Chemistry of CO2 Neutralisation - Can weight be used as an indicator of scruber life?

diveoceanos

The chemical composition of soda lime is pretty much known. It is a mixture of NaOH (or KOH) at a percentage of around 4% and Ca(OH)2.

These are both alkalis. CO2 is a known acid so the reaction that is taing place in the scrubber is a neutralisation reaction.

After my first dives I noticed the water condensation inside the scrubber and elsewhere inside the loop. I thought that definitely a source of it is water vapour coming into the loop from our lungs. However as soon as the atmosphere inside the loop becomes saturated in water vapour no more significant water loss through the lungs should be justified. Therefore without having researching the issue in depth I suppose that the major source of all this water is not our lungs.

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?

Packhorse

I dont know jack but I would guess it would be far to unreliable due to water being exhaled/ inhaled and entering/exiting thru the dsv.

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Sutty

There's your problem! Scrubber water content will vary enormously and drying it is not an option. I suspect there is still considerable amount of water lost from the lungs because although the gas we inhale is warmer than OC its nowhere near 37C and so we will add more water to it in our lungs before breathing out and forming further condensation in the loop.

Neil

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NEDIVER

Well, It has been a little since I have done Quantitative Chemical analysis on a regular basis. So, with out taking out the calculator and assuming that our calculations are right, lets look at a few things.

The first is that you are trying to quantify the weight of gasses and solids within a system that allows for the reactants and byproducts to be removed and added. Even with the help of our good friend Avogadro it becomes increasingly complicated.

To Neil's point, this is just the first variable that does not allow you to calculate "practical" weight, yet alone the specificity needed in quantitative analysis.

It's not until you look at the amount of polyprotic acid-base equilibria reactions that are taking place with in the body that you realize that this is a lofty goal to try to calculate via simple acid/base. After all the affinity of Hb to O2 and to CO2 rely on that pKa in the lungs. Resulting in a different amount of CO2 moles for each diver.

As that diver is metabolizing everything in the body, there are three more variables being added to your reaction. The first is the amount of moisture (water) that is being added to the system from the exhalation breath. This can be more or less depending on the level of hydration of each diver (think osmosis) and the humidity in the loop.

the second and third additions to your reaction are the diluent on descent/flush and the O2 that is added from metabolism and on ascent. As you can see you would have to calculate the volume being added to the loop in value of moles to even begin this calculation.

There are many biochemical engineers and scientists more intelligent than I, but I do not see how you could calculate weight in such an "open" system (in chemical terms, not breathing).

But, don't let me discourage you from trying, some of the greatest inventions were from those who tried to do something that everyone else said can't be done.

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diveoceanos

Thanks all for your answers to this question.

It seems that yes, you are all correct.

Indeed if such a parameter i.e. weight difference of the scrubber before and after the dive should be an indicative factor of how much scrubber has been used during a dive would be of any use, then a great deal of assumptions would have to be made. These assumptions could be minimised by experimental data.

No matter what the source of water is, inside a canister, a carefully designed experiment with the proper statistical analysis tools, would give us some interesting information related to this issue with an acceptable confidence interval.

I agree that the system is open but then again you have the formation of salts inside the canister that more or the less stay inside the canister. Again I am making the assumption that there is no significant exchange of sodium carbonate and calcium carbonate with the environment. These can be analysed and quantified in a series of experiments. Water inside a canister can also be measured at the same time easily.

It takes a lot of work though a lot of money a lot of wasted soda lime and thousands of dives by many many divers in many different diving environments and under a great variety of circumstances.

I don't know if it worths a try. I did not even get a calculator to do the maths. Just a raw idea!

ChemicalDiver

I am certainly not experienced in CCR, but I find the idea conceptually interesting, as weight is actually one of relatively few truly quantitative analytical tools in chemistry, which has been around for centuries (now we have a few new ones, but few are as quantitative as weight).
Classical example: To make a solution of HCl gas in a solvent like methanol one can pass HCl gas through the solvent and then laboriously titrate the obtained solution to determine the amount of HCl in it, or (and this only takes a few seconds) weigh it before bubbling HCl through and then after. The difference is the amount of HCl. You can then calculate quite accurately how concentrated your solution is and this technique is good enough for most purposes.

