fsardone
Solo Diver
CO2 is kind of insidious.
It creeps on you and when you are hit it is difficult to get out of it.
I will be forever grateful to my instructor when OC made me overexert and the only time I have got a CO2 hit was in traing and OC. It is bad, I was at 10 meters and overexerting with a twin set and three stages, I went up to the surface and rested breathing from nose and mouth. It took me 15-20 minutes and diving was over for the day.
Now that I know how it feels I know I have to prevent it. This is more true if I have a large deco obligation.
There are a couple of ways to get a CO2 hit: retention or lack of evacuation (my case above) or inspirating CO2 which was not scrubbed. Lets assume (and it is a big assumption) that gas density and dead space are ok and you do not overexert so you correctly ventilate out the produced CO2. Now it is up to the scrubber to fix the CO2 in the reaction bed to provide CO2 free gas.
I dive inspiration with a temp stick that works pretty well but the temp stick only shows the depth of the reaction front. I also have a CO2 sensor. I had a couple of occasions where the CO2 detector alarmed and the temp stick was OK (beginning of dive with fresh scrubber). I could not understand why but it was my fault (if you want to read about it it is in the near miss forum Three strikes and you are out - A rebreather tale). Scrubber break through can happen because there is too much CO2 exceeding the scrubber capability to fix it ot if there is a (full or partial) bypass of the reacting chemicals.
This can happen because of incorrect assembly or failure (of flap valves mostly).
In case of my reb, the oring sealing between the sorb container and the canister needs to be greased and correctly seated otherwise there might be bypass and some gas would not go through the sorb. In such case the CO2 monitor would alarm.
Also if one of the flap valves stays open there could be pendolar flow and you would rebreath gas that has not been through the sorb. In such case I very much doubt the CO2 sensor would alarm.
While the APD temp stick has been working for ages and it is quite reliable, the CO2 detector had been plagued by some issues initially (mostly false alarms). Now it also appears to be quite reliable. Also after a couple of alarm that I believed could be false, I discovered that might have been true, so maybe the reported false alarms might have been true after all.
Bottom line: the monitoring devices, although imperfect, add an additional layer of safety in the nasty business that a CO2 hit is.
We need to be aware of the failure modes and the limits of our life sustaining machines and stay away from them (limits not machines
)
Saving money on sorb or safety devices it is not wise. We need to use an aeronautical approach to safety. Adhere to limits and respect them.
It creeps on you and when you are hit it is difficult to get out of it.
I will be forever grateful to my instructor when OC made me overexert and the only time I have got a CO2 hit was in traing and OC. It is bad, I was at 10 meters and overexerting with a twin set and three stages, I went up to the surface and rested breathing from nose and mouth. It took me 15-20 minutes and diving was over for the day.
Now that I know how it feels I know I have to prevent it. This is more true if I have a large deco obligation.
There are a couple of ways to get a CO2 hit: retention or lack of evacuation (my case above) or inspirating CO2 which was not scrubbed. Lets assume (and it is a big assumption) that gas density and dead space are ok and you do not overexert so you correctly ventilate out the produced CO2. Now it is up to the scrubber to fix the CO2 in the reaction bed to provide CO2 free gas.
I dive inspiration with a temp stick that works pretty well but the temp stick only shows the depth of the reaction front. I also have a CO2 sensor. I had a couple of occasions where the CO2 detector alarmed and the temp stick was OK (beginning of dive with fresh scrubber). I could not understand why but it was my fault (if you want to read about it it is in the near miss forum Three strikes and you are out - A rebreather tale). Scrubber break through can happen because there is too much CO2 exceeding the scrubber capability to fix it ot if there is a (full or partial) bypass of the reacting chemicals.
This can happen because of incorrect assembly or failure (of flap valves mostly).
In case of my reb, the oring sealing between the sorb container and the canister needs to be greased and correctly seated otherwise there might be bypass and some gas would not go through the sorb. In such case the CO2 monitor would alarm.
Also if one of the flap valves stays open there could be pendolar flow and you would rebreath gas that has not been through the sorb. In such case I very much doubt the CO2 sensor would alarm.
While the APD temp stick has been working for ages and it is quite reliable, the CO2 detector had been plagued by some issues initially (mostly false alarms). Now it also appears to be quite reliable. Also after a couple of alarm that I believed could be false, I discovered that might have been true, so maybe the reported false alarms might have been true after all.
Bottom line: the monitoring devices, although imperfect, add an additional layer of safety in the nasty business that a CO2 hit is.
We need to be aware of the failure modes and the limits of our life sustaining machines and stay away from them (limits not machines
)Saving money on sorb or safety devices it is not wise. We need to use an aeronautical approach to safety. Adhere to limits and respect them.
