Scrubbing out the CO2 is used by Divers since 1726, when Stephen Hale designed the first scrubber: a flannel liner, soaked in sea salt and tarter, used in a helmet for mine disasters. 1876 Henry Fleuss, an English merchant marine officer (from Germany) , began to develop an oxygen rebreather and since then a typical Loop looks like that:
As the human physiology does not provide a reliable CO2 pre-warning system it is mandantory not to risc a scrubber breakthrough. Until now all attempts to build reliable CO2 Sensors for submersiable Rebreathers failed due to the humid atmosphere and the changes in the Diver Orientation. The break through is (depending on National Standards) defined as the Point when 0.005 bar partial pressure of carbon dioxide reach the Inhale Point of the System. Be aware that % Values might spook arround: 0.5% at the Surface means 0.005 bar partial pressure and the second is what is physiologic important for us!
Mostly CO2 absorbtion is done by "breathing lime". Chemically this is
H2CO3
Na2CO3 + 2H2O
CaCO3 + 2NaOH
when written down without the catalytic part the formular becomes
CaCO3 + H2O
The absorption process is influated by
The Speed of the Chemical Reaction is dependant to the Temperature, 10°C warmer means doubling the speed, and so decreasing the needed Masstransferzone. So if the lime is too cold it will earlier "break through". In addition colder Lime means less Evaporation and too the presence much Reaction-Water (instead of steam) means that it covers the Lime-Surface and so "downs" it. Deeper dives mean that more "Coolant" (O2 / N2 / He) per CO2 will cause a less effective Scrubber, so it is clear why some manufacturers supply tables that relate Lime-Change Intervals to Dive-Depths. Too much Heat causes too much evaporation, that's why a scubber-fill might last less long in warm shallow water, as the Water also is a needed Catalytic.
Increased concentrations lead to increased chemical reactions, leading to increased temperature, so that as long as there is sufficient fresh lime close to the inlet-area to the crubber that is a quite active region.
As You see: prediction of canister behavior is not simple.
Calculated increase of average imhaled ppCO2 as Function of "Scrubber-Size" and Time at a given average CO2-Massflow.
Mathematically modeling an absorber was done 1993 using an Atari-ST and a Basic Program. That was no big deal, it's simple finite element mathematics feed with basic chemical knowledge. Until you start screwing on all the Parameters mentioned above You will not need really need a Number-Cruncher or a lot of time. Fetch some books and try it at home, you might like it.
You will see that the ppCO2 will travel like a wave into the Scubber, decreasing the further it comes. In Phases where no Gas Flows the Pattern is reduced again. Nowadays you might form a film from it, for me the simple Picture above was sufficient to understand.
If the Scrubber is "too small" because the Lime-Fill is used too long / was "drowned" / is too cold / was mixed / the Diver is working too hard / has a bad breathing-pattern, and so on (all of that points mean lower 'line numbers'), the "tail" reaches the End of the Scrubber and forms peaks (Needles of time) where the ppCO2 is increased. The more the Problem grows the higher the per-breath average of the inhaled ppCO2 is. You will surely see that peaks if you can manage to measure the ppCO2 leaving a Scrubber at the Begin of the Pre-Breath, when the Lime is still too cold for diving it.
You also see that the average level of carbon dioxide will rise exponentially when reaching the end of the canister's life, simply stretch the line...
Accept the Manufacturers Warnings, there were not put out to fool You, they are based on Measurements. You have seen how drastically the ppCO2 increases when the Scrubber is overused or abused just a little bit too much.
http://Rebreather.de/rebreather/CO2.htm © Karl Kramer, 27.07.1998
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