Liberty, change O2 sensors all together or not?

[kensuf]

I'm really not a fan of putting all of your eggs in one basket (i.e. running the same batch for all of your cells). I tend to mix my cells and replace every few months.

There's a new manufacturer of cells that came to market about 1.5 years ago. Approximately 9 months ago, they were having some major manufacturing problems and cells would start dropping out. Because of the shortage of O2 cells due to the pandemic, many people wound up buying these new on-market cells, including myself and a few friends. We noticed cells would drop out periodically throughout a dive, I only had one of these cells but it seemed quite flakey and a friend of mine with a (new to him) JJ was having very regular problems with his cells.

There was some heated debate about whether it was his cells or his JJ head, with one person even going so far as to recommend he through his JJ in the garbage. So without knowing about the manufacturing problem, and while trying to help my friend debug the situation, I took his 3 cells and put them in my JJ. During a dive I had a good cell voted out by two bad cells. A DIL flush confirmed that the good cell was voted out and that the bad cells were reading LOW. A bailout, proper exit, and disposal of the cells into the round receptacle were the solution. We replaced the cells in his JJ with another brand, and it's been flawless since.

 

[kensuf]

The 18 month rule I got from my instructor. I do not know where he got this from.

Until now my cells lasted for 18 month. I had one exception, 1 cell died after 1 month. It worked fine for 1 month, then suddenly died from one day to the other.

Yes, changing them when necessary leads to staggering. Unless they survive 18 month, that's what they did until now.

There used to be a time when people could (and did) run cells way past their expiration date. There also used to be a time when all divers were told to open their valve all the way, then back it a quarter/half turn.

Both of these practices are now discredited by people that are paying attention to accidents and cell failure modes.

IMHO, don't run cells past the expiration date. Replace them when any of the following occur:

1. They are past their expiration date
2. They've been in your unit for more than 12 months
3. They fail to calibrate
4. The mv in air is below 9
5. They act "current limited" (technically it's voltage limited) and fail a linearity check.

This is tech diving, and with a very expensive piece of equipment on your back. If the cost of replacing your cells is prohibitive then sell your rebreather. I know, the global cell shortage has caused some problems, but this is tech diving - planning is a part of it, plan your replacements in advance and take care of your gear, otherwise it won't be able to take care of you.

FWIW, I just threw out a cell that was five months old because it hit #4 on the list (it read 8.3 in air).

 

[cathal]

IMHO, don't run cells past the expiration date. Replace them when any of the following occur:

1. They are past their expiration date
2. They've been in your unit for more than 12 months
3. They fail to calibrate
4. The mv in air is below 9
5. They act "current limited" (technically it's voltage limited) and fail a linearity check.
.

Thank you for this very useful and concise list of grounds for getting rid of a cell and thereby ensuring you always have healthy cells when diving. My only observation is that Point 2 - "They've been in your unit for more than 12 months" would appear to go aginst the rEvo Manfacturers training. In fairness rEvo(rayemakkers) has published a statistical analysis of cell behaviour that justifies rEvo's approach but I would appreciate your thoughts/observations on their cell replacement policy.

Regards

Cathal

 

[kensuf]

And I hope you can respect that with the litigious nature of North American society, I'm not going to comment on what Revo recommends except to say that if you are a Revo diver, follow their recommendation.

 

[Tassi Devil Diver]

The rEvo paper on cell replacement is an excellent guide. I replace a cell in my rEvo every 6 months in line with the paper recommendations, I put cells 1 new, 2 and 3 on my petrel controller 4 older and 5 older on the dream.
By the time the oldest cell leaves my unit it is 2.5 years old. In 7 plus years I have been diving my rEvo (500hrs) I have only had 3 cells fail before being fully cycled through the unit. I use exclusively the rEvo B cell made by Vandagraph except the cell has been independently tested by rEvo before getting the rEvo label .

