Quote: (Originally Posted by
diveoceanos)
Going into closer look at the chemistry of CO2 neutralisation inside the canister I searched the internet and found two reactions. One of them is sound and chemically correct while the other one simply does not follow any chemical rules and can not happen in reality. This is the proposed reaction that gives in its products a NaOH. Anyway what my knowledge tells me about the chemistry inside the canister is the following (simply):
2NaOH + CO2 ----> Na2CO3 (salt) + H20 + Heat
Ca(OH)2 + CO2 ----> CaCO3 + H20 + Heat.
Various attempts have been made to find a solution to monitor the scrubber life among which temperature monitor inside the loop has been one. However this method is not reliable and therefore can not be used with safety.
Looking at this condensation again I thought that Chemistry inside the canister is a major source of water. Also in these reactions the weight of the products is definitely different that the weight of the reactants (Soda Lime and CO2).
Quantitative Calculations.
Lets suppose that Megalodon Scrubber has been tested to be good for 4 hours at 0 degrees and with a flow of CO2 at 4 LPM.
That is approximately 1850 g of CO2.
Every 80g of NaOH can Neutralise 44g of CO2 and give 106g of Salt and 18g of Water (32% increase in dry weight)
Every 74g of Ca(OH)2 Neutralise 44g of CO2 and give 100g of salt and 18g of Water. (35 % increase in dry weight)
Given that we have 2,5 Kg of Soda Lime - 4% NaOH and more than 75% Ca(OH)2 we would expect (given the analogies) to have an increase of dry weight around 34% at the end of Scrubber life.
My question is if we could use an accurate weight scale to calculate the weight of the canister (before and after the dive) and estimate how much of this scrubber has been used. Did anyone make any such tests? Are there any data available?
I would say your way of looking at the scrubber chemistry is incorrect. My understanding is that the strong base is not consumed one way but that it is continously reformed as long as there is sufficient Ca(OH)2 present (and temp, humidity).
I don't think the weight approach is neither safe nor practical or even informative of the state of the scrubber.
OK, considerable amounts of water is formed during the scrubber reaction, but some is added from the users lungs, some could also be introduced from the outside from leaks. How do you determine these fractions?
Some of that water can also end up in basically every part of the unit depending on how the diver moves around and on the water/loop temp. Would you then collect all this water?
Finally what is the point with this thing? It would still not say anything about the reaction path inside the scrubber. To me the total scrubbing capacity of the scrubber is not very interesting unless you also have information on the profile of CO2 penetration and reaction path. A scrubber could still have alot of remaining scrubbing capacity while being completely unusable (especially at depth/high workload) due to an intolerable breakthrough of even a
comparatively small fraction of CO2.
So as long as CO2 sensors remain elusive in rebreathers I think empirical rules of thumb, the temp sticks and pH indicating dyes are the only things that are practical and give some degree of guidance.
BTW, the dye used in some sorbs actually gives a measure of the remaining strong base and the distribution of the used material. So as far as I can see that method (with its limitations) is already superior to any field attemps to use weight to measure scrubber life.