caustic cocktail

Sutty

I don't think detection is a huge problem, incorporating it somewhere so that you get adequate warning reliably may be more problematic. eg partial flood undetected moving with change of attitude in the water. Ok so you could detect a bit of caustic water eg below the scrubber on a kiss - may just be condensation with sorb dust in it or minor flood, may need multiple sensor points to give more info - not intrinsically difficult, could even have the system open the BOV automatically (eg solenoid-activated pneumatic valve) on sensing low-resistance fluid in the inhale hose.

Neil

__________________
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Genesis

The point would be to raise hell if low-resistance fluid is detected at the outlet of the cannister.

It doesn't have to give you MUCH warning - just enough to stop breathing in and bail! If the rig has OTS lungs this should provide quite a bit of warning but even a BM lung rig will give you enough time to alert you that your're about to have a VERY bad day.

It has to be a near-instant response (100ms or faster, I'd think) but if it goes off you've got a "loop unsafe" condition.

The K1's outlet is in the center of the can which makes it pretty tough to get enough fluid in the can that can end up in the inhale side unless you go severely-head down. And I've yet to find any free water in the can after use. But an alarm, if you went head-down and it went off, would still likely give you enough warning to bail with an OTS rig that is going to shunt any flood into the inhale C/L first - if that sucker sounds you get the hell off the loop first and ask questions later!

As far as I know, NO commercial unit currently monitors for this. The next cut of the K1 controller will.

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hrdubwd

Picking up a few points scattered all over, some chemistry, if I may:

1 Sorb is meant to be damp at all times to work - i.e. water is present.

2 There is no reaction of sorb with water (that was done in manufacture) - see 1.

3 There are no fumes from the sorb. Getting it soggy can make no difference. - see 2. Smell comes from dust and reaction of alkali with other things.

4 There can be no early warning from taste or smell - see 3.

5 It is exceedingly unlikely that gases (toxic or otherwise) could be released from ptfe under normal low. temp circs, and certainly not in concentrations to worry about. It is used internally, surgically, for its inertness and lack of toxicity.

6 I shall be kind and refer to Tom's cited website with one word: twaddle. Two words: chemical nonsense. Three words: please stay away. Sorry, Tom, that is not the way to learn anything useful about biochemistry - splitting water, indeed. When a site promotes itself through physical impossibilities, beware.

Dust from the top of the stack ought to be suppressed by any means possible - O-rings, membranes - to prevent inhalation. Lung irritation from even a small particle is to avoided. Inspection of inhale hose and valves suggests that migrating particles are very common otherwise, even if mostly trapped on wet surfaces.

I have had two floods on my KISS: WoB rose steadily as the base of the stack saturated - very obvious. BO uneventful. Distinct advantage in not being close to horizontal (! - sorry, purists) - and certainly not inverted. Cocktail never reached the top of the stack, but would have gone into the inhale CL first.

BWD

RonMicjan

maybe instead of just calling Bull$shit you could actually post some references, places to go to read more, specific reasons why? Are you a chemical professor? or amateur kitchen chemist? Sea water vs. fresh? sorb manufactured with a water content is not the same as water rolling around the loop, no? Sorb in the scrubber bed is a good thing, sorb moving around the loop with water as a transport mechanism is a bad thing, and small amounts moving around the loop IS a warning of impending flood and you WILL taste/smell it (closely related senses eh?), so it may not have been stated quite that way, but the end result and warning is sufficient to possibly avoid disaster.

Nice to say dont dive horizontal (try this in a cave/wreck) and never go inverted (try being a photographer). Or maybe just try being a something other than a PADI DM. I guarantee that a flood will slosh right past that inhale CL on your CK if you didnt have your fins in the mud m8.

__________________

hrdubwd

>post some references ... specific reasons why?

I'd be at it all day! Really, it is dreadful stuff. Deuterium! Please...

>Are you a chemical professor?

Yes.

>Sea water vs. fresh?

Would make no difference.

>water rolling around the loop

Liquid not good, I agree, but I was referring to some earlier queries and comments that needed clarification.

>small amounts moving around the loop IS a warning of impending flood

Inhale hose? Too late. Exhale, normal: condensation, if not incompetent lips.

>and you WILL taste/smell it (closely related senses eh?)

No advance warning, though. (Not really, but often confused).

> warning sufficient to possibly avoid disaster.

I think it would be hard to detect liquid in the inhale hose without actually inhaling it, or at least it reaching the mouth.

