Deco calcs & SCR operation

[Padowan]

I've been thinking long and hard about the best way to accurately calculate deco obligations on the Box when running it in SCR mode, and I would like some info on what strategy others adopt. Ideally so that I could either cut backup tables for this or input it into the VR3, and let it vaguely work it out.

Obviously you are flushing the loop with dil on a regular basis, so you could take OC Diluent as a base point but because the loop PO2 is dropping all the time, then this would under approximate the commitment, potentially by quite a lot.

Best conservative solution I could think of was that you assume that whatever breath cycle strategy you adopt (4 breaths per loop, 1 minute per loop, or something more complicated and depth compensated) is determined such that the loop PO2 should say above some threshold value (hopefully equivalent to >0.15bar at the surface) so you could use OC 15% O2.

This weekend I plan to do some tests with different breath strategies and monitor how the PO2 behaves before deciding on which one to adopt in a SHTF-both-handsets-dead-no-choice-but-SCR-cos-I-dont-want-to-go-OC situation. I'm convinced that I can use a depth compensated strategy to maximise the gas efficiency, initial thoughts are something like 3 breath cycles or 30 secs for each bar ambient pressure, so at 30m (4 bar) I could breathe a loop of diluent for 2 minutes, before flushing again. I know the PO2 would certainly drop well below the OC equivalent PO2 (hence the interest in a deco strategy for this), but it would still be well in the life supporting range (~ >0.15bar) ...

Any feedback on others thoughts on this would be appreciated.

Obviously a 4th cell, or some other independant PO2 reading would be preferable, but I'd like a "brain-only" method instead/aswell because I'm like that

[jptaylor9]

[Padowan]

Originally Posted by jptaylor9

Isn't the most conservative approach to put the VR3 into open circuit mode on whatever the diluent is. The SCR operation is just been used to make the bailout supply last longer. Could select OC 99% if you went to O2 rebreather mode @6m or less.

(Assuming you have a VR3)

No, this isn't conservative at all. As you rebreathe the loop full of diluent, the PO2 will drop, also implying that the PP(Inert) will rise - so you will ongas faster the lonnger you rebreathe the gas for.

Also running the unit as an O2 rebreather you don't really need to apply SCR thinking, as you are running fully closed again, 100% in the loop is always 100% in the loop no matter how much you metabolise (but if you're decoing, which will effectively always be the case, there will be an inert increase over time however as you offgas inerts back into the loop)

[Drmike]

Run semiclosed at constant depth and using your chosen cycle.

Your PPO2 will drop from your setpoint down to just below the dil ppo2. After a while your ppo2 will stabilize at a fixed ppo2, make a note of the value and use that in your semiclosed bail calcs.

Repeat for each dil and level.

I do this occasionally to keep in practice and to gain confidence to run/turn a dive semiclosed in case of full electronic failure (my preferred option over OC bail on a very deep dive)

[Padowan]

Thanks for the input Mike, a couple of questions...

Originally Posted by Drmike

...Your PPO2 will drop from your setpoint down to just below the dil ppo2.

Surely the PO2 will drop from the dil ppo2, to some level possibly considerably lower, depending on how long your SCR breathing strategy keeps you on the same loop of gas?

Originally Posted by Drmike

After a while your ppo2 will stabilize at a fixed ppo2, make a note of the value and use that in your semiclosed bail calcs.

Again, it'll surely cycle up and down between the dil ppo2 at flush point, to a lower level based on your breath strategy - so you would use the ppo2 of the "point at which you flush"?

It'll be a fun weekend playing with this...

[Drmike]

Why would you flush before running semiclosed unless your doing so due to CO2.

Lets say your on the bottom and your electronics die. Because you have been monitoring you know that the ppo2 cant be much below setpoint, say 1.3.

Now chances are your dil ppo2 may be below your loop ppo2 at the monent you start.

This is a good thing as you want to try to maintain a good ppo2 as long as you can. It will drop (surprisingly slowly) when it drops to around your dil ppo2 (or when min loop kicks in) you start venting out every x no of breaths . After a while it will stabalise at just below the dil ppo2. From then on the ppo2 will cycle up and down marginally but not significantly unless you screw up the cycle or your cycle is too long - thats where the practice comes in. EG if you take it to extreams and vent every 2nd breath your loop ppo2 will not fluctuate much, if you choose to vent every 6th breath it will.


Ok that was long winded - to answer your question I use the average loop ppo2. I can do this as I vent often enough for the variation not to be an issue.

[Padowan]

OK, I gettcha.

I was thinking that you'd be turning over the whole loop volume each flush/addition cycle, but in reality it'll only be a lungfull, and the loop, with all the non-compressible parts like the cannister, has considerably more volume than that.

And...because of this, it'd certainly not be a good idea to drop the loop PO2 considerably below the dil ppo2, for the same reasons, if you have a whole loop full of very low po2 gas, and you only turnover a lungful, the loop is then still well below your dil po2 and if you're basing your breath strategy on starting with the loop po2 close the the dil, you might well go hypoxic eventually.

Valuable change in thought process here... glad I asked the question.

[Andy Phillips]

So am I, I like the fact that there is a bunch of us just starting on RB's. People keep asking questions where I had not worked out that there was a question to ask.

This is definatly something I will play with in Dahab, in fact I have started a list of things I want to do in the warm, clear waters of the Red Sea (anyone jealous yet )

Andrew

[NotDeadYet]

There is a spreadsheet somewhere on the net for passive SCR dive planning, which is essentially what you are doing. You enter your gas, depth and the dump ratio and it works out the ppO2.

No idea where to find it, perhaps ask Michael (db8us)?

[Juz]

http://dg8fz.dyndns.org/~karl/dg8fz/rebreather/scr.htm


http://www.therebreathersite.nl/rekenen_met_de_scr.htm
SCROLL DOWN THE PAGE FOR ENGLISH VERSION



Now I just have to read, re-read and understand the content.

If anyone beats me to it - and I suspect you will - please post in laymans terms.

Juz