Sensors: which gas shall I have in the loop when not in use?

[Wibble]

Right. And if one is behaving differently then it probably is because of a faulty cell, not because of the age.

And if one fails in a batch, what's the chances of another failing in the same way?


It's getting a bit circular now. Although you'll find that the circle containing people who defer their cell changes to use different batches will be a lot more than the circle of people who change all cells at the same time.

 

[tbone1004]

Right. And if one is behaving differently then it probably is because of a faulty cell, not because of the age.

Sure a cell is more prone to fail as it gets older but that's again why you don't find anyone recommending to change all of the cells at once. As all of the cells age they all become more prone to fail. If you replace them as they fail then there is a mixed bag in the head. One cell fails it's not big deal, no reason to come off the loop, but if you have multiple cells start to do janky things at the same time you now have to bail out.
There is a reason no one recommends replacing them all at the same time...

 

[Gareth J]

@Agro , a lot of the 'rules' (or guidelines), that we have in diving have been learnt through bitter experience (people died).
There is a similar story with cave diving (not my forte).

I lived through the lessons (rules) learnt when using CCR. A lot of people died in a very short period of time. The rules (guidelines) we use now, are based on those accidents. Training was changed to incorporate these lessons. The introduction of CCR diving was far more brutal than Nitrox, or mixed gas, a lot more people died.

I used to throw a twinset in the boot (trunk) of the car at the end of the weekend. During the summer, I would drag it out the following weekend still wet, for the next weekend of diving. Some times it need the regulator need a slap, but it worked. I would NEVER do this with a CCR, they BITE, they need to be treated like a racing car, they are delicate, temperamental, and need love and affection - even then they have a tendency to bite.
O2 sensors are the most unreliable part of any CCR (other than the operator), they fail. I have had cells fail after a month, a year, 15 months. I try to change them between 12 and 15 months. My last cell failure was after 11 months (it was just over 12months since I had bought it - so out of warranty). Any newly changed cell is suspect, until it is at least a month old. Any drift between the cells means they are all suspect until they have been check against a DIL flush. My normal rule, is switch back to a low setpoint, reduce the high setpoint down, then see how they behave. Most (not all) cell issues result in voltage limiting, or lack of linearity as they age.

There are people on the forum who have had years of experience using CCR, the advice they give is often hard-won. Listen and learn. Most instructors, hope (often against hope), that they can stop new divers making the mistakes they have made. We hope that we can stop you making the mistakes we made, that is the basic philosophy of diver training!

 

[Agro]

@Agro , a lot of the 'rules' (or guidelines), that we have in diving have been learnt through bitter experience (people died).
There is a similar story with cave diving (not my forte).

I lived through the lessons (rules) learnt when using CCR. A lot of people died in a very short period of time. The rules (guidelines) we use now, are based on those accidents. Training was changed to incorporate these lessons. The introduction of CCR diving was far more brutal than Nitrox, or mixed gas, a lot more people died.

I used to throw a twinset in the boot (trunk) of the car at the end of the weekend. During the summer, I would drag it out the following weekend still wet, for the next weekend of diving. Some times it need the regulator need a slap, but it worked. I would NEVER do this with a CCR, they BITE, they need to be treated like a racing car, they are delicate, temperamental, and need love and affection - even then they have a tendency to bite.
O2 sensors are the most unreliable part of any CCR (other than the operator), they fail. I have had cells fail after a month, a year, 15 months. I try to change them between 12 and 15 months. My last cell failure was after 11 months (it was just over 12months since I had bought it - so out of warranty). Any newly changed cell is suspect, until it is at least a month old. Any drift between the cells means they are all suspect until they have been check against a DIL flush. My normal rule, is switch back to a low setpoint, reduce the high setpoint down, then see how they behave. Most (not all) cell issues result in voltage limiting, or lack of linearity as they age.

There are people on the forum who have had years of experience using CCR, the advice they give is often hard-won. Listen and learn. Most instructors, hope (often against hope), that they can stop new divers making the mistakes they have made. We hope that we can stop you making the mistakes we made, that is the basic philosophy of diver training!

Gareth, so you recommend changing cells
1) as they fail (tbone's recommandation)?
2) one after the other after a certain time?
3) all at once (that's what I do)?

After all I've heard and learned about it I will not follow rule 1. I will not wait until one of my cells fails. Any cell can fail at any time, no doubt about that. But waiting until this happens reduces safety, at least that's my opinion. I will change a cell after 12 month of use, even if it seems to be fine.

Still I don't like 2, I prefere 3. Reason: Inspiration accident I described.

 

[barth]

Sure a cell is more prone to fail as it gets older but that's again why you don't find anyone recommending to change all of the cells at once. As all of the cells age they all become more prone to fail. If you replace them as they fail then there is a mixed bag in the head. One cell fails it's not big deal, no reason to come off the loop, but if you have multiple cells start to do janky things at the same time you now have to bail out.
There is a reason no one recommends replacing them all at the same time...

