Liberty, change O2 sensors all together or not?

[Big-swede]

Of topic.. how does the voting logic work on the liberty if you only use 2 cells?
Since they promote it as being "redundant" and that you only need one working side.

 

[tbone1004]

Very interessting indeed, thanks.
I will not start staggering replacement. Right now I put 4 new cells, I will have an eye on them like I always do. I will replace them when necessary or at least in 18 month. Even if they are still OK, I do not feel fine if I know I have "old" cells.

So when one cell goes bad, you'll replace all 4? That seems very risky to replace known good cells with unknown cells

Of topic.. how does the voting logic work on the liberty if you only use 2 cells?
Since they promote it as being "redundant" and that you only need one working side.

The Liberty has 2 cells per side. If one side "dies", then the voting logic is done and it has to average the two remaining cells, no different to a Shearwater voting one cell out. It won't vote a second one out because it doesn't know how. The difference with the Liberty though is you can manually vote a cell out. Say you're diving EAN32 dil and diving to 30m/100ft. The left O2 board failed and you only have the 2 cells from the right. You do a dil flush to make sure everything is OK like you are supposed to and you get 1.27 on cell 1 and 1.01 on cell 2. Any normal CCR would average those two to 1.15 and try to maintain whatever setpoint you have.
In the other breathers you have to drop the setpoint and run it manually off of cell #1 and ignore cell #2. With the Liberty you go into the menu and tell it to disable cell #2. In the cell menu it will tell you excluded if it was voted out, or disabled if you turned it off.

If you are diving trimix dil, then you can turn it to He mode and it can do some fuzzy logic with the helium sensor to back into the ppO2 and it will fire the solenoid/s based off of that.

 

[barth]

So when one cell goes bad, you'll replace all 4? That seems very risky to replace known good cells with unknown cells

When you buy a new rebreather all of the cells are new and from the same batch. Or will revo or other manufacturers mount different batches of cells in their new rebreather?

(When you have 4 cells of the same batch, and one cell will fail you would replace it. When 2 weeks later another cell will fail I think it would be a good thing to replace also the two cells who seems to be ok.)

( Honestly when the first cell would fail after 10 months I would replace all of them. I wouldn’t wait till the next cell will fail. The JJ has 2 batteries for the solenoid. When one battery would fail I would replace both of them and I would use the same batch.)

 

[tbone1004]

When you buy a new rebreather all of the cells are new and from the same batch. Or will revo or other manufacturers mount different batches of cells in their new rebreather?

(When you have 4 cells of the same batch, and one cell will fail you would replace it. When 2 weeks later another cell will fail I think it would be a good thing to replace also the two cells who seems to be ok.)

( Honestly when the first cell would fail after 10 months I would replace all of them. I wouldn’t wait till the next cell will fail. The JJ has 2 batteries for the solenoid. When one battery would fail I would replace both of them and I would use the same batch.)

on a new unit they're obviously from the same batch and if 2 weeks later a second cell goes I would at least have the next one ordered, but I have never experienced that. This is very different than a battery which has significantly more predictability than an O2 cell.

 

[Agro]

So when one cell goes bad, you'll replace all 4? That seems very risky to replace known good cells with unknown cells

No, I will replace one cell if one cell goes bad. And I will replace all cells after 18 month.

 

[barth]

on a new unit they're obviously from the same batch and if 2 weeks later a second cell goes I would at least have the next one ordered, but I have never experienced that. This is very different than a battery which has significantly more predictability than an O2 cell.

Honestly it doesn’t make sense to sell a rebreather with 4 cells of the same batch and tell the users of the rebreather that it’s not safe the replace all the cells at the same time. I think that’s what revo does or am I wrong?

I think both methods have positive and negative things. I think the older cells get the higher the risk will be that a cell will fail. With a brand new cell the risk will be low, with 5 years old cell the risk it will fail will be much higher. I do understand that JJ tells the users to use a cell maximum 15 months after manufacturing and use it maximum 12 months in a rebreather.

 

[tbone1004]

No, I will replace one cell if one cell goes bad. And I will replace all cells after 18 month.

Very interessting indeed, thanks.
I will not start staggering replacement. Right now I put 4 new cells, I will have an eye on them like I always do. I will replace them when necessary or at least in 18 month. Even if they are still OK, I do not feel fine if I know I have "old" cells.

These conflict with each other, if you are replacing them when necessary then they will naturally end up staggered. Where did you come up with the 18 month rule?

Honestly it doesn’t make sense to sell a rebreather with 4 cells of the same batch and tell the users of the rebreather that it’s not safe the replace all the cells at the same time. I think that’s what revo does or am I wrong?

I think both methods have positive and negative things. I think the older cells get the higher the risk will be that a cell will fail. With a brand new cell the risk will be low, with 5 years old cell the risk it will fail will be much higher. I do understand that JJ tells the users to use a cell maximum 15 months after manufacturing and use it maximum 12 months in a rebreather.

There is only so much they can do when they build the new ones since they have to buy the cells in bulk, but I think the point is to not replace cells that don't need to be replaced "just because".
I do not see any positive in replacing all cells at once, only negatives, nor do I see any negatives in replacing them "as-needed"

 

[barth]

These conflict with each other, if you are replacing them when necessary then they will naturally end up staggered. Where did you come up with the 18 month rule?



There is only so much they can do when they build the new ones since they have to buy the cells in bulk, but I think the point is to not replace cells that don't need to be replaced "just because".
I do not see any positive in replacing all cells at once, only negatives, nor do I see any negatives in replacing them "as-needed"

If you only replace sensors as-needed, I think you will wait till you can’t use a sensor anymore and then replace it. That can be at home for me when preparing my rebreather. That wouldn’t be a problem for me.

It could be at the dive site before I would dive that could mean I would be to late for the dive and couldn’t dive.

It could also be underwater, then I should abort the dive...

It would be nice if I could replace a sensor before it would fail.

 

[tbone1004]

If you only replace sensors as-needed, I think you will wait till you can’t use a sensor anymore and then replace it. That can be at home for me when preparing my rebreather. That wouldn’t be a problem for me.

It could be at the dive site before I would dive that could mean I would be to late for the dive and couldn’t dive.

It could also be underwater, then I should abort the dive...

It would be nice if I could replace a sensor before it would fail.

that's the point though, any cell is prone to do that.
Keep one spare cell in your save a dive kit. I keep one spare, when one dies, I use the spare and then buy another one. If they pass all of the checks in the morning during assembly, it is highly unlikely they're going to fail at the dive site, and unless you are on a boat, it shouldn't cost you a dive, though depending on your comfort level you could always dive it with 2 cells.

It would be nice if we didn't have cells that failed like this, and I hope Poseidon gets their sensors to the rest of the market. Right now those sensors are around 200E/$250USD but are good for around 10 years and don't have weird failure modes since they're LED's. They will still be replace at failure items because electronics fail randomly, but it will be a lot less frequent than the galvanic cells

 

[Agro]

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.