Why the high PO2 on CCRs?

[padiscubapro]

Genesis once bubbled...
with no deco required?

How's 'dat?

PO2 of 1.3 @ 130 feet is about 26% O2. The EAD is still 120' on that percentage..... which definitely doesn't give you much NDL time.... (like 10 minutes!)

When you get done with the first hour and a half you certainly do have a serious deco obligation if I'm doing the math right... Do you carry enough bail-out gas to meet that obligation if something goes wrong with the CCR at that point?

That's a heck of a lot of gas in an OC bailout, is it not, especially considering that you'd have deep stop obligations being down that long....

Assuming you hold a PO2 of 1.3 at 50 fsw, you'd be running about a 50% mix (actually 51.7%, but who's counting at that point)

Yes, you'd offgas a lot during that hour and a half, but do you really have no deco obligation remaining at the end of that 90 minutes?

I'm curious about the OC bailout requirements to do this safely...

(I'm not a rebreather diver, but am trying to understand them... I understand the gas consumption advantage, but am trying to get my arms around the elevated mix....)

Would it not be better to run the lower PO2 at the bottom, and then crank it up when you do your deco? That is, something adaptive to get you the offgassing advantage when shallow and doing your deco, but removing the O2 clock issue when deep?

The long 3 hr deep dives I discussed, I only do with another CCR diver.. Yes, after the deep part there is considerable deco reqired, but believe it or not you alter the deep stop requirement during the 100 foot leg..

I Plan these dives (and deeper) where the ccr TEAM has atleast 1.5 the gas required to get 1 diver home... in this case much more

If you look at the calculated profiles shown later in this thread, th 10 ft stop is equivalent to 100% oxygen, the 20ft stop is equivalent to 81%, oxygen.. You can run both of these off the 100% if you use 50% times are only slightly increased as does the deeper stops.. On most dives with this level of decompression switching from 50% to o2 doesn't take much time off of requirements, its usually only a few minutes difference for me one 40cf 50% bottle under most circumstances would suffice (besides I would have at least another 20cf of diluent available)..

We will start the dive with our diluent bottles filled (aprox 23 cf), and one diver will carry 50% and bottom mix the other will cary 50% and oxygen. The cylinders are usually 40cf each. 160cf of bailout gas is more than enough for these dives..
I have a very low RMV (I recently did an OC dive to 130 for 45 mins and used less than 100cf of bottom gas).

I always build bailout tables, and dive with 2 CCR capable dive computers.. My 1st choice would be to dive the computer in case of failure, but I still carry the tables just in case.. I dive an abysmal explorer and a VR3 as a backup on mix dives, or an Explorer and a cochran commander as backup on "air" dives.. The explorer and VR3 both can be switched from CCCR mode to OC underwater.

Run the profile on something like the GAP, If you use the default gradient factor you have a slight deco requirement, I reduce the GF a bit to give shorter deco times(in this case none - if you use an unmodified buhlman you were clear before the 1.5 hours), I find I don't need to be overly conservative when on my CCR.

The o2 clock issue really isn't a problem at 1.3 your limit is 3 hours for a normal exposure.. On longer dives you can take low PO2 breaks to simulate "air" breaks to extend the time.. The 1.3 is definately an advantage when doing multilevel dives since it increases o2 as you get shallower starting offgassing when OC divers are usually ongassing.

I also find that "personally" my o2 tolerance seems to increase as a week of diving progresses. On deep deco dives I'll do my 20 ft stop with 100% flushing the loop to keep it as close as possible, on the first few dives I get the usual congested feeling seen by divers using 100% oxygen, within a few days Its totally gone..

 

[padiscubapro]

here is an output from vplanner with conservatism set to +2.

