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

[northernone]

Being a math nerd, I had noticed that as well. I just figured I didn't know enough about the subject.

It seems like doing these calculations for you would be a feature someone would add to a CCR dive computer.

I mean, if you calibrate at 0.21 ATA, then check it with 1.0, the computer could have an alternate screen to go to to show you calculated versus metered mV values and also show linear deviation. Ditto for checking at 1.6 ATA.

Or something like that. Bottom line: It's a straightforward process and simple arithmetic (or so it seems), so why not have your computer do it for?

I agree. That is logical and safer as this thread demonstrated. And it is indeed a feature in some computers. Probably for the unfortunately demonstrated reason above. Memorizing a math formula is one thing, trusting life altering decision based on correctly applying it is another.

Frighteningly sometimes we see what we are looking for. I'm vicariously embarrassed I didn't pick up the error in the example myself after boldly stating I understood and was taught the material presented on the page.

Regards,
Cameron

 

[v101]

Yes I was taught how to perform this test. I feel its wise to do, to know what condition my cells are in before the dive. I better understand the math and logic after doing the checks a few times.

 

[stuartv]

As someone who may embark on CCR training soon, I am curious how common the problem is of having new or new-ish cells that are not linear, and how much is a common amount of deviation?

For example, if you a batch of new cells that you were going to install in your CCR, if you put a cell in and calibrated in surface air, then flushed the loop with O2 (on the surface), you would want to see it read 1.0, right? So, how common would it be to see 0.95 or 1.05? How common to see 0.90 or 1.10?

Is it even a thing that there would be deviation on the high side? Or is linear deviation always an error in the low direction? So, you might see 0.95 or 0.90, but you would never see 1.05 or 1.10?

I'm specifically asking about deviation here, not about how common it is to find new cells that are limited.

 

[kensuf]

Yes, but my instructor was Ted.

When I teach Mod 1, I go over the first part showing the mv readings in air and what to expect in 100% as part of my explanation on checking cell linearity. I don't extend it out to 1.3 though.

 

[Bobby]

As someone who may embark on CCR training soon, I am curious how common the problem is of having new or new-ish cells that are not linear, and how much is a common amount of deviation?

For example, if you a batch of new cells that you were going to install in your CCR, if you put a cell in and calibrated in surface air, then flushed the loop with O2 (on the surface), you would want to see it read 1.0, right? So, how common would it be to see 0.95 or 1.05? How common to see 0.90 or 1.10?

Is it even a thing that there would be deviation on the high side? Or is linear deviation always an error in the low direction? So, you might see 0.95 or 0.90, but you would never see 1.05 or 1.10?

I'm specifically asking about deviation here, not about how common it is to find new cells that are limited.

If you calibrate then you will only see 1.0 because you calibrated. Without doing the math you have no way of knowing if the cell is linear.

It is, IMHO & experience, very rare to have a cell that is not within 1-2% fresh out of the bag. The problem is when the odd thing happens such as a bad batch of cells from a manufacturer or simply one cell that made it through QT yet is out of spec when it comes out of the bag to be used. More common is for a cell to have too much linear drift in a short amount of time. This I would say, from certain manufacturers, has been a 1 in 10 or so experience for me.

Cells can also have linear drift on the high side as well as the low side. I have never personally experienced a cell that had so much drift on the high side that I would bin it however at least one manufacturer has a cell that tends to go positive just before it dies completely. Good new cells tend to be 1-2% on the high side until they have some run time on them. I've seen a cell as high as 8% above, I ran it, however it died within a couple of months. That one time is my only experience so it doesn't really say anything. Interestingly it didn't just go out, it quickly moved to more than 10% linear drift on the low side so I removed it.

 

[kensuf]

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.

 

[Bobby]

I disagree with the mv range in pure O2.
If the range on 8-13mv is used the range of mv in O2 should be 38 - 62mv (38.02 to 61.88 for you pedantic types)

Dave,
Are you comfortable with you or your students accepting a 30% or higher linear drift? That is why I don't agree that a range of mv should be used and that the math should be performed every time.

Without a pressure pot or 02 check at 20'/6m we can't verify cell limiting either.

 

[wedivebc]

As someone who may embark on CCR training soon, I am curious how common the problem is of having new or new-ish cells that are not linear, and how much is a common amount of deviation?

For example, if you a batch of new cells that you were going to install in your CCR, if you put a cell in and calibrated in surface air, then flushed the loop with O2 (on the surface), you would want to see it read 1.0, right? So, how common would it be to see 0.95 or 1.05? How common to see 0.90 or 1.10?

Is it even a thing that there would be deviation on the high side? Or is linear deviation always an error in the low direction? So, you might see 0.95 or 0.90, but you would never see 1.05 or 1.10?

I'm specifically asking about deviation here, not about how common it is to find new cells that are limited.

Although the majority of my CCR experience is on a meg which up until the most recent release used a 2 part calibration sequence that corrected the offset between the low and high point calibration so I was not that aware of minor offsets but in the last few years also diving P2 which has only oxygen calibration I can say I have not seen any cell that was grossly non-linear.

 

[stuartv]

If you calibrate then you will only see 1.0 because you calibrated.

Maybe what I said didn't make sense based on how you really do things. What I was talking about was calibrating in surface air, meaning calibrating to 0.21. So, then you would hope to see 1.0 on an O2 flush. But deviation (or limiting) would mean seeing some other value.

Anyway, I think you, kensuf, and wedivebc have answered my questions. Thank you to you all.

 

[wedivebc]

Dave,
Are you comfortable with you or your students accepting a 30% or higher linear drift? That is why I don't agree that a range of mv should be used and that the math should be performed every time.

Without a pressure pot or 02 check at 20'/6m we can't verify cell limiting either.

Absolutely not. Where do you see any evidence I would? You content that most cells show 12mv in air and that is false. I contend (as per factory documentation) that acceptable cell limits are 8-13mv in air. That means if I have a cell sitting at 8mv in air it will show 38mv in O2 which I will accept as a perfectly linear cell. If I have a cell showing 13mv in air and 62mv in O2 I will be comfortable with that cell as well.
To be honest I have not found any cells that were noticeably non-linear. This is the reason ISC dropped the 2 part calibration sequence when it released APECS 4 because it's not really a common issue and it can be easily picked up by a diver doing proper pre-dive checks.