Hi Jon,
The only way to know the true airflow over the surface is to measure the
flow entering or exiting the unit. The true CFM is not only a function of
the fan rating but also restrictions to flow in a particular scrubber design
and type of fan used (radial/axial).
Where a flow meter when you need one?!
Greg C
-----Original Message-----
From: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of Jon Wallace
Sent: Thursday, August 11, 2011 11:40 AM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Scrubber CFM
Just another variable that needs to be considered as part of the testing
process. Too large airflow and the air will be flowing too quickly for
good CO2 transfer efficiency across the chemical. The fan I used to
convert the ExtendAir canister may be way too big since the lowest
setting is 20 cfm. Might be able to turn it down with a rheostat.
Jon
On 8/11/2011 11:07 AM, Greg wrote:
I was talking with Tom Daley over at Micropore this morning about proper
airflow through their "Extendair" cartridges (specifically the "SR0101C",
which is a popular size used in a rebreathers) He did some quick
calculations and came up with a value of 125 LPM (assuming 70 % efficiency
of the cartridge)to meet the needs of the standard man at rest. I thought
that this sounded surprisingly low- but Tom reminded me that calcium based
scrubbers can easily dry out with too much airflow, specialty if the
ambient
humidity is low.
I then referred back to Phil Nuytten's White paper and noted that he also
specified a fan with a flow rate of only 160 LPM (page 5). 160 LPM
translates to less than 6 CFM- a VERY small fan indeed. Most small CPU
fans
flow 30 CFM or more. Apparently Phil chose this size carefully (why am I
not
surprised?)
Of course, the good news is that a 5 CFM fan also consumes very little
power, which comes in handy in a long duration dive or emergency. Tom also
suggested that a lung powered option might be best in an emergency, not
only
to save power but also because breathing into the canister supplies
additional humidity. Of course, the optimum humidity range for Extendair
cartridges may be different than the range needed using granules.
Greg C
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