I allow for a 3 minute decent @ .7po2 this will actually add a bit more safety since you will be ongassing less in reality, since anything deeper than 77 feet adding diluent PO2 will be higher.
86 minutes is the minimum time@ 50 ft to clear deco requirements, 60minutes@50 only requires an 8 minute stop@10ft, 30 minutes requires 4:20 @20, 16 @10.
if the 50 ft level is ommitted the stops after the 100ft level are
Stop at 40ft 5:00 (101) on Diluent Air, 1.30 SetPoint, 6ft END
Stop at 30ft 9:00 (110) on Diluent Air, 1.30 SetPoint, 0ft END
Stop at 20ft 10:00 (120) on Diluent Air, 1.30 SetPoint, 0ft END
Stop at 10ft 15:00 (135) on Diluent Air, 1.30 SetPoint, 0ft END
Asc to sfc. (135) on Diluent Air, -30ft/min ascent.

with only the 130ft leg
Stop at 60ft 1:40 (67) on Diluent Air, 1.30 SetPoint, 31ft END
Stop at 50ft 4:00 (71) on Diluent Air, 1.30 SetPoint, 18ft END
Stop at 40ft 5:00 (76) on Diluent Air, 1.30 SetPoint, 6ft END
Stop at 30ft 6:00 (82) on Diluent Air, 1.30 SetPoint, 0ft END
Stop at 20ft 8:00 (90) on Diluent Air, 1.30 SetPoint, 0ft END
Stop at 10ft 12:00 (102) on Diluent Air, 1.30 SetPoint, 0ft END
Asc to sfc. (102) on Diluent Air, -30ft/min ascent.

Off gassing starts at 87.5 ft
OTU's this dive: 148
CNS Total: 56.1%


Full Planned Dive

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

DIVE PLAN

Surface interval = 2 day 0 hr 0 min.
Altitude = 0ft
Conservatism = + 2

Dec to 130ft (2) on Diluent Air, 0.70 SetPoint, 50ft/min decent.
Level 130ft 0:24 (3) on Diluent Air, 0.70 SetPoint, 144ft END
Level 130ft 60:00 (63) on Diluent Air, 1.30 SetPoint, 120ft END
Asc to 100ft (64) on Diluent Air, 1.30 SetPoint, -30ft/min ascent.
Level 100ft 30:00 (94) on Diluent Air, 1.30 SetPoint, 82ft END
Asc to 50ft (95) on Diluent Air, 1.30 SetPoint, -30ft/min ascent.
Level 50ft 90:00 (185) on Diluent Air, 1.30 SetPoint, 18ft END
Asc to sfc. (187) on Diluent Air, 1.30 SetPoint, -30ft/min ascent.

OTU's this dive: 274
CNS Total: 103.7%

 

[padiscubapro]

Genesis,

I can't speak for other RBS but I can tell you I have well over 300 hours on an INSPIRATION. I stopped tracking my time at that point.. My shortest dives are around 1 hour my longest is almost 4 hours..

In all these dives I have had only 3 dives with any problems, none required me to abandon the loop.. All three dives were in a row.. I had a 1 sensor that I had trouble calibrating with, and 1 sensor that momentarily showed a value higher than it actually was.. The one sensor I had calibration tbls was at the end of its life but once calibrated it tracked ok... the sensor that had intermittent high values was due to a plugged equalization hole (it was a problem with a batch of sensors) it would moementarily show a value .1 to .2 higher than actuality when the o2 solenoid fired but within 2 or 3 seconds read fine.. changing these sensors after the first 2 dives with problems caused problem #3.. I didn't have the bulk head tight enough (it passed pos pressure test though) after changing sensors and allowed water intrusion into the loop. Using proper teqnique I was able to stay on the loop, and complete deco (at the end of the dive I had the entire bottom of the canister filled with water).

I consider this system highly reliable with the proper care but still enter the water each and every time with the attitude this device can kill me without warning.. I always carry sufficient bailout and hope I never have to use it..

Training is the key to safe RB diving.. There appears to be 2 different camps when it comes to dealing with problems.. 1 believes when in doubt bail out.. the other (which I believe in) unless its a catastrophic loop failure, I don't abandon the loop ( I may give it up momentarily under certain circumstances)

I regularly practice all my emergency and bailout drills.. a RB is no place to be complacent, it requires a little bit more diligence but I will never go back to OC without being forced.. I can fly my unit blindly without any electronics help and maintain my setpoint consistently within .2 and usually within .1. I can fly the unit confortably as an SCR and stretch the onboard 3l cylinder to at least that of a 15 l cylinder (and stay near my planned deco requirements). If I accept maintainging a minimum PO2, I can stretch it much farther..


I especially like the added safety it allows me when training OC technical students.. I don't have to worry about my gas, and have 40cf of bottom gas and 40 cf of deco gas that can be handed off to a student in an emergency.