Calibration question

[doctormike]

I have been in the habit of calibrating my controller and NERD when I build the unit, when the cells are dry, which is usually around once a week during dive season. I track the mV of the cells over time to watch for drift, and I calibrate the NERD simultaneously with the controller. This was as per my training, and as per the JJ manual:

"The oxygen sensors should be calibrated regularly. This should ideally take place prior to every dive and at least once daily with multiple dives.... Calibration of the HUD should take place immediately after calibrating the controller! It is recommended that the HUD and controller always be calibrated at the same time!"

On the other hand, I have read a lot about how excessive calibration can mask sensor output drift. In a few threads, people suggest only calibrating once a month or so. In one RBW thread from 2012, there is a post from Bruce Partridge saying:

"I would almost never calibrate the HUD and the main display at the same time. Even when I calibrate every dive, I would alternate calibration and verify at 6 m. ... Calibration compensates for cell drift. If you calibrate one unit, but not the other, the cell drift is apparent. If you calibrate both, you mask the drift."

So, what do you think? Am I calibrating too much? JJ and Shearwater, two awesome companies. Which to believe...?

 

[joshk]

I don't calibrate every dive. I calibrate if there's a change in the mV air readings from last calibration or if verification at depth (20') changes. I track my cells religiously. Every time I prep the unit I check the mV readings and write it down along with surface pressure. If I calibrate I write that down as well.

If the numbers are the same, why calibrate? What are you changing?

One could argue that the mV in air is a bad value to check and should do an O2 flush and compare that on the surface. I check at 20' going down and coming back up, so I think that's about the same.

Where this gets really interesting is tracking cell mV and comparing values from a "wet" (post-dive) cell to a "dry" cell. In my limited experience dry cells (head aired out and not dove for a few days) tend to have a higher output. This means if I calibrate (say a new cell was installed) and get 12 mV in air and 57 mV in O2 that's typically not well aligned with what it will read after a few days of diving. It'll typically read lower (say 11 mV in air and 50 mV in O2) right after a few hour dive. Now that's only 13% off, but at 20' you'll get a max reading of 1.4 compared to the 1.6 you'd expect. This also gives you different decompression obligations. Right now my NERD is calibrated ~10% low compared to my controller, so it gives me a slightly longer decompression obligation in comparison.

 

[MegDiver792]

The new vandagraph cells are so stable, I only calibrate if they dont read 21 in air.

 

[doctormike]

I don't calibrate every dive. I calibrate if there's a change in the mV air readings from last calibration or if verification at depth (20') changes. I track my cells religiously. Every time I prep the unit I check the mV readings and write it down along with surface pressure. If I calibrate I write that down as well.

If the numbers are the same, why calibrate? What are you changing?

One could argue that the mV in air is a bad value to check and should do an O2 flush and compare that on the surface. I check at 20' going down and coming back up, so I think that's about the same.

Where this gets really interesting is tracking cell mV and comparing values from a "wet" (post-dive) cell to a "dry" cell. In my limited experience dry cells (head aired out and not dove for a few days) tend to have a higher output. This means if I calibrate (say a new cell was installed) and get 12 mV in air and 57 mV in O2 that's typically not well aligned with what it will read after a few days of diving. It'll typically read lower (say 11 mV in air and 50 mV in O2) right after a few hour dive. Now that's only 13% off, but at 20' you'll get a max reading of 1.4 compared to the 1.6 you'd expect. This also gives you different decompression obligations. Right now my NERD is calibrated ~10% low compared to my controller, so it gives me a slightly longer decompression obligation in comparison.

Thanks! I just got a cell checker, so I'm looking forward to being able to get an accurate mV reading for high setpoint, and then writing that on my controller to check during the dive. I guess the question is what amount of drift or decay would you consider "the same". All cells are going to slowly lose output over time, so is there some standard delta at which you would re-calibrate?

To eliminate that question, I could just do what the manual says and calibrate every diving day. The only downsides of regular calibration would be the small amount of O2 that I used, as well as the possibility of masking a dying cell (but that's why we track mV, right?).

So are you doing what Bruce says and calibrating your controller when the cells are dry, but calibrating your NERD when the cells are wet?

 

[doctormike]

The new vandagraph cells are so stable, I only calibrate if they dont read 21 in air.

