To start off, just couple of quick words to refresh the little gray cells about how things happen in the nature. Imagine standing in the middle of a forest and breathing in. This very easy sounding activity triggers a very complex chain reaction. Part of the oxygen in the gas you inhale (78% nitrogen, 20,9% oxygen, 0,9% argon, 0,04% carbon dioxide and a bit of water vapor) gets metabolized by your body for the process of energy production and the other way around the “used” gas that you exhale contains about 4-5% less oxygen and the same amount more carbon dioxide.
The best scrubber ever designed is actually surrounding us and is called photosynthesis. It is a process used by plants and other organisms to convert the light energy captured from the sun into chemical energy that can be used to fuel the organism’s activities. Carbon dioxide in combination with water is used by plants to synthesize carbohydrates to feed themselves on their own photosynthetic products. Luckily this process also releases a waste gas named oxygen and thus completes the cycle of “natural rebreather”.
Unfortunately the process stops working underwater so we need a replacement to get rid of excess CO2 and get some extra O2 on board to function properly.
Skipping the theory of a CCR course at the moment we’re now at the point where we are assembling a rebreather and have a carbon dioxide scrubber canister in our hand. So what’s in your scrubber canister?
Looking more closely at it you see either a material in granular form or a preformed roll. That is a chemical mixture that reacts with carbon dioxide in a divers expired breath. So, note one: the scrubber is not an actual filter, it’s a chemical reaction that cleans your breathing gas.
Let’s look at now what’s going on inside the canister, how it works and maybe even more importantly how it doesn’t work.
Sodalime in the scrubber canister is usually a mixture of calcium hydroxide and sodium hydroxide and it reacts by absorbing carbon dioxide by converting it into calcium carbonate (surprise, surprise you can find that also in the nature, it is a big constituent of limestone) and producing water and heat. In reality divers never see crystals of calcite in the scrubber canister because sodalime granules are never completely converted to calcite. Typically, no more than 50% of the granules react completely with exhaled CO2. Note two: heat from the canister is a positive indication that the CO2 absorption process is working.
For the chemical reaction to take place in the canister we need two crucial components: contact between the gas and the absorbent material and time. The exhaled CO2 along with other gases travels from the counterlung to the canister and that’s where things start to happen. The CO2 comes in contact with the granules that hold water containing dissolved hydroxides and is taken into solution forming carbonic acid, which reacts with the hydroxides to form sodium carbonate. This process regenerates the water and the sodium carbonate reacts with the hydrated lime to form calcium carbonate, caustic soda and caustic potash.
If chemistry wasn’t as important as the opposite sex for you in high school then try to remember only the following. The process works best when there’s the right amount of liquid involved. To little will not allow the reaction to occur, too much will dissolve the cranules by turning them into a paste and thus closing the pores and stoping the reaction. Note three: remove all exess water from the canister to make the reaction as efficient as possible.
In short that’s basically what happens in the canister of your rebreather’s canister when you’re underwater, feeling good and doing your thing. Only three highlighted notes for the whole unit you might ask? Well, the rest You’ll find out when You visit us in Thailand for some CCR diving.