Computer & variable point PO2

rwwiggins

Hello, I am an experienced closed-circuit diver and am considering my first rebreather purchase. I have a question about variable PO2 rigs like the KISS series which lack a fixed-point controller. Lacking something like a VR3 with real-time PO2 monitoring, how do you accurately monitor your bottom time / nitrogen absorption? Would my current dive computers (Niteks, FWIW) be of any use and how do you account for the PO2 jumping around a bit even at the same depth?

I'm truly perplexed by this and have found no existing info on this outside of the aforementioned VR3. Ultimately, the reason I ask is if there are cumbersome work-arounds but the invetiable solution is a VR3 I want to factor that extra cost in my decision if & what to buy. Thanks.

nurseling

I beleive that you progarm your ppo2 on your nitek at 1.1 anf then run 1.2
and it give you a fudge factor.

whaam68

Hi
the ppo2 doesn't "jump around" at a constant depth it drops relatively slowly from the "set point" dependent on your work rate. You bump up the PPO2 when you get to the bottom (doesn't need much as the increasing pressure will have taken it up a fair way just like on O/C)
If you change depth/work rate there are fairly obvious clues that you need to check the handsets & inject a little more O2 like the OPV firing on ascent or the ADV firing at constant depth because you've metabolized a lot of O2 finning hard. It's very intuitive with a little practice. For dive planning either use GAP or some other other deco programme to cut a table for your "set point" only be conservative so plan for 1.2 say & bump it up to 1.3, before it gets anywhere near 1.2 you will have noticed & bumped it back up a bit no sweat, if you do for whatever reason take your eye off the ball you can always compensate by raising the PPO2 above "plan" on the deco. Some dive computers like the HS or VR2/3's or a nexus on air dil run constant PPO2 settings & the first two can be integrated to read deco in real time from one of the O2 cells in the rebreather.
I hope that helps.
Mike

viscya

If you want to use the Nitek, then calculate your equivalent Nitrox for the setpoint and depth.

fraction O2 = set point / pressure ( ata )

For example, if using a set point of 1.2 and depth of 99ft

f O2 = 1.2 / ( ( 99 / 33 ) + 1 )

f O2 = 1.2 / 4

f O2 = 30%

Set your computer to 30% and keep the PO2 just above 1.2.

For no Deco dives this is close, but conservative. If you want to do Deco, then use tables or constant PO2 computer

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Ben Field

Shearwater

see- www.rebreather.ca

Like VR3 or HS Explorer except monitors all three cells like AP Vision.

Previous used Vyper set to bottom PPO2 (just pick the nitrox to match bottom theoretical mix as stated above) and/or PC based software tables- DDPlan, Proplanner etc etc.

PPO2 management and deco provision on the KISS is easier than you'd think, if its changing quickly then you are changing depth or "something" just happened- if you don't know what just happened (it wasn't ADV or you adding O2) then you have an issue...

Long story short- its works well and shouldn't be a major concern, to start with any computer, even and air one will suffice for deco so long as you know its not tracking O2 exposure and to start with that shouldn't be an issue either.

BEN

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Beware Fridge Suck

EBT

Play around with some deco software, you soon reach the conclusion that as long as your PO2 is roughly close to your intended it doesnt make a massive difference to your deco.

I dive tables + mental fudge and so far everything has been fine.

If you want to use a complicator, just figure out your FO2 based on a conservative PO2.

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rwwiggins

Thanks for info. I think I have a much clearer picture of what's involved. And for starters I meant to kick off the thread by saying I was an experienced 'open-circuit' diver not 'closed-circuit'. Gotta get used to the all this 'rebreather' terminology... Thanks again.