[PSUBS-MAILIST] Calculating Scrubber Residence/Contact Time

[Steve via Personal_Submersibles]

Thanks. That helps. I will be curious to see how the actual test data with me in my sub compares to the calculated values.
Originally I wanted the scrubber capacity to closely match my O2 capacity without needing to refill the media (60hrs.) but decided that wasn"t the best management strategy. A smaller scrubber with spare media is a better use of my space.
Steve

Sent from my Verizon Motorola Smartphone
On Nov 23, 2017 9:48 PM, River Dolfi via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>
> Rick- I don't believe any added humidity from mounting a CO2 sensor at the outlet of your scrubber will cause any issues. 
>
> Everything inside a sub (that doesn't have A/C) should be designed with the expectation that the internal atmosphere will be at or close to 100% RH, and that condensation is to be expected. I've utilized a cheap NDIR CO2 sensor in an enclosure outside and have never had any condensation issues. 
>
> NDIR (Nondispersive infrared) CO2 sensors function as mini spectrometers. They use infrared waves to energize gas molecules, which give off light at particular wavelengths based on the type of gas. Unfortunately, the wavelengths given off by CO2 are rather close to the wavelengths given off by H20, so water vapor would theoretically interfere with a measurement.
>
> For this reason, most (if not all) NDIR CO2 sensors include a humidity sensor so that they can compensate for this interference. Nothing to worry about there.
>
> I say go for it. A properly designed scrubber prior to breakthrough should be outputting close to 0 CO2. If the test is in the sub, just beware that that low reading at the exhaust might not be reflective of the well mixed air in the cabin that you are actually breathing.
>
> Hank- The Sodasorb CO2 absorbents manual states that most anesthesia CO2 scrubbers have a length-to-diameter ratio of 1.5:1. That seems to be the practical limit for designing a scrubber with a pressure drop suitable from being human lung powered. Definitely important if you're building an anesthesia machine or a diving rebreather, but less so if you're building a fan powered mechanical scrubber.
>
> Steve- A method to calculate the pressure drop across an axial scrubber is described in the previously mentioned Sodasorb manual:
>
> "The specific resistance, K, of 4-8 mesh soda lime is of the order of 1 mm of water pressure per centimeter per liter per minute (mmH2)•cm•L-1•min). The pressure drop (P) across an absorber packed correctly, will depend upon the flow rate (V), length (L), specific resistance and effective cross sectional area (A) of the absorber. This relationship may be expressed by the equation:
> P= (KLV)/A
> For an absorber with a soda lime compartment of 2 liters, at a flow of 60 liters per minute, P is less than 1 centimeter (cm) of water pressure if the length of the combined chambers is less than 18 cm and the diameter greater than 12 cm, a ratio of 1.5 to 1. At a flow rate of 100 liters per minute, which is the maximum respiratory flow expected in anesthetized adults, the pressure across the Sodasorb compartment would not exceed 1.5 cm water pressure. These values do not take into consideration the resistance effect of breathing circuit, endotracheal tubes, directional valves, and fittings. In modern equipment, resistance offered by these components usually exceeds that of Sodasorb absorbent."
>
> The maximum pressure for your blower should be listed on it's spreadsheet. I would start with the blower pressure, punch it into that equation to find flow rate, then finding the dwell time should be easy once you know the flow rate.
>
> I've been thinking alot about scrubber design lately. I definitely recommend anyone interested in designing a scrubber to read through that manual. Might try to formalize the dwell time math for axial and radial scrubbers.
>
> Thanks,
>
> -River J. Dolfi
>
> rdolfi7 at gmail.com
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