Oxygen Sensor Fundamentals

[stuartv]

Stuart,
Everything you said is spot on except one issue, which may or may not be in play for you and the types of dives you do. Yes you can check and verify with FO2 of dil and today a lot of our dive computers will even compute that for us. When doing deeper dives though and wanting to raise the PPO2 deeper to accelerate of gassing we need to verify that cells are not limited and/or humidity limited. Even then you would not "have to" use mV, I do gas switching with my CCR so that I have a FO2 at an appropriate decompression stop. A solid dil flush will show me where my cells are and how high they will show, then I simply keep a couple of points below that threshold and I know that the PPO2 is right.

Where learning to think in mV really helps is in tracking and knowing your cell health. All cells will change in linear deviation as they age. How much linear deviation is unacceptable for the brand of cells you are running and the type of diving you are doing? When is it time to replace them? I have heard things like "just rotate them out every six months".

I view cells like the brakes on my car. I get over 100,000 miles on a set of brakes however my wife gets far less. Why should we both replace our brakes every six months? That would be crazy. I pull 18,000 pounds with my truck regularly, have an exhaust brake and heavy duty brakes, and drive mostly on the highway. She drives mostly in town and has a mid size car.

Cells age depending on the brand (type of vehicle) and how they are used (type of driving). By tracking cells linear deviation and limiting, over time, with mV you will know how your cells are aging and their health. If you simply calibrate when things are off you don't know the percentage of linear deviation. By learning to think in mV you will be able to verify how a cell is acting at any time during a dive, I wish that I could leave the screen on mV, it would be easier for me. Cells dance and react completely different than the PPO2 that is displayed. The cells actual mV output is a much more accurate representation of what is happening with the gas PPO2. We already do a ton of rounding in diving and PPO2 is yet another layer of rounding to "smooth" out what we are seeing. We become comfortable and reliant on that rounding which leads us into a false sense of security. By thinking and watching mV we can see more granular detail and have a more clear picture of what is happening at the moment and over time.

Hey, Bobby. I have to thank you. As always, you are very generous and patient to put up with my incessant questions and giving me solid and thoughtful answers.

After reading this latest from you, I'm not prepared to say that I fully understand the importance of frequent monitoring of mV. But, I can definitely say that you've piqued my interest and I intend to start paying a lot more attention to the mV than I have been, so I can develop my own feel for how those numbers behave and how they correlate to the ppO2 readings that are shown.

Thank you! I am still on the steep part of the CCR learning curve and I appreciate any and all the help I get.

 

[Bobby]

Hey, Bobby. I have to thank you. As always, you are very generous and patient to put up with my incessant questions and giving me solid and thoughtful answers.

After reading this latest from you, I'm not prepared to say that I fully understand the importance of frequent monitoring of mV. But, I can definitely say that you've piqued my interest and I intend to start paying a lot more attention to the mV than I have been, so I can develop my own feel for how those numbers behave and how they correlate to the ppO2 readings that are shown.

Thank you! I am still on the steep part of the CCR learning curve and I appreciate any and all the help I get.

Stuart,
Thank you and you're welcome, brother. It is all about discussion and sharing knowledge and experience. If you start tracking linear deviation every dive day you will find that you will see a reliable curve to judge cell health. I also perform a 1.6 at the start and end of every dive and track that as well. What that tells me is what to expect, for different duration and profile dives, regarding the amount of humidity limiting from each cell position. Different positions react differently and every type of unit is different as well. Different manufacturer's cells will age and die differently as well. Tracking will give that experience also. It is a perpetual learning curve for all of us sharing, questioning, and evaluating information is key to continued progress, IMHO.

 

[JonG1]

I "think" the most important element that I learned when I looked at all this was that if for example the linear deviation was at 95%, the PO2 reading might show 1.3 but the actual the PO2 would actually be 1.365, not a major issue, but at 1.6 on deco it could be 1.7 plus the effects of humidity potentially.

Might be way off tho!

 

[stuartv]

I "think" the most important element that I learned when I looked at all this was that if for example the linear deviation was at 95%, the PO2 reading might show 1.3 but the actual the PO2 would actually be 1.365, not a major issue, but at 1.6 on deco it could be 1.7 plus the effects of humidity potentially.

Might be way off tho!

That would not be correct if you calibrate using 100% O2 at 1 ATA.

If you do that, then the 5% is already corrected for at 1.0. So, when it shows 1.3, it's only off by the 5% of just the 0.3 part. So, if it's showing 1.3, it's really 1.315, not 1.365.

 

[JonG1]

I am on a SW so it corrects for altitude/climactic pressure and therefore can be quite a bit lower than 100% showing in the unit.

