave dive) and rechecked PO2. It was high again, so this time I stopped and just watched it over a few minutes, "yep... its climbing... must be an o2 leak". Given the way the kiss is laid out the only option for me then was to feather the tank valve. So far so good, now is when I was less than smart. I had shitloads of bailout (so did my buddy) and it was a relatively shallow dive....so i carried on the dive.

What did I learn?

1. keep shit simple so its easy to diagnose.
2. 'something feels wierd' is just as valid an alarm as something beeping.
3. never underestimate your ability to rationalise a reason to carry on the dive.

If Im honest I'd probably do the same today, on the basis that I was carrying more than double the bailout I needed and was in there with someone else who was similarly equipped.

EBT
02-05-12, 10:02 AM
ps. timely post on another forum.

APD Solenoid Shut Off Valve - CAUTION - Rebreather World (http://www.rebreatherworld.com/inspiration-evolution-rebreathers/42585-apd-solenoid-shut-off-valve-caution.html)

look at all the layers of kit added to fix the initial problem, each requiring checking and bringing their own risks. *IF* the shut off was really needed why not have a nice obvious quarter turn burn valve (eg like a swagelock sk series), or find a valve that locks into position....

Heres a quarter turn valve than Ron M put on his home-buggered draeger (years ago, just shows theres nothing new...)

http://www.tmishop.com/Gen1_files/image032.jpg

matthewoutram
02-05-12, 10:34 AM
ps. timely post on another forum.

APD Solenoid Shut Off Valve - CAUTION - Rebreather World (http://www.rebreatherworld.com/inspiration-evolution-rebreathers/42585-apd-solenoid-shut-off-valve-caution.html)

look at all the layers of kit added to fix the initial problem, each requiring checking and bringing their own risks. *IF* the shut off was really needed why not have a nice obvious quarter turn burn valve (eg like a swagelock sk series), or find a valve that locks into position....

IMHO this is a crazy solution to a problem that can simply be managed by taking a spare bottle of O2 and a whip.

Matt.

EBT
02-05-12, 10:47 AM
Agreed entirely, hence the *IF* ;)

On a leaky orifice ccr the quarter turn valve can be appropriate for other reasons.....

Major Clanger
02-05-12, 11:16 AM
Liking the spare O2 and whip solution.

Paul r s
02-05-12, 12:31 PM
Gary that's what I was practicing last time we was diving with the additional O2 cylinder.

Major Clanger
02-05-12, 01:39 PM
Gary that's what I was practicing last time we was diving with the additional O2 cylinder.

Looked bulky, does it fit in the new box?

Paul r s
02-05-12, 01:54 PM
Only bulky cos it's a 1.5L. Get a .85L and attach it to your back plate.

matthewoutram
02-05-12, 02:05 PM
http://i1214.photobucket.com/albums/cc497/MatthewOutram/YD/259092_2186796751300_1288475652_32640253_34142_o.j pg

Major Clanger
02-05-12, 05:14 PM
http://i1214.photobucket.com/albums/cc497/MatthewOutram/YD/259092_2186796751300_1288475652_32640253_34142_o.j pg

Don't know much about the inspritation. I assume the white cylinders are both three litres, one a spare O2, is the other a small bailout/spare dil.

gobfish1
02-05-12, 05:26 PM
http://i1214.photobucket.com/albums/cc497/MatthewOutram/YD/259092_2186796751300_1288475652_32640253_34142_o.j pg

think i know that diver ,, lol you still only got wips on the drop tanks matt ,,:zip:

Gtzav
02-05-12, 05:39 PM
As all will be aware that own a meg, the O2 feed is to the inhale lung. There is talk about a possibility that a faulty valve could allow the injection of low levels of oxygen in to the inhale lung, the build up of which was undetected by the cells as breathed straight to lungs, casuing 02 toxicity. I emphasise that this may just be speculation. If so, what can be done to avoid this. I was thinking of feeding the O2 hose to the offboard valve on the exhaust lung or having a freeflow type switch and leaving it off unless needing manual inject. Thoughts?

i did move my O2 feed to the exhale lung shortly after getting my Meg.
This has been discussed exnensivly in the past and I think ISC is still firmly insisting that inhale is better.
The two alternatives have pros/cons.
A. Main advantage of having it on inhale is that in case of "near hypoxic event " you add and breath the O2 immediately (this is more applicable near the surface of course).
B. Main disadvantage of having it on inhale is that in case of stuck O2 manual button (that is something not unheard of - I have personally heard many Meg divers reporting a stuck O2 button) or a slow leak at great depth could lead to a very highinspired PO2.

