[PSUBS-MAILIST] Calculating Scrubber Residence/Contact Time

[River Dolfi via Personal_Submersibles]

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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