Oxygen Sensor Fundamentals

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@JonG1 and @stuartv
I do not run hot bottom mixes often. Most of my diving is cave oriented so it's long duration and I usually run 1.1. I usually run EAN32 and my dil ppO2's will often be 1.2 in certain caves. That does mean that a dil flush raises my ppO2 and I have to breathe it down. For reference that is usually 4ata diving with EAN32. In contrast, a more "traditional" configuration would use air for that dive and the dil ppO2 would be .84. With 0.84, you get to see the dil drop all the way down and then watch how the cells respond as you breathe it back up. For cave diving, the hotter dil mixes also give us the flexibility to use SCR mode in the event of either electronics or O2 supply failure without serious risk of hypoxia or a wicked premium to pay in deco time.
If diving EAN32, you would write down your expected mV's based on calibration data and linearity, and when you were at 4ata's, you would dil flush and validate both the ppO2 and mV's against what you have in wet notes.
Why is this important. IMO you should only dive up to a ppO2 that you are able to actually validate which is your dil ppO2. The 1.6 check at the beginning of the dive is critical, however what it can't do is show you how your cells are doing after 2-3 hours on the loop where they may start to behave differently than when they were nice and cool/dry at the start of the dive. They have now been fully saturated with humidity and are often times at a much higher temperature than they were at the beginning depending on surface conditions. The computer tells you the ppO2 that you are supposed to be at on the dil or if you're diving archaic readouts you can always just write it down somewhere, and then you have the mV's to check against just to make sure everything is behaving. With single point calibration units as said above, I think the mV validation is not nearly as critical as it used to be where you had two opportunities to screw up a calibration, but my Meg 2.7 does still require 2 calibration points, and you do not know that the calibration was correct until you do that 1.6 check. If the air cal point was wrong and the cells are acting super goofy, you at least have the mV's written down and can run by those.

On the subject of what @Bobby was getting at. If you are only running around 1.0 for the bottom, but you intend to run at 1.6 for deco, then you need the ability to plug in a 1.6 gas to the unit before you should feel comfortable running around at that high of a ppO2. I don't use onboard bottles because they're basically useless for cave diving and I firmly believe in the dilout concept. Onboard bottle does make a most excellent inflation bottle though for either suit, wing, or suit & wing depending on what you're doing. You have to have the 1.6 bottle for your planned OC deco ascent anyway and I have the ability to plug all of my bottles into something. Depending on your setup and preference that would either be a LPI or a QC6/CEJN on a regulator hose. The ascent schedule then has the same gas switches as you would for an OC ascent where you unplug your backgas and plug in the deco bottle following the same gas switch procedure as you would if you were doing an OC dive. I will sometimes make an exception to that for O2 stops where I will often stay on the onboard O2 if it's the last dive of the day. If it's first dive, I usually plug in since the O2 flushes on deco tend to kill the small bottles pretty quickly.
Why can't you just run a 1.3 and use O2 to bump it up? You have no earthly idea how much O2 to add. If the cells are limited, and heaven forbid they all are, you can keep adding O2 until the cell says 1.6 but the actual ppO2 may be well higher and deep into oxtox territory.
 
If you run a real lean dil mix that some people argue for, then you don't have the ability to do any ppO2 validations via dil flush, only the ability to see if they're moving.

I don't understand this comment, can you explain?
 
I don't understand this comment, can you explain?

That was a bit confusing. It doesn't have the ability to give any valuable ppO2 validations. If your bottom ppO2 is 0.8, who cares unless you are diving at 0.8? If you're trying to run a 1.2, then a 0.8ppO2 validation does not do you a bit of good. It can tell you how fast they respond to 0.8 and back up to 1.2, but does nothing to validate calibration/linearity/current limiting. I'm a firm believer that dil should be a minimum of 1.0 since it at least allows you to validate calibration, but should ideally match the ppO2 that you intend to run.

One of the main reasons that people argue for dil flushes is to make sure the cells are doing what they're supposed to. What is better than a validation of your calibration at 1.0 or a validation that your cells are doing what you expect them to do at the ppO2 that you want to be? Fringe benefits of dil flushes. Gets humidity out of the loop which can be a bit irritating to your respiratory system, cools the whole loop down if you're in warm water, can help to dry out cells depending on where the addition point is *obviously unit specific*, I'm sure I'm forgetting several others.
 