As i have experienced a O2 stuck button at a very bad moment (I realized it by the buoyancy change) I realized that had this occurred at 100+ meters I would have a very very large PO2 spike I decided to move the hose to the left.
At the same time I always always run my loop 100% O2 at the beginning and end of each dive from 6m and up so argument A. is not so strong for me at least...

but I guess this is a personal choice ....

gobfish1
02-05-12, 05:44 PM
i did move my O2 feed to the exhale lung shortly after getting my Meg.
This has been discussed exnensivly in the past and I think ISC is still firmly insisting that inhale is better.
The two alternatives have pros/cons.
A. Main advantage of having it on inhale is that in case of "near hypoxic event " you add and breath the O2 immediately (this is more applicable near the surface of course).
B. Main disadvantage of having it on inhale is that in case of stuck O2 manual button (that is something not unheard of - I have personally heard many Meg divers reporting a stuck O2 button) or a slow leak at great depth could lead to a very highinspired PO2.

As i have experienced a O2 stuck button at a very bad moment (I realized it by the buoyancy change) I realized that had this occurred at 100+ meters I would have a very very large PO2 spike I decided to move the hose to the left.
At the same time I always always run my loop 100% O2 at the beginning and end of each dive from 6m and up so argument A. is not so strong for me at least...

but I guess this is a personal choice ....

only way to dive a ccr m8 ,, good point ,,

Major Clanger
02-05-12, 05:55 PM
Excellent replies throughout and plenty to think about. I'm minded to change the O2 hose to the exhale cl and use an extra O2 bottle and whip.

nigelH
02-05-12, 06:07 PM
As i have experienced a O2 stuck button at a very bad moment (I realized it by the buoyancy change) I realized that had this occurred at 100+ meters I would have a very very large PO2 spike I decided to move the hose to the left.
At the same time I always always run my loop 100% O2 at the beginning and end of each dive from 6m and up so argument A. is not so strong for me at least...
Hang on.
I'm having difficulty working out how the point in the loop where you inject too much O2 make any difference other than a slight delay in it getting to you. One of the things about gases like this is they don't actually mix much, they just march round like a platoon of solders doing drill. Even going through the scrubber the forward progress is pretty even. Making slow moving gases mix is actually quite difficult.

Timw
02-05-12, 06:12 PM
The big question for me is whether there is really a big difference in ppo2 between an o2 leak when fed the inhale or exhale lungs. Is there any empirical evidence either way?

I have my off board split to provide my deep bail out gas as dil if I need it - that feeds my exhale lung.

I carry a 1L O2 on my backplate on the same side as my Main O2. I run a longer blue hose down the inside of the lung so it is easy to identify if I need it. I've swapped hoses to the manual add at 80m as practice and it is pretty easy, even in 5mm gloves.
I added this after Charley lost his O2 when his opv failed.

matthewoutram
02-05-12, 09:04 PM
Don't know much about the inspritation. I assume the white cylinders are both three litres, one a spare O2, is the other a small bailout/spare dil.

Yup, they're both 3L, the one on my left is suit-feed (air) the other is O2.


think i know that diver ,, lol you still only got wips on the drop tanks matt ,,:zip:

Yes Steve, just a whip and an OPV on those ones. I think that's the Zephyr off Plymouth (one of my many not-the-stockforce dives).

Major Clanger
02-05-12, 09:46 PM
Hang on.
I'm having difficulty working out how the point in the loop where you inject too much O2 make any difference other than a slight delay in it getting to you. One of the things about gases like this is they don't actually mix much, they just march round like a platoon of solders doing drill. Even going through the scrubber the forward progress is pretty even. Making slow moving gases mix is actually quite difficult.

The difference I think is in how long the cells take to indicate the higher ppo2 level if a fault develops maybe, which becomes increasingly significant with greater depth. Problems with the o2 feed on the inhale side should be detected by the cells but gas is directly inspired beforehand, whereas problems with an O2 feed on the exhale lung will be detectable by the cells before inspiring gas. The time difference may be slight but the impact of that time differential increases with depth. The flip side is that a hypoxic problem may take a little longer to address with an O2 inject to the exhale cl. That's how I read it.

