How major of NDL benefits?

[teknitroxdiver]

I'm doing some research on CCRs. One thing I'm trying to get a handle on is how much NDLs and deco times are improved. Lets say on a 130' dive. On OC air, I'm looking at basically 6 minutes. What do I get on the breather at 1.2 PPO2?

How about a deco dive? Lets say 200' for 20 minutes. How does OC compare to the CCR there? I have no doubts that gas savings are enormous, how about time savings?

thanks!

 

[Mr.X]

I'm doing some research on CCRs. One thing I'm trying to get a handle on is how much NDLs and deco times are improved. Lets say on a 130' dive. On OC air, I'm looking at basically 6 minutes. What do I get on the breather at 1.2 PPO2?

How about a deco dive? Lets say 200' for 20 minutes. How does OC compare to the CCR there? I have no doubts that gas savings are enormous, how about time savings?

thanks!

Depends on a variety of factors. Your mix (diluent -use of helium), preferred setpoint and personal preferences regarding decompression. In the end, human physiology is what it is - regardless of whether you are on OC, or CCR. One thing I can say is having done the deep stuff on OC that I will never go back to using heavy tanks rigged with travel mixes, bailouts and deco. gases. My back thanks me.

 

[Diver Mike]

From V-planner

V-Planner 3.74 by Ross Hemingway,
VPM code by Erik C. Baker.

Decompression model: VPM - B

DIVE PLAN
Surface interval = 1 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2

Dec to 130ft (2) Diluent Air 0.70 SetPoint, 50ft/min descent.
Level 130ft 13:24 (16) Diluent Air 1.20 SetPoint, 124ft ead
Asc to 40ft (19) Diluent Air 1.20 SetPoint, -30ft/min ascent.
Stop at 30ft 1:00 (20) Diluent Air 1.20 SetPoint, 0ft ead
Stop at 20ft 2:00 (22) Diluent Air 1.20 SetPoint, 0ft ead
Stop at 10ft 2:00 (24) Diluent Air 1.20 SetPoint, 0ft ead
Surface (24) Diluent Air -30ft/min ascent.

Off gassing starts at 77.9ft

OTU's this dive: 30
CNS Total: 10.7%


200 foot dive on CCR


DIVE PLAN
Surface interval = 1 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2

Dec to 200ft (4) Diluent 15/50 0.70 SetPoint, 50ft/min descent.
Level 200ft 16:00 (20) Diluent 15/50 1.20 SetPoint, 68ft ead, 86ft end
Asc to 130ft (22) Diluent 15/50 1.20 SetPoint, -30ft/min ascent.
Stop at 130ft 0:40 (23) Diluent 15/50 1.20 SetPoint, 32ft ead, 57ft end
Stop at 120ft 1:00 (24) Diluent 15/50 1.20 SetPoint, 26ft ead, 53ft end
Stop at 110ft 1:00 (25) Diluent 15/50 1.20 SetPoint, 21ft ead, 49ft end
Stop at 100ft 1:00 (26) Diluent 15/50 1.20 SetPoint, 16ft ead, 45ft end
Stop at 90ft 1:00 (27) Diluent 15/50 1.20 SetPoint, 11ft ead, 41ft end
Stop at 80ft 1:00 (28) Diluent 15/50 1.20 SetPoint, 5ft ead, 37ft end
Stop at 70ft 1:00 (29) Diluent 15/50 1.20 SetPoint, 0ft ead, 32ft end
Stop at 60ft 3:00 (32) Diluent 15/50 1.20 SetPoint, 0ft ead, 28ft end
Stop at 50ft 3:00 (35) Diluent 15/50 1.20 SetPoint, 0ft ead, 24ft end
Stop at 40ft 3:00 (38) Diluent 15/50 1.20 SetPoint, 0ft ead, 20ft end
Stop at 30ft 6:00 (44) Diluent 15/50 1.20 SetPoint, 0ft ead, 16ft end
Stop at 20ft 7:00 (51) Diluent 15/50 1.20 SetPoint, 0ft ead, 12ft end
Stop at 10ft 10:00 (61) Diluent 15/50 1.20 SetPoint, 0ft ead, 8ft end
Surface (61) Diluent 15/50 -30ft/min ascent.

Off gassing starts at 150.2ft

OTU's this dive: 78
CNS Total: 27.7%

Same DIve on OC


DIVE PLAN
Surface interval = 1 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2

Dec to 200ft (4) Air 50ft/min descent.
Level 200ft 16:00 (20) Air 1.48 ppO2, 200ft ead
Asc to 110ft (23) Air -30ft/min ascent.
Stop at 110ft 1:00 (24) Air 0.91 ppO2, 110ft ead
Stop at 100ft 1:00 (25) Air 0.85 ppO2, 100ft ead
Stop at 90ft 2:00 (27) Air 0.78 ppO2, 90ft ead
Stop at 80ft 2:00 (29) Air 0.72 ppO2, 80ft ead
Stop at 70ft 3:00 (32) Air 0.65 ppO2, 70ft ead
Stop at 60ft 3:00 (35) Air 0.59 ppO2, 60ft ead
Stop at 50ft 6:00 (41) Air 0.53 ppO2, 50ft ead
Stop at 40ft 6:00 (47) Air 0.46 ppO2, 40ft ead
Stop at 30ft 10:00 (57) Air 0.40 ppO2, 30ft ead
Stop at 20ft 15:00 (72) Air 0.34 ppO2, 20ft ead
Stop at 10ft 25:00 (97) Air 0.27 ppO2, 10ft ead
Surface (97) Air -30ft/min ascent.

