Oxygen Sensor Fundamentals

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@tbone1004

Okay, so my Predator controller and my NERD2 both show ppO2 in hundredths. And the rEvo is a single-point calibration. So, you're in agreement that there really is no use for me to be checking mV readings during a dive?

I do check mV readings, usually when I calibrate. If a sensor used to output 50mV in O2 and now it's calibrating to O2 at only 45mV, I want to know that, so I can be aware that it might be going off. But, other than that, I have not found a reason to look at actual mV outputs. Nor do I recall being instructed to pay special attention to mV during a dive (vs just paying attention to ppO2) during my training.

Watching Joe's presentation left me wondering if I was really missing something. I'm glad to know that (apparently) I was not.

Thank you for the clarification.
 
Joe Citelli is awesome. Thanks for posting this presentation.

I have a couple of questions:

In the middle part, where he first talks about linearity, he goes through an example of a cell being non-linear by 10%. So, in his example, your computer is telling you you're at a ppO2 of 1.0 and you're really at 1.1.

So, I don't know about any other CCRs, but the Shearwaters on my rEvo all calibrate to O2. So, no matter what the linear deviance is of my sensor, if the computer says I'm at 1.0, then I'm at 1.0. Right? And if it says 1.1, then I'm really at 1.11, and if it says 1.3, then I'm really at 1.33. Right?

Second, during the summation at the end, Joe talks about putting a piece of tape on your controller and writing down the expected millivolts for each sensor at a ppO2 of 1.0 (and possibly other ppO2 values). He didn't go into great detail on how to use that, but if I understood him correctly, the idea would be that during your dive, you can check your displayed ppO2 against the millivolt readings.

For example, if your expected millivolts are 47 for a ppO2 of 1.0, and you're going along on your dive, you adjust your loop until the ppO2 reading (of the sensor you're checking) is showing 1.0, then you (if it's a Shearwater) tap the right button a few times to check the millivolt reading and confirm that it is showing 47.

I don't understand how that makes sense. Isn't the displayed ppO2 simply the current millivolt reading multiplied times whatever factor was set during calibration? So, after calibration, if it shows ppO2 of 1.0 and 47 millivolts, then won't it ALWAYS show ppO2 of 1.0 for a reading of 47 mV, until it gets calibrated again?

I don't understand how noting the expected millivolt readings and checking them during a dive is of any use at all. If I understand how these things work correctly, then the ppO2 reading will always directly relate to the mV reading, so you can just check that (the ppO2). And, the only way to check it (the ppO2) for accuracy is to either do a dil flush and compare against the Dil pO2, or do an O2 flush (if you are shallow enough). Anything else is just looking at an unknown gas and comparing the displayed ppO2 against the displayed mV reading (which will always match, for any given calibration).

I feel like I must be missing something here. :-(

This is all presuming single-point calibration, as that is the only thing I have experience with.

You are correct in your thinking. The new megs no longer do 2 point calibration and haven't for a few years now. I believe APECS 2.7 was the last iteration of the 2 point cal. The PO2 value will be a number based on the MV multiplied by the conversion factor established during calibration. The most value I see in recording the MV is to check the number against air then confirm the value in O2 is 4.76 times the air value which assures me I obtained a reasonable O2 saturation of the cells before commiting the calibraion value to the memory. It is highly unlikely a CCR will lose it's calibration during a dive and if it did the number would likely be so ridiculously out that you would immediately notice. There was a flaw in the early APECS 2.5 computers where a small number of them would lose the calibration but then the reading would go to zero.
 
You are correct in your thinking. The new megs no longer do 2 point calibration and haven't for a few years now. I believe APECS 2.7 was the last iteration of the 2 point cal. The PO2 value will be a number based on the MV multiplied by the conversion factor established during calibration. The most value I see in recording the MV is to check the number against air then confirm the value in O2 is 4.76 times the air value which assures me I obtained a reasonable O2 saturation of the cells before commiting the calibraion value to the memory. It is highly unlikely a CCR will lose it's calibration during a dive and if it did the number would likely be so ridiculously out that you would immediately notice. There was a flaw in the early APECS 2.5 computers where a small number of them would lose the calibration but then the reading would go to zero.

Thank you for that.