@JonG1 and @stuartv
I do not run hot bottom mixes often. Most of my diving is cave oriented so it's long duration and I usually run 1.1. I usually run EAN32 and my dil ppO2's will often be 1.2 in certain caves. That does mean that a dil flush raises my ppO2 and I have to breathe it down. For reference that is usually 4ata diving with EAN32. In contrast, a more "traditional" configuration would use air for that dive and the dil ppO2 would be .84. With 0.84, you get to see the dil drop all the way down and then watch how the cells respond as you breathe it back up. For cave diving, the hotter dil mixes also give us the flexibility to use SCR mode in the event of either electronics or O2 supply failure without serious risk of hypoxia or a wicked premium to pay in deco time.
If diving EAN32, you would write down your expected mV's based on calibration data and linearity, and when you were at 4ata's, you would dil flush and validate both the ppO2 and mV's against what you have in wet notes.
Why is this important. IMO you should only dive up to a ppO2 that you are able to actually validate which is your dil ppO2. The 1.6 check at the beginning of the dive is critical, however what it can't do is show you how your cells are doing after 2-3 hours on the loop where they may start to behave differently than when they were nice and cool/dry at the start of the dive. They have now been fully saturated with humidity and are often times at a much higher temperature than they were at the beginning depending on surface conditions. The computer tells you the ppO2 that you are supposed to be at on the dil or if you're diving archaic readouts you can always just write it down somewhere, and then you have the mV's to check against just to make sure everything is behaving. With single point calibration units as said above, I think the mV validation is not nearly as critical as it used to be where you had two opportunities to screw up a calibration, but my Meg 2.7 does still require 2 calibration points, and you do not know that the calibration was correct until you do that 1.6 check. If the air cal point was wrong and the cells are acting super goofy, you at least have the mV's written down and can run by those.

On the subject of what @Bobby was getting at. If you are only running around 1.0 for the bottom, but you intend to run at 1.6 for deco, then you need the ability to plug in a 1.6 gas to the unit before you should feel comfortable running around at that high of a ppO2. I don't use onboard bottles because they're basically useless for cave diving and I firmly believe in the dilout concept. Onboard bottle does make a most excellent inflation bottle though for either suit, wing, or suit & wing depending on what you're doing. You have to have the 1.6 bottle for your planned OC deco ascent anyway and I have the ability to plug all of my bottles into something. Depending on your setup and preference that would either be a LPI or a QC6/CEJN on a regulator hose. The ascent schedule then has the same gas switches as you would for an OC ascent where you unplug your backgas and plug in the deco bottle following the same gas switch procedure as you would if you were doing an OC dive. I will sometimes make an exception to that for O2 stops where I will often stay on the onboard O2 if it's the last dive of the day. If it's first dive, I usually plug in since the O2 flushes on deco tend to kill the small bottles pretty quickly.
Why can't you just run a 1.3 and use O2 to bump it up? You have no earthly idea how much O2 to add. If the cells are limited, and heaven forbid they all are, you can keep adding O2 until the cell says 1.6 but the actual ppO2 may be well higher and deep into oxtox territory.

This is all well and good on those 110ft max caves.
But completely falls apart on 1) wreck dives where you aren't at the deepest point or 2) caves that arent basically flat tunnels at a consistent depth. Sure in FL your 32% is a great cave dil. And its fine in MX because even on a 40-60ft deep cave there is no reason to dive greater then 1.0 anyway as you wont have any deco anyway.

But staying close to your area, this doesnt work in Eagles Nest nor Indian nor anything else that goes up and down beyond the nitrox range. It also falls apart in non-tourist caves because you have no idea what the max depth might end up being. I laid almost 2000ft on line last summer at a max depth of 132ft on 18/45 because I had no idea if the max was going to be 130 or 150 or 200. And given the size of the cave and the water temps (4C) I decided 200ft was about as deep as I was willing to go. It was still a 3hr dive including 45mins of deco.

Have you ever done a 50% gas switch on CCR and where was that? I haven't even on 250ft cave dives.
 
