Negative (or Positive) Test using O2 sensors

[tbone1004]

@stuartv rewrite your checklist to what suits you, nothing says the manufacturer has given the ideal checklist. What is after your O2 flush that a positive or negative would hold you up on?

O2 cells respond differently in different conditions, so if the unit says 0.95 but the cells are really cold vs. a calibration when they were closer to diving conditions is going to give you a different calibration result and also risks calibration mistakes which are quite easy to do. Most manufacturers say to calibrate every time, but most of the time the rebreathers don't need it.

 

[stuartv]

@stuartv rewrite your checklist to what suits you, nothing says the manufacturer has given the ideal checklist. What is after your O2 flush that a positive or negative would hold you up on?

O2 cells respond differently in different conditions, so if the unit says 0.95 but the cells are really cold vs. a calibration when they were closer to diving conditions is going to give you a different calibration result and also risks calibration mistakes which are quite easy to do. Most manufacturers say to calibrate every time, but most of the time the rebreathers don't need it.

My totally layperson opinion is that checklist design is just as much a product of solid engineering as the unit itself. In other words, rearranging the checklist seems to me to be just as serious of a task as, for example, deciding to move where the O2 sensors are in the loop. i.e. not something a person like me (i.e. NOT a CCR engineer) should be doing.

The Build checklist for rEvo starts with checking the O2, including sensor validation/calibration.

After that, it moves over to checking the dil, including validation of the mix, and ending with the negative test.

One product of that is that your rEvo does not sit around indefinitely with O2 in the loop, consuming the O2 sensors more quickly than necessary. At the end of the Build checklist, the unit is full of your dil (with a little bit of air sucked in at the end of the negative test, when you open the DSV/BOV).

After the Build checklist, your unit is ready to be transported to the dive site - however long that might take (could be a few days). Then there is the Pre-jump checklist to do right before you splash.

As far as O2 cells being cold or some other condition that throws them off, yes, the user is certainly responsible for understanding those things and using some good judgment with regard to the O2 sensor readings being seen during the O2 portion of the Build checklist. But, that sidestepped my question.

If my calibration is off (which carries the presumption that I've accounted for things like cold sensors), I'm going to calibrate it. But, if it is "on", what harm is there is storing the same calibration value again, overwriting the (same) value that is already stored? I think there is none.

So, the only real issue is that you are saying that there are ways you can screw up the calibration. E.g. calibrate with really cold sensors. So, it's not really that one shouldn't calibrate every time. The issue is REALLY that you have to make sure you do your calibration correctly every time. Which includes all kinds of things, one example of which would be to not perform a calibration with really cold sensors. Another example might be to not perform a calibration with a brand new sensor that you JUST took out of a sealed package.

 

[rddvet]

I was never taught to watch the MV like tbone suggested during the negative, but I plan to start trying it. My wife recently had to call a dive before we got in the water because her unit failed a negative at the dive site but was fine the night before. But in retrospect she wondered if the original negative was actually as truly accurate as she thought it was. It seemed the negative didn't catch the small leak initially. So we're both going to start seeing how it works.
The best thing I learned in my sidewinder class from Edd was to watch MV and wait for them to stabilize before calibrating. I wasn't taught that initially, I was just taught to "eyeball" when you felt the loop was fully flushed with oxygen. Now I use the MVs to tell me when the cells are fully flushed with oxygen

 

[tbone1004]

@stuartv the sensors are designed for a 42 month life in 100% O2 at ambient pressure. 10 extra minutes a day in O2 vs air is going to do **** all to their longevity.
Also there are no "CCR engineers" most of these things were garage designed and don't put anything into the checklists more than it's a list of stuff you have to do in a reasonably logical order. I.e. there is no strong reason to do O2 or Dil first when analyzing the mixes.
Why is the unit full of dil at the end of the build? What is the downside of doing dil first and having O2 in the loop at the end of the build right before you splash?

On the cells being cold. I was building my unit right around New Years in cave country when it was near freezing and the head was outside which caused the cells to be really low and sluggish. If I had calibrated during assembly I would have killed myself on the next dive. Knowing not to calibrate it was critical and the cells were perfectly fine in the water. Passed their 1.6 checks at 20ft and dil was spot on at 100ft. The same applies to really warm/wet cells that may act a bit weird, though the membranes are a lot better than they used to be on the moisture front and if you're doing that, then you may want to calibrate after breathing on the unit for 10-20 minutes when everything has stabilized in order to give some predictability to how the cells are going to behave if you're doing a huge dive the next day.

You can't account for any of the environmental factors when calibrating though which is scary, so that's why I teach/preach/believe that you should only calibrate when you need to. On that trip over New Years, the unit was actually still on the calibration from Thanksgiving believe it or not.
Overwriting it is not a problem on any of the Shearwater units in terms of risk of screwing it up, but on the old Megs it was hugely problematic because they had to be 2-point calibrated which is a righteous PITA, though admittedly quite useful to have, but it's also just extra button presses that aren't necessary. I can't tell you how many times I have had to restart the calibration process on the Meg because it wasn't perfectly stabilized in air and it messed the 2-point up.

@rddvet that is rather scary that no one told you to look at the mV's during your initial course to watch if they were stable... The Divesofts actually tell you when the readings are stable enough to confirm calibration and if they're wandering it won't let you calibrate.

 

[stuartv]

@stuartv the sensors are designed for a 42 month life in 100% O2 at ambient pressure. 10 extra minutes a day in O2 vs air is going to do **** all to their longevity.
Also there are no "CCR engineers" most of these things were garage designed and don't put anything into the checklists more than it's a list of stuff you have to do in a reasonably logical order. I.e. there is no strong reason to do O2 or Dil first when analyzing the mixes.
Why is the unit full of dil at the end of the build? What is the downside of doing dil first and having O2 in the loop at the end of the build right before you splash?

