Tracking maintenance

[tbone1004]

You can manually do a two point calibration on every unit you may remotely consider diving. I'm 99% positive your CCR instructor will teach you how. He will also teach you how to track the health of the cells over time.

manually doing the 2-point calibration is the linearity check, which should be taught by every instructor. That is outside of the computer though on most every CCR out there including I think everything on the market now except the Liberty.... ISC did away with it, Shearwater has never had it, and I don't know about the Defender's SG-1 thing.

2-point calibration is y=mx+b
single point calibration is y=mx
y=ppo2
m=the adjustment factor
x=cell output mV
b is the adjustment of the line intercept required for 2 point calibration

Fun fact, the Divesoft is working on the Liberty so it can also do a 3 point quadratic calibration which is super interesting though I'm not entirely sure it's necessary or something I would want to use. It can calibrate at air, o2, and then pump O2 up to 1.6 and fit a line between the three. Realistically I don't think anyone cares about what the cell does much below 1.0. It's not going to be so far off at .7-.8 that it will be an issue, so calibrating in air really isn't useful. You really need that line to be drawn between 1.0 and 1.6-1.8. That to me is a HUGE plus to the Liberty right now

 

[kensuf]

I don't have time to write a book explaining it, and Ted will explain it to you next month when you're here, but when you do a linearity check you are manually doing a 2 point calibration and you are manually validating that the slope of the line falls within an acceptable range.

 

[tbone1004]

I don't have time to write a book explaining it, and Ted will explain it to you next month when you're here, but when you do a linearity check you are manually doing a 2 point calibration and you are manually validating that the slope of the line falls within an acceptable range.

I'm aware of that, I have been doing that for years with analyzers. It sounded like you were saying that the handsets could be calibrated manually vs automatically which was my point of confusion

 

[joshk]

Fun fact, the Divesoft is working on the Liberty so it can also do a 3 point quadratic calibration

You've got three points and none of them line up. How are you going to “fit a line between the three?”

How many hours have you put on the Meg?

 

[tbone1004]

You've got three points and none of them line up. How are you going to “fit a line between the three?”

How many hours have you put on the Meg?

you can do it one of two ways, both of which are more accurate than two point.
You can fit two separate lines based on the linearity between each set of points.
In this case linearity may be 95% between air and O2, and 90% between O2 and 1.6

The other and what DiveSoft said they did was use a quadratic curve will put an arc between the three points to allow the drift to be more natural. Is this beneficial? I'd say no, but it is going to be technically more indicative of how what the output actually looks like

and on the meg I only have about a dozen hours from several years ago. Wasn't warranted for the dives I was doing and couldn't justify the investment. Things have changed recently in my diving so it is now much more beneficial

 

[joshk]

and on the meg I only have about a dozen hours from several years ago

I don't want to dampen your enthusiasm for the subject, but you should obtain some more real-world experience before jumping into every thread or other people's replies.

 

[tbone1004]

I don't want to dampen your enthusiasm for the subject, but you should obtain some more real-world experience before jumping into every thread or other people's replies.

you asked about meg hours, not about total ccr hours, or whether I've built one or not, or the fact that I'm an engineer and have quite an in depth knowledge of how galvanic oxygen cells work and the pros and cons of single point and multi-point calibration. The answer to those questions are quite a bit different.
Real-world experience on a specific unit, or even rebreathers in general has no bearing on calibration and function of o2 cells. The real-world experience gives you how the cells behave after being exposed to high humidity, or how long they will last and what failure modes warrant replacement. It has no bearing on understanding of linearity or calibration procedures.

You want proof of that, I can give you at least 3 instructor trainers that do not understand linearity, one of which specifically teaches not to do a 1.6 ppO2 check and cites safety reasons for not doing it. Despite that check being the one check that you can do, and only have one chance to really do it on a dive to safe your butt if you have a limited cell.

Comments like what you made above only add to the "voodoo" that surrounds rebreather diving because the vast majority of these instructors and instructor trainers know how to work a rebreather, but they really don't know the why behind the how.

 

[DA Aquamaster]

you can do it one of two ways, both of which are more accurate than two point.
You can fit two separate lines based on the linearity between each set of points.
In this case linearity may be 95% between air and O2, and 90% between O2 and 1.6

The other and what DiveSoft said they did was use a quadratic curve will put an arc between the three points to allow the drift to be more natural. Is this beneficial? I'd say no, but it is going to be technically more indicative of how what the output actually looks like

I have not formally plotted the output of my cell tests, and I'm not inclined to mess with a serial port to output the data to a computer, but looking at the numbers as the PPO2 rises from .21 to 1.6, the resulting percentage change from the actual PP02 in the pot is clearly a curve, and the rate of change clearly increases the higher the PPO2 gets past 1.0.

People talk about "linearity" but clearly any line used will just approximate the actual curve, and a line between 3 points will be a better approximation than a line between just two points.

.../
/... or the fact that I'm an engineer and have quite an in depth knowledge of how galvanic oxygen cells work and the pros and cons of single point and multi-point calibration. The answer to those questions are quite a bit different.../

/....Comments like what you made above only add to the "voodoo" that surrounds rebreather diving because the vast majority of these instructors and instructor trainers know how to work a rebreather, but they really don't know the why behind the how.

I chuckled a bit when you were "Ken splained" to about manual 2 point calibration. I wondered whether he didn't know you are an engineer, just forgot you are an engineer, or just figured it didn't matter.

I agree with you that way too many CCR divers, CCR instructors, and CCR instructor trainers come up short on theory. They know drills and procedures but seem to come up short on the chemistry and physics involved. For example, I suspect many instructors and perhaps a majority of CCR divers won't understand how you could stay on the loop in full CCR mode after a total electronics failure, as long as you were at and continued to maintain minimum loop volume and don't change depth.

 

[joshk]

you asked about meg hours, not about total ccr hours,

You said you dive a Meg:

meg 2.7 that I inherited

Not “a meg, and other homebred stuff.”

fact that I'm an engineer and have quite an in depth knowledge of how galvanic oxygen cells work and the pros and cons of single point and multi-point calibration. The answer to those questions are quite a bit different.

An engineer with a lot of experience with galvanic oxygen cells?

You have a dozen hours on a Meg “from several years ago” and this thread is about tracking maintenance, implying for a rebreather someone actually dives. If you have other questions about oxygen cells, maybe bring it up in a different thread.

 

[tbone1004]

@joshk you were the one who brought up the clarification of tracking actual ppo2 vs displayed ppo2, not me. I have a full understanding of linearity of oxygen cells, different calibration methods, and the downsides of calibrating with intent to use outside of the calibration range.

number of hours on a specific unit, or even rebreathers in general has no basis on ones understanding of how something functions, especially cells since they are not unique to rebreathers. Case in point the instructor trainers who have thousands of rebreather hours and are adamant that linear deviation is not something that happens to cells. Yes, they exist.

In terms of tracking maintenance, again, not something unique to rebreathers or something you need hundreds of hours on. I have a maintenance department that reports to me, we have hundreds of machines and sensors that need to be maintained and calibrated on a regular basis. They are maintained in databases similar to what you indicated you use. This is also how I track all of my dive gear maintenance