Liberty, change O2 sensors all together or not?

[Agro]

Until now I used the 4 sensors for about 18 month, checked them at 6, 12, 13, 14, 15, 16, 17 month up to 3.5 bar. No problems so far.

Now I was told this is dangerous because all the 4 sensors come out of one serie. It is better to change 2 sensors after 9 month and the other 2 sensors after 9 more month. So I have different series of sensors. In case of faulty serie I always have 2 good sensors.

I am not sure if this makes sense:
- are there bad series?
- will I detect a bad serie by overpressure testing to 3.5 bar?
- I have 2 young sensors, 2 older sensors, they behave differently. The unit believes the young sensors are wrong and kicks them out. Until now it is OK. Now let's assume one of the older is dammaged, now I have a serious problem. In case of 4 equal sensors I have no problem, if one is dammaged.

I know about an accident on a AP Inspiration. The diver changed one ot the 3 sensors every 6 month, so the age was:
No 1: 6 month
No 2: 12 month
No 3: 18 month
No 3 died during the dive because of its age, it gave wrong measurement.
No 2 was dammaged for any reason. Unfortunately it behaved exactly as No 3.
The unit kicked out No 1, the only working sensor. The diver died.
Not exactly the same situation as on a Liberty, it has 4 sensors. But the problem is similar.

How do you change sensors on a Liberty?
How do you change sensors on any other unit?

 

[tbone1004]

Alright, that was a bit of a ramble.

The Liberty has a safeguard against the AP incident. In the middle of the dive you can tell the Liberty to ignore cells, i.e. it can vote cells out, but you can also choose individual cells to remove from the voting logic.

Age should not have anything to do with when you replace cells.
Step 1-is the cell stable between the manufacturers mV range in air. Typically 7-13mV, but check the specific cells as they can be different between models and manufacturers
Step 2-is the cell behaving within the linearity parameters set by the manufacturer, +-2% *or if you accept a wider variance, say 5%, within that*
Step 3-is the cell behaving within the speed parameters, 6 seconds to 90% of final value. I.e. when you calibrate it, it shouldn't take more than 6 seconds to get from air to 0.9

Are there bad series-yes, but they will fail the checks above. Linearity is the one that you want to pay attention to most, and it is the one that very few every do.

I would not overpressurize them to 3.5bar. I would send them up to 2.0, but there is no reason to go higher than that since you won't ever use them that high. What they do above 1.6 really doesn't matter to you. I assume you're using the Liberty head adapter to pressurize the head and in that case the unit will check all 3 of the steps above.

If the unit believes the 2 young sensors are wrong, they may well actually be wrong. Until you do the 3 checks above, you don't know which ones are right. See why age doesn't matter? Age increases the risk that the cells are going to fail the checks, but it doesn't mean that the young cells are going to be good and the old cells are going to be bad. I just changed an 8 month old cell in one of my CCR's and the other two are over a year old. It failed speed and linearity, so it got voted off the island.

Replace the cells as they need to be replaced, don't mass replace.

 

[Agro]

Hmmmmhhhhh, I dont' quite agree. If a cell is old the risk of failure becomes higher. I may test it today, it is OK. In one week it is not OK anymore just because it made the step from OK to KO within 1 week because of its age. I change a cell if the test in Liberty cell adapter tells me to do so or if I find it necessary because of strange behaviour. AND I change a cell after 18 month anyway, no matter how good it seems to be (or how good it is).

I always thought it is a very good idea to test the cell up to 3.5 bar. If they can handle 3.5 bar they can easily handle 1.6 bar. If they can handle 3.5 bar today, they are able to handle 1.6 bar even if they get worse (older) within the next weeks. But perhaps I am wrong in this point?

So tbone you change cells it they do not pass the tests anymore? Not earlier? So you always have cells of different age, no time-rule for changing them?

 

[broncobowsher]

For the most part you are using redundancy to guard against any single failure. Start stacking multiple failures at the same time and it gets real difficult to plan for. Loose 2 out of 4 cells, which 2 are the bad ones?

As for staggered replacments, I agree with that. I've had light bulbs that were all installed at the same time and would burn out at almost the exact same time. Mass production results in very consistent batches. If a batch is flawed it is easy for all of them to be equally flawed.

Revo had a nice write up describing the reason for the multiple cells. Ignore the sales pitch for the 5 cells the rEvo uses but the rest of it is worth a read. Somewhere on the rEvo site.

 

[Agro]

So Bronco you practice staggered replacement?

 

[vioch]

Revo had a nice write up describing the reason for the multiple cells.

This one?

 

[tbone1004]

Hmmmmhhhhh, I dont' quite agree. If a cell is old the risk of failure becomes higher. I may test it today, it is OK. In one week it is not OK anymore just because it made the step from OK to KO within 1 week because of its age. I change a cell if the test in Liberty cell adapter tells me to do so or if I find it necessary because of strange behaviour. AND I change a cell after 18 month anyway, no matter how good it seems to be (or how good it is).

I always thought it is a very good idea to test the cell up to 3.5 bar. If they can handle 3.5 bar they can easily handle 1.6 bar. If they can handle 3.5 bar today, they are able to handle 1.6 bar even if they get worse (older) within the next weeks. But perhaps I am wrong in this point?

So tbone you change cells it they do not pass the tests anymore? Not earlier? So you always have cells of different age, no time-rule for changing them?

Correct, I change when they need it. It is typically between 12-24months depending on how much diving I'm doing on that unit and how mean I am to the cells. Remember that the cells are basically batteries, so when you push them to 3.5bar you are stressing them considerably beyond their design criteria. I don't believe that does any good and only serves to accelerate their degradation. Unfortunately that is the pressure that the Liberty uses, so it is what it is. I would only use that when I had a suspicion that a cell was misbehaving.

Unfortunately cells are not predictable and even if it works today, it may not work tomorrow. They fail in all sorts of unique ways which is why you still need to do all of the checks on every dive.

 

[Bobby]

A cell may not last an hour, a day, a week, a month, or a year. Linear deviation and limiting are the two most common failures by a long shot. Changing a cell at X date just doesn't make sense. Testing the linearity and limiting of the cell along with response time is the only way to know if the cell is within specification. Every day that the cells are going to be used they should be checked. Even with doing all of that a cell can go way out of specification during the dive as well. That is why diluent flush checks and other procedures are in place to deal with such a failure.

Voting logic, on almost all CCR's, is a poor way of determining which cell (s) have failed during a dive. There have been many incidents that are attributed to one good cell being voted out against two bad cells. I had this happen myself back in the early 2,000's when a bad batch of cells made it onto the market. Recently, in the last couple of months, a cell manufacturer had a bad batch of cells that were released onto the market, it happens.

 

[rddvet]

I agree with Tom on this one. I stopped changing cells after 1 year or the manufacturer sell by date. My personal limit is 18 months. After that they get changed, but to change a perfect cell at 12 months just due to age is silly in my book. I also drink milk a couple days out of date if it smells and tastes fine. I've changed a cell at 12 months just to put a new one in that sucked. If they pass my checks for linearity and current limiting regularly, then I don't change them. I also understand staggering changing them. I'll stagger if I need to change a cell at a certain time, but otherwise I'll cahnge all 3 at the same time with zero concerns.
My one new hard rule, I will only use AST cells if I have no other choice. I don't like their cells and don't trust them. I have two in case of emergency backup needs since my current cells are harder to find. Otherwise I won't use AST

 

[Agro]

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.