>Nice to say dont dive horizontal (try this in a cave/wreck) and never go inverted (try being a photographer).

Oh, I know - quite understood. I could say TIC. Awareness is all.

>just try being a something other than a PADI DM.

Sorry, was than an insult?

>I guarantee that a flood will slosh right past that inhale CL on your CK if you didnt have your fins in the mud

And that was my point. If it is suspected ...

BWD

AD_ward9

Of course. The quantity of water is important. None at all means no reaction, too much is a caustic cocktail.
The Calcium Hydroxide dissolves in water. That means when there is too much water, the Calcium Hydroxide comes out of the carrier holding it, into the water, which then enters the breathing hoses.
Agree.
You are missing an important piece of information. Send me a PM with an email address, and I can send some detailed information on this. Off gassing is a serious problem. SINTEF have done considerable work on this, because diving umbilicals get contaminated. What happens in diving is the helium migrates into the porous plastic and then migrates out again bringing toxic chemicals with it. Furthermore the high PPO2 accelerates the ageing of these plastics and some of the breakdown products give such serious cause for concern that many organisations ban them or campaign to ban them. Such as the US Navy banning many of these plastics in hyperbaric and rebreather systems.

On your point 6, with all due respect, I would examine the facts first. Starting with SINTEF's publications on this topic. Considerable work has been done in this area that you should read up on.

Alex

hrdubwd

Ca(OH)2 is not very soluble (1 - 2 g/L). "Lime water" is not nice to inhale, but the problem is from the "soda" part - much more soluble, much more aggressive.

OK, will do. Always glad to see evidence and correct my understanding.

6? Hardly. I suppose you mean 5.

Thanks,

BWD

AD_ward9

Thanks for the PM with your address. I sent a reply. I did mean 6, where you suggested gassing was a boulder dash.

However, you are right in that it is sodium hydroxide that dissolves fastest.

On the off-gassing, this is a physical phenomena rather than a chemical phenomenon.

For example, if I asked you "Does aluminium off-gas?", as a chemist you would guess, and judging from your post, say of course not. Actually it does off-gas so badly that companies offering high vacuum processes turn down jobs using it.

For example, IonBond offer a diamond like carbon ion assisted implant process but state that many grades of aluminium are not suitable due to the off-gassing from contaminants in the metal. That is also why high vacuum parts are not made from aluminium. A vacuum is a pressure different of less than 1 ATM, causing this off-gassing. In diving the situation is much worse, with helium being used to pressurise the system then the pressure differential coming back up inside the structure can be 10 ATM or more even when the parts appears to be at ambient pressure.

Most materials are slightly porous. All materials under strain are porous to gasses, which is why high vacuum components are made from such thick metal. In a dive system, the strains are quite low, except for the first stage and parts at intermediate pressures, but relative to a vacuum system the strain on a rebreather component relative to ambient (just 30mbar) is actually very high because the thickness of the material is much less and the materials are plastics rather than vacuum grade stainless steel. As another chemist pointed out on another thread, some of these plastic off-gassing components are extremely toxic.

On the health risks, some identified offgassing components are highly toxic. Others are not known very well. Over time, chemicals tend to be found to be more toxic than first thought. A good example is hydrogenated fats, put into food for decades, but a few years ago found to be a major reason why people from Northern Europe and the USA die from coronary diseases at a much higher rate than in Southern Europe and Japan where these fats are little used. That was something as innocent as a fat. Now try the breakdown products of PTFE, some of which are effective as a human nerve gas weapon. Just not a good idea to put much of that into a breathing loop used for diving.

On the medical implants, these too are going away from PTFE to Polyvinylidene Fluoride (PVDF, or Kynar) for implants because of the safety concerns, and polyphenylsulfones for non-implanted structures.

Anyhow, the upload of the doc should just about be finished and welcome your comments if you could send me an email identifying yourself so I know who it has gone to.

Alex

hrdubwd

We're getting in a tangle - that was 5. (nice malaprop...)

Context is important. I try not to be rash. But physics is my domain as well as materials science in general. As the devil said, "You have to specify, Stanley."

I think you confuse bulk transport with partial pressure.

Too broad: some yes, He in particular. That does not mean that anything and everything goes through regardless. There are some surprising things, but let's not go mad on the generalization, if one can they all must. This is not true. The activation energy for some diffusion processes is so high the material would be melted first.

