Calibration question

[doctormike]

While you're correct, all you'd be checking is that your calibration is correct and your computer is still multiplying X * Y to give you PPO2. I can't think of a single thing checking mV underwater will tell you that the PPO2 display won't tell you.

I was referring to your scenario of Calibration -> Dive 1 -> Calibration ->Dive 2.

In that case, what you would be checking is how that factor changed, i.e. how your cell output changed at standard conditions, which is what "calibration" uses to generate that factor. The relevance would be that wet recalibration can potentially mask a very large dropoff in output, which may or may not be relevant in predicting failure. On the other hand, as you pointed out, what you really want to calibrate for is real world conditions, which change over the course of a dive.

The alternative way of getting that information (i.e. the change in the factor between dives) is what you suggest, flushing with dil or O2 at a known depth, and then comparing that from dive 1 to dive 2. As discussed upthread, that depends on (1) a perfect flush (at least in the area where the cells are), and (2) and accurate depth reading. A better way is using the pressure pot, assuming that the idiot who bought it didn't blow the head off and potentially damage the gauge.

 

[njdiverjoe]

Does anyone have a spec sheet or datasheet available for their cells? I know a lot are advertised as operating within spec between 0-99% humidity, how the cell output varies in different levels of humidity would be something I would expect to see there.

DeepLife did a good amount of cell testing and has some documents available: Deep Life Design Team: Selected Design Validation Reports for DL & Open Safety Equipment Ltd's Rebreathers

As long as we are talking calibration and summer is in full swing I wanted to re-emphasis temperature. If you check out some of the Deeplife testing docs they found that increased heat has a rather drastic (and possibly permanent) effect on cells in temperatures that are easily attainable in a hot summer car. I don't believe it's common but I have seen guys leave their unit to bake in a hot car .

 

[joshk]

The alternative way of getting that information (i.e. the change in the factor between dives) is what you suggest, flushing with dil or O2 at a known depth, and then comparing that from dive 1 to dive 2. As discussed upthread, that depends on (1) a perfect flush (at least in the area where the cells are), and (2) and accurate depth reading. A better way is using the pressure pot, assuming that the idiot who bought it didn't blow the head off and potentially damage the gauge.

If you flush (maybe several times) and the controller doesn’t read 1.6 you’re calibration is off or you’re not using 100%.


You could certainly use the pot between dives to check how a dry vs. wet cell responds, but how do you take that and adjust calibration so your PPO2 readings are accurate during the dive does? To my knowledge calibration is done to the DiveCan system in the head, not the handset.

 

[doctormike]

If you flush (maybe several times) and the controller doesn’t read 1.6 you’re calibration is off or you’re not using 100%..

Sorry, losing track of the thread. You mean the 20 foot O2 flush? Yup, if you accurately at 20 feet and get a good flush with 100% O2, the controller should read 1.6. I think that we keep saying the same things, but that's OK because I like talking about rebreathers as much as I do diving them.

You could certainly use the pot between dives to check how a dry vs. wet cell responds, but how do you take that and adjust calibration so your PPO2 readings are accurate during the dive does?

I don't know if I would want to pull it all apart between dives, but tbone gave one way of accounting for that upthread....

To my knowledge calibration is done to the DiveCan system in the head, not the handset.

The handset is just a readout, the brains are in the head, from what I understand. This may be unit specific, but you may have seen Leon's memorable demonstration of that....

 

[sea_ledford]

I do wish that there was some way of "locking" the mV display on the Shearwater controller. It times out too quickly, and that means many button presses for each pressure level when you are testing. I don't see why that would be a safety issue. They did incorporate that 0.19 setpoint that I use to keep the solenoid from firing when I'm uploading log data or doing this sort of test.

Send Tyler at Shearwater an email asking for that feature. He has a list of things people have asked for and the more popular ones get added. This one is already on the list. I would like it as well so I can see the actual mv readings rather than the filtered calibration readings of the po2. Not on a normal basis but for trouble shooting and general cell health awareness, I would like it.

Also keep in mind for O2 flushes at the end of the dive that even if you get a perfect flush, you are actively offgasing into the loop, so it will still drop. I haven't even been able to get 1.6 at 20 feet at the end of a dive. High 1.5 maybe, but never 1.6. Maybe that is the moisture issue as well, but a couple hundredths isn't a worry anyway.

 

[joshk]

I can see the actual mv readings rather than the filtered calibration readings of the po2. Not on a normal basis but for trouble shooting and general cell health awareness, I would like it.

On the surface this makes sense. When would you use this underwater?

 

[rjack321]

Send Tyler at Shearwater an email asking for that feature. He has a list of things people have asked for and the more popular ones get added. This one is already on the list. I would like it as well so I can see the actual mv readings rather than the filtered calibration readings of the po2. Not on a normal basis but for trouble shooting and general cell health awareness, I would like it..

On the SW petrel its right button x3 and you get mV along the bottom row.

 

[rjack321]

Who fills out checklists?

CCRs divers in the game for the long haul

 

[joshk]

On the SW petrel its right button x3 and you get mV along the bottom row.

I think the ask is that the mV display doesn't timeout back to the main display after ~15 seconds.

CCRs divers in the game for the long haul

 

[DA Aquamaster]

I was referring to your scenario of Calibration -> Dive 1 -> Calibration ->Dive 2.

In that case, what you would be checking is how that factor changed, i.e. how your cell output changed at standard conditions, which is what "calibration" uses to generate that factor. The relevance would be that wet recalibration can potentially mask a very large dropoff in output, which may or may not be relevant in predicting failure. On the other hand, as you pointed out, what you really want to calibrate for is real world conditions, which change over the course of a dive.

The alternative way of getting that information (i.e. the change in the factor between dives) is what you suggest, flushing with dil or O2 at a known depth, and then comparing that from dive 1 to dive 2. As discussed upthread, that depends on (1) a perfect flush (at least in the area where the cells are), and (2) and accurate depth reading. A better way is using the pressure pot, assuming that the idiot who bought it didn't blow the head off and potentially damage the gauge.

I agree with you that you want to take some care not to re-calibrate to mask the effects of a dying and current limited cell - but that's a potential issue with wet or dry recalibration.

I check our cells with a cell checker just before the trip to verify they are not current limited and that they are linear. On the trip, I then calibrate the cells in the RB after the build at the start of dive day #1. Then, presuming nothing unusual happened on the dive (no flooded loop, etc that may have abnormally impacted the cells) I recalibrate the cells in the RB after Dive #1 while they are "normal moist". The end result on dive #2 (whether that is dive day #1 or dive day #2) is almost always cells that read very consistently with each other, with a diluent flush at depth and a with an O2 flush at 20'. When I 02 flush at 20' (with a PPO2 usually around 1.54 to 1.55) I also expect to see a PPO2 of 1.0 at the surface if no diluent has been added on ascent.

Provided you control for potential current limiting with old cells, my opinion is that you get cell readings that more accurately reflect the actual loop PPO2 under actual dive conditions if you recalibrate just after your dive with moist cells.

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I've also noted that the Mv readings at the surface with 100% O2 should be above 47 for each cell. With AI cells I've noted that surface 100% o2 calibration readings less than 47 usually result in that cell lagging and reading low at depth, relative to the other cells.