Clearly there may be other indicators of a problem, such as buoyancy issues but in this scenario we're looking at less than easily identifiable O2 feed problems and the possible impact. Much hair splitting I know. I'm coming to the conclusion I'd be more comfortable switching the o2 feed to the exhale cl. What I don't know is what other make of breavers also feed the o2 to the exhale cl and the thinking behind it for them.

nigelH
03-05-12, 06:12 AM
The time difference may be slight but the impact of that time differential increases with depth.
Definitely no.
Your breathing rate in L/min does not change with depth nor does the loop volume hence the mean speed round the loop for any molecule does not change. There are just more of them.
Also bleeding an extra 'surface litre' of O2 into the loop changes the ppO2 of the loop by the same amount regardless of depth.

There may be some factor I haven't thought of yet but I'm struggling to find one.

Major Clanger
03-05-12, 06:45 AM
Definitely no.
.

I was thinking more with respect to the speed of onset of toxicity increasing with relatively higher ppo2 levels at increasing depth for the same mix.

petlowe
03-05-12, 08:07 AM
I was thinking more with respect to the speed of onset of toxicity increasing with relatively higher ppo2 levels at increasing depth for the same mix.

I am with Nigel on this one, decay and addition are perceptible both from sensing and buoyancy. Inhalation prior to the cells will only result in a breath or 2's delay while it moves through the exhale loop then the cells will see it. It's not like you took more than your 1LPM just because there is now more than that coming into the loop. On the rEvo during calibration I can see that a manual add on my exhale will take 2 breaths to reach my cells then another 3 to bottom out my lungs again.

This should be very easy to bench check if you want hard and fast proof.

PS. You can breath a PPO2 of 3 bar, just not for very long, long enough however for the cells to spot it and even a fairly inattentive diver to do so as well.

PPS. I know that this is the result of a rather worrying event but I would be looking more at causative factors and events that would have lowered his tox limits because surely a leaky reg is easy to diagnose after the event?

matthewoutram
03-05-12, 08:44 AM
I think we have no real clue how one particular individual may react at depth to high ppO2. The rest is speculation. If o2 leaks from the inhale side then it is directly inspired. At 60m this is a ppO2 of 7. I have no clue if one lung full of O2 at ppO2 of 7 may cause a problem but my gut tells me not to try it. I suspect that in some people on some days, perhaps with hypercapnia in the mix, that this could well be a problem. 7 is a big number for me in terms of ppO2.

I also have no clue if it is possible to breath the neat O2 from the failed injector, blow it into the exhale bag and have it mixed before it hits the cells. But either way having neat 7 bar ppO2 in my lungs sounds bad.

Matt.

gobfish1
03-05-12, 08:44 AM
i seem to look at the o2 feeds on my unit like this id rather have them open and free of sliding gizmos .
the odd,s of them (STUCK ON / slow feed of o2 ) is low ,, if they do go south i should notice it in 4 dif ways
im still not dead , (yet ),, o2 is my friend,

On the other hand if it all go,s north , ie no o2 comming in .. id only notice in 2 way,s or im dead ,,

seems like a no-brainer to me ,,, left or right ,,

NotDeadYet
03-05-12, 08:57 AM
i seem to look at the o2 feeds on my unit like this id rather have them open and free of sliding gizmos .
the odd,s of them (STUCK ON / slow feed of o2 ) is low ,, if they do go south i should notice it in 4 dif ways
im still not dead , (yet ),, o2 is my friend,

On the other hand if it all go,s north , ie no o2 comming in .. id only notice in 2 way,s or im dead ,,

seems like a no-brainer to me ,,, left or right ,,

I had a look at that thread on Burgerworld. That shutoff valve... It looks like something I'd built. Sounds like a brilliant idea, some valve that simultaneously shuts off the O2 to the solenoid and the MAV held together with a bit of yellow plastic. I cant see any downsides to that...

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petlowe
03-05-12, 08:59 AM
I also have no clue if it is possible to breath the neat O2 from the failed injector, blow it into the exhale bag and have it mixed before it hits the cells. But either way having neat 7 bar ppO2 in my lungs sounds bad.

Matt.

But that's a boom and is self evident from the aftermath, we are debating a dribble only. A boom is bad in this config because as you rightly say it's bail off, don't breathe but aren't we taught that anyhow?

matthewoutram
03-05-12, 09:13 AM
But that's a boom and is self evident from the aftermath, we are debating a dribble only. A boom is bad in this config because as you rightly say it's bail off, don't breathe but aren't we taught that anyhow?

