Why does depth matter for scrubber lifetime

York

I think he was referring to the sport KISS, not CK

caveseeker7

It was written by Gordon Henderson (author of DDPLAN).

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Dutchy

I'm puzzled too. For simple reactions the reaction speed is a function of presure, temperature and concentration.
The concentration thing I understand because at identical pPCO2 the fraction is more diluted at depth. But this is only the case if the first step in the reaction is the limiting factor. From what I understood so far it isn't. Alternatively it would account for the ALLEGED effect if it were to influence the Sodiumhydroxid recovery or the final conversion to Calciumcarbonate but those take place in an aquatic environment so it can't be that either.
ANy chemist here that can explain from that perspective?

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Mark Chase

Oh your right, sorry


ATB

Mark

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DanDunfee

I'll not pretend to understand all the factorials that go into it...few if any folks understand it completely.
However, correlation of observed scrubber breakthrough time data, mostly based directly on, or normalized to, the EC test parameters, the data indicates:

* A quite-consistant scrubber breakthrough time reduction of ~ 14 % per ata of increasing depth. For
simplicity, I'll call it 15 %. Such a simple factor should be very useful to typical Rebreather divers in pre-dive planning.

* The effect is quite consistant over a relatively wide range of scrubber charge weights (~ 2 to 4 Kg), and scrubber designs.... axial, doughnut radial, pancake radial, etc... including the vane-modified Boris.

I'll do an Article writeup, which shows the base data and the correlations, as a 'winter project' for posting in the Rebreather World Library.
The correlations also indicate the surprisingly (to me) large ambient water temp effects esp. WRT smaller scrubber charges.

Meanwhile, if any one has deduced a significantly different factor, based on correlations of equivalent scientific test strength (not ancedotal deductions & conclusions) and are willing to share the data, I'd like to communicate with you personally by PMs.

Hope to see many of you at DEMA.

Dan

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York

Dutchy,
as has been pointed out in other threads, no-one knows exactly what happens, and up to today I found more potential chemical pathways of what is actually happening than people quoting them. I also found, having worked for a manufacturer who also produces absorbent for rebreathers and having talked to their chemist-in-charge, that (as is often found in other industries) the manufacturer themselves might not be the most knowledgeable in the area. This is not meant as critique, but as simple observation. I mention this because I recently saw a chemical pathway proposed by a manufacturer that was completely different and potentially in conflict with measurement results - you draw your own conclusions. I personally believe that the role of sodium hyrdoxide is overrated, but then who am I.
I personally agree with you that reducing the reaction based on a single, rate-determining step may be an oversimplifcation, yet it may be powerful enough to model the process and make it comprehendable.

Allow me to draw your attention to another factor, independant of the chemistry or diffusion in the scrubber: gas temperature. Lets assume a scenario where the water in the shallows and at depth has the same temperature. Furthermore, lets assume that the diver breathes the same minute-volume in the shallows and at depth. Both at depth and in the shallows, breathing gas leaves the diver's lungs at the same temperature (37C). In the shallows, the gas has less mass (due to less density), hence cools faster while travelling through the hoses and the counterlungs. At depth, cooling of the increased mass would take longer, meaning that at depth, the gas reaches the scrubber warmer. Very simply, warmer gas means faster diffusion and chemical reaction, means better scrubber reactivity.
This, ironically, counteracts the diffusion-barrier due to increased gas density. Measurement results published in the 80s show this battle of these two conflicting effects, where the optimal scrubber performance was actually reached at around 10m and not on the surface.
Now put on top of that water condensation, water being necessary to support the reaction (while, at the same time potentially serving as another diffusion barrier around the pellet), and you end up with THREE mechanisms pointing in different directions - I don't think anyone can make a certain prediction based in theory alone how a scrubber is going to perform. Another factor is the flow field, mainly influenced by the mechanical layout of the scrubber, but also to a lesser degree by local temperature profiles. There are quite a number of additional influencing factors (radial dispersion coefficients, local heat transfer coefficients influenced by moisture-content and water-droplets, etc.) and you end up with a scrubber which performs perfectly at a certain depth at one breathing pattern, but insuffciently at a slightly changed breathing rate.

