Scrubber Pressure Lapse Rates

jqn

Okay this is a question for our chemists.

The number of molecules of CO2 does not change with pressure but the density of the gas changes. Therefore, the higher the pressure the lower the probability of a CO2 molecule encountering scrubber material and being reacted out of the gas.

now if a particular scrubber has a depth rating, say 60M, then I assume that the the probability of a CO2 molecule getting through the scrubber, while non-zero is not high enough to be significant from 0M to 60M.

In the same example, if one were to go from 60M to 100M then there would be an increase in pressure of 11b/7b. Is there anything in general we can say about how that changes the probability of a CO2 molecule getting through?

Another way to look at this problem is: what is the formula for scrubber efficiency with respect to gas pressure given a constant No. of moles of CO2 being exhaled?

Jerry

OceanOpportunity

the calcs for scrubber duration are found in 'Life Support Systems Design" by Nuckols et al.

__________________
Michael Lombardi
Oceans of Opportunity
www.oceanopportunity.com

Elected Director, Society for Human Performance in Extreme Environments

MN'07, The Explorers Club

Project Manager, Diving a Dream

York

the diffusional blocking of CO2 by inert gases is only one of the mechanisms affected by depth. The other is heat retention, which also influences diffusional speed and rate-of-reaction. The higher the ambient pressure, the more time is required for the exhaled gas to be cooled in the exhalation hoses/lungs due to higher mass. If breathing stays the same shallow and at depth, the exhaled gas will reach the scrubber warmer. The overal result is a superimposition of hampered rate of diffusion, and increased diffusional speed and reaction rate - to my knowledge it is not generally predictable which one of those mechanisms wins at any given moment. To make it even more complicated, there is water condensation, and that fights on two sides of the fence. Measurements by Nuckols overal demonstrate that slight changes in breathing will change scrubber effectiveness - in both directions. For example, one of the series published shows maximum scrubber efficiency at 2 bar, which shifts when breathing rates are altered. Additionally, the problem with "break-through-curves" is that they generally presume a cut-off at 0.5%SEV, but I am not aware of measurements as to the rate at which the canister goes through the 0.5% SEV value ("buffer effect").
Unless you are a breathing machine with a set rate, trying to come up with a rule-of-thumb of duration vs. depth is probably not a healthy approach. But then again - we dive it anyway, don't we? So it must be working ...

€0.02
Joerg
(not a chemist)

OceanOpportunity

the scrubber discusion will always be an interesting one. the thing that gets me is the perception that there are any fixed 'rules of thumb' as far as duration goes. there are too many variables. When scrubbers are tested, it is typically at a fixed depth (relatively shallow) with fixed parameters. This provides a baseline to 'rate the duration' but doesnt account for the realities of the dive...best thing we can do is be conservative from the baseline. THE Nuckols equations also show that duration is significantly affected by depth. Since most of us make surface to surface excursions with short times at depth, this is somewhat irrelevant. under a deep and long saturation exposure, breakthrough would come much more quickly.

__________________
Michael Lombardi
Oceans of Opportunity
www.oceanopportunity.com

Elected Director, Society for Human Performance in Extreme Environments

MN'07, The Explorers Club

Project Manager, Diving a Dream

York

>under a deep and long saturation exposure, breakthrough would come much more quickly.

While I agree with everything else you said, I think that over-simplifies it - to my limited understanding. What I do agree with you is that, since we are not at depth for a long time, you could "gain" some more scrubber time on ascent.

Perhaps the best way of looking at it is the old-fashioned "reaction zone" - often quoted, poorly understood (by me at least).
If for a moment we agree that the reaction zone widens with depth, that still leaves the speed by which the reaction zone travels along the (assuming axial) scrubber the same and solely dependant on CO2 production rate. Adverse conditions require more scrubber-granules (wider zone)- wide zone means more granules involved as reaction is slower, but once the zone has established its length in a steady-state process (quasi-, because pulsed), the speed forward is determined by CO2-rate. CO2 is contained in that zone, the zone then travels forward as scrubber-granules are used up, leaving behind "dead" granules.
The question whether scrubber time is much shorter depends on the ratio of reaction zone to scrubber length. Assume for a moment two axial scrubbers of same diameter but different length.
example calculation (type: back of napkin, no details included, only for demonstration purpose):
The short canister is lets say 30cm long, the other 60 cm (double)
Zone travels with 10cm/hr (arbitrary number, sounded good)
In the shallows, the zone is 10 cm wide
At depth, the zone is 20cm wide.

Duration (until zone reaches canister end):
Short canister shallow: 2 hours
Short canister deep: 1 hour
long canister shallow: 5 hours
long canister deep: 4 hours

Let's take both canister really deep, shall we?


If there is one thing that I think can be generalised it is that a bigger scrubber gives you more time, and is also more "forgiving" once you reach its end. "Going minimal" is more likely to cause problems - which is, unfortunately the way we as recreational Rebreather-diver tend to go.

Again, assumptions, assumptions. Any one of those can be argued against under specific conditions. What is not considered in napkin-calculation above is the time required for the zone to spread in the beginning.

Joerg

Freef

Does scrubber particle size have any bearing on the matter?

I have used Divesorb, 797 2-5mm and 797 1-2.5mm in my Dolphin [I have some Spherasorb to try out as well]. The larger size 797 seems to have a more diffuse hot spot than the smaller, which is more akin to the Divesorb one.

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David.

Currently owner of two differently sized ankles.