Topic: psychosomatic hypoxia (Pilots)

[SkeptiKiwi]

I cannot imagine how a mask could be producing CO. They can cause retained CO2. It is my understanding that CO is only present as a product of incomplete combustion.

I have used Denitrogenation systems and they required a compressor to force the air (at about 300psi) through the molecular sieve or membrane that removes some of the N, the system I used only produced enriched gas at up to 40% O2.

If the compressor was poorly maintained or the wrong type of oil used then you could get CO produced but inline detectors are not rare in such systems. I'd be amazed if they did not have them in flight systems.

This system takes Bleed air from the compressors on the Jet and passes it through the system.  Compression ratios on these reach as high as 44:1.  Even at very high altitudes, this should provide sufficient compression to run these O2 concentration systems adequately, again provided they don't fail.

Regarding CO production.  I was only speculating.  I do not know the technical details of how these 'molecular sieve's' operate.

Aha - it has to be the lubricants in the air compressors.

j/k, but it reminded me of the care you have to take with diving equipment not to use the wrong o-ring lubricants, for example.

Well, to be honest, when I say compressors, Realize I'm talking about the spinning blades at the front of the engine.  There is no lubrication at the point where the air is 'bled' off.



In the above illustration, the 'bleed air' comes from somewhere in the blue area on the left.  The closer you are to the combustion chamber, the greater the pressure and thus the greater the bleed pressure.  There's no lubrication involved, all of that is either in the center sealed off, or further aft where it couldn't possibly affect bleed air.

I forgot they'd have all the compressed air they'd want from the engines.

I know how bad the maintenance costs are for a standard dive compressor, I'd hate to see what it is for a Jet..

[Citizen Skeptic]

I dive using a Rebreather and when diving deep use a hypoxic gas mix to keep the ppO2 below a safe level at depth.

For recreation?

Yeah for recreation. It would take the fun out of it to be paid for it.

 

That's very cool. Out of curiosity, have you fooled around with helium mixtures too? Is it the convenience of a rebreather that you prefer? It must be odd not listening to bubbles.

[SkeptiKiwi]

Yes Helium is essential remove the narcotic effects of Nitrogen at significant depths. It also reduces the work or effort required to shift the gas around the breathing loop. Your lungs do all the work in the system and high effort leads to increased co2 retention and risk of a hypercapnic event.

Of course Helium being much smaller than Nitrogen means that it on gasses into your body tissues faster leading to longer decompression schedules. But well worth it for the clear head and ease of breathing.

Typical mixes I use are (typically expressed as O2/helium)

21/30 for wrecks to a depth of about 45 metres. For shallower than 30 metres and non overhead environments I don't use helium as its not needed.

8/70 for much deeper and various mixes for in between.

Being on a rebreather makes for, as you point out, no bubbles but it also optimises decompression as you control the ppo2 at all times so minimise on gassing and maximise off gassing. It's also warmer and very efficient in gas usage.

I realise I may be assuming alot in my replies any questions just ask.

[Citizen Skeptic]

I'm a certified diver but have only done recreational diving in shallow (<90 feet) water and it has been a very long time. I haven't kept up with the technology at all.

[Plastique]

Yes Helium is essential remove the narcotic effects of Nitrogen at significant depths. It also reduces the work or effort required to shift the gas around the breathing loop. Your lungs do all the work in the system and high effort leads to increased co2 retention and risk of a hypercapnic event.

Of course Helium being much smaller than Nitrogen means that it on gasses into your body tissues faster leading to longer decompression schedules. But well worth it for the clear head and ease of breathing.

Typical mixes I use are (typically expressed as O2/helium)

21/30 for wrecks to a depth of about 45 metres. For shallower than 30 metres and non overhead environments I don't use helium as its not needed.

8/70 for much deeper and various mixes for in between.

Being on a rebreather makes for, as you point out, no bubbles but it also optimises decompression as you control the ppo2 at all times so minimise on gassing and maximise off gassing. It's also warmer and very efficient in gas usage.

I realise I may be assuming alot in my replies any questions just ask.

Rebreathers are pretty uncomfortable to use aren't they? Someone told me they're really difficult to breath through.

[SkeptiKiwi]

I'm a certified diver but have only done recreational diving in shallow (<90 feet) water and it has been a very long time. I haven't kept up with the technology at all.

It's still very much a minority activity using a Rebreather. But if you dive very deep or do a lot of wreck diving then they make sense as you have many more options if the crap hits the fan.

If you only dive occasionally or shallow then diving a Rebreather (RB) makes no sense either financially or from a risk/benefit perspective. The risk of dying on an RB is 100 times that faced by someone on open circuit scuba. There is a push toward getting the masses into RBs but I think that is bound to end in failure.

[SkeptiKiwi]

Yes Helium is essential remove the narcotic effects of Nitrogen at significant depths. It also reduces the work or effort required to shift the gas around the breathing loop. Your lungs do all the work in the system and high effort leads to increased co2 retention and risk of a hypercapnic event.

Of course Helium being much smaller than Nitrogen means that it on gasses into your body tissues faster leading to longer decompression schedules. But well worth it for the clear head and ease of breathing.

Typical mixes I use are (typically expressed as O2/helium)

21/30 for wrecks to a depth of about 45 metres. For shallower than 30 metres and non overhead environments I don't use helium as its not needed.

8/70 for much deeper and various mixes for in between.

Being on a rebreather makes for, as you point out, no bubbles but it also optimises decompression as you control the ppo2 at all times so minimise on gassing and maximise off gassing. It's also warmer and very efficient in gas usage.

I realise I may be assuming alot in my replies any questions just ask.

Rebreathers are pretty uncomfortable to use aren't they? Someone told me they're really difficult to breath through.