To your point about checking the mV readings to confirm a good saturation, I feel like I am achieving the same check by way of how my checklist goes. I calibrate to O2 first. Then, later in the check list, I do a full dil flush and check that the handset is showing the expected ppO2 for my dil. I *think* that gives me the same confirmation that the sensor had a proper saturation of O2 at calibration.

In addition, I always look at my ppO2 reading for my O2 before I calibrate, and I only calibrate if it's off. But, if it's way off, then I know something is wrong. I.e. if my sensors are showing 0.96 or 0.97, I will do a Calibrate (which calibrates to 0.98). But, if they were showing 0.93 or something, then I would know something was wrong - like possibly that the sensors were not properly saturated yet.

Does that sound reasonable to you?
 
Thank you for that.

To your point about checking the mV readings to confirm a good saturation, I feel like I am achieving the same check by way of how my checklist goes. I calibrate to O2 first. Then, later in the check list, I do a full dil flush and check that the handset is showing the expected ppO2 for my dil. I *think* that gives me the same confirmation that the sensor had a proper saturation of O2 at calibration.

In addition, I always look at my ppO2 reading for my O2 before I calibrate, and I only calibrate if it's off. But, if it's way off, then I know something is wrong. I.e. if my sensors are showing 0.96 or 0.97, I will do a Calibrate (which calibrates to 0.98). But, if they were showing 0.93 or something, then I would know something was wrong - like possibly that the sensors were not properly saturated yet.

Does that sound reasonable to you?
Yes, I alway equate calibration to tuning my guitar. If I strum a couple of chords and they sound good I don't tune it. Doppler did an article a while back about the level or precision we use in technical diving and whether close is close enough (spoiler alert, it usually is)
 
Okay, now I feel REALLY lost.

I get to 6m and do an O2 flush and you're telling me that I won't be able to detect a sensor that is current limited to the equivalent of 1.2 ATA ppO2?

I thought that was the whole point of the O2 flush at 6m. If a sensor doesn't read around 1.55 ATA ppO2 or better, then it is current limited.

Sorry you are correct, I blame lack of diving / coffee / adult supervision.
 
Thank you for that.

To your point about checking the mV readings to confirm a good saturation, I feel like I am achieving the same check by way of how my checklist goes. I calibrate to O2 first. Then, later in the check list, I do a full dil flush and check that the handset is showing the expected ppO2 for my dil. I *think* that gives me the same confirmation that the sensor had a proper saturation of O2 at calibration.

In addition, I always look at my ppO2 reading for my O2 before I calibrate, and I only calibrate if it's off. But, if it's way off, then I know something is wrong. I.e. if my sensors are showing 0.96 or 0.97, I will do a Calibrate (which calibrates to 0.98). But, if they were showing 0.93 or something, then I would know something was wrong - like possibly that the sensors were not properly saturated yet.

Does that sound reasonable to you?

StuartV,
You are correct in what you said and how you are looking at it.

The reason for looking at mV is to ensure that you are calibrating correctly from the very start then checking your calibration there after. Assuming the calibration is correct is what gets a lot of CCR divers in trouble from the start. Calibration is the most critical and easiest to get your PPO2 output wrong. If your unit is properly calibrated then there is no need to calibrate it for a dive. When people don't look at mV, then they calibrate, and have linear deviation their PPO2 output will be off. The 1.6 check verifies that you have proper calibration by checking the mV. Many CCR divers also either under or over saturate their cells during calibration, looking at mV verifies that the diver has proper O2 saturation of their cells.

Many people are taught to simply calibrate for every dive and they never look at their mV or check for linear deviation and/or limiting. It is also common for people to not be taught to do a 1.6 check at the start and end of the dive, which will verify calibration, verify linear deviation of a cell (s), and show cell limiting. On deep dives a gas switch with a known gas of 1.6 with a full dil flush with verify cell deviation and/or limiting when wanting to raise your PPO2 to optimize decompression. For these reasons it is suggested to note the mV at 1.6 to verify. Many people just crank up their PPO2 setting on their controller and assume that all it good. Linear deviation can cause a much higher PPO2 than displayed and limiting can cause constant O2 inject on an ECCR or over addition of O2 on an MCCR. Humidity limiting is also very real and has gotten many CCR divers into trouble with elevated PPO2's on longer/deeper dives.
 
