Question O2 sensor calibration

[mr_v]

Does anyone have an experience where an O2 Flush at 20 feet resulted in the discovery of a significant issue with one or more oxygen sensors? In other words, has anybody gotten valuable from this exercise?

In my experience, not really. But flushes may confirm prior concerns. For example, if a cell is slower to respond on a dive and spikes slower than the rest during O2 flushes, I mark it as a replacement candidate.

@justinthedeeps - yes, continuous PPO2 flushes may accelerate deco, but the practice is debatable, especially with long deco schedules. What is helpful is having a loop full of O2 anytime you're above 20'/6m. Remember, you're always diving hypoxic, especially if you're sure you're not ;-)

 

[Alekseolsen]

I would partly agree, though going 100% loop O2 (vs say ~87% O2 @1.4 @6m) can shorten final stop lengths in a deco planner (small and unnecessary in many cases--especially when helium off-gassing is driving deco, unimpeded by 20 or 50% nitrogen in a nitrox deco gas)

So agreed the flush makes sure the sensors will register a high O2 condition, but there is actually nothing magical about the 1.6 number for that, anything between 1.4+ and 1.8 would do (latter for the hardcore 9 metre flush peeps who don't mind a lip tingle or rare catastrophic seizure.)

For that matter, a really easy way to check for 1.6+ might be to vent the loop, go onto open circuit, and blast oxygen through the loop at a deeper depth. It's not science, but that's what dry land oxygen pressure pot testers were made for.

I learned that, for checking, you could simply do your O2 flush at 6m, change to OC and decend to 7 meters. Pretty similar as you described but allows O2 flushing with loop open

 

[justinthedeeps]

yes the degree of non-linearity will show up to a greater extent at 1.6 ppo2 than at 1.5 ppo2 and therfore, you know to replace the cell, if it can't get to 1.6 at all, you know you really have a problem.

Ok well like I said for those who really feel the need to see values way above what's relevant, you could easily test this by going off an emptied loop to OC and flushing oxygen deeper (some of us have even done it "accidentally")

The whole ritual of feeling like something's wrong when not getting 1.60 with a handset @ 5.99 metres on ~99% is kind of unnecessary though

Proper linearity checks involve a dry land oxygen pressure pot

 

[rddvet]

Also remember if coming off of trimix to start deco it is a little harder to maintain a high pO2 because you’re off gassing helium so rapidly. Of all the things that amazed me on ccr is watching how coming off of helium vs nitrox effected pO2. You can even kind of guesstimate where you are in deco based off of how your ability to maintain the pO2 changes.

 

[rddvet]

Does anyone have an experience where an O2 Flush at 20 feet resulted in the discovery of a significant issue with one or more oxygen sensors? In other words, has anybody gotten valuable from this exercise?

Yes. A couple times I caught a limited cell. Both times that cell was a little wonky compared to the rest so I already knew there was an issue.

 

[davehicks]

Yes. A couple times I caught a limited cell. Both times that cell was a little wonky compared to the rest so I already knew there was an issue.

That is pretty much my experience. The underwater calibration didn't tell me anything i didn't already know from observation during a dive and dry calibration to 100%

 

[rddvet]

That is pretty much my experience. The underwater calibration didn't tell me anything i didn't already know from observation during a dive and dry calibration to 100%

Yep, really only time it was helpful was when I had a good calibration and then the sensor was funny on a dive. It always seems to go that way for me and its not condensation. I'd say 80% of cells i replaced were good, then bad on the dive, then bad when I retested dry

 

[inquis]

The underwater calibration didn't tell me anything i didn't already know from observation during a dive and dry calibration to 100%

Diving at altitude, l can only see up to a PO2 of 0.8 pre-dive, which is simply not close enough to the setpoint. My pressure pot is at 27 ffw.

 

[justinthedeeps]

I guess I will stop counterpointing and agree...a bit
Information could be gained if sensors all reach 1.5ish in unison, but 1.6 was expected

The actual math on this is rarely done while diving though, and enough error (including those noted by OP) could be coming from other causes that I doubt it counts as a linearity check.

It sounds like this check is even more useful for seeing whether any particular sensors act differently, or are slow on the flush check, which is observable over a range of ppO2 that are not simply ==1.6

 

[justinthedeeps]

I wonder if/when CCR dive computers will track/store/test/alert to more information about sensor behavior, beyond just instantaneous ppO2 readings.

Beyond just flagging or voting out sensors based on ppO2 (or mV) alone, a modern dive comp could sense when a sensor is out of spec, responds slower than it did on the previous dives, or seems blocked--e.g. less mV change expected during depth change with no solenoid injection--or has an mV pattern that matches more complex known problems (with x% confidence), or track/profile/flag a more detailed history of individual sensor data over their whole lifetimes.

Many of us are already doing this ("keeping an eye on sensor #3...")

A few thousand detailed CCR dive logs with the relevant data could easily generate a 'baseline' profile for what normal sensor data should look like. Predicting outliers in realtime is increasingly trivial machine learning/classification

I *hope* this is included in whatever Shearwater is doing with the new default/mandatory clouding of any dive logs imported into its apps