O2 Flush @ 19-23 ft / 6-7 m

[Heat Miser]

By way of background, I have about 80 hours on a rEvo rebreather micro expedition (CE Lung upgrade), which I bought 14 months ago and I have completed TDI Helitrox CCR Mod 1. So I've made a few mistakes in my rebreather journey I have flooded the unit twice, (my fault not unit error) but so far I have only had to replace 2 cells.


Specifically in the unit I have had 3 cells that where flooded twice (14 months old) and 1 cell once (8 months old) and 1 cell never (2 months old). Being paranoid and not having a pressure pot, I do a 02 flush at 6-7 meters after the majority my dives but at least once a month.


If I’m doing the 02 flush @ 7m and only getting the 5 cell readings to spike to 1.55-1.57 should I be worried that I can’t get them to spike to 1.6 on O2 that reads 99.5% at surface.


I find the dreams respond quickly to 02 flush, but I have my old cells in both of those dreams, and I have my newer cells in the shearwater petrel controller. The oldest cell (14 months and number 2) in the controller takes the longest to adjust but eventually 2-3 seconds get to 1.55.


No cells - not even the brand new one hits 1.6. thoughts please

 

[grantctobin]

Is this at start or end of dive?

 

[Whitrzac]

Its next to impossible to get all the nitrogen out of the cl. A single o2 flush on my kiss gets me about 1.5, a 2nd on will get me to ~1.55.

 

[Heat Miser]

Is this at start or end of dive?

end of dive

 

[Heat Miser]

Its next to impossible to get all the nitrogen out of the cl. A single o2 flush on my kiss gets me about 1.5, a 2nd on will get me to ~1.55.

so assume its two X seven second flushes....

 

[kensuf]

If the O2 flush is at the end of the dive, keep in mind you'll be off-gassing, which will be adding inert gas into the loop. This is especially true when diving helium mixes, you can watch your PO2 rapidly decrease the first few minutes when you hit a stop.

 

[grantctobin]

The other consideration is that your cells are wet, and doubly so if you’re pushing dive length. This is one of the reasons to avoid high PO2 on a CCR on long exposures and a reason to run linearity checks. You don’t know if 1.4 on cells = 1.4 breathing.

(Yes you can use richer external deco dil to validate, but outside of scope of use for average diver)

 

[Capt Jim Wyatt]

If I’m doing the 02 flush @ 7m and only getting the 5 cell readings to spike to 1.55-1.57 should I be worried that I can’t get them to spike to 1.6 on O2 that reads 99.5% at surface.

There is more to the story.

What do the cells read at 1 ata in air? lets assume they read at least 10mV--you can predict what they should read in 1 ata of O2 -- If they fall short of that prediction the cell may be current limited. Do the below test and see what you get.

1 atm / .21 atm = 4.76
eg. Cell 1 reads 11 mV in Air, Cell 2 reads 10.1 mV in Air, Cell 3 reads 12.5 mV in Air
To predict what each cell will read in 100% O2
11 X 4.76 = 52.36 mV for Cell 1.
10.1 X 4.76 = 48.0 for Cell 2.
12.5 X 4.76 = 59.5 for Cell 3.

 

[tbone1004]

search posts by @Bobby regarding cell linearity. 1.55/1.6=97% linear which is pretty bloody good and I wouldn't worry about it but you need to understand how linearity works as it is a critical piece of information about the health of your cells.
One thing to think about, particularly if you are running a ppO2 more than about 1.1-1.2 is doing a 1.6 check on descent to make sure the cells aren't current limited. What I do is O2 flush at the surface before I descent so the loop and my lungs are full of O2. Leave the ADV turned off and when you hit 20ft you check for 1.6, then open the ADV and add dil as normal on descent. Not practical on hot drops in the ocean, but it adds a lot of warm fuzzies to my diving.

 

[Capt Jim Wyatt]

you need to understand how linearity works as it is a critical piece of information about the health of your cells.

Absolutely agree.

It was taught to you in your rEvo class, at least it was supposed to be. The rEvo website has two good articles on "Understanding O2 Cells".