It finally happened - my CCR tried to kill me

[wedivebc]

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?

any idea what the VO2 Max would be on a diver breathing at 1.5 cuft/min?

 

[tbone1004]

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.

it also depends on what your definition of "pass" is. After a minute you are going to know if they are stable or not by watching the ppO2 readouts on the handsets. If it's stable after a minute, it is probably going to be stable for an hour. Stability is what really matters and is why you need to watch the ppO2

 

[tbone1004]

any idea what the VO2 Max would be on a diver breathing at 1.5 cuft/min?

not off hand, but they're injecting ~1.3lpm of CO2 and say that that is 85% of vO2 which is 4% of RMV so I'm sure you could back calculate it if you're so inclined. I'm not sure if vO2 max is really the right thing to look at though vs the actual breathing rate. The flow rates are what determine not only dwell time in the scrubber itself but also the resistance to flow and experienced work of breathing that contributes to CO2 retention.

 

[wedivebc]

not off hand, but they're injecting ~1.3lpm of CO2 and say that that is 85% of vO2 which is 4% of RMV so I'm sure you could back calculate it if you're so inclined. I'm not sure if vO2 max is really the right thing to look at though vs the actual breathing rate. The flow rates are what determine not only dwell time in the scrubber itself but also the resistance to flow and experienced work of breathing that contributes to CO2 retention.

I am trying to get my head around a comment you made regarding outbreathing vs overbreathing. I am assuming that one means exceeding the ability for the scrubber to come in contact with CO2 due to high velocity of the gas (dwell time) and the other is the inabilty to completely convert the CO2 due to shear volume. In other words I assume one relates to VO2 Max and the other relates to dwell time.

 

[tbone1004]

I am trying to get my head around a comment you made regarding outbreathing vs overbreathing. I am assuming that one means exceeding the ability for the scrubber to come in contact with CO2 due to high velocity of the gas (dwell time) and the other is the inabilty to completely convert the CO2 due to shear volume. In other words I assume one relates to VO2 Max and the other relates to dwell time.

Since vO2 is based on the subjects weight it's not actually a good measure of scrubber capacity since my vO2 max could be the same as someone half my weight but I am producing 2x the CO2.
People talk about overbreathing a scrubber, as in you are generating CO2 faster than the rebreather can remove it. That is typically thought to be related to dwell time in the scrubber itself. My issue with that claim is that the scrubbers are tested to breathing rates much higher than normal divers are going to achieve.

For reference. 3cfm, the max that they normally test and what the Navy says all but the most in shape can barely maintain is equivalent to you breathing almost your full lung volume 30x per minute. Try doing that for a minute without passing out.... 1.5cfm is obviously about half that, whether that is 30x half size breaths, or 15x full breaths. Probably doing 30x half breaths in an "excited" situation, vs 15x full breaths when trying to kick really fast if you're in good shape and using rhythmic breathing.

The point of all of this is that "overbreathing" the scrubber is going to be much more related to volume of gas moving through the scrubber which increases the WoB vs the quantity of CO2 in that gas. When you are breathing that hard, the WoB gets worse, when the WoB gets worse, you are going to retain more CO2, when you retain CO2 you start breathing harder. See the failure loop? Once you hop onto OC, the WoB gets better and you are able to get rid of all of that CO2. People talk about that phenomenon as overbreathing the scrubber, as in we are getting CO2 pass through, when the reality is that you are likely just retaining CO2 in your lungs due to your inability to expel it. @Bobby is big about talking about the "rebreather cough" where it's a good idea when you start feeling that shortness of breath coming to try to cough into the rebreather to get rid of that residual lung volume that is holding the CO2.

Now, what is curious to look at is the duration tests for the O2ptima that I linked above. Those tests are stopped when the CO2 hits 0.5% which is considered normal levels of atmospheric air. It appears that the EAC is unable to scrub as much CO2 at depth and this could well be due to scrubber "breakthrough" as a function of gas density and dwell time where the reaction front appears to get more conical instead of more flat at lower ambient pressures causing the CO2 levels to trip faster.

Clear as mud?

 

[wedivebc]

Now, what is curious to look at is the duration tests for the O2ptima that I linked above. Those tests are stopped when the CO2 hits 0.5% which is considered normal levels of atmospheric air.

Actually the EN14143:2003 standard considers a rebreather to have experienced breakthrough when it reaches 0.5% CO2 so that standard is not specific to the EAC. EN also tests further to 1.0% CO2 which it considers reserve capacity.

 

[tbone1004]

Actually the EN14143:2003 standard considers a rebreather to have experienced breakthrough when it reaches 0.5% CO2 so that standard is not specific to the EAC. EN also tests further to 1.0% CO2 which it considers reserve capacity.

correct, though the values quoted are at 0.5%. I think that was talked about on the DGX page? unsure, can't remember, but probably is. I didn't open the actual graphs though since it's not relevant to my units

 

[stuartv]

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

Hopefully, that new knowledge will save me from flooding my new battery box at some point in the future...

 

[Zebra 1]

Since vO2 is based on the subjects weight it's not actually a good measure of scrubber capacity since my vO2 max could be the same as someone half my weight but I am producing 2x the CO2.
People talk about overbreathing a scrubber, as in you are generating CO2 faster than the rebreather can remove it. That is typically thought to be related to dwell time in the scrubber itself. My issue with that claim is that the scrubbers are tested to breathing rates much higher than normal divers are going to achieve.

For reference. 3cfm, the max that they normally test and what the Navy says all but the most in shape can barely maintain is equivalent to you breathing almost your full lung volume 30x per minute. Try doing that for a minute without passing out.... 1.5cfm is obviously about half that, whether that is 30x half size breaths, or 15x full breaths. Probably doing 30x half breaths in an "excited" situation, vs 15x full breaths when trying to kick really fast if you're in good shape and using rhythmic breathing.

The point of all of this is that "overbreathing" the scrubber is going to be much more related to volume of gas moving through the scrubber which increases the WoB vs the quantity of CO2 in that gas. When you are breathing that hard, the WoB gets worse, when the WoB gets worse, you are going to retain more CO2, when you retain CO2 you start breathing harder. See the failure loop? Once you hop onto OC, the WoB gets better and you are able to get rid of all of that CO2. People talk about that phenomenon as overbreathing the scrubber, as in we are getting CO2 pass through, when the reality is that you are likely just retaining CO2 in your lungs due to your inability to expel it. @Bobby is big about talking about the "rebreather cough" where it's a good idea when you start feeling that shortness of breath coming to try to cough into the rebreather to get rid of that residual lung volume that is holding the CO2.

Now, what is curious to look at is the duration tests for the O2ptima that I linked above. Those tests are stopped when the CO2 hits 0.5% which is considered normal levels of atmospheric air. It appears that the EAC is unable to scrub as much CO2 at depth and this could well be due to scrubber "breakthrough" as a function of gas density and dwell time where the reaction front appears to get more conical instead of more flat at lower ambient pressures causing the CO2 levels to trip faster.

Clear as mud?

Well stated and practical / useful information, in your opinion how does a "rebreather cough" compare to doing deep exhales and inhales?

 

[tbone1004]

Well stated and practical / useful information, in your opinion how does a "rebreather cough" compare to doing deep exhales and inhales?

You can't get rid of the volume without coughing. Exhale all the way to where you don't think you have any more. Then cough. You can still get some gas out, and that's the stuff at the very bottom of your lungs and where the CO2 is going to keep building if you aren't breathing properly.