deco plan when running semiclosed

Steve

Have some green Mark. This is the take home nub of this discussion in my eyes.

__________________
WARNING: I contain occasional coarse language, extreme sexual references, nudity, and adult themes, which may offend some people - Usually churchy types.

GuyW

I'm not sure if this has been properly answered because I can't be bothered reading all the crap being thrown back and forth.

So here is my two bobs worth:

1. As has been said, practice SCR mode and get an idea of the ppO2 drop from your diluent. Assuming you breath out every 4 breaths or so then the drop will be less at depth than at the surface. This is important and useful.

2. At depth, when you can ill afford to go to OC due to the gas usage, go to SCR. The ppO2 drop may be negligible (like <1%) so COULD be ignored initially especially if you have important things to do like ascend, find the drop line etc.
Important Rule: "avoid breathing water". Preserving your gas is more important than being 5 mins out on deco. So get your butt up the line and don't fiddle with the VR3 until the first stop.

3. As you get shallow the ppO2 drop will become more significant, maybe 2-3%. But it's effects are really felt most on the long shallow deco stops. So you can bump the VR3 down by these few percent, or now that you are shallow go to OC bailout. Finish off on some nice cool oxygen which makes up for a host of sins.

Guy

lizardland

Exactly. People do things they don't have paper qualifications to do. I bought my KISS and my 2nd dive was a 65m cave dive with a 3hr run time. I flooded the Rebreather pretty much right at the furthest penetration with the beginnings of a CO2 hit and a primary light failure for good measure. Should I have been taught how to deal with that on Mod1 even though that was what I was planning to dive?

You get taught up to a certain level, what you do beyond that is up to you just as is how you manage that risk. Don't do dives you can't bail out from until you know how to bail out from them. Don't blame instructors or course standards for not including things they aren't supposed to include.

My driving instructor never taught me the correct procedure for driving a 4x4 head on into Ford Mondeo in the snow. He should have known that I was going to do it and told me to join the AA so I didn't have to hitch home in a blizzard.

Cheers,

Stuart

__________________
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

Azerbaijani Association of Technical Divers Publicity Officer and Goat Wrangler

DepthCharge

Is this true? Doesn't a PO2 of 1.2 have the same number of molecules whether at the surface or at 100 meters? So that the drop should be the same regardless of depth?

__________________
Later,
John

It's better to beg forgiveness than ask permission

wedivebc

Yes but in SCR you are dealing with a fixed fraction of gas not a fixed PPO2

__________________
Cheers,

Dave....

Man is the only animal burdened with the knowledge he will eventually die

Hanssing

Hmm, a loop with 1.2bar PO2 has the same amount of molecules.
But when running SCR your breath of DIL contains an amount of O2 wich is dependant on the FO2 of the DIL, and the depth your are at.
Simplified example:
Say youre comfortable with breathing AIR once at surface.
Then at 10m your free volume will twice, so you can breathe it twice.
At 20m your volume will be... Yes thrice, so you can SCR-breathe it 3 times.
So in orther words, the drop in PO2 in the loop ie. from DIL_FO2*AbsPressue will be relatively larger at shallower depths assuming you keep the same SCR-rate.

Another explantion:
Dolhins, KISS, rEvo et al, uses constant massflow orifices wich has a constant mass flow. Ie. the flow of O2/Dil is independant of depth.
But manual SCR in a bailout scenario is a flow wich is proportional to absolute pressure (tidal-volume*pressure/SCR-ratio), and therefor dependant on depth.

So for any given bailout you could in theory run a ratio wich is depth dependant - assume drop in O2 of 0.05bar pr. breath, then an EAN25 could be breathed twice on surface, 4:1 at 10m, 6:1 at 20m, 8:1 at 30m etc. Though while ascending ratio should be kept lower than theoretical limit.

Edit: If bailing out due to CO2-issues this is probably a BAD IDEA, however if bailing out due to O2-supply-issues or PPO2-monitoring issues its the best way to get maximum use of your bailout.

Now the bad part:
Im comlety untrained, and have no Rebreather-experience, so I'm probably out in the woods
So if anyoe writes I'm of my rocker - belive them, not me

Regards
Nicolai

__________________
Woohooo - I can change my rEvo!
Its going to be bitchin' tricked out piece of gear

Hanssing

Just to point out my misunderstandings:
"conventional" SCR as found in Dolphins, Submatrix and others, are fixed MASS-flow.
SCR-bailout techniques with FIXED breath-ratios are 'relative flow SCR', wich is NOT THE SAME. So unless you use a breath-ratio wich is dependant on depth (See previous post), you will NOT HAVE fixed FO2!
FO2 will vary with depth, and it will be closets to the bail-out dil at depth than at shallow (assuming fixed breath-out ratios).
- or as I wrote before I'm out in the woods....

Will be nice to test with my own Rebreather

Nicolai

__________________
Woohooo - I can change my rEvo!
Its going to be bitchin' tricked out piece of gear

DepthCharge

Just to clear up matters........What I was saying that taking 4 breaths from a PO2 of 1.2 on the surface, will see the same amount of drop as breathing 4 breaths from 1.2 at 100m. IE your metabolism doesn't change so you pull off the same number of molecules, irregardless of depth, dropping it by the same amount. Things do change when talking FO2, but I believe the original statement was concerning PO2

__________________
Later,
John

It's better to beg forgiveness than ask permission

divetheworld

You probably missed it but Mike G. mentioned/covered this a bit earlier in another practical way. Rather than repeat I have copied below.

Pressure is a factor. Probably more elequant than I could have explained anyway.

Brent

__________________