Calibration question

[kensuf]

I don't necessarily press the "calibrate" button, but I do verify every time I do a unit build. On more than one occasion I've encountered a cell that's reading correctly in air, but is current limited in a high PO2 environment. My most recent one was a cell that was only 4 months old. I'd rather catch that kind of problem while I'm assembling the unit rather than at the dive site as I'm trying to get in the water.

I also do an O2 flush at 15-20' at the tail end of every dive to validate the cells are still functioning and not limited.

 

[joshk]

I've encountered a cell that's reading correctly in air, but is current limited in a high PO2 environment.

You've had cells from good to current limited below 1.0? Most of the current limited failures I've hard about (never experienced) is cells reading “fine” on the surface (range 0.21-1.0), but limiting at 1.1 or 1.2 during a dive. Cribbing from a previous diagram:

 

[doctormike]

The depth measurement is precise. The NERD is typically mounted higher being on the loop than the controller which is mounted on your arm. I typically see a ~1 foot difference but can easily make them agree by holding my arm at the same level as the NERD unit.

Nah, I don't think so. In horizontal trim, not much depth difference between the NERD sensor and the controller, and I have seen a significant difference in readings (more than 1 foot) in that position. Maybe one of them needs servicing? In any case, I think that our depth measurements aren't as precise as we think they are.

I can get a very efficient flush on the SF2, but even if you can't drawing a slug of O2 slowly across the cells (slow inhale after adding O2) should have a similar effect.

Some of this discussion is unit specific. For example, in the JJ, the O2 goes into the exhale T-piece, and the sensors are just before the inhale portion of the loop. Again, we tend to assume that at our 20 foot check we are replacing all the gas in the unit with O2, but I don't think that's the case. And it also may be dependent on technique, that could be something that improves with practice.

I'm interested to see your results. Assuming that (1) your depth and flush is precise (you indicated above it might not be?) my hypothesis is that you'll see a lower mV simply due to cell strength between a dry rested cell and a wet active cell. Still, I'm curious to see your results and hopeful that there's some helpful information there..

Will email you the cell checker spreadsheets. The issue that I would be checking for by writing down target mV for high setpoint would be how degraded the output becomes during a dive due to water on the sensors, etc...

I can rule out current limiting by making sure I can hit 1.8+ with a touch of O2 deeper, or by adding O2 coming up from my 30' stop. I might have misunderstood, but if you have a properly calibrated unit, draw a 100% O2 loop, are at 20', and have a controller reading 1.6 across the board, is that not validation?

Again, depends on the accuracy of the flush and the depth reading. But if you can read 1.6 at 20 feet, yes, that would imply validated cells and a good flush, unless of course the depth reading was significantly inaccurate and low.

Do you mean on the surface pre-dive? Have you checked surface pressure? I find most variations in mV readings are explained by surface pressure differences, to the point where my cell log system has a field for that built in and bases calculations around that value.

Yes, that's a good point, and something that I am now incorporating. Can you send me a template of your log?

In a nutshell yes, this is what I do. I'll also calibrate if there's a new cell or if a cell is responding weakly at 20' (reading low). I'm very much still Figuring This **** Out™ but so far I'm happy with the approach of calibrating less often and religiously tracking what my cells are doing compared to what a perfect cell would do.

Thanks! I really appreciate your insights. See you Tuesday...

 

[doctormike]

On more than one occasion I've encountered a cell that's reading correctly in air, but is current limited in a high PO2 environment. My most recent one was a cell that was only 4 months old. I'd rather catch that kind of problem while I'm assembling the unit rather than at the dive site as I'm trying to get in the water.

You mean that you use a cell checker before every build of the unit?

 

[doctormike]

Re: cell checker, I'll throw my cells in, check values for PO2 and mV in air, flood the checker with O2, bleed the pressure off and calibrate at 1ATA with O2, check PO2 and mV. It all gets logged as I pressurize up to 2ATA. It's good for checking linearity at high PO2, and the spreadsheet graphs its linearity. I'll purge with air and run it up to 3ATA to just for funzies to check what behavior the cells are experiencing during a simulated dil flush.

My Pelagian and SF2 are great for validation, the Pelagian because of the location of the O2 and dil feed into the head, the SF2 is just really efficient at doing a flush. Pushing a high PO2 during the dive works well and validating the behavior of the cells during a high PO2 flush with the behavior of the cells in the cell checker gives me a little piece of mind.

I like the Narked mini cell checker. I've got backs for both molex and the SF2 male SMB jack, plugs right into my Predator secondary.

As for calibrating, if I'm doing a weeks worth of diving, I'll calibrate at the beginning of the week and as long as the cells are performing as expected, I won't touch it. If I notice an issue at the beginning of the week, I'll typically recalibrate once and see how things shake out, sometimes cells need a bit to wake up, but I'll keep a close eye on their behavior and act accordingly. Longer than that and I'll recalibrate about once a week, although typically if my cells are stable, it doesn't accomplish anything.

I know that interface type shouldn't make a difference, but I've been really impressed with the stability and linearity of the cells that SF2 sources, moreso than the molex cells I pick up from DGX.

