Question How to check an analyser O2 cell?

[rjack321]

For a rebreather, having a cell that reads above one atmosphere partial pressure is a must.
Not sure why you need that overpressure for an analyzer?

well typically you don't...

you check the mV output in 100% 1ATA O2 against the predicted output based on [air mV]/0.209 and the cell specifications. Checking at >1 ATA is not part of how you check analyzers and at 15 months old this cell just needs to be replaced.

 

[Agro]

For a rebreather, having a cell that reads above one atmosphere partial pressure is a must.
Not sure why you need that overpressure for an analyzer?

1. A good cell has linear reading from 0 to 3 bar.
2. An old cell often is linear from 0 to 1 bar, non linear from 1 to 3 bar.
3. A very old cell is non linear from 0 to 1 bar.

If you test a cell from 0 to 1 bar you only detect situation 3, not situation 2.
No 2 tells me, that No 3 will happen within the next time.
If I test my analyser cell at 3 bar and get situation 1 I know cell is good and will probably still be good from 0 to 1 bar within the next time.
If I test my cell at 3 bar and get situation 2 I know cell is good from 0 to 1 bar but may fail within the next days.
If I have a cell which sometimes gives good results, sometimes bad results, the cell might be between situation 2 and 3. By testing it at 1 bar I might get result 2, then I believe everything is fine. By testing it at 3 bar I will detect the problem.

I my opinion it is a good idea testing a cell at 3 bar even if it is used in an analyer only. But I am very open to other opinions.

 

[Agro]

and at 15 months old this cell just needs to be replaced.

Maybe this cell needs to be replaced. After a 3 bar test I hopefully KNOW if it has to be replaced or not.

I already had brand new cells which did not work properly after 2 weeks. After 3 weeks they were fine, after 4 weeks they failed. First reaction: can not be the cell, it is brand new. I was looking for other mistakes: calibration, poor gas blending...... In this situation a 3 bar test is helpfull I guess, it will show for example linear reading from 0 to 0.9 bar, nearly linear reading from 0.9 to 1 bar, non linear reading from 1 to 3 bar.

Again, this are my ideas. Are they right? I don't know.

Thanks for you opinions.

 

[Agro]

I produced a cable entry into Liberty's head. The plastic part is the original Liberty overpressure valve which you use during 3 bar test. The M3 thread rods are contacts + and -. Between cable lug and cell a Molex connector is missing.

This way I can simply run the usual 3 bar Liberty test and by the way test a 5th cell. I simply measure the voltage at thread rods. Hope this works....

 

[broncobowsher]

If the cell is current limited at a PPO2 of 1.5 that could be very bad in a rebreather. That is where I see testing in pressure pot is a good thing. That cell should not be used in a rebreather. But it still has a lot of life left in an analyzer. The most an analyzer will ever see is a PPO2 of 1.0

I don't see the value of a pressure pot to test analyzer cells to test if they are linear to 3 bar. You only need them linear to 1 bar. What difference does it make (to an analyzer) if they are linear from 1.1 to 3.0 bar?

 

[Jimchris]

@Agro

Are you referring to the Divesoft He/O2 analyser ?

I usually change the O2 sensor annually, they don't appear to last long in my part of the world, even when sealed in a "Pelican" style box.

That's been my experience as well.

 

[Agro]

What difference does it make (to an analyzer) if they are linear from 1.1 to 3.0 bar?

If it is linear from 0 to 1.1 then the cell is very old and will soon not be linear anymore at 1.0 or 0.9.
If it is linear from 0 to 1.5 then the cell is old but will work for a while.
If it is linear from 0 to 3.0 it is new and will work for a long time.

 

[rjack321]

If it is linear from 0 to 3.0 it is new and will work for a long time.

I don't know where you got this idea but cells can fail at any time for various reasons. Just because it's linear today doesnt mean it won't have a compensation circuit failure or perforated membrane tomorrow.

 

[Agro]

Of course a cell can die from one day to the other, similar to hard attack.

But as far as I know the normal behaviour is as described: first the cell is linear up to 3 bar, then linearity drops down and down and down. At least that's what I find on my rebreather cells.

 

[nadwidny]

Maybe this cell needs to be replaced. After a 3 bar test I hopefully KNOW if it has to be replaced or not.

It's 15 months old. Just replace the effing thing and quit mucking around.