volume or weight of absorbant?

OceanOpportunity

Hey gang,

From a practical standpoint, would you prefer to see scrubber specs based on volume of the cannister, or weight of absorbant held?

it seems to vary out there

REgards
ML

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Michael Lombardi
Oceans of Opportunity
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Elected Director, Society for Human Performance in Extreme Environments

MN'07, The Explorers Club

Project Manager, Diving a Dream

caveseeker7

Volume makes more sense as weight depends on the sorb used (i.e. Sofnolime vs. Sodasorb, 4-8 vs. 8-12) as far as the fill capacity on the rebreather is concerned.

Weight seems to be much more popular and common though, the stuff is sold by weight, makes some calculations easier (cost per fill, fills per container). Hence more practical I think.

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tibby

I was actually just wondering about this and I was thinking if you go by weight and the sorb has been allowed to lose some of its water content (accidentally) then you'd end up with a more conservative result (maybe?).
If you go by volume and the sorb is "dryer" than it should be you would reach breakthrough faster that calculated (maybe?)
Since sorb manufacturers say a dryer sorb is less efficient I'm thinking it would help to measure by weight which would mean you make up the missing weight in water through extra volume.
Also, it's easy to check the weight of the scrubber after packing (provided it's packed the same way) and see potential differences in weight from batch to batch (again, maybe?).
This is all speculation of course, I'll have to try this and see if it's true.
Tibby

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nigelh

The cannister volume is the only fixed item.
That is the only thing that is set by the manufacturer.
The moment you quote weight you are 'assuming' the density of a product you do not control.

Also: I fill by volume. By all means quote the 'expected' weight but do any of us measure out 3.2Kgs of 'sorb and try to fit that in the can?

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nigelh

OceanOpportunity

so, from a unit marketing standpoint, you are saying you'd rather see cannister volume, yes? From a performance standpoint then, it would be beneficial to quote approximate weight of absorbant. (for example, 'our 2L cannister holds 1.5kg of absorbant'...or something to that effect.)

reason I'm trying to sort through this is that I have spent a good deal of time working out the theoretical bedlife calculations proposed in Nuckols et al for both sodasorb and sofnolime based ona fixed set of environmental and metabolic variables and am working out a table that might aid in determining a basic level of scrubber performance. This excersize has provided some very useful and interesting data that we'll be publishing shortly. We're a bit up in the air on whether to present the data with Kg or L.

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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

nigelh

I tend to feel we define the rebreather by volume but does the 'sorb effect depend on the weight?

Somewhere a density number gets in but is the change in density due to water content variations less than the change in density slapping it down? The trouble with weight is it is less helpful to us when we compare two different fills. If I switch from Sofnoline to Dragersorb on holiday the volume is the same so I want to know if the same volume gives me more or less time so a weight number sends me off on a quest for density values and complicates matters.
If I change rebreather either will do as I want to ratio my normal usage times.

I'd go for volume. Weight is easy to measure for an experement but less useful in real life.

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nigelh

OceanOpportunity

actually, as the math works out, cannister duration has everything to do with the weight of the absorbant itself, and is independent of intergranular space (assuming that velocity, dwell time, temp, etc are all constant). The 'density' of absorbant is usually listed around .9g/cm3. HOWEVER, this is the density of a cm3 of absorbant pellets, not the solid chemical. Actual absorbant capacity is defined by the chemical weight/density, which off of the MSDS sheet is roughly 2g/cm3.

now, the scrubber's actual performance is dependent on the physical characteristics of the pellet size and shape, as this factors in to dwell time and gas velocity.

so practically speaking, 3L of sofnlime vs 3L of dragersorb are very different animals, but this is directly related to the weight of absorbant required to fill each of these spaces, and each's varying makeup (it's slight, but does have some effects)

it's actually been quite an interesting excersize to work though the precise physics mathematically and froma chemistry/physics perspective.

__________________
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