I'm just saying it's dangerous to make these assumptions. In a relaxed diver with a low tidal volume we're talking figures as low as 500ml (the APOC thread gave us something about this at least). I agree that's more than a dribble, but is it enough to cause a problem? I don't know. That's my point.

Matt.

matthewoutram
03-05-12, 09:14 AM
I had a look at that thread on Burgerworld. That shutoff valve... It looks like something I'd built. Sounds like a brilliant idea, some valve that simultaneously shuts off the O2 to the solenoid and the MAV held together with a bit of yellow plastic. I cant see any downsides to that...

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There's another thread about making the block QD. Perfect for pushing salt-water into the solenoid. Disaster waiting to happen with that mod.

Matt.

Major Clanger
03-05-12, 09:22 AM
I think we have no real clue how one particular individual may react at depth to high ppO2. The rest is speculation. If o2 leaks from the inhale side then it is directly inspired. At 60m this is a ppO2 of 7. I have no clue if one lung full of O2 at ppO2 of 7 may cause a problem but my gut tells me not to try it. I suspect that in some people on some days, perhaps with hypercapnia in the mix, that this could well be a problem. 7 is a big number for me in terms of ppO2.

I also have no clue if it is possible to breath the neat O2 from the failed injector, blow it into the exhale bag and have it mixed before it hits the cells. But either way having neat 7 bar ppO2 in my lungs sounds bad.

Matt.

I'm thinking similar. In normal use I see no issues with using the MAV for manual operation. ISC instructions make it quite clear to use short bursts and time it with an exhale. It's the unquantifiable levels inspired if there's a fault with the MAV if not quickly identified at depth that's got my attention. Like I said, I'm moving the O2 hose to the exhale lung unless there are greater reasons not to.

EBT
03-05-12, 09:36 AM
I had a look at that thread on Burgerworld. That shutoff valve... It looks like something I'd built.

To be fair, I doubt it would've passed the NDY review team, given the lack of gaffa tape and cable ties ;)


Like I said, I'm moving the O2 hose to the exhale lung unless there are greater reasons not to.

Every rig has its foibles, but this 'o2 in the inhale lung' has always made me scratch my head. Then again I dont park my car on my bereaver, or turn into edward scissorhands on a dive, so i guess Im not the megs target market....

NotDeadYet
03-05-12, 09:39 AM
There's another thread about making the block QD. Perfect for pushing salt-water into the solenoid. Disaster waiting to happen with that mod.

Matt.

It really sounds perfect for me. Can I get some saltwater into my O2 cylinder as well? That's the clincher for me, if it's a yes I'm ordering.

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Major Clanger
03-05-12, 09:43 AM
Then again I dont park my car on my bereaver, or turn into edward scissorhands on a dive, so i guess Im not the megs target market....

I clearly was. Dropped it from a four foot high shelf within days of buying it.

johnkendall
03-05-12, 10:10 AM
Coming at this with not much in the way of CCR knowledge, but might one reaction of a leak into the loop be to breathe out of the nose to maintain buoyancy? If it's an O2 leak in the inhale lung then there would be a chance of the user breathing in a higher PO2, and exhaling at least part of it through the nose due to the increase in buoyancy. If he's exhaled part or all of the gas through his nose then the sensors will not see the increase.

However as I said, I've not got much knowledge about these things, so would welcome corrections to my assumptions.

Thanks
John

matthewoutram
03-05-12, 10:26 AM
Coming at this with not much in the way of CCR knowledge, but might one reaction of a leak into the loop be to breathe out of the nose to maintain buoyancy? If it's an O2 leak in the inhale lung then there would be a chance of the user breathing in a higher PO2, and exhaling at least part of it through the nose due to the increase in buoyancy. If he's exhaled part or all of the gas through his nose then the sensors will not see the increase.

However as I said, I've not got much knowledge about these things, so would welcome corrections to my assumptions.

Thanks
John

It's a fair comment John. And the more the leak the more likely the scenario you describe.

Matt.

Timw
03-05-12, 05:03 PM
Like I said, I'm moving the O2 hose to the exhale lung unless there are greater reasons not to.

The only reason I can see is that your dil and O2 are injected on the same side. I like to carry the "right rich" rule for all my gases. It also makes my off board o2 easy to route in parallel to my onboard hose.

A leaking MAV or solenoid won't give you a lungful of pure O2 in the space of one breath. I'd say that unless you watch your ppo2 at every breath, you wouldn't notice the difference between adding to the inhale or exhale side.

I also think it is easier to problem solve if the O2 feed is on the O2 side of my kit.