02 sensor in analyzer

[MaxE]

How can you tell when an O2 sensor needs to be replaced? Will you start to see low readings, high readings ?

 

[Mobulai]

Low for sure

While in “storage” mode and no access to o2 tank > 21%, it started for me by seeing low % that I dismissed and just clicking the calibration button — I assumed it was the humidity or stuffiness of where I stored it

But then I tried exhaling onto it and also tried some soda stream gas on it and it didn’t budge. (Should’ve went lower)
Seemed very fishy
It only took a month till I was at an LDS and I hooked it up to my tank , no matter how high I flowed air on it it was at 6%.. that was around the same time I learned about o2 sensors in rebreather and that they have a shelf life.. it was such an “aaah” moment for me

 

[Mobulai]

Of course this is completely anecdotal and mostly happening in a dark/unobserved incident, so this presentation might help with more “hard facts”

 

[Mobulai]

In rebreather predive checks we check the generated voltage (mV), not the ppo2 (or fo2) of the known gas; that’s because these cells are tiny batteries that have “fuel” reacting to oxygen
Once you go below a certain mV level in air you know they are dying out
Maybe there are analyzers out there that show mV, would be pretty cool actually

 

[landphil]

When my Palm O2 sensor went bad it would read something like 9% in air when it should have been 21- didn’t read much higher with higher O2 either. It would also error when you tried to calibrate it to air. Replacing the sensor got it all working again.

Your best check will be analyzing your tanks when you pick them up at the shop, then comparing with your analyzer to see if it gets close.

 

[gmerick]

The O2 sensor reacts with oxygen to produce electricity. There is only so much "fuel" in the sensor to react with that oxygen. Store the sensor in it's sealed wrapper. Protect the sensor from air when not in use. My analyzer came with a plastic plug and o-ring to seal the sensor air tight. I can tell the plug works, because when I turn it on with the plug in place, it reads very low O2 content. Remove the plug and calibrate with ambient air it's back to normal. I know the sensor is reaching end-of-life when the calibration knob has to be turned to near it's rotation stop to get to 21% on the display. I verify the shop fill with my own analyzer just as a check against the shop's "known good" sensor. (and twice it has turned out that my sensor was the good one, and the shop sensor was due for replacement)

 

[broncobowsher]

Cal in air, 21%
Test against your O2 source (I have 100%). Reads 100%, you are good. Actually close to 100% is fine. Any air calibration error will be multiplied in pure O2. Depending on humidity, etc. you could actually read over 100% on pure dry O2. If just running an oxygen concentrator, you will be reading how good you are actually making O2.

When the O2 cell starts dying, the upper end of the readings is what gets chopped. You can get a reading in air, and a little richer than air, but the readings will flatten out. Once you reach where the cell is current limited (I know it makes voltage that you read, but it really makes current that is converted to voltage internal to the cell) That is when it fails. It may read air fine, 30% OK, even show 40%, but on pure O2 it may never show over 50%. This is the failure point that is so bad for CCR. If you are trying to run a set point of 1.2PPO2 and you are reading 0.50 PPO2 (50% O2 at ambient) the CCR diver, or the CCR computer, will add more O2 to try and bring the number/percent up. Ending up with a very high PPO2. Now a bad cell reading 0.50 should be easy for a trained CCR diver to see. The issue comes when they are trying to run a setpoint that is right at the current limited point. Often a CCR diver will do a spike of O2 on a dive just to make sure all the cell will read well above the normal running setpoint.

That idea of the CCR diver spiking O2 to test cells during a dive is the same as a Nitrox blender testing the analyzer on pure O2. If the analyzer will read air and 100%, it is still a good cell for reading a blend well below the 100% that it has been checked to read.

 

[stiebs]

The easiest way to test your analyser is putting a multimeter across the terminals. Depending on the analyser model, in air it should read somewhere between 8mV and 25mV. When it's new, take a reading in air, and that's your benchmark. as the sensor deteriorates, the mV reading will drop, eventually down to 0mV.

My home made analyser shows the mV voltage reading:

A(nother) DIY o2 analyser

After being shelved a couple of times, earlier this year I finally got enough enthusiasm and motivation to complete the design and build of my own O2 analyser. In my usual style, it is was more over-engineered than it needs to be, but it certainly got the interest of some of the dive crew on my...

scubaboard.com

 

[GF99/99]

How can you tell when an O2 sensor needs to be replaced? Will you start to see low readings, high readings ?

For me I know my analyzer cell is going when it starts to take forever for it to read pure O2. As others have mentioned this is the current of the cell becoming limited.

You will start to notice that when you are trying to analyze O2 you getup to 95-97% pretty quick but then it takes what feels like an eternity to finally get up to 99% or 100% or it might now even fully get up to 99%/100%. For me once the cell starts to "slow down" like this that means its time to go. IMO if your using cells that can't even read air properly or 50% etc. than you have already waited waaaaay to long.

I always just throw my old rebreather cells in my analyzers every year when I change rebreather cells.

 

[stiebs]

As others have mentioned this is the current of the cell becoming limited.

Voltage, not current.