Could someone please sum up what was said. (if possible)
Heard rumours about discussions on negative effects of back mounted counter lungs....
Can someone fill me in?
Cheers
Roger
Classic KISS, Megalodon
Could someone please sum up what was said. (if possible)
Heard rumours about discussions on negative effects of back mounted counter lungs....
Can someone fill me in?
Cheers
Roger
Inspo, Hammer Head, KISS rEvo
Ill try.
Dr Mitchell can use this response to see how well he got though to the thicko's like me :D
The main topic was C02.
The core principle of the body dealing with C02 is that the body must be able to vent at least the same amount of C02 as it produces.
If you fail to fully vent the surplus C02 with each breath, the residual will re enter the body and start a viscous cycle of accumulation.
Unless immediate steps are taken to break this down ward spiral you will eventually pass out.
In such cases divers believe it is the scrubber or some unit failure that is causing the c02 but this may not be the case. The C02 spiral may be the result of work of breathing.
The Alveoli in the lungs are best described as an elastic balloon connected to a rubber tube. The c02 builds up in the balloon and the elasticity of the balloon forces it out down the rubber tube.
Due to the laws of physics the pressure inside the balloon and at the entrance of the tube is slightly higher than the pressure inside the exit point of the tube.
If this pressure balance becomes too big the tube will collapse restricting the ventilation of gas to near zero. QED the diver can no longer expel the C02.
Causes of tube collapse:
1: Gas density.
The denser the gas the higher its friction on the side of the tube. The higher the friction the greater the pressure imbalance at the beginning and end of the tube.
Air is denser than Trimix. Trimix is denser than Heleox.
The more helium in the mix the less dense the gas.
2: Work Of Breathing
If the diver has to force the outward breath then they use the muscles of the diaphragm to compress the lungs rather than letting the lungs work in the normal way.
This causes a pressure to build up around the balloon used in our first analogy. This pressure plus the elasticity of the balloon is now trying to force the gas out.
However it also introduces a pressure outside the rubber tube. This pressure combined with the afore mentioned pressure imbalance from the entrance to the exit of the tube causes it to collapse further. In doing so restricting the breathing.
This restriction causes the diver to try harder to breath. At its extreme the diver will cough on exhale in violent muscle spasm trying desperately to vent the c02.
This just increases the pressure outside the tube and it collapses even more.
By this dive the diver is near death.
Having front mounted counter lungs causes a slightly positive pressure on the situation which will help to avoid the collapse of the tube. Having rear mounted counter lungs will do exactly the opposite.
In theory if the situation arose where breathing becomes labored the rear mounted counter lung diver would be better off rolling on to his back.
All of the above can occur with a perfectly functional c02 scrubber in the unit.
The talk was presented with a back drop of Dave Shaw's tragic last dive. Toward the end you could hear Dave coughing in an attempt to expel gas. All the time he had the presence of mind to do diluent flushes but they were not helping. He knew he was having a C02 hit but he could do nothing about it because his work of breathing was too high.
His high work of breathing was due in part to gas choice (Heleox would have been better) and the rear mounted counter lung design of his unit.
However the key issue was an after market moisture absorbent pad used in his Mk15.5 CCR. This pad covered vital air ways and restricted gas flow. Whilst this was not causing noticeable effect on shallow dives it was enough to tip the balance at 265m.
The conclusion I took from the talk was that If a C02 hit occurred the diver must use all their will power to concentrate on deep breathing without forcing the outward breath. The diver should also get on to the lowest WOB possible. Id go for OC and trickle purging the reg into my mouth.
Units must be running at their optimum WOB and the diver should maximize the helium in the gas choice.
Finally the dive its self should try and keep the work load to an absolute minimum.
Hopefully i didn't miss anything too serious or make too many mistakes in the above. If i did I apologize and hope someone will correct them for me.
ATB
Mark
Classic KISS, Megalodon
Thanks a lot for taking time with this!
Very interesting indeed...
Cheers
Roger
That's a pretty bold statement..... was this unit put through its paces in the lab? Or is this theorizing based upon post incident inspection....?Originally Posted by Mark Chase
Mempilot, Padipro and I got into a fairly heated argument concerning the recent fatality of a friend of ours, specifically the amount of 'value' or credit we place on supposition, postulation and deduction vs. data and smoking guns.
