Were you trained on Oxygen sensors with this level of detail?

[DA Aquamaster]

What's the benefit?

But that's exactly what the mV readings are only divided by whatever you calibrated at 1.0 with. If each sensor gave out 60mV at 1.0 then you'd have 73.8, 72, and 70.2 mV out for the display in your example.

Exactly. The problem is if the cells are "current limited", that's exactly what it means. The fuel cells, when exposed to a given elevated PPO2, are no longer generating enough current for the calibrated computer to reflect the correct PPO2. Whether it's expressed in millivolts or PPO2 is a moot issue - it's still no more accurate than the cell providing the millivolts.

I'll normally see this as an issue in an older, weaker sensor that may calibrate just fine (i.e. over 47 mv on 100% O2 at 1 ATA in my case) in nice, dry, pre-dive conditions but will start to lag behind and read lower than the newer sensors as the dive progresses, and/or the membrane gets wet from condensation, etc. Yes, if I page through the menu and look at the actual millivolts it will confirm what the PP02 numbers are telling me - that one of the cells is showing a lower millivolt reading than the others.

In order for this to be useful information for me to have however, I have to know if the low reading is excessive low for that cell at a known PPO2. In order to do that, I have to have a loop PPO2 of 1.0 (a known PPO2, not one measured by the sensors, which means a thorough diluent flush at a depth where the diluent PPO2 = 1.0), then read the current millivolts at that time/depth/PPO2, and then compare the suspect cell's current millivolt reading with what I had for that cell when it was calibrated at a PPO2 of 1.0. A drop compared to the reading I had at calibration would confirm the cell's performance is impaired (but not necessarily why - moisture, a bad connector or splitter, some caustic water draining off some of the cells out put through the connection, etc).

However, it's mostly a lot of extra work for nothing. The reality is that if I doubt the other 2 sensors, I'm going to do a diluent flush and compare the diluent PPO2 at that depth with the readings on the sensors to see which sensors are reading accurately - and then just stop there. I really don't care about the actual millivolts in that case either.

The assumption is that if you get a minimum of x millivolts in calibration at 1 ATA on 100% O2, the cell will have enough potential to be accurate up to the maximum 1.6 PPO2 you'll use on the dive - but that's often just an assumption when moisture is also involved.

In practice, from dive to dive, day to day, week to week, I gain a pretty good understanding of what is "normal" for my sensors, and if sensor number 3 consistently drops it's PPO2 reading by .1 on every dive after 3 hours, or responds a little slower late in the dive, then it's less of an issue than if a sensor shows a drop in PPO2 during a dive, for the first time, outside it's usual pattern.

In other words I can tolerate one cell that calibrated in the 45-47 mv range and then lags a bit on the dive, or even reads .05 or even.1 low after 3-4 hours, - or for that matter I can tolerate a sensor that reads high. However, I can't tolerate 2 inaccurate sensors because of the method used by the computer to compute the deco. I can run the PP02 based on the good cell, but I'll want to use my SA computer for the deco calculations - and replace both inaccurate sensors before the next dive.

 

[tarponchik]

can you show me the graph of that sensor please?

I do not have one at hand but I've seen those. The reason the sensor ages like that is in anode depletion. At some point of aging, the anode surface is not sufficient to provide for the higher current values at high O2.

 

[DA Aquamaster]

can you show me the graph of that sensor please?

I don't have a graph per se, but I can show you the results from a recent test of my current sensors.

I have 3 sensors, and 1 and 3 have been in service since May, while sensor 2 was placed in service in January.

At a theoretical PPO2 of .8 (based on calibration of the cells at 1 bar pressure), the cells all indicate a .8 PPO2, with an error range of about .1%.

At a PPO2 of 1.0 (achieve by an increased in pressure in the test chamber) the cells still all give an indication of PPO2 that rounds to 1.0 with errors for each cell ranging from 1 to 1.5%.

At a PPO2 of 1.2, the error range increases to between 2 and 2.5% per cell, and the PPO2 reading are now starting to lag slightly with readings of 1.17 for the older cell and 1.18 for the newer cells. Still not an issue however.

At a PP02 of 1.4 the error range is now between 4% and 6% for the cells and the old cell is reading 1.31 while the newer cells (six months in service) are reading 1.37.

