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

diveoceanos

Hi Alex. That would be more than enough! I would say ideal for our practical reasons! We just follow manufacturers recommendations and try to figure out at the same time if our findings could be in line with what we allready know and use for scrubber life!


What we have to loose apart from a couple of moire minutes. After all rebreathers take a lot of time! What are two more minutes?

diveoceanos

Tom,

The ultimate idea is to develop a simple method to be able to answer the very difficult question: Is my scrubber good for my next dive or shall I have to use fresh soda lime?

The hypothesis is that salts are being formed inside the scrubber. As the scrubber is being consumed its weight is being increased. There is a certain maximum in the scrubber content weight above which the scrubber is useless.

The question we are trying to answer is what that weight is in terms of percentage (compare to the weight of fresh scrubber)?

How are we going to meaure the weight? What procedure should we follow in order to have a standard method? We need to think about it and as a team to have an input on this idea.

My idea is to measure the scrubber cartridge empty (we need to do this just once). Then pack the cartridge with a given soda lime and record what grade that soda lime is. Then weight the cartridge before the dive.

Record dive profile, oxygen consumed, temperature inside the loop (if possible), temperature outside the loop (water temperature), record any major leaks in notes. Record any Carbon Dioxide poisoning symptoms in notes if occured.

After the dive, open the canister remove the cartridge, wipe off the excess water droplets around the cartridge neck and body, weight the cartridge in 5 minutes after opening.

Then weight it again before the next dive as some water might have been evaporated within this interval and start over again with recording the next dive.

Eventually when we have enough data we might try to approach the question again and try to see if we can reach to any usefull conclusions.

How is that?

Sotos

NEDIVER

Sounds Great! What software program do you want to use, we can add in info for each dive and share data with a common program.

Do you use Excel?

I suggest we call it; "The Sotos Experiment"

Here is a suggested "data table";

Pre-Dive Data
brand of absorbant (granual size)
weight of absorbant (kg/g)
pre dive time on loop

Dive conditions:
ambient temp
water temp at depth
time on loop (including pre dive)
max depth
bottom time
deco depths and time

Post dive analysis
leaks noticed
hypoxic conditions observed?
condition of scrubber
Weight of absorbant (5 min)
weight of absorbant (10 min)
Weight of absorbant (pre second dive and time)
O2 used (psi/bar)

Let me know your thoughts.

__________________
-Tom-

"slow is smooth, and smooth is fast" TM
"Insanity is repeating the same thing over and over again, expecting different results" Albert Einstein

diveoceanos

Here is a suggested data base in excel form. A great part of it is your input.

rebreather world experiment.zip

What do you think?

NEDIVER

That looks great, I have just thought of one more bit of data that might be of use. Temp stick indicator predive and post dive. (nothing quantitative with this but could be useful in determining amount of unused scrubber material, and/or time to break through.)

I still think it should be called The Sotos experiment

__________________
-Tom-

"slow is smooth, and smooth is fast" TM
"Insanity is repeating the same thing over and over again, expecting different results" Albert Einstein

wreckdiver68

This looks like an interesting idea. It looks like it's already been mentioned but water absorbsion of the media from exhaled gas is going to be problematic to account for.

One way to help account for it might be to run some control tests with expended sorb (non diving obviously) to remove the variable of the reaction. This should allow you to get a rough idea, probably still underestimating, of the amount of weight increase due to the amount of water absorbed by the media.

By assembling your rig with the inhale hose disconnected you should be able to expose the scrubber to exhaled gas w/ 100% humidity without the risk of hypercapnia.

The only other variables should be the specific rig/canister, and ambient temperature. After drying out a bunch of used sorb and packing a scrubber, one would only need to weigh it at set intervals, dry, then maybe every 10 minutes, until until the weight levels off.
Repeating the test with the same sorb a couple of times will help insure that your not actually measuring weight increase due to CO2 absorbed.

In actual use I would imagine you would get more weight due to condensation but you might be able to get a little closer to what you want while still maintaining a reasonable safety margin.

