iainwilliams
Contributor
Hi - I have a question (well several questions interwined) regarding partial pressure of oxygen, EAN mixes and repetitive diving. I have seperated them for ease of reading (I hope).
(1) We know that the NOAA CNS % exposure table recommends a maximum PO2 of 1.6 atmospheres. We also know that EAN mixes have been calculated on the basis of equivalent air depth (EAD), and NOT the PO2 100% which at 1.6 atmospheres is 6m / 20 feet. (NOTE: I prefer to use 1.4 for conservatism)
(2) QUESTION: If I use an EAN mix above (less than) 1.6 atmospheres (say 1.4 atmosheres to be safer) on a no decompression dive, well within EAN32 or EAN36 table limits, then at the safety stop (5-4 m) decide to breath 100% O2 for say 5-10 minutes to outgas N2; will I be risking a lowering of CNS tolerance and possible convulsion? I know what CNS% level I am from the mix, depth and bottom time (from the table or computer) but will the extra 100% oxygen increase my CNS too much at the safety stop?
(3) Furthermore, will this intake of 100% O2 add to my CNS debt at the surface during my surface interval, if I decide to do a repetitive dive? Im sure it will - but by exactly how long? How can I calculate the extra uptake to the exisiting CNS accumulation from the dive?
(3-a) If I am using nitrox tables, say EAN32 in the above scenario, how should I calculate my surface time interval? How much do I add for the extra accumulated O2 at 4 m? Or is the added CNS debt acquired from my 100% oxygen spell at 4 m hardly worth the worry? My opinion is that it should be added and remain a constant during the surface interval for added safety . What about moving up into the next bracket on the NITROX surface table as a fudge factor?
(5) I know that I can do a dive on normal air, swim to the safety stop and outgas on 100% oxygen at 4 m (I do this regularly to offset against the chance of DCS). If I am planning a repetitive dive on AIR after a surface interval, do I have to take into account the small CNS uptake I accumulated at 4 m 100% ) O2 from my previous dive? Obviously, I will outgassing at 4 m again on 100% O2 from the second dive.
I hope this all makes sense. Thank you. Iain
(1) We know that the NOAA CNS % exposure table recommends a maximum PO2 of 1.6 atmospheres. We also know that EAN mixes have been calculated on the basis of equivalent air depth (EAD), and NOT the PO2 100% which at 1.6 atmospheres is 6m / 20 feet. (NOTE: I prefer to use 1.4 for conservatism)
(2) QUESTION: If I use an EAN mix above (less than) 1.6 atmospheres (say 1.4 atmosheres to be safer) on a no decompression dive, well within EAN32 or EAN36 table limits, then at the safety stop (5-4 m) decide to breath 100% O2 for say 5-10 minutes to outgas N2; will I be risking a lowering of CNS tolerance and possible convulsion? I know what CNS% level I am from the mix, depth and bottom time (from the table or computer) but will the extra 100% oxygen increase my CNS too much at the safety stop?
(3) Furthermore, will this intake of 100% O2 add to my CNS debt at the surface during my surface interval, if I decide to do a repetitive dive? Im sure it will - but by exactly how long? How can I calculate the extra uptake to the exisiting CNS accumulation from the dive?
(3-a) If I am using nitrox tables, say EAN32 in the above scenario, how should I calculate my surface time interval? How much do I add for the extra accumulated O2 at 4 m? Or is the added CNS debt acquired from my 100% oxygen spell at 4 m hardly worth the worry? My opinion is that it should be added and remain a constant during the surface interval for added safety . What about moving up into the next bracket on the NITROX surface table as a fudge factor?
(5) I know that I can do a dive on normal air, swim to the safety stop and outgas on 100% oxygen at 4 m (I do this regularly to offset against the chance of DCS). If I am planning a repetitive dive on AIR after a surface interval, do I have to take into account the small CNS uptake I accumulated at 4 m 100% ) O2 from my previous dive? Obviously, I will outgassing at 4 m again on 100% O2 from the second dive.
I hope this all makes sense. Thank you. Iain
