Calibration question

[rjack321]

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.

If you lose the flash memory of the calibration (it happens) you can still use the cell mV as a proxy for ppO2.
In my MOD1 we calculated the mV at ppO2 1.2 and wrote it on a piece of white duct tape stuck to the handset. I haven't had to do that personally, but I know people who have used it.

 

[joshk]

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.

This is only if your cells are start with reading more than 10mV in air at sea level.

 

[joshk]

If you lose the flash memory of the calibration (it happens) you can still use the cell mV as a proxy for ppO2.
In my MOD1 we calculated the mV at ppO2 1.2 and wrote it on a piece of white duct tape stuck to the handset. I haven't had to do that personally, but I know people who have used it.

You'd trust the electronics after it lost a few bits of data to get you out of there with any accuracy? This is where bailout or SCR mode would be far far preferred (for myself) than doing math underwater.

 

[JohnnyC]

I definitely wouldn't trust the electronics if it lost calibration. It's indicative of some other problem. SCR is annoying, but it's better than trying to correlate potentially inaccurate mV readings. Bailing out would be an even better option.

I dive with both handsets plugged in, so it would have to be a deal if both of them got torched. That's why I always wear a watch and a backup depth gauge when I'm with my rebreather. Having things all connected is great for accuracy, but also has the potential for something to take out all of your data.

 

[rjack321]

You'd trust the electronics after it lost a few bits of data to get you out of there with any accuracy? This is where bailout or SCR mode would be far far preferred (for myself) than doing math underwater.

shrugs, even in SCR mode do you want some hint of ppO2 or just ignore the mV?

Especially if you are ascending knowing your now SCR loop isn't dangerously hypoxic is a plus IMO.

Losing flash memory and the resulting calibration is actually fairly easy to do. That does not negate the volt meter functions though. I would use the mV output and wouldn't be afraid to add O2 to bump them up, especially when I see them dropping when I expect them to drop (ascent). If they are all crazy up/down/left/right/sideways when I am at a stable depth I am more likely to suspect a broader issue like a flood.

 

[joshk]

Losing flash memory and the resulting calibration is actually fairly easy to do. That does not negate the volt meter functions though.

DiveCan models are all digital with the calibration being done to the head and the handsets displaying what the head says. mVs aren’t being read directly. I don’t know if those are separate subsystems on the bus or not. Either way, if the PPO2 display zeros out I’m not inclined to trust the electronics.

How is losing flash memory “easy?”

 

[rjack321]

DiveCan models are all digital with the calibration being done to the head and the handsets displaying what the head says. mVs aren’t being read directly. I don’t know if those are separate subsystems on the bus or not. Either way, if the PPO2 display zeros out I’m not inclined to trust the electronics.

How is losing flash memory “easy?”

Beats me but I've seen it happen a few times now - on analog units. I really don't know anything about divecan stuff. I have 2 analog units myself and none of my regular buddies have divecan units either.

 

[tbone1004]

DiveCan models are all digital with the calibration being done to the head and the handsets displaying what the head says. mVs aren’t being read directly. I don’t know if those are separate subsystems on the bus or not. Either way, if the PPO2 display zeros out I’m not inclined to trust the electronics.

How is losing flash memory “easy?”

mV's are read directly, just not by the handsets *fun fact, you can actually plug and play handsets between units underwater and since the calibration is in the head not the display you can continue using the deco profile from computer 1 while you swap to a new rebreather if you are doing stupid long dives. I think it was Ted McCoy and Jon Bernot that figured that one out by accident.
The beauty of the digital system though is that the mV's only have to travel a couple of inches on the O2 board itself. What it does there is then convert the actual mV into a signal that corresponds to that mV and "tells" the main board something along the lines of "hey, port 1 is 12.1mV, port 2 is 11.8mV, and port 3 is 12.7mV" and it does that at some interval in the programming *many times per second I would imagine*. The head then has a calibration factor in place from calibration and then relays that info to the displays, relatively simple.
While it is much more complex, it is prone to much less signal degradation than running teeny tiny analog wires several feet, through bulkheads, that are getting fatigued from moving around, subject to voltage loss due to the length of the wires *albeit small loss since it's tiny voltage and negligible current*, and all sorts of interference to get to the handsets.
What @DA Aquamaster had issues with is not something a digital bus is subject to which is nice if you want to run multiple displays. The Meg had to do it with a clunky system with independent primary and secondary boards to keep them from interfering with each other but that's needlessly complex compared to the digital bus system that also has the advantage of being able to report failures to you. I.e. if the O2 board is starting to go weird, it can tell you if it's a cell acting weird or the board sending a signal that it isn't supposed to since there is 2 way communication and validation. With an analog unit you don't know if the cell is bad or the wire is bad.

 

[joshk]

mV's are read directly, just not by the handsets

I meant the handsets/displays. “mV's aren't being read directly [by the Shearwater Petrel]”

Beats me but I've seen it happen a few times now - on analog units. I really don't know anything about divecan stuff. I have 2 analog units myself and none of my regular buddies have divecan units either.

What PPO2 monitor do you use? What unit is this?

I'm imagining a KISS with a Shearwater Petrel on a Fischer cable, which would mean the Shearwater “lost” the calibration. I'm curious to know what the symptoms are if this happens.

 

[rjack321]

Meg 2.7 primary handset
Rev C+ hammerhead (I think)
You can get 0.0 on ppO2 (on one or more cells) and still get a mV output on that cell plus the other.

If for instance you have:
1.19, 0.0 , and 0.0 on ppO2
and yet have
49, 52, and 51 mV on three cells

Are you going to bail, SCR, or feel fairly safe running manually that you are at roughly ppO2 1.2?

For me on an OW trimix dive I would bail and just go up. Its not worth mucking with on the bottom accumulating deco time.
For me on a 32% dil cave dive (i.e Ginnie or Jackson Blue) I would dil flush to confirm the cells are "alive" and responding, then stay on that loop and run it manually as I exited. Watching not just cell1 but that cells 2 and 3 were comparable too. While I could go SCR, that wouldn't really make me feel any safer, especially coming up shallower.

If the cell values were all crazy up and swinging all over the place I'd bail - since in my experience that indicates the cells are flooding. But a flat zero (ppO2 & mV) indicates a wiring issue, whereas a flat zero with a mV output still is a good hint of lost calibration, flash memory failure, or internal processor issue. But I am not especially skeptical that those 52 and 51 mV values are way off base. Plus I can double check that they are actually responding by doing the dil flush.