Oxygen sensor failure

[northernone]

You guys are forgetting that cells have an established mV starting point, for most analyzers its 9-13 mV. You can't just start with "any" voltage reading and somehow play with the dial enough to get it to calibrate. Sure mathematically you could put a 1mV cell in there are turn up the potentiometer to the max and get it to read 0.21 and the cell wouldn't be capable of reading past 0.25 and that would be "dangerous". But the reality is that doesn't happen because while a 8mV in air cell could calibrate to 0.21 and read (perhaps) up to 0.95 once that cell falls to 5 or 6 mV in air its not going to read 100% at all. Instead of thinking your deco mix is off you suspect the cell, in part because you cranked up the potentiometer just to get it to read 0.21. By the time a cell won't read 0.32 or 0.36 its so far gone that you can't even get it close to 0.21 at the start, so even with a "one point" calibration you notice the issue. The reality is that OC divers aren't being killed by failed analyzers, its when they don't use an analyzer at all.

This would be an important point. Any tests showing the potentiometer range in the common o2 analyzers? I quite certainly can crank up a low cell in the few I've tested.

 

[Storker]

By the time a cell won't read 0.32 or 0.36 its so far gone that you can't even get it close to 0.21 at the start,

I believe that the data quoted in @northernone 's post #15 contradicts your claim.

 

[northernone]

I believe that the data quoted in @northernone 's post #15 contradicts your claim.

I've only seen it once in 24 o2 cells so it's fairly rare I think?

It's the non linear failures an oxygen cell is capable of that frighten me. If they all always just progressively read lower and lower outputs in a linear fashion until they current limited I think we'd have less dead rebreather divers.

Maybe it's rare enough to be irrelevant for improving safety on OC?

 

[Storker]

If they all always just progressively read lower and lower outputs in a linear fashion until they current limited I think we'd have less dead rebreather divers.

I'm not a rebreather pilot, but of all rebreather fatalities caused by faulty cells, how many are caused by hypoxia and how many are caused by oxtox? If the RB cells fail due to current limiting, the breather control system would underestimate the pPO2 in the loop, inject too much O2 and hyperoxia/oxtox would be the reason for dying. I don't know which other non-linearity failures one should expect in an O2 cell, but if it's non-linear with a positive 2nd derivative, hypoxia (or DCS) would be expected.

 

[Storker]

But in any case, us rec OC divers would be reasonably safe from most O2 cell failures by analysing our tanks with two different analyzers. And IMNSHO analyzing just after filling for marking the tank and then analysing at the site is good practice anyway since it also provides a bit of a safety margin towards faulty tank marking, relying on our memory or someone tampering with the tank between filling and diving.

 

[rjack321]

This would be an important point. Any tests showing the potentiometer range in the common o2 analyzers? I quite certainly can crank up a low cell in the few I've tested.

I dunno but mine (3 analysers although one has a dead cell atm) are all a fairly narrow range, based on the % output plus minus ~3 mV.

I believe that the data quoted in @northernone 's post #15 contradicts your claim.

Perhaps but I focus on what actually harms divers, not the mathematical threat. Clearly this cell was flagged as old or "weird" or something else (which is why it's on the bench) before it actually harmed anyone. Based on accident reports I've generally followed here and in the yahoo and FB groups for over 15 years, there have been zero OC deaths due to failed sensors in analyzers that were actually used. Seems like a non-problem.

 

[rjack321]

I'm not a rebreather pilot, but of all rebreather fatalities caused by faulty cells, how many are caused by hypoxia and how many are caused by oxtox? If the RB cells fail due to current limiting, the breather control system would underestimate the pPO2 in the loop, inject too much O2 and hyperoxia/oxtox would be the reason for dying. I don't know which other non-linearity failures one should expect in an O2 cell, but if it's non-linear with a positive 2nd derivative, hypoxia (or DCS) would be expected.

Pretty much all cell related CCR deaths are due to having multiple (eg 2 out of 3) old current limited cells leading to hyperoxyia

It is possible to have a cell fail high (eg a perforated membrane or faulty temp compensation circuit). BUT this is both rare and not a function of age so pretty much never happens to 2 of 3 cells at the same time. And they still have some output so if (a huge if) 2 of 3 cells were somehow to fail high and have a similar enough failure that they were within 10% of each other so the machine voted them as "good" together your true ppO2 might be low but is exceptionally unlikely to be so low that you have hypoxia. You're inerts are just way higher than you think so you'd get bent not pass out.

 

[Storker]

Clearly this cell was flagged as old or "weird" or something else (which is why it's on the bench) before it actually harmed anyone

AFAIU the cell had been used in a rebreather, and rebreather cells have higher demands than rec OC O2 analyser cells have. Which means that RB pilots should have stricter cell test procedures than rec OC divers need. Most of us rec divers calibrate single-point against ambient air (20.9%) and are good with that. That particular cell would have passed that test while being unable to tell me that the tank I thought I'd filled with 28% really contained some 36-40%. Which might well have turned out nasty if my dive was planned to 40m.

It might well be a minor problem in real life, perhaps partly due to the fact that you don't necessarily die a gruesome death if you exceed 1.6 bar pPO2 for a short time, but I personally prefer to be quite certain that I'm breathing the mix I think I'm breathing. If that's overthinking it, I can live with that.

 

[Storker]

Pretty much all cell related CCR deaths are due to having multiple (eg 2 out of 3) old current limited cells leading to hyperoxyia

Thanks

 

[Johnoly]

There are sensor troubles and also analyzer component troubles.....

Ohh I have one that is even W I L D E R that you will love.

I was getting crazy drop readings when an air stream sample was being measured like we normally do to check tanks.

Background info:
....Sensor is original equipment replacement, not aftermarket imitation.
....Sensor is about 15 months old, came in a sealed bag.
... Analyzer machine is about 5 years old
... Everything sent back to the OE for testing, all replaced at discounted price.

Still never found out the final failure, but suspect a 'loose' component solder joint on the sensor pcb that flexed when air pressure introduced.

Here's the 1 minute youtube crazy video of it.