 

[JonG1]

Bobby I'd be interested in your gas switching approach, bit OT so could be a new thread, and not where my diving is presently but lots of questions pop up about CNS, how much it affects run time, whether potentially its clinically more efficient at the deco stress level, whether the main motivation is the dil PPO2 for verification or other, what PO2 you use at depth if switching on deco, what lead to that approach given that it's quite different to the norm etc.

 

[Bobby]

Bobby I'd be interested in your gas switching approach, bit OT so could be a new thread, and not where my diving is presently but lots of questions pop up about CNS, how much it affects run time, whether potentially its clinically more efficient at the deco stress level, whether the main motivation is the dil PPO2 for verification or other, what PO2 you use at depth if switching on deco, what lead to that approach given that it's quite different to the norm etc.

JonG1,
I personally think that gas switching on a CCR dovetails well with the presentation, IIRC I added a paragraph about it at the end of the presentation however I'm not sure if Joe kept it or not. A lot of people contributed to make that presentation. Gas switching is a personal choice that I have decided is worth doing and adds safety to my dive. I would not be comfortable raising my PPO2 without verifying cell humidity limiting.

I generally don't get too concerned about gas switching from depth until past 100m or long duration dives deeper than 60m. The reason it that I tend to keep my PPO2 low enough that I'm not concerned with humidity limiting until my run times on deco get long enough. So lets take a "long enough" dive to 100m that I will do a gas switch, this will allow me to verify the amount of humidity limiting that I have at the time of the gas switch. I want to have a few minutes of deco at the switch depth to have time to fully flush and see where I stand, so for a 100m dive I will setup a switch gas at 40m that gives me 1.6. Usually it will be 32/35 or 32/40 depending on what my END was for the 100m gas. The more task loaded the dive plan the lower I'll push my END. Lets say that I see 1.5 out of my cells, I'm going to do this in PPO2 to keep it simple as an example, then I'll push my PPO2 on shallower stops to 1.3. I keep two points from my max to keep conservative, if I could only get 1.4 then I would go down to 1.2.

If the total run time is over 5 or 6 hours I would likely throw in another gas switch at 21m as well. At 1.3 or so on a 5-6 hour dive I can shave a good amount of deco and I personally believe that I am promoting better off gassing as well. I keep my decent and bottom PPO2 low, 1.0 or less depending on dive profile, so my lungs don't get burnt. I generally don't run above 1.0 except for deco with long run times. I also always bump up to 1.6 @ 6m even with no deco obligation, this allows me to check 1.6 again at the end of the dive.

 

[tbone1004]

@JonG1 to make an additional point from @Bobby 's post above for less "extreme" dives, this is also one of the reasons that I prefer to run the same bottom ppO2's that I would on OC. 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.

 

[JonG1]

Thanks Tbone can you maybe run through an example, I take it you are maybe using 1.4 for the bottom portion then? Would you also gas switch on deco?

 

[stuartv]

@JonG1 to make an additional point from @Bobby 's post above for less "extreme" dives, this is also one of the reasons that I prefer to run the same bottom ppO2's that I would on OC. 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.

Okay, I was biting my tongue to try and stay on OT and also to not out myself by saying anything TOO stupid. But, I can't contain the stupid any longer. I have to ask some questions that will probably sound plain dumb.

Why do you have to run a hot dil? If you're on the bottom, running a SP of 1.3 (or whatever your dumb ole Meg HUD has brow beaten you into running ) and you want to see if you have any cells that have become limited (which could be from humidity or from just age), why can't you just pump a quick squirt of O2 into the loop manually and verify that you can manually fly it up to 1.5 or 1.6 or whatever you want to look for (or the mV equivalent, of course)?

You're really not expecting that all of your sensors will be limited, are you? I mean, I've had a sensor get squirrely on me on my rEvo a number of times, that I have attributed to moisture/humidity. But, I have never had more than one do that. So, if you assume they're not all going to be limited, then a shot of O2 will let you check them and also (by having at least one good sensor) avoid pumping it up to 2.0 while you're trying to check them. Right?

And even if all your sensors were limited, you should be able to tell by experience if you have pumped enough O2 in and are not seeing the appropriate response, thus, in that case, telling you that all your sensors are limited. I.e. be able to tell well before you've pumped so much in that you're now somewhere past 1.6. Right?

Are you saying that the actual linearity can change (significantly) during a single dive? I understand that substantial limiting can occur during a single dive, but is there a real concern about the linearity itself changing by a significant amount? If so, then I can see how you would potentially want a known mix (i.e. a hot dil) to use to actually test against on the bottom.

Regarding doing CCR gas switches, I assume what you and @Bobby are talking about is diving with a dilout setup, so you can just unplug your dilout hose from your bottom mix and plug it into a stage cylinder. So, for example, you plug into a bottle of 50% when you get to 70' (this is just as an example - I understand you'd likely do it deeper), then do a dil flush and confirm you're getting a 1.55 - 1.6 reading on all your sensors. Correct?