Caution is admirable, but random horror stories do not help much. I can count about a dozen polymers in my loop in various places, not many of which would I be happy to implant. However, do we want to argue about the toxicity of nickel, chromium, copper, zinc, aluminium ... just because they are there and could corrode and could be ingested? I think we have to maintain some perspective. We get exposed to more in an office. Gosh, you would love the chemistry of silicone rubber! Don't forget, skin absorption is just as good, what with neoprene, a dozen dry suit materials, natural latex rubber (you want a scare story?). Stay away from any adhesives, trees, perfume, hay ...

OK, back to earth. We are taking risk/benefit, cost of error vs. practicality. There are some things we should avoid, and we should do our best to get reliable information. Not sure what effect it has on everyone else to say that all polymers in use in diving equipment emit noxious substances. Can you smell it when new? 'nuff sed.

Will do (a bit later)

Back to work ... !

BWD

AD_ward9

Absolute pressure difference is due to gas being absorbed at depth and then released during compression, similar to human decompression. The the migration of gas through the structure which is due to the strain on the structure: I go back to the vacuum analogy to show how little strain is needed for this. As another demonstration, fill a helium balloon up with gas, and it goes flat in a few days. Fill it with gas, then spray a thin lacquer onto it and it stays up for a month: the only strain on the lacquer is from thermal expansion and contraction.

The PPO2 issue only relates to the oxygen being more active in breaking down the plastic.
All gasses migrate through plastic. In fact, even water migrates through plastics. The speed depends on the gas, the temperature, pressure differentials and the strain. The plastics are porous: it is not a normal diffusion process. The gas channels through the lattice of the material under strain, which plastics are particularly acute because they shrink unevenly during extrusion or moulding. In some cases, water pours through thick plastic just because it cooled fast during extrusion: see some of the posts people have put on Rebreather World with their DIY handset problems as an example.
Silicone is actually very stable and we use it in breathing loops. The process is interesting. I was at the plant that makes silicone a couple of weeks ago, in Barry just outside Cardiff. The second biggest silicone plant in the world. Takes inert compounds, puts it through a dangerous process (SIL 2), and produces an inert product. Where gasses are concerned, the primary risk is inhalation. The skin is a very good barrier and is orders of magnitude slower than for a gas in the lungs.

On nature's toxins, they are are well contained, otherwise the animal or tree that produces them eliminates too much of its environment to survive. Box jellyfish, some clams, some snakes, some frogs even have very strong poisons, and most people should stay away from them. Ergots (the black blemish on wheat) has wiped out entire villages in the past. The localisation of the toxin is very good: walking through a forest or swimming in the sea does not expose a person to any toxins that are anything like as poisonous as can be obtained from PTFE under the right conditions - you are talking about 8 orders of magnitude difference in toxicity here! Fundamental to toxicity is the need for a minimum dose. There is no health issue with a forest walk. Breathing a closed loop giving off known toxins for hours at a time, under conditions of variable pressure and in the presence of helium means one has to take special care because toxins can build up enough to cause both short and long term health problems.

As regards metals, the metals you mention are metals the body requires for enzymes to operate (trace elements). Your body does not need the breakdown products of PTFE or PVC. The recent poisoning case in London shows some other metals are astonishingly poisonous, both chemically and radiotoxic. Plutonium for example is a very toxic metal, but only mildy radiotoxic. Plutonium is less toxic than some of the breakdown products of PTFE.

Latex is not allowed in the breathing loop in EN14143 because it produces a common allergic reaction. On the skin most people tolerate it, but breathing through it produces a stronger reaction.

On the question of are these things real: offgassing products have been identified by SINTEF to be the main cause for saturation divers feeling nauseous during a dive. They use a topside reclaim system which gets rid of these products better than in a rebreather. This means the issue is a real one rather than a vague possibility that a rebreather designer might brush aside. These chemicals have long term health issues, more than short term, so SINTEF and others finding short term effects raises the bar.

As you rightly say, it is all a risk/reward issue. I hope we would concur in that it would be impossible to support a safety case using PTFE or PVC in a rebreather: there is significant risk and no reward - other plastics do both a better job and do not carry the risk.

Alex

NB: Time to split the thread? The hydrophobic membranes one would use in a rebreather to prevent a cocktail are not the membranes made from PTFE. They are made from Kynar or Nylon 12.

This is an interesting debate. Incidentally, you have a 6, to which I referred. http://www.rebreatherworld.com/116287-post43.html