Were you taught this math in your entry level CCR class?

[wedivebc]

In the last year I saw ~5 or 6 cells from AI that went super high when they died. I mean like ridiculously high mV readings.

If you check those cells a day or so later they will likely read zero as cells sometimes go super high just before the electrolyte burns through.

 

[Bobby]

Dave,
I asked if you would, which I actually assumed that you would not. It was not meant as an attack on you, I can understand that it may have come across that way, my intent is to use your response to make a point. That point is that people need to understand the range and how one end of one side of the range affects the other side and end. Too many people are simply taught that X range is acceptable without the why and how.

I do not contend that most cells are 12mv in air. I simply use that number regularly to make a point. If you have a cell that is sitting at 12 or 13mv in air and it shows 38mv in O2 then there is a problem. I know you understand this, what I question is how many others actually understand it along with the why. My experience is that when I take myself out of teaching mode and put myself in asking questions mode, I have found that the majority of CCR divers don't understand the why and don't understand how the air range affects the O2 range. Far too many believe that a calibration ensures that everything is OK.

 

[wedivebc]

Dave,
I asked if you would, which I actually assumed that you would not. It was not meant as an attack on you, I can understand that it may have come across that way, my intent is to use your response to make a point. That point is that people need to understand the range and how one end of one side of the range affects the other side and end. Too many people are simply taught that X range is acceptable without the why and how.

I do not contend that most cells are 12mv in air. I simply use that number regularly to make a point. If you have a cell that is sitting at 12 or 13mv in air and it shows 38mv in O2 then there is a problem. I know you understand this, what I question is how many others actually understand it along with the why. My experience is that when I take myself out of teaching mode and put myself in asking questions mode, I have found that the majority of CCR divers don't understand the why and don't understand how the air range affects the O2 range. Far too many believe that a calibration ensures that everything is OK.

I don't feel attacked, this is just a discussion.
This is a snip from your blog
"...Another common way of avoiding linearity checks is to simply toss a cell once it goes below 10mv. Cells are rated at 12mv (the majority of CCR cells, the math is the same for any O2 cell) with many of them coming out of the bag a bit above that. ..."

So if you're wondering where I get the idea you believe most cells are 12mv in air.

If you calibrate a cell in O2 and it shows .99atm and it returns to .21atm when exposed to air it is linear. If it gives you something different then either the calibration was faulty or the cell is not linear. In my experience 100% of the time the calibration is faulty but some simple arithmetic will prove that out. I teach my students how to do the math and encourage them to continue to do it when there is a discrepancy but I don't do the calulation every time I calibrate. If the PO2 comes out right I'm satisfied.

 

[_Slacker_]

Devil's Advocate:
Is there a practical example that the math would catch something but a diver skipping this step would not have multiple other ways to notice the issue (spiking O2 at 20ft or at depth, comparing behavior to other cells at any depth other than 1.0 ppO2, dil flush at depth), and it be problematic?

In your example above if one calibrates a cell in 100% O2 @ 38mV and then looks at the computer in air it would read >0.30 (12mV instead of the expected 8mV). No math done, know nothing about mV, understand almost nothing about cells, haven't compared the reading to other cells, and still see the problem before hitting the water. That example perhaps was an extreme error. Of course less error is less noticeable, but also becomes less of an issue going unnoticed.

 

[wedivebc]

Devil's Advocate:
Is there a practical example that the math would catch something but a diver skipping this step would not have multiple other ways to notice the issue (spiking O2 at 20ft or at depth, comparing behavior to other cells at any depth other than 1.0 ppO2, dil flush at depth), and it be problematic?

In your example above if one calibrates a cell in 100% O2 @ 38mV and then looks at the computer in air it would read >0.30 (12mV instead of the expected 8mV). No math done, know nothing about mV, understand almost nothing about cells, haven't compared the reading to other cells, and still see the problem before hitting the water. That example perhaps was an extreme error. Of course less error is less noticeable, but also becomes less of an issue going unnoticed.

The calibration of a rebreather is the point where you take a mv reading which is related to actual measured PO2 and it gets converted into a PO2 value A bad calibration can result in an error in PO2 reading and that is where the math can be important. When a diver calibrates the CCR they are exposing the cells to oxygen and telling the CCR computer this mv value equals pure oxygen. When they then expose the cells to air a reading of .21 is what is expected. If that is not the case. either the calibration was at fault or the cells have a linear drift. My experience tells me the most likely scenario is the cells were not actually exposed to pure O2 as can happen when an incomplete flush of the CCR occurs. There is a possibilty that the calibration was good and the cells are faulty and that is what the math will determine.

 

[Bobby]

Dave,
How much linear drift do you feel is acceptable? How much off on PO2 do you allow before you actually do the math? Why not simply do the math from the beginning so that you don't have to repeat dil and O2 flushes? It seems, to me, like more work to have less accuracy. Being a tenth off on PO2 could have as much as 7.4% linear drift. IMHO, if CCR divers don't fully grasp linear drift they have less chance of understanding limiting. I've also found about the same amount of people that don't check linearity don't check for limiting to 1.6 either. Unfortunately it also seems that these same divers are the ones that also run the higher PO2's.

We now have two polls that show, IMHO, far too many CCR divers were not shown, do not understand, and are not checking linearity. Yet the industry continues to keep its head in the sand and say there is no problem. It is simple math and there have been deaths from not doing it, along with checking for limiting to 1.6. If we scratched just a little deeper I am confident that we would find that a far higher percentage, than the polls show, really don't understand linear drift and limiting. Hopefully future students will see these threads and ask deeper questions of their instructors which may bring some change to the industry.

 

[Capt Jim Wyatt]

We now have two polls that show, IMHO, far too many CCR divers were not shown, do not understand, and are not checking linearity.

Yes for sure...that is my biggest/most significant takeaway from this conversation.

 

[stuartv]

I hope y'all don't forget to post the poll results here and in the other thread so that those of us who didn't vote can see them. Or edit the poll and add an option for "I don't dive CCR but I want to see the results."

 

[Capt Jim Wyatt]

Non voters can see the poll now.

 

[wedivebc]

Yet the industry continues to keep its head in the sand and say there is no problem. It is simple math and there have been deaths from not doing it,

You're going to have to cite some examples of deaths caused by linear drift because I'm calling BS on this. We're not talking about current limiting here, you're saying linear drift and either we are talking about 2 different things or you have a whole different data source than I do because I have never heard of a CCR fatality attributable to linear cell drift.
Whether the display swings between 1.0 - .021 or 47mv and 10 mv what's the difference? There is no linear drift. If it goes from .21 in air to 1.0 in oxygen how is it not linear.

When I am teaching CCR my students ask how often do I calibrate. I use this analogy. I pick up my Gibson Les Paul and strum a chord. If it sounds right I play a few licks, if not I tune it.
If I turn on my rebreather and my cells show 0.21 and when I flush with O2 it shows 1.0 what more do I need to do? How is this process not safe. Please explain.