What I am trying to say with this rather lenghty post is that the chemistry itself is only one part of the many required to understand the scrubber performance. Overall, it is a simplified assumption on the safe side that scubber performance decreases with depth. There is enough data showing that this is however not the case in reality.

Joerg

Shawn

So then if I understood correctly that break through is the largest determining factor when determining the scrubber life using calcium hydroxide then would (on deep dives) lythium hydroxide be a better alternative? (sorry about the spelling)

And if alot of this problem is negated by the use of radial scrubbers, how come Rebreather's don't come with a radial scrubber instead of the axial scrubber?


Thanks


(wow...I just went back and re-read some of the posts and ..I'm a little out of my league) But still very good stuff to know.

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Abbo

Sounds like somebody has just been watching the movie 'The Cave'! Lithium hydroxide absorbs more CO2 per unit volume than calcium hydroxide does, and it has lower density too. It is used in submarines and a tub of it became a major factor in the lives of the Apollo 13 crew. Its higher efficiency would make it very attractive for scrubbers in rebreathers, and mountaineers would absolutely love it too, if it were not for the fact that it is exceedingly caustic. A caustic cocktail from normal sorb is life threatening, but one from lithium hydroxide would be far, far worse. If a rebreather could be designed where there were no risk of that caustic cocktail, then lithium hydroxide, though expensive, would become attractive. In addition to having almost double the absorbent capability, the reaction is highly exothermic, which would be a positive on cold water dives.

Radial scrubbers are highly efficient right up until they're not. They are better on work of breathing and are more efficient. However, when breakthrough occurs, the amount of CO2 getting through rises far more suddenly with a radial.

Tom Rose

Boy is that right, and the problem with spikes (over breathing) is worse in a radial scrubber as they are currently designed. I had not been nearly as aware of those issues until I started putting CO2 monitors in units testing my monitor.

While I have a decent background in Chemistry and a better background in physics, combining the two in physical chemistry measurements of air pollutants for over 30 years, it has become more and more obvious to me that the chemists look at the world much differently than the physicists and both are necessary in this area of scrubber analysis, as are the physiologists.

Dan as a generalist might just be the one to take a good crack at the description. He for one might not be trapped by looking at it from one side.

"If the only tool you have is a hammer, you see every problem as a nail"

Great discussion material at DEMA....

Tom

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DanDunfee

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Joerg

I certainly agree that there are many, many chemical, thermal, and physical factors and functions which go into determining scrubber breakthrough times for a given set of conditions. It's a complex problem... some of the individual functional component effects are additive, and some offsetting, to a point of being rather confounding. Extensive computer modelling has helped understand some of the major individual component effects, However, efforts to model the whole integrated function, from the grass roots up have been, at best, marginally successful. None that I know of have yielded a simple set of parametric factors which are usable, even by divers who are especially skilled in the arts, for making on-site, pre-dive estimates of probable scrubber duration under prevailing dive conditions.

This many-faceted scrubber duration challenge is, in many ways, analogous to the complex thermo-chemical and physical function factors in a solid-propellant rocket motor. In dealing with these types of complex problems, especially those which defy comprehensive detailed understanding and modelling of each functional contributor,.. One often can, by reverting to simplified parametric correlation of the total observable effect, deduce simplified but valid functional factors which arch-over the complex interactivity and yield reasonable computational factors which are useable by skilled, working-level users (on site divers). Thats a principal goal in the studies I spoke of in my previous post, but this remains to be documented and validated to the Rebreather diver community. Till then I believe that the 14 % per ata of depth is a reasonable approximation of the depth-dependent effect on a scrubber of a given charge weight (given a previously validated scrubber duration, at analogous dive and work conditions, but another depth).

Re the red-highlighted statement above ....
Au Contrare !! There are substantial bodies of both lab and manned testing data showing 'irrefutibly', in principle, that scrubber breakthrough times decrease as a function of depth, at least down to 100 m. I know of no credible scientific test data to the contrary. The question, as above, is simply how great is the effect, and can reasonably reliable computational factors be deduced to support on-site, pre-dive estimates. Anything would be welcome to fill the vacuum that exists now.
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Lithium Hydroxide has seen significant use in rebreather diving equipment, including SCUBA-type rebreathers.... just not significantlly in the recreational or civilian technical diving arenas.

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