Yeah, I noticed a big difference between them and the AI cells. But relying on the mV in air doesn't take into account linearity drift, right?

 

[joshk]

So are you doing what Bruce says and calibrating your controller when the cells are dry, but calibrating your NERD when the cells are wet?

Briefly from my phone, it is the opposite. NERD was calibrated with dry stable cells (not dove for a week or more). Controller was calibrated a few weekends ago after a cell failed at Dutch, almost immediately post dive.

 

[joshk]

Thanks! I just got a cell checker, so I'm looking forward to being able to get an accurate mV reading for high setpoint, and then writing that on my controller to check during the dive.

Why do you need a cell checker? I wouldn’t expect a dry stable mV reading to be the same as a wet one. Why are you checking mV during a dive? Calibration links PPO2 to mV values, why not look at your controller during a Oxygen flush?

I guess the question is what amount of drift or decay would you consider "the same". All cells are going to slowly lose output over time, so is there some standard delta at which you would re-calibrate?

This is a good question and I’m still feeling this out. I tend to not calibrate if a cell is reading slightly higher than before, e.g. calibrated when it was reading 10.1 and now it’s reading 10.4 on the surface in air. I’ve had several dives where I validated at the end of the dive the cell was back to performing at the 10.1 “strength” so it seems to me I shouldn’t recalibrate to a “stronger” reading which is likely a misrepresentation of the cell’s actual state.

Now if a cell jumped 1mv or more I would be suspicious. If this was across all cells I’d expect higher surface pressure contributing to some anomaly.

Historically I’ve calibrated when a cell has dropped 0.4mV or more compared to the last measurement once you account for surface pressure differences or if it doesn’t validate at 20’. I do validate my cells underwater on the way down and the way up to ensure they are correct and note when a cell is drifting downwards. Currently my cell #3 is reading low and has only dropped 0.2mV on the surface.

 

[doctormike]

Why do you need a cell checker? I wouldn’t expect a dry stable mV reading to be the same as a wet one. Why are you checking mV during a dive? Calibration links PPO2 to mV values, why not look at your controller during a Oxygen flush?

I do that too, but an O2 flush is never going to be as accurate as a pressure pot, because you probably aren't completely flooding the loop with O2, and depth measurement isn't so precise (my NERD and my controller often differ more than I might expect).. The idea was to put a target mV on the controller and confirm that there is no significant drift - during the dive, from the last time I checked the cells dry. I got the idea from something posted by Richie Kohler, but maybe I misunderstood his point. Also, the pressure pot lets you check linearity in the region of interest (around high setpoint) more easily than a one point flush.

Historically I’ve calibrated when a cell has dropped 0.4mV or more compared to the last measurement once you account for surface pressure differences or if it doesn’t validate at 20’. I do validate my cells underwater on the way down and the way up to ensure they are correct and note when a cell is drifting downwards. Currently my cell #3 is reading low and has only dropped 0.2mV on the surface.

I don't know how you can really "validate" at 20'. What you can do is rule out current limiting. I also find such variability in surface measurements that I don't think that native cell output is the only culprit. I'm assuming that humidity (also hard to measure) is a bigger factor.

So basically, you watch mV on air to see if your output is drifting down, and then calibrate again once any of the cell drops more than 0.4 mV? That is a simpler approach...

 

[joshk]

I do that too, but an O2 flush is never going to be as accurate as a pressure pot, because you probably aren't completely flooding the loop with O2, and depth measurement isn't so precise (my NERD and my controller often differ more than I might expect)..

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.

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.

The idea was to put a target mV on the controller and confirm that there is no significant drift - during the dive, from the last time I checked the cells dry.

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.

Your point about checking linearity above 1 ATA is also very valid, if you had a flaky or questionable cell it would be an accurate and simple test to validate it's function.

I don't know how you can really "validate" at 20'. What you can do is rule out current limiting.

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?

I also find such variability in surface measurements that I don't think that native cell output is the only culprit. I'm assuming that humidity (also hard to measure) is a bigger factor.

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.

So basically, you watch mV on air to see if your output is drifting down, and then calibrate again once any of the cell drops more than 0.4 mV? That is a simpler approach...

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.

 

[JohnnyC]

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.