At a PPO2 of 1.6, the error range is now between 6% and 8% and the PPO2 readings on the cells are 1.40 for the old cell and 1.54 for the new cell. The newer cells are hanging in there ok but the old cell is lagging a full .2 low. This is one reason you don't want to maintain a PPO2 of 1.4 on the dive. The CNS issues at bad enough even before you start getting into issues of your cells reading low. If I were on a cave trip and did not have access to a spare cell I'd keep it in play, but pay close attention to it - mostly because I do non deco dives at 1.0 and deeper deco dives at 1.2, then deco out at 1.4, so I'm never going to have 1.6 in the loop. But it's days as a functional cell are pretty much over as performance just goes down hill from here.

As an example I tested the two cells retired in May (cells 2 and 3), along with a rejected cell purchased at the same time (Cell 1) to show the current limiting that follows with cells 1, 2 and 3

.8 PPO2
Cell 1 = .85
Cell 2 = .76
Cell 3 = .41

It's clear already that cell 3 is severely current limited, while cell 2 is lagging slightly. Cell one is reading high.

1.0 PP02
Cell 1 = 1.12
Cell 2 = 1.0
Cell 3 = .39

Cell 3 is still current limited and in fact fading, and while cell 2 is reading 1.0 but took 17 seconds to get there. Cell 2 is still reading high

1.2 PPO2
Cell 1 = .1.26
Cell 2 = .1.18
Cell 3 = .35

1.4 PPO2
Cell 1 = .1.44
Cell 2 = 1.38
Cell 3 = .36

For both 1.2 and 1.4 Cell 2 is still reading 6% over the expected reading, cell 3 is running on fumes and cell 2 is reading ok, but still has an excessive lag time.

1.6 PPO2
Cell 1 = 1.66
Cell 2 = 1.57
Cell 3 = .38

Cell1 is still 5.6% high, cell 2 is still only 7.5% low, but it's got a 20 second lag time and cell 3 is still showing PPO2 that would give me a "low PPO2" warning on my computer if it were installed in my CCR on a dive.

These cells are all out of service for different reasons. Cell 1 reads high enough that it rounds up to the next PPO2 on the unit. (1.3 rather than 1.2 for example). It performs better when calibrated with 100% O2, but the slope of it's graph is still wrong, so it varies from low to high and only reads well right around a PPO2 of 1.0. It was annoying so it got replaced pretty quickly. Cell 2 gives an acceptable reading but lags at 15seconds behind the other cells. Cell 3 is severely current limited. At the time it was replaced however it was reading like the soon to be replaced noted above - reading 1.3 when the PPO is really 1.4.

-----

Now...here's the kicker. The weaker cell currently in my unit is less than 12 months in service and less than 18 months old over all - yet it is reading .1 low at 1.4 PPO2. If you subscribe to the theory that you should replace all your cells at once, you could be diving a unit where all three of your 10 month old cells are reading 1.4 when you're at an actual loop PPO2 of 1.4, and that could be a problem. The fact that three old cells from the same production batch are very consistent with each other doesn't mean they are accurate.

As evidenced above, you can also have a cell that reads high and or just doesn't have the same slope in terms of actual PPO2 and millivolt output.

Some form of linearity testing is vital. You can test for current limiting at 20 ft with a thorough O2 flush. You can then also test for linearity with three data points via a slow ascent from 20 ft and a pause at 10 ft where the PPO2 will be approximately 1.3 at 10 ft, and then an ascent to a PPO2 of 1.0 at the surface. It still works to some extent if you can only get a less than perfecO2 flush - you're just going to have to verify the numbers at 10 ft and at the surface to confirm they are correspondingly low compared to your PPO2 at 20 ft.

The problem with not getting a 1.6 at 20 ft, is that you'll also have to be very consistent in the less than perfect O2 flush and recognize what the best effort looks li on your unit, so you can tell whether 1.51 is good enough or an indication of current limiting. For example, if you end with an actual 1.0 PPO2 at the surface and only had 1.51 at 20 ft, your cells are current limited. Ifyou end up with .93, the question the is do you end up with .93 every time? The only way you'll know that is to do an O2 flush at 20 feet on a regular basis.