I take the point of the CCR not being strictly a closed system (i.e. in addition to the gas you have water from the breath etc), I suppose if the diver were to jump on the scale before and after (toweled off with hair blow dried...) along with the scrubber the moisture from the diver transferred is irrelevant. Bt you would not be allowed to pee or sweat or burp? while diving. But then this would be too inaccurate.

More seriously, I wonder however, how much the moisture transfer from the diver to the scrubber will vary from one dive (say in dives in similar temperature water and similar activity) to the next. And in any case, if water leaks in from elsewhere by mistake, it will just make it safer as the scrubber will be heavier and therefore seem more used.

You are not trying to figure out scrubber life in seconds, but more like a fuel gage on a car (half full, quarter full?), so I wonder whether weighing will provide a "yes, as I estimated the weight also confirms that the scrubber is still good". If it is heavier than anticipated and typically found in your diving, change sorb?

Perhaps it would be interesting to plot scrubber weight across time dived for multiple sorb fills and see how much variability is. A good digital kitchen balance should work and be accurate enough? Is it worth getting the data?

Tomorrow is April 1st after all.... anyone going diving?

Alex

Outlaw

Hey guys,

i take it from Sotos here and would invite all of you to put an effort here and lets try to make a forum experiment. i will personally weight my scrubber, as it is packed and ready and will do 2 dives tomorrow, and will weight it after just to start keeping data.

we need to have a datasheet that i guess one can make, but lets all use the same one. i.e depth, duration, setpoint, temperature would be good points to keep track of. if we can all contribute to this, then i am pretty sure that very soon, there will be a clearer picture regarding this issue.

well done Sotos for bringing this one up. green for you.

Thanks,

Spyros

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diveoceanos

Hi Tom,

Some thoughts related to your valuable input:

Let us study just the system of scrubber cartridge content! This is more related to the scrubber life than the full loop.

Water amount inside the scrubber cartridge is considerably lower than the total amount of water inside the loop. Water vapour from our lungs or even water added by the DSV or by any other leak is largely trapped in the water traps below the canister. I wouldn't expect a great amount of water to bypass this trap under normal circumstances (i.e. there has not been a partial or fully flooded loop). Again this is an assumption that needs to be verified. Given that the assumption is correct the amount of water inside the scrubber cartridge could be standardised in an easier manner. After all water can be removed from the scrubber with the proper drying technique just to validate the assumptions! If we focus just on the scrubber cartridge weight rather than the whole loop water content we might be able to minimize the effect of water content in our calculations.

Biochemistry within the body is an issue for oxygen utilisation and carbon dioxide release indeed. However what happens inside the scrubber has to do with the the amount of CO2 that is being neutralised inside the scrubber cartridge. Biochemistry and individual characteristics may affect the rate of CO2 that passes through the scrubber, yes, but the scrubber life is only depended on its ability to neutralise CO2. We should rather look at its ability to do this rather than the body metabolism.

I am not sure if I can see how O2 level inside the loop may cause any significant change in scrubber life. There is definitely a water output with every diluent flush or other gas release and water input with any leak from the enviroment inside the loop. But let's don't focus on water. Let's focus on salts formation inside the scrubber cartridge. This would make the equation more simple.

Thanks
Sotos

diveoceanos

It only takes a couple of extra minutes of time to weight the scrubber cartridge before and after the dive. We might even try to get rid of the extra water droplets around the scrubber neck before we do the weighting. We might even try to built a data base with other fileds like for example the oxygen being used during the dive, the loop temperature during the dive and the water temperature during the dive. Other fields could be the duration and depth of the dive, more or less the dive profile.

Oh and it needs an accurate weight scale with accuracy of grams, you know!

NEDIVER

Sotos,

I think all of your points are well received. I think we need to know from here what the hypothesis is?

Are you simply looking for average by product weight post dive?

Are we setting up permitters for an international experiment? If so, we need to standardize all of the data. I would start by suggesting that, to your point, we weigh the scrubber post drying. To be more specific, we need to agree on a drying time and way of drying to be most accurate.

Secondly, I would make a suggestion that it be recorded in Kilograms/Liter. This way it doesn't matter the size of the scrubber, it can all be calculated into a constant.

I can't believe that this is starting to form into an actual experiment

I look forward to your thoughts and guidance on the hypothesis (after all it was your idea to begin with).

.

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