Divesoft He/O2 cell life

[broncobowsher]

I have a Divesoft analyzer that is over 2 years old. I know they claim the cell will last up to 3 years. It has been stored indoors, all the right conditions. In air it is now in the low side, but still above 7mV. I recall that 7mV is the min spec for the cell. Don't know where I heard that.

I have been calibrating this with a 2-point scale. Put the flow limiter directly on the O2 bottle. Now I have been thinking about this. It will show me a number for O2. All the way up to 100%. What happens if the cell is getting current limited on the top end? I have tried to think this through. An 80% mix would show the same as 100% (or fairly close since the current limiting flattens out the top end of the read curve).

How would you know that the O2 cell is current limited? Analyze an old bottle of 50% from a year ago and if it is getting richer, your 2-point cal is flawed? What if I don't have a known bottle of mix?

I hope that Divesoft knows the O2 cells enough to reject a flattened cal curve, but I don't know.

So that was this mornings thinkings.
And is there a dedicated time to change the cell in the Divesoft analyzer? I didn't see anything except "should last 3 years".

 

[tbone1004]

calculate linearity. If the cell is 7mV in air, then it should be 7*1/.21=33.33mV. There is some room for linearity in there so if it's above probably 0.95% linear, so 33.33*.95=31.6mV then you're fine.

 

[Agro]

Yes linearity is the point.
By the way the divesoft analyser also allows a 3 point cal, I always use this option: He, air, O2
Do they say the O2 cell will last for 3 years? 2 years is already good for a cell, 3 years is optimistic. The divesoft cells are common O2 cells, they can not last any longer in a Divesoft unit then in other analysers.

 

[rjack321]

Typically you don't notice analyzer cells losing linearity until you put it on an O2 bottle and you can only reach 93% or something like that. You can't have 2 unknowns (cell and the gas) and somehow deduce that the cell is failing.

This is why analyzers are only used to confirm what you already know - because you followed good mixing practices in the first place.

 

[JahJahwarrior]

I cheated to avoid doing math very often, but to keep me in the habit of doing exactly what Tbone1004 and people like Bobby have written about, and it will work for your analyzer as well. Only problem is, I set the minimum limit at 8mv as that's the limit specified in my o2ptima manual and I'm used to cells well above that. Changing the limit is easy, but I personally would be wary of a cell with that little output. Maybe I don't have reason to be, though. My answer would depend a lot on how the cell was when it was new. If it was a 12mv cell that's recently dropped, I would toss it. If it was an 8mv cell to begin with that's declined slowly to 7mv over time, maybe a different story. I've just never seen a cell that low.

www.divebhb.com/gas-planning

 

[broncobowsher]

Typically you don't notice analyzer cells losing linearity until you put it on an O2 bottle and you can only reach 93% or something like that. You can't have 2 unknowns (cell and the gas) and somehow deduce that the cell is failing.

This is why analyzers are only used to confirm what you already know - because you followed good mixing practices in the first place.

The 93% over time I would see it as just needing a cal done. Since I have adaptors to attach the flow limiter directly to the O2 bottle, that is where I set the upper number. Without trying to figure what was left in the hose.

T-bone has it right. I forgot to even think about a basic linearity test. For quick checks, read air and multiply by 5. That should be real close. If it doesn't look right, time to do the actual math.

Guess it is time to open it up and put in a year old rebreather cell. Not that it has done much this past year.

 

[rjack321]

The 93% over time I would see it as just needing a cal done. Since I have adaptors to attach the flow limiter directly to the O2 bottle, that is where I set the upper number. Without trying to figure what was left in the hose.

You can continue to recalibrate (which was my point, you need the known 100%) but eventually you'll max out and won't be able to anymore. Ideally you would replace it before that point, in practice just order a new cell once you can't calibrate anymore.

If you are reusing CCR cells then you're doing good to get 2 additional years out of them.

 

[Ouvea]

Your Divesoft O2/H3 analyzer will give you an error message requiring you to replace the O2 sensor. I forget the exact warning but it prevented me from performing any analysis. Ordered a new sensor from Divesoft, although DGX sells similar ones.

@tbone1004 @broncobowsher , can you guys explain the voltage reading on the cell and how this indicates a worn cell?

 

[KDAD]

@Ouvea I will take a stab at answering your question but if I am wrong someone will be along to correct me. *I don't have the specs for cell output don't base your determination of a good cell off of my numbers - I worked with what was provided above and this is only an example.

The cells are electro-chemical so as they react chemically with the oxygen they will output an electrical voltage based on the oxygen percentage. That electrical output can degrade over time as the chemical reaction weakens. So, at 21% O2 the sensor should put out a certain milliVolt but when that output drops to 7 mV the sensor is considered depleted enough and the units that they are used in may not be able to calibrate at that level and the sensor should be replaced. In other words, the units will reject a calibration in air if the sensor is outputting below 7 mV resulting in the error.

The same may be said if analyzing 100% O2. Using @tbone1004 example above for a linear cell, if the cell output is 7mV in air, then output should be 7*1/.21=33.33mV in 100% O2. Cells may not be exactly linear in their output so accepting a 5% linearity error would give an output of 33.33*.95=31.6mV when in 100% O2. If the cell outputs that it can be considered outputting correctly (linear) but at the bottom end of what an acceptable cell is.

When you do a calibration the units measure the sensor voltage, say 8 mV, and you are telling the unit that that voltage corresponds to 21% O2. The units will adjust all other measurements based upon that calibration. Some units allow a two-point cal (21%, 100%) or even a three-point cal (0%, 21%, 100%). The additional points will correct for the linearity of the sensor. The linearity of the sensor can be determined by calculating what the expected voltage is based on a known input gas (21%, 100%) and comparing that to the actual voltage.

 

[tbone1004]

Your Divesoft O2/H3 analyzer will give you an error message requiring you to replace the O2 sensor. I forget the exact warning but it prevented me from performing any analysis. Ordered a new sensor from Divesoft, although DGX sells similar ones.

@tbone1004 @broncobowsher , can you guys explain the voltage reading on the cell and how this indicates a worn cell?

what @KDAD said is correct.

On this analyzer in particular. Output in air at standard atmospheric conditions are 8.5-14mV. Anything outside of that is considered out of spec and the cell should be discarded per AII.

PSR-11-39-MD | AII Inc