PO2 drop ?

Devon Rob · April 3rd, 2006, 01:52 PM

Has anyone got a good LAYMANS explanation for handling the PO2 drop on SCR by ascent? All the technical explanations are making my head spin

and yes I do know it has to do with the reduced ambient pressure!

canucksubmariner · April 3rd, 2006, 02:24 PM

Quote: (Originally Posted by Devon Rob)

Has anyone got a good LAYMANS explanation for handling the PO2 drop on SCR by ascent? All the technical explanations are making my head spin

and yes I do know it has to do with the reduced ambient pressure!

something along the lines of, The deeper you go the less oxygen by % in the gas you breath will be needed to sustain life. That is why deep dives on trimix will have an Oxygen percentage below 21%, the normal amount of oxygen in the atmosphere, Many trimix rebreathers (NOT DIR BTW) will provide an optimal mix as you go deeper, reducing the oxygen (and nitrogen) content as you go deeper and adding helium, as you go up those values will be increased and the helium reduced (helium % can be looked at a few ways depending on how the diver chooses/the machine is programed). If the rebreather fails to increase the oxygen on assent then the diver may well find they do NOT have enough oxygen to survive and thus expire. It is a very insidious way of dying. Rebreathers have a few incidious ways of killing a diver.

They sure save money on Helium however!

*JohnKendall* · April 3rd, 2006, 02:38 PM

Quote: (Originally Posted by Devon Rob)

Has anyone got a good LAYMANS explanation for handling the PO2 drop on SCR by ascent? All the technical explanations are making my head spin

and yes I do know it has to do with the reduced ambient pressure!

Ok, I'll give it a go, however I am not rebreather trained, so please take everything that I write on the subject as being a best guess.

On some SCRs (Like the halcyon) you have a system whereby a percentage of the loop volume is dumped overboard on each exhale. When you breath in again there is not enough gas, and so a regulator injects gas to fill up the space. This new gas contains O2, so the O2 level stays pretty constant (although lower than the % in the drive gas itself)

Now when you ascend, the gas expands, and so the regulator doesn't inject, and so no new O2 is injected. This causes the PPO2 to drop. The way around it is to manually dump more of the gas (for example by breathing the loop out through your nose)

Or at least that is how I understand it works.

J

lizardland · April 3rd, 2006, 03:31 PM

Hi John,

yes that is how it works. You may also need to switch drive gases on the ascent too as some gases will not be breathable in the loop even with regular flushes in shallow water but the same problem exists with hypoxic gases on open circuit, it's just on a passive SCR it amplifies the problem. The other alternative that some people do is ascend on open circuit but I don't know if that is what GUE teaches though.

April 3rd, 2006, 01:52 PM	#1 (permalink)
Devon Rob(Offline) Can´t remember-member Breakout Champion! Join Date: Mar 2006 Location: Lyme Bay.Devon.UK formerly Hamburg, Ger. Posts: 316	PO2 drop ? Has anyone got a good LAYMANS explanation for handling the PO2 drop on SCR by ascent? All the technical explanations are making my head spin and yes I do know it has to do with the reduced ambient pressure! __________________ this mail was checked for viruses,worms & bad breath Last edited by Devon Rob; April 3rd, 2006 at 01:55 PM.

April 3rd, 2006, 02:24 PM	#2 (permalink)
canucksubmariner(Offline) New Member Join Date: Mar 2006 Location: GA Posts: 72	Quote: (Originally Posted by Devon Rob) Has anyone got a good LAYMANS explanation for handling the PO2 drop on SCR by ascent? All the technical explanations are making my head spin and yes I do know it has to do with the reduced ambient pressure! something along the lines of, The deeper you go the less oxygen by % in the gas you breath will be needed to sustain life. That is why deep dives on trimix will have an Oxygen percentage below 21%, the normal amount of oxygen in the atmosphere, Many trimix rebreathers (NOT DIR BTW) will provide an optimal mix as you go deeper, reducing the oxygen (and nitrogen) content as you go deeper and adding helium, as you go up those values will be increased and the helium reduced (helium % can be looked at a few ways depending on how the diver chooses/the machine is programed). If the rebreather fails to increase the oxygen on assent then the diver may well find they do NOT have enough oxygen to survive and thus expire. It is a very insidious way of dying. Rebreathers have a few incidious ways of killing a diver. They sure save money on Helium however! Last edited by canucksubmariner; April 3rd, 2006 at 02:30 PM.

April 3rd, 2006, 02:38 PM	#3 (permalink)
JohnKendall(Offline) GUE Instructor/DIRX Moderator Join Date: Nov 2005 Location: Cambridge, UK Posts: 1,099	Quote: (Originally Posted by Devon Rob) Has anyone got a good LAYMANS explanation for handling the PO2 drop on SCR by ascent? All the technical explanations are making my head spin and yes I do know it has to do with the reduced ambient pressure! Ok, I'll give it a go, however I am not rebreather trained, so please take everything that I write on the subject as being a best guess. On some SCRs (Like the halcyon) you have a system whereby a percentage of the loop volume is dumped overboard on each exhale. When you breath in again there is not enough gas, and so a regulator injects gas to fill up the space. This new gas contains O2, so the O2 level stays pretty constant (although lower than the % in the drive gas itself) Now when you ascend, the gas expands, and so the regulator doesn't inject, and so no new O2 is injected. This causes the PPO2 to drop. The way around it is to manually dump more of the gas (for example by breathing the loop out through your nose) Or at least that is how I understand it works. J __________________ John Kendall http://www.guetraining.com/ GUE Instruction, Santi and Halcyon Equipment NEW - Online Santi Shop

April 3rd, 2006, 03:31 PM	#4 (permalink)
lizardland(Offline) LCS Join Date: Nov 2005 Location: Evidently Chickentown Posts: 591	Hi John, yes that is how it works. You may also need to switch drive gases on the ascent too as some gases will not be breathable in the loop even with regular flushes in shallow water but the same problem exists with hypoxic gases on open circuit, it's just on a passive SCR it amplifies the problem. The other alternative that some people do is ascend on open circuit but I don't know if that is what GUE teaches though. __________________ Can you imagine drifting along in the sea with your mouth open and a load of f***ing plankton going in? You'd like it, would you? www.westons-cider.co.uk The Lot isn't the only place to dive: http://www.lulu.com/content/613554