Manual Version 19 / 01-2019 II Page 11 of 44
SF2 ECCR OPERATION MANUAL
Chapter 2 - The soda lime
The filtering out of the carbon dioxide (CO2) in the rebreather takes place with the
help of the soda lime through a chemical reaction. The soda lime reacts with the
CO2 in several stages and produces calcium carbonate (lime), water and heat. The
moisture and heat in turn enable the calcium reaction.
Put simply, CO2 and calcium hydroxide ultimately result in calcium carbonate. This
requires water (approx. 14-18% moisture) and a little sodium hydroxide (approx. 3%)
as a reaction accelerator.
Carbon dioxide reacts with water to form carbonic acid. Therefore, a certain basic
moisture content of the lime (as well as the moisture in the breathable air) is
necessary to start the reaction.
CO
2
+ H
2
O H
2
CO
3
(carbonic acid)
In a further step, the carbonic acid reacts with the caustic sodium hydroxide to form
sodium carbonate. This creates water again.
H
2
CO
3
+ 2 NaOH (Natriumhydroxyd) Na
2
CO
3
(Natriumkarbonat) + 2 H
2
O
Ultimately, the sodium carbonate reacts with the calcium hydroxide to form lime and the
sodium hydroxide is regenerated again.
Na
2
CO
3
+ Ca (OH)
2
(Calciumhydroxyd) CaCO
3
(Calciumcarbonat)+ 2 NaOH
The reaction substances water and sodium hydroxide are constantly renewed during
the process and are included in the reaction again. Only the calcium hydroxide is
consumed and therefore limits the absorption capacity (service life) of the lime. To
understand: The soda lime does not bind the CO2 in a gaseous state, e.g. Filter
cartridges in the compressor. Due to the completely different type of binding, filter
cartridges release the filtered CO2 back into the ambient air over time. The chemical
reaction of soda lime, on the other hand, leads to a complete conversion of the
gaseous CO2 from the air you breathe into a solid component, calcium carbonate.
No gaseous CO2 is present in fresh, wet or used lime.
The exothermic (= heat-releasing) chemical reaction generates heat. You can tell
that the scrubber is lukewarm after a dive, even in icy water. This heat prevents the
breathing gas from cooling down and significantly minimizes heat loss through the
lungs. The lime is used up in layers. The CO2 from the exhaled air flows into the
fresh soda lime and immediately reacts to calcium carbonate. If the lime has already
been used up a bit, the reaction zone (reacting layer of soda lime) moves further
forward accordingly. The soda lime is used up when the reaction zone nears the end
of the scrubber. The thickness of the reaction zone depends on various factors such
as the type of lime (particle size), reactivity (reaction temperature) and gas flow rate
(gas density).
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