Very happy with the mini-checker, thanks for your recommendation BTW. I do think that a lot of this stuff is unit specific.

I just did the first couple of runs this weekend, and I noted that most of the readings on the six cells that I tested were more than 5% off expected, but they were fairly linear. Here is a sample run, I was wondering if the air mV reading was too high for some reason? Maybe I should have waited longer after connection? Although they did return to close to the initial output after the pressure run.

I guess I should ask John at Narked@90, but I'm wondering if the expected mV are calculated from the air reading AND the 2.0 ATA O2 reading, since the cells don't populate until you put that last entry in (#12). But it looks like just Expected mV = Air mV x 4.76 x ATA..?

 

[kensuf]

You've had cells from good to current limited below 1.0? Most of the current limited failures I've hard about (never experienced) is cells reading “fine” on the surface (range 0.21-1.0), but limiting at 1.1 or 1.2 during a dive. Cribbing from a previous diagram:

Yes.

I've also seen some output really high voltages right before they die.

 

[tbone1004]

@doctormike also remember that your specific barometric pressure is going to change the air calibration which is why I don't worry if my cells are spitting out .19-.23 in air.
the cell checker I have can actually go up to enough pressure to calibrate at 2.0 using air and I've found that the cells behave a little different when under that much pressure than they do if I flush with O2 and send them to 2bar which is interesting. Readings are the same, but you can get some speed checks as you're going up there

 

[doctormike]

@doctormike also remember that your specific barometric pressure is going to change the air calibration which is why I don't worry if my cells are spitting out .19-.23 in air.
the cell checker I have can actually go up to enough pressure to calibrate at 2.0 using air and I've found that the cells behave a little different when under that much pressure than they do if I flush with O2 and send them to 2bar which is interesting. Readings are the same, but you can get some speed checks as you're going up there

Right, and I guess if you knew that, you could just ignore the deviation from expected and the pass/fail column as long as it wasn't too far off. My concern was that a minor inaccuracy in the air mV baseline reading would translate to a greater than 5% deviation, even if the cells were good. The fact that mine are linear up to PPO2 of 2.0 is reassuring, even if they are all reading low compared to the expected line. I suppose that's what calibration is for!

 

[tbone1004]

Right, and I guess if you knew that, you could just ignore the deviation from expected and the pass/fail column as long as it wasn't too far off. My concern was that a minor inaccuracy in the air mV baseline reading would translate to a greater than 5% deviation, even if the cells were good. The fact that mine are linear up to PPO2 of 2.0 is reassuring, even if they are all reading low compared to the expected line. I suppose that's what calibration is for!

that's why we calibrate at 1.0 instead of .21 though, the deviation grows as you get farther from the calibration point. What the cells do in air is irrelevant if you're running at 1.0-1.6. In the cell checker, know the mV of your setpoints and plot, then in your wetnotes/wrist slate wrist down the deviation. I have a laminated chart in my wetnotes for that. It's really simple, I've got 1.1 thru 2.0 in .1 increments as rows. Columns are 90%, 95%, 105%, 110% and what the equivalent is. i.e. 1.6 in the 95% column is 1.52 so if the cells are 95% linear and I want to run a 1.6, I need to have the setpoint at 1.5. If the cells are 105% linear, then I need to run a 1.7. It's all "close enough" though. I check pre-trip with the cell checker so I know which column I "should" be in, then at the 1.6 check I know which one I'm "actually" in and can plan accordingly. I run a 1.1 setpoint for just about everything because I have the dual-LED meg hud and anything else is annoying to keep track of. On deco I'll obviously go up to 1.6 and that's when the chart matters.

What is super interesting on the linear deviation though is that because I have a Meg, there is a 2-point calibration on the handset, but a single point calibration on my Predator. There will often be a slight disagreement between what the handset says my ppO2 is and what the Shearwater says when I start to get outside of the .9-1.1 range because the Meg can factor in linearity. Does it matter? Not IMO and I'm glad that ISC removed that feature in the Meg15, but it's interesting to see it real-world.

 

[doctormike]

that's why we calibrate at 1.0 instead of .21 though, the deviation grows as you get farther from the calibration point. What the cells do in air is irrelevant if you're running at 1.0-1.6.

Right, it just occurred to me that it might be forcing a "fail" in the Narked@90 spreadsheet. If that is just working on one point for the expected values, then a minor inaccuracy in the air mV will result in a bigger delta down the line. The spreadsheet actually compensates for barometric pressure...

Calibration at 1.0 on the bench pre-dive is probably more accurate in terms of what the cells are seeing than is the O2 flush at 20 feet, even thought the 20 foot flush is more relevant for current limitation checking and divergent linearity above 1.0 (really the same thing). But when I run auto-calibrate, I'm assuming that the JJ engineers have set up that process to give me close to 100% O2 near the sensors - i.e. as accurate as I can get without a pressure pot.

In the cell checker, know the mV of your setpoints and plot, then in your wetnotes/wrist slate wrist down the deviation. I have a laminated chart in my wetnotes for that.

Cool. Care to share a link or a PDF?