@rjack321 the idea is that you would at least choose the same dil gas that you would choose if you were diving OC vs. many that go from air dil and then straight to something like 12/70. I do believe in standard gases vs. best mix so obviously in a cave like Peacock or most all in Mexico, or a myriad of ocean dives that you would choose EAN32, you aren't really going to be diving some obscure mix.
The argument however is that you don't dive a ppO2 higher than either the ppO2 that you can validate with your dil, or 1.0 that you calibrated against. In most of those shallow caves, you aren't going to have deep stops where you would want to crank the ppO2 up to 1.6 until you get to your 20ft stop where you can flush with O2 and the whole argument goes away once you hit 20ft.
In your example, the decision there is to look at the gas mix you have and what its bottom ppO2 will be, 0.9 in this case, and say that because you can't validate any ppO2's above that, you're not going to run a 1.4 bottom ppO2 because you just don't know. ppO2 stays around 1.0, I'd personally be comfortable up to about 1.2 on a cave dive, and call it a day. If the deviation is less than 1 which is usually is, then your ppO2 is higher but not by an amount that will likely be toxic and for decompression it is rounding to safety. At 132ft with only 45mins of deco, you probably didn't have any really deep stops where you would be pushing the ppO2's above that to save any amounts of deco so I wouldn't swap dil on that dive.

I have done 50% gas switches, but not really out of necessity. My GF choices and run times haven't really gotten to the point that a difference in 1.2 vs 1.4-1.6 for the deeper than 20ft stops makes a real difference in TTS. Bobby was doing some extreme dives in terms of depth and duration, all of which occurred before the big shift in GF's where the deep stop durations where really excessive. If I had a dive plan where the difference between 1.2 and 1.4-1.6 for my deeper than 30ft deco stops amounted to better than probably 15mins for TTS then I would switch.
 
That was a bit confusing. It doesn't have the ability to give any valuable ppO2 validations. If your bottom ppO2 is 0.8, who cares unless you are diving at 0.8? If you're trying to run a 1.2, then a 0.8ppO2 validation does not do you a bit of good. It can tell you how fast they respond to 0.8 and back up to 1.2, but does nothing to validate calibration/linearity/current limiting. I'm a firm believer that dil should be a minimum of 1.0 since it at least allows you to validate calibration, but should ideally match the ppO2 that you intend to run.

One of the main reasons that people argue for dil flushes is to make sure the cells are doing what they're supposed to. What is better than a validation of your calibration at 1.0 or a validation that your cells are doing what you expect them to do at the ppO2 that you want to be? Fringe benefits of dil flushes. Gets humidity out of the loop which can be a bit irritating to your respiratory system, cools the whole loop down if you're in warm water, can help to dry out cells depending on where the addition point is *obviously unit specific*, I'm sure I'm forgetting several others.
So if I understand correctly doing a dil flush when your dil PO2 is closer to your desired set point is better than doing a dil flush when when your target SP is much greater than your dil flush will result in? Is that where you're going with that? If so I agree with the exception of the fact that a dil flush to validate cells is more of a sanity check than an actual calibration and is one of those cases where close is close enough.
 
So if I understand correctly doing a dil flush when your dil PO2 is closer to your desired set point is better than doing a dil flush when when your target SP is much greater than your dil flush will result in? Is that where you're going with that? If so I agree with the exception of the fact that a dil flush to validate cells is more of a sanity check than an actual calibration and is one of those cases where close is close enough.

correct. Agreed wholeheartedly that close enough is close enough, however I think that 1.4 is close enough to 1.5 but not close enough to 1.6. The farther you get from calibration point, the bigger the deviation will be, so you need to understand that and choose what is acceptable or not.
 
One thing about millivolts that seem to be missing (or maybe I read too fast) is writing down the mv value when calibrating in air and with oxygen. Follow how it changes over time. If there is a jump in value something is wrong.

During diving I don't see any benefit of using mv instead of pPO2.
 
This is a presentation by Joe Citelli of IANTD on the fundamentals of O2 sensors. This is information that the majority of CCR instructors and vast majority of CCR divers do not understand. Presentation was birthed by @Bobby who has had passionate discussions about this with many of us and Joe decided to run with it. It's a long presentation, but I urge everyone who is actively diving CCR's or interested in diving CCR's to truly listen to this and absorb it. We all hope for a day when we have reliable and robust solid-state sensors where we no longer have to worry about these limitations of galvanic cells, but that day is not here yet and we all need to truly understand how our current sensor technology works.


Excellent presentation, thanks for sharing.

Picking on one of the point of the presentation, I would like to hear how others are doing preventive maintenance on the connectors to avoid oxidation.

This is what I do. Once or twice a year I am using Deoxit to clean the 5-pin wet-mate connections and then dip them in Molykote 111 for protection.
  • Are others also using Deoxit on the cables connections to the sensors inside the head?
  • Any other type of maintenance?
 

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