StuartV,
You are correct in what you said and how you are looking at it.

Many CCR divers also either under or over saturate their cells during calibration

Thank you, Bobby.

To your additional comment there, I never really considered this until just recently. Meaning, I do my check/calibration exactly by the rEvo checklist and it seems like a highly reliable way to do it.

But, I was talking to another rEvo diver recently. One who is even newer than I am, having just completed his first training in Feb, I think he said. But, he's already got 40 hours on the loop, so I figured he must have his sh!t together pretty well by now.

He mentioned having a problem with his unit (which his instructor custom ordered for him, so it was brand new when he got it). He described some wild O2 readings when he descended down to 15' at the start of his recent dive.

It turned out that the way he says he was trained to calibrate his unit is to do an O2 flush and close the loop. Then turn his O2 off. Then, when ready to actually calibrate his controller (a Petrel 2), briefly open the loop (to let the pressure in the loop drop to ambient) and close it again. THEN perform the calibration.

I was, well, shocked. That process seems like a perfect example of what you're talking about. SO prone to result in oversaturation of the sensors during calibration.

I would never have guessed that people would be doing their calibration like that. I'm always happy to learn something new, though! Even when it's a case of learning a new way to NOT do it.... :D
 
@stuartv
over saturation occurs when the loop itself is pressurized during calibration. His instructors procedure is fine. Do a full O2 flush and close the O2. This makes sure that the unit is flooded, but also allows you to do your positive pressure checks on the unit. Briefly opening the loop is there to remove any built up pressure in the loop that may have occurred due to a leak or being overzealous during the O2 flush.
That shouldn't have given any wonky ppO2 readings. If he had wonky readings, it would have likely been due to a bad calibration, most likely due to an inadequate O2 flush. This is where checking the mV's is important because you have expected mV's based on no deviation or on historical deviation from the cells. That's what you have to look at during the O2 flush to make sure that the mV's stabilize.
 
@stuartv
over saturation occurs when the loop itself is pressurized during calibration. His instructors procedure is fine. Do a full O2 flush and close the O2. This makes sure that the unit is flooded, but also allows you to do your positive pressure checks on the unit. Briefly opening the loop is there to remove any built up pressure in the loop that may have occurred due to a leak or being overzealous during the O2 flush.
That shouldn't have given any wonky ppO2 readings. If he had wonky readings, it would have likely been due to a bad calibration, most likely due to an inadequate O2 flush. This is where checking the mV's is important because you have expected mV's based on no deviation or on historical deviation from the cells. That's what you have to look at during the O2 flush to make sure that the mV's stabilize.

Sure. Except that the rEvo has a CMF. So, what happens when you close the loop and then turn off the O2? The HP line bleeds down into the loop, creating an overpressurization.

Then what happens when you "burp" the loop (as he put it), but don't leave it open long enough for the pressure inside to fully drop to ambient?

What happens when you turn off the O2 and then burp the loop before the HP line has finishing bleeding down to ambient inside the loop?

And, what happens if you miss one step and don't turn the O2 off? That is what the guy told me thought he had done. He *thought* he forgot to turn the O2 off before he burped the loop and then calibrated his computer.

The point is not whether the process works when done exactly correctly. The point is how vulnerable it is to errors.

In contrast, the rEvo check list is:

- suck the loop down to a negative
- flush the loop with O2 until the OPV pops
- repeat 2 more times
- close the loop after the 3rd time, while it still has positive pressure
- reopen the DSV just a crack
- check/calibrate computer

Much less vulnerable to errors, in my opinion. I can't see why anyone would do it any other way. Even if someone is worried about not "wasting" O2, the whole process is so quick that the difference between turning off the O2 before or after doing the calibration is literally negligible.
 
@stuartv and that again is where if he had paid attention to the mV's, then he would have caught that. You can see the voltage move with pressure. With a CMF though it certainly does increase the risk of an issue with calibration. My Mod1 course included ignoring the actual ISC checklist and making my own as we assembled the unit. After I thought I had it sorted, we compared against the ISC checklist and adjusted as necessary. My checklist changes every once in a while as a function of missing something that I didn't think needed to be on there. Every time I miss something, it gets put on the check list.
It sounds like your buddy didn't have that on his checklist, or didn't actually follow it properly.
 

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