Please can you explain this to a newbie like me.
I thought a CCR would give you the perfect mix for your deco PPO2 wise. How can the bailing out have less deco ? Is that because the ascent is faster ?
Below about 60-65m you actually have more deco than you would on OC for the same bottom time. Plug in some numbers into multideco and you'll see the trend. At 100m you have waaaay more deco on CCR than you would on OC for the same bottom time. The "less deco" for constant ppO2 only really applies to multilevel dives less than about 60m. Below that depth you end up with a lot more deco on CCR and bailing to OC reduces your time to surface.
The reason is the lack of gas switches. You can do diluent switches but they are only so-so popular, can be complicated, and you never know exactly what you are breathing because your new dil is mixing inerts with your offgassed inerts. They are a bit of a mathematical artifact and for a 70m dive highly unlikely you would try doing dil switches even if you didnt have another failure going on.
Here's 70m example with a 17/46 backgas/dil (It's a bit hot, was just trying as an example and I dont know what is in common use in the UK) and constant ppO2 of 1.2 vs bailing to 17/46 and EAN50 as a deco gas. After a 30min bottom time the bailout plan is actually 10mins faster to the surface. Don't be afraid of bailing.
DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = GF 50/75
Dec to 230ft (3) Diluent 17/46 0.70 SetPoint, 60ft/min descent.
Level 230ft 26:10 (30) Diluent 17/46 1.20 (1.32), 92ft ead, 106ft end
Asc to 120ft (34) Diluent 17/46 1.20 SetPoint, -24ft/min ascent.
Stop at 120ft 0:25 (35) Diluent 17/46 1.20 SetPoint, 30ft ead, 57ft end
Stop at 110ft 1:00 (36) Diluent 17/46 1.20 SetPoint, 24ft ead, 53ft end
Stop at 100ft 2:00 (38) Diluent 17/46 1.20 SetPoint, 19ft ead, 48ft end
Stop at 90ft 2:00 (40) Diluent 17/46 1.20 SetPoint, 13ft ead, 44ft end
Stop at 80ft 3:00 (43) Diluent 17/46 1.20 SetPoint, 7ft ead, 39ft end
Stop at 70ft 4:00 (47) Diluent 17/46 1.20 SetPoint, 2ft ead, 35ft end
Stop at 60ft 5:00 (52) Diluent 17/46 1.20 SetPoint, 0ft ead, 31ft end
Stop at 50ft 7:00 (59) Diluent 17/46 1.20 SetPoint, 0ft ead, 26ft end
Stop at 40ft 8:00 (67) Diluent 17/46 1.20 SetPoint, 0ft ead, 22ft end
Stop at 30ft 12:00 (79) Diluent 17/46 1.20 SetPoint, 0ft ead, 17ft end
Stop at 20ft 8:00 (87) Diluent 17/46 1.20 SetPoint, 0ft ead, 13ft end
Stop at 15ft 41:00 (128) Diluent 17/46 1.20 SetPoint, 0ft ead, 10ft end
Surface (128) Diluent 17/46 -20ft/min ascent.
OTU's this dive: 167
CNS Total: 59.4%
Gas density: 5.6g/l
DIVE PLAN - BailOut
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = GF 50/75
Dec to 230ft (3) Diluent 17/46 0.70 SetPoint, 60ft/min descent.
Level 230ft 20:10 (24) Diluent 17/46 1.20 (1.32), 92ft ead, 106ft end
Level 230ft 2:00 (26) Trimix 17/46 1.32 ppO2, 90ft ead, 109ft end
Asc to 120ft (30) Trimix 17/46 -24ft/min ascent.
Stop at 120ft 0:25 (31) Trimix 17/46 0.77 ppO2, 38ft ead, 49ft end
Stop at 110ft 1:00 (32) Trimix 17/46 0.72 ppO2, 33ft ead, 44ft end
Stop at 100ft 1:00 (33) Trimix 17/46 0.67 ppO2, 29ft ead, 38ft end
Stop at 90ft 3:00 (36) Trimix 17/46 0.62 ppO2, 24ft ead, 33ft end
Stop at 80ft 3:00 (39) Trimix 17/46 0.57 ppO2, 19ft ead, 28ft end
Stop at 70ft 3:00 (42) Nitrox 50 1.53 ppO2, 32ft ead
Stop at 60ft 3:00 (45) Nitrox 50 1.38 ppO2, 26ft ead
Stop at 50ft 4:00 (49) Nitrox 50 1.24 ppO2, 19ft ead
Stop at 40ft 7:00 (56) Nitrox 50 1.09 ppO2, 13ft ead
Stop at 30ft 10:00 (66) Nitrox 50 0.94 ppO2, 7ft ead
Stop at 20ft 6:00 (72) Nitrox 50 0.79 ppO2, 0ft ead
Stop at 15ft 46:00 (118) Nitrox 50 0.72 ppO2, 0ft ead
Surface (118) Nitrox 50 -20ft/min ascent.
OTU's this dive: 99
CNS Total: 36.4%
Gas density: 5.6g/l
I don't understand that either. First off, the CCR is not going to do anything perfectly if the computer is dead. Bailing out at 65M would be to a 21% mix or less, maybe switch to a richer mix as you approached 6m assuming it was available.
It's tragic and sad this happened to a diver who had adequate bailout and chose to stay on loop with a broken eCCR. Yet another reason for mCCR in my book.
It would be bailing to 16/45ish. Then 50%. You would never do a 70m dive without deco gas on your person.
Orifices can clog and you are then running full manual like this diver. You'll go hypoxic on ascent without LOTS of o2 addition too.