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abowie · 24th November 2006, 11:45

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.

24th November 2006, 11:45	#4 (permalink)
abowie Fighting Girl Current Rebreather/s: Sport Kiss Other Rebreather/s: Join Date: May 2005 Location: Land of Oz Posts: 574	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. __________________ Andrew Bowie Rebreather-friendly Buddy
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