Rebreather Question

[JohnnyC]

What davehicks posted isn't wrong, it's just incomplete. Calibrating the cells at 100% and observing the cells reaction down to air PO2 is entirely appropriate to ensure that your cells are capable of calibrating correctly. Coupled with an understanding of the millivolt readings that are being output from the cells can give you a quick idea re:health of the cells. However, this is separate to checking for current limited cells, and doesn't guarantee linearity, but if you hit both values with accuracy, your variation is minimal. Personally I wish more rebreather manufacturers would implement a 2-point calibration routine available like the older Megs have.

There's nothing wrong with a cell linearity check and calibration on the surface, however it is not complete, and doesn't give you a full picture of cell health. The omitted second half of the check, is validating the cells will accurately read a PO2 higher than atmospheric, that they are not current limited below the practical PO2 range. That requires either a pressure pot, or purposely creating a loop PO2 higher than the max PO2 you will be using, typically higher than 1.6. Descend to say 8m with a loop FO2 of 100% and you can validate that the sensors are not current limited within the acceptable range, and are capable of accurately reading loop PO2.

Personally, I calibrate with 100%, check that they return to the correct PO2 in air, checking millivolt values of each cell, and validate they are not current limited by descending beyond 6m with a fully flushed loop full of O2 at the start of the dive. Realistically, I don't care if they're current limited at 2.0, as long as they are accurate beyond 1.6. On occasion I'll throw them into my cell checker and validate at 2.0 just to be sure. As soon as they start to show signs of current limitation, they either get tossed, or thrown into an analyzer.

 

[rjack321]

If you are at 66ft on an air dil dive with widely divergent cells there are better strategies than a dil flush.

Go up to about 20-25ft and do an O2 flush. Are all your cells hitting 1.5 to 1.7? This will tell you far more about cell health than any dil flush. Yes its possible the 0.8 is reading higher than it should due to something like a perforated membrane. That'll show up with an O2 flush as 1.8+ when the max possible at 25ft on 100% should be ppO2 1.75.

If the 0.6 values don't go to 1.5 or more they are bad. Obviously they all should have been at 0.21 at the surface and you can surface and check that low point. Between the surface and 25ft you can check the entire linear range of your cells (in theory).

 

[joshk]

cell current limiting is the number one killer of rebreather divers!

Do you have data behind this claim?

 

[fsardone]

Do you have data behind this claim?

While I do not have hard data compiled, but if you limit the accidents to equipment failure (thus excluding medical and human) it is true: just read a few incident reports. Wrong ppO2 is either the cause or a contributing factor in eccr incidents.
Sorry but I do not have the database. Fact is many reb divers have a distorted perception about the sensor/computer way of translating microamperes into millivolt into ppO2 which drives in turn the solenoid and their failure modes and how to handle them.

 

[joshk]

While I do not have hard data compiled, but if you limit the accidents to equipment failure (thus excluding medical and human) it is true: just read a few incident reports. Wrong ppO2 is either the cause or a contributing factor in eccr incidents.
Sorry but I do not have the database. Fact is many reb divers have a distorted perception about the sensor/computer way of translating microamperes into millivolt into ppO2 which drives in turn the solenoid and their failure modes and how to handle them.

Current limited sensors—which is not the only failure mode—would mean a hyperoxic loop. I remember one very public incident on CCR about that, I'd like to know if there were others. I'm not asking generally about “wrong ppO2” here, I'm explicitly asking about data to support “cell current limiting is the number one killer” claim.

 

[rjack321]

Current limited sensors—which is not the only failure mode—would mean a hyperoxic loop. I remember one very public incident on CCR about that, I'd like to know if there were others. I'm not asking generally about “wrong ppO2” here, I'm explicitly asking about data to support “cell current limiting is the number one killer” claim.

Nobody is tabulating reliable data (DAN comes close, don't even think of posting that bogus excel table from the UK). I know a couple dozen hyperoxic loop fatalities: Washington State, Florida, Australia, Thailand, Egypt, UK, New Zealand ones come to mind. A fewer hypoxic loop fatalities, Florida and Massachusetts come to mind.

If you think some other agent is causing more fatalities than hyperoxia what would that something be? hypoxia? CO2? fatal bends?

 

[joshk]

If you think some other agent is causing more fatalities than hyperoxia what would that something be? hypoxia? CO2? fatal bends?

I've heard about an order of magnitude more hypoxic loop deaths/incidents than hyperoxic ones. That's obviously a very subjective way to gauge something.

That's why I'm asking if there's any underlying data behind the statement.

 

[rjack321]

10x more? I'd be surprised if any cause is 10x more than another. If a single risk factor was that noteworthy designing protocols to address it would be much easier.

 

[joshk]

10x more? I'd be surprised if any cause is 10x more than another. If a single risk factor was that noteworthy designing protocols to address it would be much easier.

Hypoxia for any reason over hyperoxia by reason of current limited cells.

 

[rjack321]

While not a complete list, hypoxia and hyperoxia are about par with medical events far exceeding both.
Dive fatalities - DAN Annual Diving Report 2012-2015 Edition - NCBI Bookshelf