I'm sure Simon had some other humorous slides in as well... his PFO talk a few months ago was a trip...
Simon gave that presentation first time at Oztek 2007, great speaker!
Seems there was some evidence, pictures of the moister foam etc..
conc positive pressure, an easy way to achieve that with back-mounted units, is to take a bit more vertical position in the water.
and then do slow and DEEP breathing in and out
paul
www.rEvo-rebreathers.com
...."Yes you have to pre-breathe to activate the scrubber sorb, anyone who says different doesn't know what they are talking about!"...
.... to get more accurate CO2 injection in the breathing machine we put 2 mass flow controllers in series ...
.... The noise is a few tens of nano-volts, so DL were able to reduce the output voltage ...
.... radial scrubbers give longer dwell time than axials...
.... the earth is flat and ...
Inspo, Hammer Head, KISS rEvo
I have done my best to give a summery of what I took in from the talk. More detail will need the input of someone with a much better understanding and recall than me or input from Simon himself.
The pics we were shown of the Mk15.5 pad and the pad that Dave was using the pad covered recessed tunnels toward the center of the dome and half covered the exhale port.
The pad was apparently a more dense material than the pad specifically designed for the 15.5.
Dr Mitchell pics were from his own 15.5 so he knows the unit intimately
He was quick to point out Dave was unable to get hold of the correct pads despite numerous attempts.
Perhaps Dr Mitchell will read this and put the pics up for us to see.
ATB
Mark
Last edited by Mark Chase; 22nd November 2008 at 17:15.
Mark's summary was pretty accurate. The physiology means that at a certain point you are unable to get gas out of your lungs any faster however much extra expiratory effort you put in as the extra effort closes the airway (within the lung).
If you need convincing that this can happen it is well known that the maximum expiratory volume reduces if it is very forcible due to airway closure limiting gas loss from alveoli. Add high gas density and it can happen more easily.
It is possible but Not guaranteed, that a negative static lung load could exacerbate the effect in divers - I would anticipate this being a relatively small effect though. Remember we are talking about Extreme depth. Avoiding BMCLs won't make the problem go away!
Simon showed some photos of Dave Shaw's moisture absorbent pad Vs the official one, it was hard to see that Dave's version could have done anything but make WOB worse.
Neil
Drager
Roger et al,
You might find some of the subject matter presented @ Eurotek in the DAN Tech Conference presentation he gave DAN Divers Alert Network
Its just a pity all of his presentations don't get recorded for publication.
Regards
Brad
You can seek therapy or anger management about that heated argument issue. Group sessions can be cheaper, look for a group discount. Please avoid directing your anger at yours truly, seek help. Thank you.
Rather than allow arguments to occur over such conjecture, it is always best to keep these issues on file for future reference. There is the possibility that some trend emerges over time or that some elucidation occurs to shed light on a particular component to advance the state of the art.
In many cases, we can all recollect things we have read or heard that remained plausible but somewhat lacking in empirical or controlled test data. Over time, a definite collective conscience emerges. One can think of the use of the SPG, equipment redundancy, conservative decompression and much more.
This is part of the excitement and risk of being on the cutting edge. As Guinea pigs, it is incumbent upon us to be mindful of all logical, plausible, theoretical, hypothetical causes of accidents. It is no easy job to sift through the chaff and the intangible elements in these events.
It is undeniable that WOB issues exist with all RBs, with position and attitude dependencies inherent to all machines, every factor must be scrutinized. This is especially the case given the autonomy and depth capability of machines that result in extreme physiological stresses, far more than has hitherto been experienced or recorded using OC.
I saw Si's talk at oztek 2007 twice, and its definitely worth hearing.
The full paper has been published, I know its been posted on here a couple of times, but for those who haven't seen it already, its a free download, and well worth the read.
Fatal Respiratory Failure During a “Technical” Rebreather Dive at Extreme Pressure
Authors: Mitchell, Simon J.; Cronjé, Frans J.; Meintjes, W. A. Jack; Britz, Hermie C.
Source: Aviation, Space, and Environmental Medicine, Volume 78, Number 2, February 2007 , pp. 81-86(6)
Publisher: Aerospace Medical Association
Available from
IngentaConnect Fatal Respiratory Failure During a Technical Rebreather Dive at E...
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