Off gassing starts at 137.9ft

OTU's this dive: 42
CNS Total: 18.4%

189.8 cu ft Air
189.8 cu ft TOTAL

 

[teknitroxdiver]

Wow. 38:40 minutes deco vs 74 minutes deco.

For bailout on a deco dive, do you basically plan to be able to complete the dive even if total RB failure happens at the point of max deco obligation?

 

[Diver Mike]

That is and always should be the planning that goes into the dive.

Some buddy teams use the team bailout concept, I am not a big fan of that one and am more of the "If it all goes pear shaped" I want to be able to get to the surface, safely.

Mike...

 

[ianr33]

Diver Mike: Are you in the habit of doing 200 foot dives on air with no deco gas
If you run the same dive (20 minutes at 200) using sensible gas choices it comes out like this

DIVE PLAN
Surface interval = 1 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2

Dec to 200ft (4) Trimix 18/40 50ft/min descent.
Level 200ft 16:00 (20) Trimix 18/40 1.27 ppO2, 91ft ead, 107ft end
Asc to 120ft (22) Trimix 18/40 -30ft/min ascent.
Stop at 120ft 0:20 (23) Trimix 18/40 0.83 ppO2, 48ft ead, 59ft end
Stop at 110ft 1:00 (24) Trimix 18/40 0.78 ppO2, 43ft ead, 53ft end
Stop at 100ft 1:00 (25) Trimix 18/40 0.72 ppO2, 38ft ead, 47ft end
Stop at 90ft 1:00 (26) Trimix 18/40 0.67 ppO2, 32ft ead, 41ft end
Stop at 80ft 2:00 (28) Trimix 18/40 0.62 ppO2, 27ft ead, 35ft end
Stop at 70ft 3:00 (31) Nitrox 50 1.56 ppO2, 32ft ead
Stop at 50ft 1:40 (33) Nitrox 50 1.26 ppO2, 20ft ead
Stop at 40ft 3:00 (36) Nitrox 50 1.10 ppO2, 13ft ead
Stop at 30ft 5:00 (41) Nitrox 50 0.95 ppO2, 7ft ead
Stop at 20ft 5:00 (46) Oxygen 1.60 ppO2, 0ft ead
Stop at 10ft 10:00 (56) Oxygen 1.30 ppO2, 0ft ead
Surface (56) Oxygen -30ft/min ascent.

Off gassing starts at 148.4ft

OTU's this dive: 72
CNS Total: 34.7%

99.7 cu ft Trimix 18/40
18.2 cu ft Nitrox 50
12.6 cu ft Oxygen
130.5 cu ft TOTAL

Wow,faster than your rebreather profile!

I am sure rebreathers have many advantages,but greatly reduced deco times on 200 foot dives is not one of them !

 

[PerroneFord]

He's also running a VERY conservative setpoint. I wonder what the runtime would be on the rebreather with a 1.3 setpoint on the bottom, and a 1.6 from 70ft up.

Yes, I'm too lazy to run it in VPlanner myself!

 

[silent running]

Hello teknitroxdiver, no matter which deco alg/plan you use, CCRs will give you the much more efficient deco than OC. Because they maintain a constant partial pressure of O2, the inert gas uptake is a smooth curve, with an increasing percentage of O2 as you ascend. The use of CCRs by civilians was pioneered primarily by divers who do lots of deco. I got into CCRs to have better wildlife interactions and the increased NDLs and efficient deco are another great benefit.
Depending on how much diving you will do in a day-O2 exposure-I would almost certainly do those dives on 1.3 and do the last deco stop on 1.5/6 as a precausion. My first 100 meter dive I did on 1.4, but that was it for the day, so no O2 exposure issues. If I remember correctly 10 minutes of bottom time gave me a 75 minute run time with RGBM alg, I don't have GAP to check it right now. -Andy

 

[cameron]

I don't really notice the benifit on square profiles like the ones illustrated, but consider a nice mulit level profile.

Maintaining a 1.2 while slowly working up a wall is where the real benifits of constant PO2 diving are seen.

 

[aue-mike]

He's also running a VERY conservative setpoint. I wonder what the runtime would be on the rebreather with a 1.3 setpoint on the bottom, and a 1.6 from 70ft up.

That's asking for trouble for a variety of reasons, IMHO.