Hybrid Rebreathers

UWSojourner

I thought it was about the same amount of time no matter what the depth (assuming same starting PO2 and O2 consumption rate)?


Are you sure?

I'm not entering the general discussion about wisdom of this setup, just have questions about the statements above.

PaulTG2

I gave this quite a bit of thought a while back. Based on my logic I believe the following is true:

Let's assume a 1atm total loop volume of 8 liters. I made this number up, but it probably isn't unreasonable to have that amount of gas in a loop (anyone have more a more accurate number?). Remember, we are talking about total volume, not just the counter lungs. This would include the volume of the gas in the entire loop including hoses, counter lungs, scrubber, etc.

Any time you have a PO2 of 1.0 your loop will contain exactly the amount of O2 as the 1atm loop volume. If your total loop volume is 8 liters then you have 8 liters of O2 in the loop at any depth you have a PO2 of 1.0 (Maybe take a little off for exhalation counter lung perhaps). To get down to a PO2 of .2 you will have to get down to a O2 volume of 1.6 in a 8 liter loop volume. That leaves 6.4 liters of O2 before heading to a hypoxic mix. At a 1 lpm O2 consumption rate that gives you 6.4 minutes of O2 before your PO2 reaches starts become hypoxic.

If you run a PO2 of 1.3 you would have 8 x 1.3 or 10.4 lts or O2. To get to a PO2 of .2 you would have to use 8.8 liters of O2 -- or almost 9 minutes at a 1.0 lpm O2 comsumption rate.

That amount of time you have is dependent on the loop volume and initial PO2 set-point. It doesn't vary with depth.

To me that's a pretty good buffer, even at higher consumption rates.

Anyone see a hole in my logic?

-p

paulraymaekers

at the same metabolic consumption, PO2 drops at the same rate, independant of depth, if you keep your counterlung volume constant (ADV). (if not, near surface PO2 will stay a bit higher but then you will hit the bottom of your counterlung faster)

anyway, the time it would take to breath out all the oxigen (if you could) is in both cases the same

reason: with the same PO2, you have the same amount of oxigen available: that's what PO2 is about

paul

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.... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials...

Brainx3

Hi,


Mike, the hybrid system can be used in parallel with the primary system of the unit OR as a backup for the primary system in case the primary system failed and you don't want to terminate the dive OR have too much DECO time in which you want to swim it around. Everything has at least 1 failing point so again I say that nothing would be perfect. I don't see a huge risk of attaching an orifice which would take over in case my solenoid is not working due to electrical or mechanical problems. I don't see it relavent to bring up sex as an example. IF you have sex with your wife then why using a condom? The same is, if you know your unit and trained on it then you would have confidence diving it (As long as you understand the concepts, have proper training and a good risk management).

Dave, you have interesting experience on different units, would you mind explaning to us how do you use the hybrid system once you are diving it?


Best Regards. Wael

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divelermentov

plus if you have to shut the eccr tank down you do not loose the manual injection in case of a hybrid utilix\zing two tanks

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paulraymaekers

Hello Wael, on all dives less than 80m, i have the standard setup: fixed MP, orifice and solenoid both active,
then depending of the type of dive, one time I fly the unit manual, controller setpoint at 0.7, pure CMF type diving, and adding ox when I get an orange blink on the HUD's, other time I fly the unit automatically, controller on high setpoint.
when, well if i have nothing to do, manual, if I have higher workload, automatically, or even, depending on the 'moud' (??)

this way the manual flying reflex is maintained, the PPO2 verifiing reflex is there..

only on + 80 m dive, I block the orifice, and take off the 'fixed MP' cap from the first stage, and then it's pure eCCR
in that case, at deco I have the setpoint low and adjust manually

so in my config I use only one ox tank (for the moment :-)

paul

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.... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials...

Brainx3

Hi,


Thanks for the information, Paul. The revo sounds an interesting unit for sure. I have never dived one so far, but would surely like to try in order to compare it with the my Vision. What kind of orifice you use? What's the IP for the solenoid side?


Best Regards. Wael

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paulraymaekers

orifice .0035, pressure 10-11 bars

paul

and before you raise the question: yes, the solenoid can handle 11 bars: it handles up to 17 bars..

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UWSojourner

Thanks for the reply. I would not call it a hole in your logic, but I would add a caveat just to close the loop (so to speak ) ...

The scenario under consideration assumes a diver ascends from 50m starting at a PO2 of 1.3 without additional gas added to the loop.

If the diver ascends at a rate of 20m/min and consumes 1 lpm of O2 along the way, the diver will surface with a PO2 of about 0.11, with some variation depending on loop size and other variables.

In this case, within just a couple of minutes (not 8 or 9) a very comfortable PO2 has become life threatening. Ascent should always be considered a time for extreme diligence.

Dave Sutton

I turn the eCCR system on, allow the system to hold setpoint, and ignore the orifice.
PP02 stability ends up better and if the pod craps out mid-expedition I can just keep diving.



Let me ask: "Hey Lass: DrMike wants to know if you're cute?"....
Yeah: She says she is and they always know best.

Weather looks crap all week though and looks like our main target might be tough to get to.



Dave



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