It finally happened - my CCR tried to kill me

[rjack321]

I would think Extend Aire would/should have done some kind of study on overbreathing their cartridges.

I'm skeptical of the anecdotal evidence with so many variables in play. At the same time I agree and think it would be foolish to ignore that evidence.

The scrim in the EAC takes up some of the space that otherwise would be active sorb. Those channels essentially create preferential flow paths for gas to bypass the active lime.

To study it, you would first have to define what overbreathing actually is - I'm assuming you mean having inadequate dwell time for the respiratory rate and/or Co2 production rate applied to the scrubber. Therein lies the problem, you have 2 variables happening at once and no way of knowing which is causing the co2 to bypass.

It is all irrelevant to the OP since the Revo does not use EACs anyway

 

[stuartv]

@stuartv I believe the O2 tank feet not seated properly and banging against the scrubber lid is the main suspect for the flood. You also mentioned the external battery box flooded. Is it possible for water to travel through the battery box into the lungs along the solenoid/rms cable and flood the rEvo?

My battery box was flooded when I arrived down in Florida. From my previous dive,

I replaced it before I did any diving. Then did 2 dives with no problems before the dive with the flood.

 

[tbone1004]

@Rose Robinson in terms of "how long" it depends on the unit, but I would not expect them to hold for the expected duration of a dive. The rebreathers are not pressurized during the dive, they remain at ambient pressure. The positive and negative pressure checks are there to find leaks and they should hold for about 5 minutes. Some units hold positive and negative better than others. The Meg does not hold particularly well because it uses a compression fit sleeve for the DSV so you can't create as much of a vacuum or as much pressure as you would normally and this is where you need to know your unit and how it behaves.


@Zebra 1 On the EAC discussion. You aren't going to overbreathe a scrubber, IF the unit has passed CE rating and you aren't some superhuman. What people call overbreathing the scrubber is actually outbreathing your lungs. What causes people to THINK they have overbreathed a rebreather is CO2 retention in the lungs due to the higher work of breathing which causes the feeling of not being able to breathe and symptoms of hypercapnia.
The WOB tests for CE require breathing rates of 40lpm which is around 1.5cfm. Most people can't maintain that for any length of time, but the rebreathers are all tested to 3cfm anyway which no normal person can maintain for any length of time without passing out.

Definitions used for the ~1.5cfm test
A physically fit diver, taking slow deep breaths while swimming hard,
can sustain an RMV of 37.5 lpm for a few minutes.
For the 3cfm test
A diver athlete in superb physical condition, doing severe work,
can sustain an RMV of 75 lpm for one or two minutes.

For the extendair testing done by NEDU
Dive Rite O2ptima Rebreather Scrubber Duration Test Results | Dive Gear Express®

 

[Dusty123]

My battery box was flooded when I arrived down in Florida. From my previous dive,

I replaced it before I did any diving. Then did 2 dives with no problems before the dive with the flood.

Just checking, you back the battery box cover off a 1/4 turn or so from fully seated?

 

[stuartv]

Just checking, you back the battery box cover off a 1/4 turn or so from fully seated?

I don't. Am I supposed to?

 

[stuartv]

Just checking, you back the battery box cover off a 1/4 turn or so from fully seated?

Duuuhhh... I noticed that my new battery box said right on it "Do Not Overtighten". I didn't read past that. The last part says to seat and then turn back 1/8 turn. Definitely good to know! My old battery box didn't have any of that on it.

 

[Dusty123]

Yup they say it now, they deform apparently...

 

[rjack321]

@Zebra 1 On the EAC discussion. You aren't going to overbreathe a scrubber, IF the unit has passed CE rating and you aren't some superhuman. What people call overbreathing the scrubber is actually outbreathing your lungs. What causes people to THINK they have overbreathed a rebreather is CO2 retention in the lungs due to the higher work of breathing which causes the feeling of not being able to breathe and symptoms of hypercapnia.
The WOB tests for CE require breathing rates of 40lpm which is around 1.5cfm. Most people can't maintain that for any length of time, but the rebreathers are all tested to 3cfm anyway which no normal person can maintain for any length of time without passing out.

Definitions used for the ~1.5cfm test
A physically fit diver, taking slow deep breaths while swimming hard,
can sustain an RMV of 37.5 lpm for a few minutes.
For the 3cfm test
A diver athlete in superb physical condition, doing severe work,
can sustain an RMV of 75 lpm for one or two minutes.

For the extendair testing done by NEDU
Dive Rite O2ptima Rebreather Scrubber Duration Test Results | Dive Gear Express®

yet the older optima with the required EAC has had quite a people complaining about overbreathing them. Since that unit never had a Co2 monitor I'm not going to speculate on the cause of that, but insufficient dwell time was certainly complained about.

 

[tbone1004]

yet the older optima with the required EAC has had quite a people complaining about overbreathing them. Since that unit never had a Co2 monitor I'm not going to speculate on the cause of that, but insufficient dwell time was certainly complained about.

The CE/NEDU tests for duration use CO2 on the outlet of the scrubber to determine the stop time. Also of note, the EAC's seem to be super sensitive to temperature and depth. If you look at the durations, it's 3.5hrs at 75F, but only 2 hours at 50F which is a massive swing. At 50F it's also 4hrs at the surface but only 2hrs at 5ata's, so it may well have been that these divers were over extending the scrubber instead of overbreathing it. Of note, all these tests were done at a constant 1.5cfm SAC rate, and when was the last time you ever saw someone do that for more than a few minutes?

 

[Rose Robinson]

@Rose Robinson in terms of "how long" it depends on the unit, but I would not expect them to hold for the expected duration of a dive. The rebreathers are not pressurized during the dive, they remain at ambient pressure. The positive and negative pressure checks are there to find leaks and they should hold for about 5 minutes. Some units hold positive and negative better than others. The Meg does not hold particularly well because it uses a compression fit sleeve for the DSV so you can't create as much of a vacuum or as much pressure as you would normally and this is where you need to know your unit and how it behaves.


@Zebra 1 On the EAC discussion. You aren't going to overbreathe a scrubber, IF the unit has passed CE rating and you aren't some superhuman. What people call overbreathing the scrubber is actually outbreathing your lungs. What causes people to THINK they have overbreathed a rebreather is CO2 retention in the lungs due to the higher work of breathing which causes the feeling of not being able to breathe and symptoms of hypercapnia.
The WOB tests for CE require breathing rates of 40lpm which is around 1.5cfm. Most people can't maintain that for any length of time, but the rebreathers are all tested to 3cfm anyway which no normal person can maintain for any length of time without passing out.

Definitions used for the ~1.5cfm test
A physically fit diver, taking slow deep breaths while swimming hard,
can sustain an RMV of 37.5 lpm for a few minutes.
For the 3cfm test
A diver athlete in superb physical condition, doing severe work,
can sustain an RMV of 75 lpm for one or two minutes.

For the extendair testing done by NEDU
Dive Rite O2ptima Rebreather Scrubber Duration Test Results | Dive Gear Express®

Hi tbone,

Thank you very much for the reply.

My question came out of Stuarts incident, and after having reviewed one manufacturers pre-dive checklist that indicated the unit only had to hold negative/positive checklist prerequisites for one minute, I considered this to be very marginal to say the least.

I have attended three rebreather clinics, and under restricted conditions, have dove three different units, and it seams to me that the rebreather should hold ''air tight'' status longer than required protocols dictate.

Having said that, as you have said, during operation, the rebreather is only under ambient pressure. although to me, one minute still seems to be too short a test period.

Again, thank you for your reply. .

Rose.