designing a diver powered etCO2 for rebreathers

[CG43]

After reading the info about the Solid State Carbon Dioxide CO2 Sensor and the posts from crewisool and Duke Dive Medizine, I wonder why the sensor has to sit in a side stream at all?

Possible reasons would be a certain volume or mass flow, pressure, temperature or humidity?
If this does not apply to available and affordable sensors, then why the bypass stream ?

 

[crewiscool]

How much power do you want to gain from the diver's breath work?
For OC, EN 250 requires a maximum WOB of 3 joules/litre.
If the diver breathes 25 liters/min., that's just 1.25 watts.
So you can gain max about 0.5 watts from the WOB.

The power you need to push your measuring volume through the CO2 sensor is also low with :
Power = Pressure * Volumen/sec.
The problem of pumping a small volume flow against a very low pressure through a CO2 sensor
should be easy to solve with a small, cheap dosing pump.

What does the CO2 sensor need?
A constant volume flow , or constant mass flow , or simply a measuring volume
that has to be replaced regularly ?

After reading the info about the Solid State Carbon Dioxide CO2 Sensor and the posts from crewisool and Duke Dive Medizine, I wonder why the sensor has to sit in a side stream at all?

Possible reasons would be a certain volume or mass flow, pressure, temperature or humidity?
If this does not apply to available and affordable sensors, then why the bypass stream ?

I am by no means a medical doctor or expert on etCO2. but I believe for these systems to be accurate you need to pull out the very end of an exhaled breath (I.E. end-tidal) so it wouldn't be effective to just stick and SS CO2 sensos in the loop some where.

 

[Mod63]

I am by no means a medical doctor or expert on etCO2. but I believe for these systems to be accurate you need to pull out the very end of an exhaled breath (I.E. end-tidal) so it wouldn't be effective to just stick and SS CO2 sensos in the loop some where.

If I remember correctly what Dr. M did write over and over again, this is the one major issue on getting correct readings in a rebreather. Measuring loop CO2 is simple

 

[Duke Dive Medicine]

I think it depends on what you want to measure and why. End tidal CO2 measured on the exhalation side close to the diver is more a measure of respiratory and circulatory function, though increasing ETCO2 readings in an otherwise healthy diver could be a late indicator of scrubber failure. Loop CO2 measured just downstream of the scrubber on the inhalation side would be a more direct way of measuring scrubber function.

 

[lostsheep]

I am not following why it is difficult to get a CO2 reading only at the end of the exhalation cycle (end of phase 3 below). An instantaneous sample at just the right time seems very tricky but surely a program could be written to identify key values using continuous monitoring.

I’m sure I’m missing something, please enlighten me.

If the plot above is representative of typical respiration, wouldn’t etCO2 just be the peak value?

 

[Mod63]

I think this is a good starting point for reading up on the subject: https://dhmjournal.com/images/IndividArticles/40Dec/Mitchell_dhm.40.4.206-209.pdf

 

[lostsheep]

I think this is a good starting point for reading up on the subject: https://dhmjournal.com/images/IndividArticles/40Dec/Mitchell_dhm.40.4.206-209.pdf

Thank you.

Sounds like the problem is WHERE the measurement is taken. Either the sample has to be moved to the sensor, which requires work, or the sensors need to be miniaturized and incorporated directly into the mouthpiece.

 

[Duke Dive Medicine]

I am not following why it is difficult to get a CO2 reading only at the end of the exhalation cycle (end of phase 3 below). An instantaneous sample at just the right time seems very tricky but surely a program could be written to identify key values using continuous monitoring.

I’m sure I’m missing something, please enlighten me.


View attachment 882250

If the plot above is representative of typical respiration, wouldn’t etCO2 just be the peak value?

It's been done. A lot of end tidal CO2 meters read out digitally in pCO2. Alaris, who manufactures the IV pumps we use, makes an EtCO2 module that attaches to the main pump body and will lock out a patient-controlled analgesia pump channel if the EtCO2 reading exceeds a certain limit.

 

[CG43]

I am not following why it is difficult to get a CO2 reading only at the end of the exhalation cycle (end of phase 3 below). An instantaneous sample at just the right time seems very tricky but surely a program could be written to identify key values using continuous monitoring.

I’m sure I’m missing something, please enlighten me.

Shorten the phases II and III which corresponds to fast shallow breathing.
Then the measured mean value decreases while the actually effective alveolar CO2 value increases.
A mean value measurement cannot provide sufficient warning in this case. Unfortunately, this is a common case. In Dr. Mitcell's paper, page 208, last paragraph, two cases are described.
I myself have experienced two cases where my buddy only sat quietly on the ground with his arms hanging down and it looked like a constant stream of small bubbles came out of his reg. The tidal volume may have been about 0.2 l, which means that the phase II and III ev. has not even existed anymore.

 

[rjack321]

Most medical side-sampling etCO2 sensors only need 30-60mL/min from my research. so you would only need about 6% efficiency. You could also pause the sampling if your if the revoir gets below a certain pressure. I also think your normal respiration numbers are a bit low for scuba diving. How many people do you know with an RMV of 8L/.28cuft/min?

If you can get one working on a 60mL/min and battery level voltage and amperage that would be something.