They can be uncomfortable. The position of the counter lungs (the bags that your exhaled breath goes into pre scrubber and where your next breath comes from post scrubber) relative to your lungs is crucial.

The further away the counterlung centroid and your lung centroid ( http://en.wikipedia.org/wiki/Centroid ) are the worse the breathing characteristic is in both a positive pressure and negative pressure, think chipmunk cheeks with positive pressure and sucking like a crack w$#@e for negative pressure.

Learning to maintain the optimal position at all times is part of the learning curve. It's one of those learning situations with really good feedback so you learn very quick!

[Plastique]

Yes Helium is essential remove the narcotic effects of Nitrogen at significant depths. It also reduces the work or effort required to shift the gas around the breathing loop. Your lungs do all the work in the system and high effort leads to increased co2 retention and risk of a hypercapnic event.

Of course Helium being much smaller than Nitrogen means that it on gasses into your body tissues faster leading to longer decompression schedules. But well worth it for the clear head and ease of breathing.

Typical mixes I use are (typically expressed as O2/helium)

21/30 for wrecks to a depth of about 45 metres. For shallower than 30 metres and non overhead environments I don't use helium as its not needed.

8/70 for much deeper and various mixes for in between.

Being on a rebreather makes for, as you point out, no bubbles but it also optimises decompression as you control the ppo2 at all times so minimise on gassing and maximise off gassing. It's also warmer and very efficient in gas usage.

I realise I may be assuming alot in my replies any questions just ask.

Rebreathers are pretty uncomfortable to use aren't they? Someone told me they're really difficult to breath through.

They can be uncomfortable. The position of the counter lungs (the bags that your exhaled breath goes into pre scrubber and where your next breath comes from post scrubber) relative to your lungs is crucial.

The further away the counterlung centroid and your lung centroid ( http://en.wikipedia.org/wiki/Centroid ) are the worse the breathing characteristic is in both a positive pressure and negative pressure, think chipmunk cheeks with positive pressure and sucking like a crack w$#@e for negative pressure.

Learning to maintain the optimal position at all times is part of the learning curve. It's one of those learning situations with really good feedback so you learn very quick!

Shit, it sounds claustrophobic.

[SkeptiKiwi]

No it really isn't, I probably describe it in a way that makes it sound worse than it is.

You get used to it and good units have very good work of breathing. If the effort is high you are probably not in the optimal position.

I love it as you can get a level of freedom in the water that just isn't possible on open circuit gear.

[Citizen Skeptic]

Oh, another question please.

How does the regulator compare with conventional presurized tanks? Do you get than same on-demand breathing feel?

[SkeptiKiwi]

No.

If you breath through a vacuum cleaner hose that pretty well captures what the sensation is like. It is not as reassuring as open circuit (OC) in that with OC there is that big differential in pressure, that gas "wants" to get into your lungs and if the regulator fails it typically fails in a failsafe manner i.e. too much gas rather than not enough.

RBs have 3 main failure modes that are pernicious.

Hypoxia, No warning as the point where you might think that something is weird is right about the point where you are powerless to do anything about it.

This is a very good example of being powerless to effect your own survival.

hypoxia

Hyperoxia, No warning (some claim facial ticks and visual disturbances but this is greatly variable) you just start convulsing and drown due to the mouthpiece coming out of your mouth.

Hypercapnia, little warning and only remedied by going to an OC source of gas and getting your CO2 levels down. If no action is taken then death is the only outcome. One of the truly nasty things is that CO2 increases breathing rate which lowers the efficiency of your breathing which increases CO2 which increases narcosis which decreases cognitive function which <insert accident here>.

There are protocols that mitigate the risks of these occurring somewhat and establishing the correct monitoring habits is an essential part of learning.

I am using a full face mask to reduce the risk of drowning should I have a incapacitating event underwater.

The proper understanding of what can go wrong and how coupled with good pre dive checks and post dive cleaning and maintenance is what keeps you alive.

There is a lot of debate over whether there is an inherently safer RB design. You can have a manual or an electronic/automatic design. The manual design has an orifice that allows the constant injection of O2 set to your metabolic rate. You are then responsible for the manual addition of O2. Some argue this engages the "I don't want to die" circuits in your brain and ensures you monitor the PPO2 in an attentive manner.

The Auto proponents like to leave the maintaining of PPO2 up to a computer and still try and keep an eye on the PPO2 as well.

The lower death rate on manual RBs seems to suggest a safer principle but the statistics are skewed a little by the reluctance of manufacturers to be entirely candid about production numbers.

I dive a manual unit. It means there can be a bit more task loading at certain points of the dive but it's ok.

[khendar]

I once saw a documentary (Human Planet perhaps) about divers in the Philippines who dive with only a half mask and a airhose attached to an industrial strength compressor normally used to power jackhammers. No mouthpiece, no regulators, no reduction valve, just a hose held in the teeth to breath through. Often they would have half a dozen divers on one compressor, with a tangle of hoses which required constant supervision to prevent them from being tangled, kinked or yanked out of the diver's mouth. The bends is very common amongst these divers.

[SkeptiKiwi]

Bends and CO poisoning from a crappily maintained machine shop compressor... 

There was a superstition that was introduced to one area by (I can't remember what locality or where I heard this from) a visiting dive physician. The problem in reduction of DCI (Decompression illness) he faced was a very low understanding of the principles of breathing physiology and causes of DCI.

So his pragmatic response was to tell the divers that it was evil water spirits that were causing the painful symptoms. If they stopped at certain times during their ascent for specified periods the spirits (who could not easily see stationary divers) would leave them alone.

It may be apocryphal.. 

EDIT: It seems the Miskito divers of Honduras and Nicaragua hold a belief of a Woman water spirit  (liwa mairin) causing DCI. This may have been where the story originated.