Oxygen sensor failure

[rjack321]

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.

Yep, CCR divers need to be:
1) replacing their cells at least 2x more frequently than in a surface analyzer
2) does not hurt to semi regularly test them for linearity above 1 bar O2 in something like this
https://www.narkedat90.com/Cell_checker_s/1839.htm

 

[rjack321]

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.

I'm guessing the actual lead anode is wiggling in the matrix when you tap or rotate that.
http://www.alphasense.com/WEB1213/wp-content/uploads/2013/07/AAN_009.pdf

 

[Storker]

You guys are forgetting that cells have an established mV starting point, for most analyzers its 9-13 mV

While my club's nitrox mixer always starts by telling me the voltage the cells are giving (and thus giving me the proper reality check on its percentage values), my personal Analox O2EII doesn't. Neither does my club's tank analyzer. So if I'm analyzing my tank's content, there's no way I can determine whether the cell in the analyzer I'm using has the mV starting point it should have.

 

[northernone]

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.

If that cell was in a personal o2 analyzer calibrated against air only prior to analyzing a nitrox tank how would it have been flagged? This is the real life situation we could
be discussing.

I caught it because I calibrate at 100% and check again against 21% (and ppo2 1.6 on the dive). Those who don't I suspect wouldn't have noticed because it was behaving perfectly fine at 21%.

As a side note:
There might be a case for the pointless nature of end user analyzing at all, due to the safety margins built in to oxygen exposure recreational limits and the rarity of a bad fill... But thats another topic. Any record of any any recreational dive fatalities due to faulty/failure to analyze whatsoever? What's the likelihood to get a bad tank and a bad analyzer to match expected % both the same dive as well as the actual gas being deadly enough to kill based on the planned dive profile... Seems a fairly limited chance.

That aside, it's a real failure state which the common practice of calibrating against air alone wouldn't reveal as a failure. I think this bares noting, even if fairly unlikely to harm.


@Johnoly love it! Thanks for posting that up.

 

[Storker]

I calibrate at 100% and check again against 21% (and ppo2 1.6 on the dive).

Geek asking a question: how do you "calibrate" against 1.6 bar pPO2 during the dive?

 

[northernone]

Geek asking a question: how do you "calibrate" against 1.6 bar pPO2 during the dive?

I misspoke, sorry. I sanity test @ ppo2 of 1.6. no adjustment is made on the spot underwater so not a calibration.

The o2 cell gets stuck in my rebreather and I drop down to 20ft on an o2 filled loop, if it doesn't give me the output I expect I know the cell is starting to go funny and it gets retired to dryland. Suppose the same could be accomplished by sticking an analyzer in a well sealed soft sided see through bag filled with o2 and take it on a dive with you... Or just built a pressure pot for dry testing o2 cells.

All that to say:

For ambient pressure analysis I am happy if I know the cell is able to operate within the range I need it to test. At least if its good at 21% and 100% my odds are better than only at 21% given a common failure is current limiting.

 

[Storker]

I sanity test @ ppo2 of 1.6. no adjustment is made

Thanks. I suspected as much.

For ambient pressure any analysis I am happy if I know the cell is able to operate within the range I need it to test.

FTFY

 

[rjack321]

SB is loaded with hypothetical risks. Sure your bizarro 21% ok but no higher than 25% sensor "could" cause a problem. But first: the cylinder would have to be filled with something higher than 25% (eg 32%) but the diver would have actually wanted 25% for the planned dive. Only then would they be in a situation where they were above the MOD of their gas.

If you had a sensor that maxed out at 32%, you would still have to have a fill that was erroneously filled with a higher %age, then used at 32% depths. A good example would be a partial pressure filled cylinder not topped off with air. But if you had a cylinder with only 400psi in it (400psi O2 + air to 3000psi = 32%) you wouldn't dive it anyway. The holes in the cheese are even less likely to align perfectly if the cylinder was filled by continuous blending since there's an analyzer on the compressor and concentrations greater than 40% are likely to explode.

This all comes back to analyzers being used to confirm what you should already know anyway, which is the rough percentage of gas in the cylinder.

 

[Storker]

SB is loaded with hypothetical risks

Some of the risks I consider in my personal risk assessment may well be hypothetical. Others may not be. It's still my personal risk assessment and decisive for which risks I'm willing to accept.

And while your personal risk assessments may well be different from mine, I still reserve the right to think (but not say) that IMNSHO you're crazy if they differ

 

[BRT]

I certainly don't analyze as many tanks as you guys do but I own a number of analyzers and have had the cells all fail over time. My experience is they act funky and it is time to get rid of them. Any odd readings are a good reason to bring in another analyzer or a new cell. I know how much O2 I am feeding my compressor so I know about what should be coming out. I analyze the input mix and the output mix. Obviously should more or less match the flowmeter. Then I check the tanks with a different analyzer when I mark them. If I have a tank that "should" be 32% and it measures low I'm going to try it on another tank that "should" be 32%. If that one reads low too I'm going to try another analyzer with a different cell. Mine have always failed by failing to calibrate, but all my analyzers have auto calibration for air. If they calibrate they have always worked to 32%, but that is all I pump. I have occasionally checked them against a tank of O2 and they never read over 96% or so.