Were you taught this math in your entry level CCR class?

[Capt Jim Wyatt]

If I turn on my rebreather and my cells show 0.21 and when I flush with O2 it shows 1.0 what more do I need to do? How is this process not safe. Please explain.

That is my approach and thinking too.

 

[joshk]

Whether the display swings between 1.0 - .021 or 47mv and 10 mv what's the difference? There is no linear drift. If it goes from .21 in air to 1.0 in oxygen how is it not linear.

Maybe it would be better graphed out. There's a potential for deviation from a linear response when you only check cells at 0.21 and 1.0. Checking for linearity past 1.0 is important because you could have a cell which does not response linearly.

If I turn on my rebreather and my cells show 0.21 and when I flush with O2 it shows 1.0 what more do I need to do? How is this process not safe. Please explain.

Is your setpoint never above 1.0? If not, how do you verify any reading past 1.0?

 

[wedivebc]

Maybe it would be better graphed out. There's a potential for deviation from a linear response when you only check cells at 0.21 and 1.0. Checking for linearity past 1.0 is important because you could have a cell which does not response linearly.

View attachment 429902



Is your setpoint never above 1.0? If not, how do you verify any reading past 1.0?

Nice graph, have you ever seen it happen in real life? A cell showing this mode of failure might get you bent, not toxed

 

[joshk]

Nice graph, have you ever seen it happen in real life? A cell showing this mode of failure might get you bent, not toxed

I have not seen a cell drift off 10%, but my sample size is a lot smaller than some other members. I'm just trying to clarify the linearity questions here and find a graph often cuts through a lot of confusion.

 

[victorzamora]

Nice graph, have you ever seen it happen in real life? A cell showing this mode of failure might get you bent, not toxed

I know this isn't in a CCR, but:

My Nitrox analyzer calibrated on air, read fine for EAN32, read fine for EAN50, and then read 75% when presented with oxygen. We double-checked it with another analyzer and the EAN32 and EAN50 readings were pretty close (within 1%FO2 if I recall correctly) but my oxygen bottle actually read 100% (or 98% or whatever it was) on the other analyzer. If it could happen there, it could happen higher...right?

 

[Jack Hammer]

I know this isn't in a CCR, but:

My Nitrox analyzer calibrated on air, read fine for EAN32, read fine for EAN50, and then read 75% when presented with oxygen. We double-checked it with another analyzer and the EAN32 and EAN50 readings were pretty close (within 1%FO2 if I recall correctly) but my oxygen bottle actually read 100% (or 98% or whatever it was) on the other analyzer. If it could happen there, it could happen higher...right?

I'd have to do some digging, this has been covered for analyzers and, IIRC, there is an article from Analox discussing this issue. Basically the cell will lose its ability over time to read higher PO2s while correctly reading lower ones. Kind of a separate issue from how cells are used in a RB and part of why they are changed more often.

 

[stuartv]

Nice graph, have you ever seen it happen in real life? A cell showing this mode of failure might get you bent, not toxed

Looking at that graph, what I'm getting is that if you have, for example, a setpoint of 1.4, by the time the cell actually "reads" 1.4, your actual ppO2 would be much higher.

So, if I understand it correctly, it WOULD put you at elevated risk of toxing. No?

 

[victorzamora]

I'd have to do some digging, this has been covered for analyzers and, IIRC, there is an article from Analox discussing this issue. Basically the cell will lose its ability over time to read higher PO2s while correctly reading lower ones. Kind of a separate issue from how cells are used in a RB and part of why they are changed more often.

I could be wrong, but I don't see how it's any different. My analyzer cell is the same style as used in CC applications. My analyzer cell went bad in a way that it was fairly linear through low PO2s, decreased its linearity, and then eventually was limited. I didn't try it in higher PO2 environments than FO2=1 at the surface as it wasn't built into a CCR for me to flood with O2 at 20ft to test it out at.

That aging/failure mode that I experienced would correlate directly to nonlinearity and limiting in CCR cells. It would easily lead to OxTox if it weren't checked at O2 (which some CCR divers don't do), and it would be a lot worse if that asymptote (for lack of a better term) were at PO2>1.0. As far as I'm aware, there's no reason why that limit should exist at PO2=0.75 but it couldn't happen at PO2=1.1 or whatever.

 

[_Slacker_]

Victor, your cell is "current limited". All the cells of this type are actually current limited, but it usually occurs at a value much higher than typical CCR ranges (greater than 1.6 but usually more) in a good/working sensor. As the fuel is consumed this limit comes into normal diving ranges and is one of the common failure modes. This is easily checked by spiking the O2 levels either at the beginning, during, or end of a dive (pre-dive if it is as low as yours or higher values in a pressure pot). Once a cell is said to be limited, it will not typically start working normally again, but a working cell can develop this condition at any time or gradually as it ages. That's why there are arguments about when/how to spike O2 levels.

 

[victorzamora]

Victor, your cell is "current limited". All the cells of this type are actually current limited, but it usually occurs at a value much higher than typical CCR ranges (greater than 1.6 but usually more) in a good/working sensor. As the fuel is consumed this limit comes into normal diving ranges and is one of the common failure modes. This is easily checked by spiking the O2 levels either at the beginning, during, or end of a dive (pre-dive if it is as low as yours or higher values in a pressure pot). Once a cell is said to be limited, it will not typically start working normally again, but a working cell can develop this condition at any time or gradually as it ages. That's why there are arguments about when/how to spike O2 levels.

Absolutely. Dave (wedivebc) asked if joshk had ever seen a cell behave like his fictitious graph in real life.

I was simply responding with a story about a cell in an analyzer (which wasn't on a CCR so I had limitations like PO2max=1.0 for testing and I couldn't take it to 1.6ata). The galvanic reaction in the cell is the same in the analyzer as it is in the rebreathers (except for the new Poseidon Solid State sensor), and all of the same mechanics and concepts apply.

My cell, not checked in oxygen at the surface, would've easily lead to OxTox due to the eery linearity below PO2=0.5 and the limit at ~PO2=0.75. However, even WITH a check in Oxygen....a similar cell could've been limited at or just above "typical" CCR setpoints but below the 1.6 or even 1.4 PO2 limits considered typical. That theoretical cell (similar to mine but current limited at a higher PO2 value) would be VERY dangerous without a check at 20ft. mV readings would also have been a very useful thing to know so I could've properly diagnosed (or confirmed the diagnosis of) the limited cell. In an analyzer, the consequences were far less dire than they would've been on a CCR....and they were fare less dire than they could've been even in an OC environment.