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Thread: Scrubber Duration Formula

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    RBW Member Bnscherm is an unknown quantity at this point Bnscherm's Avatar
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    Scrubber Duration Formula

    This thread would likely be just as home in the home built category. Anyway, is anyone aware of a formula for sorb duration that takes into account all of the different variables; scrubber dimensions, fill factor, temperature, humidity, depth/pressure, and time. I'm sure there are a few more variables I've overlooked, but a formula that accurately models the sorb duration would be a valuable tool for designing scrubber cannisters and calculating the amount of life left on a charge.

    Also, does anyone have the numbers for the density of the different mesh sorbs as a starting point for calculating duration?

    Best Regards.

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    Re: Scrubber Duration Formula

    Quote Originally Posted by Bnscherm  View Original Post
    This thread would likely be just as home in the home built category. Anyway, is anyone aware of a formula for sorb duration that takes into account all of the different variables; scrubber dimensions, fill factor, temperature, humidity, depth/pressure, and time. I'm sure there are a few more variables I've overlooked, but a formula that accurately models the sorb duration would be a valuable tool for designing scrubber cannisters and calculating the amount of life left on a charge.

    Also, does anyone have the numbers for the density of the different mesh sorbs as a starting point for calculating duration?

    Best Regards.
    No matter how one looks at it it can be boiled down to one simple rule. That being; 1 hour per pound. Duration is pretty well a fixed thing. The other factors like WOB are more shape and style based.

    As for a repetitive use guide well for a decent scrubber (Prism, Meg, Boris, Mk 15.* et al) old addage of don't dive deeper than the % of life remaining has worked well for me. 20% of scrubber time remaining = shallower than 20m and so on. Having said that, these days I tend to do long deep dives (2-7 hrs) and if it is deep enough to use mix in the dil I start with fresh sorb.

    I'm sure you could get very creative with modeling and come up with a excellent formula but there really isn't a lot of point.
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    Re: Scrubber Duration Formula

    I don't think it is as simple as 1 hour per pound - as a rule of thumb yes but one has to take into account both depth, temp and rate of work for the dive.

    Also one has to have a minimum depth (well surface area I guess) of scrubber material for the CO2 to interact with to ensure you don't get any breakthrough eg a 1 pound scrubber would not last 1/4 of the time of a 4 pound scrubber because the depth of the reaction front would be that it would break through very quickly.

    Not to mention that scrubber architecture definitely has an impact the deeper you go (look at the extra run time out of the Boris scrubber when the fins were put in)

    To summarise I believe that weight of absorbent when combined with known depth, temp and rate of work for the dive (eg Rate of Breathing and CO2 Production) will give you a formulae that should be consistent across different scrubbers subject to the scrubber architecture being such that it minimises break through.
    Last edited by schford; 24th November 2006 at 09:16.

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    Re: Scrubber Duration Formula

    WARNING: this reply contains METRIC measures! Americans beware.

    OK, that over, approximations, guesswork and poor maths coming up.

    At best, sorb will remove about 150gm of CO2 per kilo of sorb.

    Molecular weight of CO2 is 14+16+16=46gm per mol so 1kg of sorb will remove about 3 mol of CO2.

    At STP one mol of gas has a volume of 22.4l, so that's around 60 to 70l of CO2 removed per kilo of sorb.

    At a lowish level of exertion one might metabolise 700ml or so of O2 per minute, producing approximately 500ml per minute of CO2. So our kilo of sorb will last 132 minutes, or 2.2 hours.

    Now it just happens that one kilo is around 2.2lb and so indeed one might be able to say "one pound per hour" (OPPH).

    The factors affecting our mythical OPPH are:

    1. Actual CO2 production per minute
    2. Actual (as opposed to ideal) CO2 absorption by the sorb

    CO2 production will obviously vary depending on a particular diver's level of exertion. Different divers will also produce different amounts of CO2 for the same level of exertion.

    The efficiency of CO2 absorption by the sorb will depend on a lot of factors. Sorb moisture content, particle size and other physical factors such as packing, channeling and scrubber design, sorb temperature and reagent quality are just some of them and many are variable and unpredictable throughout the course of a dive.

    Sorry if I'm teaching Grand mama to suck eggs here but this is just a long winded way of saying "you're not going to find a reliable, repeatable formula". Probably the closest you could get is to do CO2 breakthrough testing with your particular scrubber design and nominated sorb so that you can give a "safeish" scrubber duration. And that's pretty much what the manufacturers do with their units.

    If you're asking the question because you want to know how big your homebuilt's scrubber needs to be, the simple answer is probably that it will likely perform about as well as the commercial scrubber it's most like in size and design. If it's nothing like anything on the market you'll just have to do the empirical testing yourself.

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