On the cells being cold. I was building my unit right around New Years in cave country when it was near freezing and the head was outside which caused the cells to be really low and sluggish. If I had calibrated during assembly I would have killed myself on the next dive. Knowing not to calibrate it was critical and the cells were perfectly fine in the water. Passed their 1.6 checks at 20ft and dil was spot on at 100ft. The same applies to really warm/wet cells that may act a bit weird, though the membranes are a lot better than they used to be on the moisture front and if you're doing that, then you may want to calibrate after breathing on the unit for 10-20 minutes when everything has stabilized in order to give some predictability to how the cells are going to behave if you're doing a huge dive the next day.

You can't account for any of the environmental factors when calibrating though which is scary, so that's why I teach/preach/believe that you should only calibrate when you need to. On that trip over New Years, the unit was actually still on the calibration from Thanksgiving believe it or not.
Overwriting it is not a problem on any of the Shearwater units in terms of risk of screwing it up, but on the old Megs it was hugely problematic because they had to be 2-point calibrated which is a righteous PITA, though admittedly quite useful to have, but it's also just extra button presses that aren't necessary. I can't tell you how many times I have had to restart the calibration process on the Meg because it wasn't perfectly stabilized in air and it messed the 2-point up.

You keep saying "only calibrate when you need to". Well, if you can tell from the environmental or other factors when you "need" to versus when it's showing a bad value because of one of those factors, then you can tell when it's okay to recalibrate versus not, right?

The rEvo has 2 checklists. A Build, that is done after you finish putting the unit together, so it is (theoreticall) ready to dive. And a Pre-jump that you do right before you splash.

The Build checklist starts with O2 and finishes with Dil. Both include a full flush and checking of the ppO2 readings to confirm you have the gas your think you do (and possibly calibrate, when checking the O2). Thus, that Build check ends with the unit being full of dil. The unit could sit for days after the Build check, so being full of dil is preferable to being filled with O2.

The Pre-jump checklist goes in the opposite order. It starts with dil and finishes with O2. At the end of the Pre-jump, the loop should be at 0.7 or higher and ready for you to step into the water.

I am very willing to believe that most CCR checklists are not truly well-engineered. However, from what I can tell, I do think the rEvo checklist is actually well designed and I am reluctant to make any fundamental changes to it. As I have gone through it so many times, I have had numerous minor epiphanies where I've thought "aha! Doing it just this way ensures that I would catch this" or "it ensures that doesn't happen." I don't remember what all those little epiphanies are. I just remember that I've had enough of them that I think changing it without understanding all the little nuances of what is really being checked, what is being avoided by design, and what mistakes are being prevented would be foolish on my part. Plus, I have not generally found a case where I think changing it would make it better.

I think a more objective, quantifiable way of doing a Negative test would be an improvement, but that's not changing the checklist. It's just refining how one of the steps is performed.

@rddvet that is rather scary that no one told you to look at the mV's during your initial course to watch if they were stable... The Divesofts actually tell you when the readings are stable enough to confirm calibration and if they're wandering it won't let you calibrate.

My Divesoft says "Stable" , but as far as I can tell, it is just a 5 second countdown. It doesn't actually monitor the mV and say Stable when they are really stable.

 

[tbone1004]

@tbone1004 if they aren't stable after 5 seconds it will fail.

I haven't looked at the pre-jump checklist, but most units don't have them that require flushes of both O2 and dil, but such is life. It also doesn't surprise me that the Revo's checklist is more thorough than most

 

[wedivebc]

@stuartv the sensors are designed for a 42 month life in 100% O2 at ambient pressure.

Citation please

 

[rddvet]

[USER=187310]@rddvet that is rather scary that no one told you to look at the mV's during your initial course to watch if they were stable... The Divesofts actually tell you when the readings are stable enough to confirm calibration and if they're wandering it won't let you calibrate.[/USER]

I don't know that I'd say scary, but it is one of the handful of things I learned differently in Edd's class that I liked. So evernthough I didn't walk away with a sidewinder I got more important things out of the experience. After Edd's class I asked a few people about it that dive both Fathom's or other units. The majority of them did not watch their MVs to fully settle down when calibrating. Obviously we all knew the MV in air and at 100% for linearity checks, but to use the MV to confirm that you're getting a good oxygen flush for calibration wasn't done by most. Since the class I actually have tried to test it a few times between my wife and I. One of us will flush the unit without looking at MV like we had been doing for calibration while the other watches the MV. The majority of the time we were stopping the flush when the MV were also stabilized. So idk if it's luck, how we were taught, or the fathom, but there wasn't a huge difference. I do now always watch the MV now because I like having an objective parameter to watch. It's definitely a better way in my book

 

[tbone1004]

Citation please

Manufacturer Technical specification. Expected life-42 months. Used in hospitals, these are often in 100% O2 environments. Either way, if you believe the 12 month replacement cycle, you'll never "wear a cell out" by leaving it in 100% O2. It's the same horsesh!t that some of the instructors teach about not doing a 1.6 check for current limiting because it wears the cells out.
PSR-11-39-MD | AII Inc

 

[tbone1004]

@rddvet does the Fathom have a cal kit? I'm not familiar with how that unit is flushed, but with a cal kit it is a lot less risky than if you are trying to flush the whole unit, especially with as much dead space as there is in the Meg and its offspring, but having something objective to look at is always a good thing!