 

[Heat Miser]

IMHO, don't run cells past the expiration date. Replace them when any of the following occur:

1. They are past their expiration date
2. They've been in your unit for more than 12 months
3. They fail to calibrate
4. The mv in air is below 9
5. They act "current limited" (technically it's voltage limited) and fail a linearity check.

This is tech diving, and with a very expensive piece of equipment on your back. If the cost of replacing your cells is prohibitive then sell your rebreather. I know, the global cell shortage has caused some problems, but this is tech diving - planning is a part of it, plan your replacements).

I'll have a better look at this #4, I think one of my 5 cells failed this last build, To be honest its not easy to spot a current limited cell on a rEvo with dreams, its OK on the 3 cells wired to the shearwater, you can do an O2 flush @ 6-7 meters (20-23 feet) and look up the results in Shearwater Cloud after the event on cells 1-3, they are graphed, but with all the bubbles flying around during the O2 flush, the dreams seem to get left off, or reviewed late in the process and I'm never really certain they aren't current limited, or that alternatively the ppO2's has been breathed down by the time I've made certain my depth and buoyancy and bubbles are under control.

 

[rjack321]

I'll have a better look at this #4, I think one of my 5 cells failed this last build, To be honest its not easy to spot a current limited cell on a rEvo with dreams, its OK on the 3 cells wired to the shearwater, you can do an O2 flush @ 6-7 meters (20-23 feet) and look up the results in Shearwater Cloud after the event on cells 1-3, they are graphed, but with all the bubbles flying around during the O2 flush, the dreams seem to get left off, or reviewed late in the process and I'm never really certain they aren't current limited, or that alternatively the ppO2's has been breathed down by the time I've made certain my depth and buoyancy and bubbles are under control.

Get yourself a pressure pot and test them under controlled conditions (100% o2 and up to 2 ATA)

The rEvo paper on cell replacement is an excellent guide. I replace a cell in my rEvo every 6 months in line with the paper recommendations, I put cells 1 new, 2 and 3 on my petrel controller 4 older and 5 older on the dream.
By the time the oldest cell leaves my unit it is 2.5 years old. In 7 plus years I have been diving my rEvo (500hrs) I have only had 3 cells fail before being fully cycled through the unit. I use exclusively the rEvo B cell made by Vandagraph except the cell has been independently tested by rEvo before getting the rEvo label .

That paper is very well written and at the same time based on complete conjecture. There is ZERO data to support the failure rates discussed in there. It does give the air of authority, but its all just hand waving and vapors based on anecdotal evidence at best.

 

[Heat Miser]

Get yourself a pressure pot and test them under controlled conditions (100% o2 and up to 2 ATA).

I started looking, I'm not terribly handy and can't see myself making a good one quickly. Are there any off the shelf products people can recommend.

 

[Heat Miser]

Get yourself a pressure pot and test them under controlled conditions (100% o2 and up to 2 ATA)

That paper is very well written and at the same time based on complete conjecture. There is ZERO data to support the failure rates discussed in there.

Thanks @rjack321 is there any off the shelf pressure pots fit for purpose? I have thought about building my own, I'm not the most hands on, and I could see it dragging out longer than it has taken me to build my unfinished o2 analyser.

On the "Zero data", there is a histogram in the paper showing testing of 1,000 cells. Just eyeballing the chart I can see the left skew and I can tell there is a greater than 10% chance that a cell becomes current limited in less than 4 months. SImilarly eyeballing an average of 18 months it means to replicate the results would take (18 x 1000)/12 years = 1,500 years, if you did the test one cell at a time.

There's lots of data in the paper, some peer review would require quite a bit of testing, but its open to anyone to replicate the results. I'll leave that one for someone with access to 1,000 cells to replicate in 2-3 years.

 

[Tassi Devil Diver]

There's lots of data in the paper, some peer review would require quite a bit of testing, but its open to anyone to replicate the results. I'll leave that one for someone with access to 1,000 cells to replicate in 2-3 years.

He is full of crap