Running the test in different ambient temeratures should allow you to create charts plotting time on the media vs. weight change due to humidity of the media/condensation at different temperatures.


Regards,
Owen

diveoceanos

I am glad to see that input by other people is making the whole thing more interesting. Well mentioned Owen!

Your idea to weight the dried S/L would definitely give more accurate and realistic results as the water factor is eliminated from our equation. However there is one drawback with this method tough. It requires, as you mentioned allready, expended sorb. This experiment is much more expensive to carry out as there will be a lot of wasted sorb.

Also inhaling with the inhalation hose disconnected is not simulating inhaling during a dive as there is always fresh air in the loop. All parameters are then affected.

Nevertheless we can design a parallel experiment to measure water content in the sorb by drying after different breathing times (actual dives) to calculate a blank reference curve. This curve i.e. a function WATER = f(breathing time) would come as a mean value +/- SD and could be used later to estimate +/- the expected water content of the sorb after a given period of time in use.

NEDIVER

I am flying down to Florida on Sat for 4 days about 3 dives per day. I will record data and send it out.

This should be at least a start!!!

Great insight Owen, we will need all the data we can get, welcome aboard!

__________________
-Tom-

"slow is smooth, and smooth is fast" TM
"Insanity is repeating the same thing over and over again, expecting different results" Albert Einstein

jaap

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.

__________________
My initials: JAAP

diveoceanos

Hello Jaap,

Since there have not been any data so far how one can be so sure that the approach is a waste of time? I would say that the maximum loss in case the outcome will be inconclussive is a waste of time recording several hundreds of dives and analyse them. No?

I see your point and scepticism! This is not definitely without a base but on the other hand is a little dogmatic. Why shall this approach be considered and treated as a taboo? There has not been any suggestion to ignore what is being allready used. And that is manufacturer's reccommendations. We keep doing what we do and at the same time we accumulate some data for the sake of analysis. There is an hypothesis and this hypothesis should be tested to be confirmed or rejected! That's all.

A few thoughts about your interesting post below:

I have only one drawback with the suggested chemistry that is being around for quite a long time now. And this is simply based on very basic principles in chemistry. A salt of strong base (Na2CO3) if put together with a weaker base (Ca(OH)2) would definitely not regenerate the strong base. This would form a buffer system but the very details of this are going beyond the purpose of the suggested experiment. Perhaps I am completely wrong here, since I took my chemistry lessons back in 1995 when I was a student in Pharmacy. Those principles have been around for centuries but yes my approach might be wrong since things are changing over time and i might have lost track of chemistry evolution. Any other opinions by other folks?

For the time being the suggestion is to put this under investigation before we end up with a conclussion like the above.

All correct here! But remember that the suggestion is to study just the scrubber cartridge, not the system as a whole! The less water is adsorbed or condensed inside the cartridge the better the conditions for the experiment are going to be. Actually we want to get rid of the excess water. We are not aiming at weighting every single droplet in the system. We don't actually care about how much water there is in the system. What we care is the weight change in the sorb itself. We are also hoping that the water quantity in the sorb will be minimal or constant percentage (well more or the less) of the weight change. Anyway this has to be shown with real measurements.

Remember the experiement is to test the parameter weight and how does this relates to scrubber life. We have for instance known starting points. What are those? Scrubber life of axial scrubber in a Meg for instance has been proposed to be good for 4 hours at 0 degrees with a CO2 flow of 4LPM. This can resemble virtually any level of work load at any depth. Correct? Or shall we decrease that in 3,5 hours? Would you feel more confortable with this? Let's start and then take it from there. If we measure the weight change for all these 4 hours (or 3,5) then we can check how weight can be related (or not) with scrubber life. No? And then if eventually we end up with a conclussive otucome we are going to have an extra point to check scrubber life or some nice story to tell to our children about an experiment that proves that weight is neither safe nor informative about the life scrubber.


Nice point above! Remember that the suggestion is to test an hypothesis! Not to erase what is being allready used in daily practice!

Cheers!

Sotos