Sorry, but I don’t agree. when I buy a jj ccr new, cells have the same manufacturing date. If it was not a good idea to use 3 cells from the same batch/manufacturing date people, shouldn’t dive with a new rebreather or should at least replace the first new sensor after 4 or 6 monthas, are people doing that?

All the gue ccr divers I dive with including instructors are replacing all their cells at the same time, the same time is 12 months after manufacturing.

I know pros and cons.

 

[Agro]

All the gue ccr divers I dive with including instructors are replacing all their cells at the same time, the same time is 12 months after manufacturing.

I know experienced divers and instructors changing them all at the same time.
I know experienced divers and instructors changing 2 cells after 6/9 month (Liberty, 4 cells)
I know experienced divers and insturctors changing 1 cell after 3/4 month (Liberty)
I know experienced divers and instructors changing 1 cell after 4/6 month (Inspiration, 3 cells)
There are pros and cons.
I don't care what others do. I am very interessted WHY people are doing what they are doing. Then I take my own conclusion. Until now my conclusion is: change them all together. But I am open for other ideas.

 

[lermontov]

For what its worth I dont change my cells unless theres a deviation of 8-10 % by comparing the MV readings at .21 and 1.0 or they are current limiting at 1.6po2 or slow to respond

You can also check deviation/variation at any Po2 when you download your dives and check the mv readings on graphs on shearwater cloud

 

[tbone1004]

Sorry, but I don’t agree. when I buy a jj ccr new, cells have the same manufacturing date. If it was not a good idea to use 3 cells from the same batch/manufacturing date people, shouldn’t dive with a new rebreather or should at least replace the first new sensor after 4 or 6 monthas, are people doing that?

All the gue ccr divers I dive with including instructors are replacing all their cells at the same time, the same time is 12 months after manufacturing.

I know pros and cons.

what are the odds that all 3 cells last the same time? You're telling me that if one fails prematurely after 3 months you are going to replace all 3 of them? A natural stagger is created as you dive the units. The manufacturers are going to put fresh cells in when you buy the unit, there is no way practical way around that, but if you have a single cell fail early and replace all of the sensors you are lulling yourself into a sense of false security. I also firmly believe that thinking about cell age instead of linearity/current limiting is promoting that false sense of security for rebreather divers and is not something that we should emphasize nearly as much as most would have you believe.

 

[rddvet]

Replace as they fail or become wonky. If that means changing all at one time because they’re all wonky or using all 3 cells from the same batch, I don’t really care. There’s so much ridiculous back and forth about what’s ideal it’s nutty. At the end of the day, the chances of all 3 cells failing at the same time is fairly low except in the case of a manufacturing flaw. Manufacturing flaws in cells are way less common than they used to be. Except for the ast sensors from oxycheq. They just suck.
Pretty much any method has it’s pros and cons. Pick one you’re comfortable with. Some people say cells should be changed after 12 months of use or the use by date the manufacturer provides. Either way most cells don’t magically die after a certain date. That’s why we verify linearity and current limiting. Even if all of your cells fail at the exact same time on a dove, you’ve got enough gas to end the dive and get back to the surface. All this conjecture about best practices is silly IMO. My viewpoint is from that of a 3 year ccr diver who also deals with oxygen cells in their job.

 

[barth]

what are the odds that all 3 cells last the same time? You're telling me that if one fails prematurely after 3 months you are going to replace all 3 of them? A natural stagger is created as you dive the units. The manufacturers are going to put fresh cells in when you buy the unit, there is no way practical way around that, but if you have a single cell fail early and replace all of the sensors you are lulling yourself into a sense of false security. I also firmly believe that thinking about cell age instead of linearity/current limiting is promoting that false sense of security for rebreather divers and is not something that we should emphasize nearly as much as most would have you believe.

It depends on the situation, when the cells are 11 months old and one cell fails, I will replace all 3 of them. When the cells are 2 months an one will fail, I will probably replace only the one which fails.

It also depends on the linearity and mV in air. If the other 2 cells are 9.3 mV in air and that should be between 9 and 13 I will also replace all 3 of them.

I know also know at least 3 divers who order 4 cells instead of 3 cells, they keep one cell spare. They buy every year 4 cells, but they are using a rebreather which is using 3 cells. Honestly that doesn’t make sense for me.

My jj is manufactured in October 2019. I did replace my set of 3 cells 2 times in 2.5 years. I have never had a cell failure and did never missed a dive in 2.5 years and 170 dives. A cell failure or issue with my rebreather could easy mean that I couldn’t dive if I’m doing a dive at high/low tide.

JJ manual is telling to replace cells at least 15 months after manufacturing date and don’t use the cells longer then 12 months in the rebreather. GUE is telling don’t use the cell longer then 12 months after manufacturing date. I try to get new cells in the same month when the were manufactured and use them for 12 months.

Cell age and linearity/current limiting are different things. I think they are all important . If you think that only age is important you could get yourself in trouble. But if you don’t care about manufacturing dates you could have unnecessary cell failures…

But I think I’m still a beginner after 170 dives in 2.5 years with my JJ. And it would be good for me to follow the manual and do what my instructor did learn me.