A larger volume would provide more dilution. Two people in a Perry 12 I'd be running the scrubberp 10-15 minutes per hour, for instance. When the newbies asked how I knew when to run the scrubber. I told them it was based on lip color: pink, no scrubber; blue, scrubber required. Easy, right?
Vance
-----Original Message-----
From: JimToddPsub@aol.com
To: personal_submersibles
Sent: Sun, Mar 13, 2011 2:54 pm
Subject: Re: [PSUBS-MAILIST] open source scrubber design
Rick,
I'd certainly agree with that; it's one reason George
Kittredge didn't bother with O2 or scrubbers at all, just depended on what was
contained in the volume of the sub. Good reminder.
I haven't yet calculated how long I could operate on test
dives on hull air before activating the O2 or scrubbers. Of course that
has to be determined on shop dives. What I was referring to is
the capacity after activating the LS systems in a relatively small
sub.
Jim
I would think it would as the larger the air
volume for a person to breath, the longer it would take to drop the C02 level
to an unacceptable level before needing to scrub??
Sent: Sunday, March 13, 2011 10:35 AM
Subject: Re: [PSUBS-MAILIST] open source scrubber
design
Rick, would that matter? It seems it would just be
based on the amount of CO2 being generated via respiration. It might
influence the way you set up the circulation system.
Correct?
JT
Cliff,
I noticed in your post below that you
mentioned "30 lbs of Sodasorb = 80 hours of life support for one man" What
internal volume are you basing that on?
Rick
Sent: Sunday, March 13, 2011 10:13 AM
Subject: Re: [PSUBS-MAILIST] open source scrubber
design
Cliff,
1. My first thought was to split the total amount of
Sodasorb into two identical canisters, but then it occurred it might cut
operating costs to have one smaller canister adequate for most
missions and a second larger canister (sealed) plumbed in for longer
missions and long term life support. That way you're not having to
dump as much unused Sodasorb.
2. In sizing canisters for one and two person subs, is
there some relatively constant ratio between diameter and length, or should
the variability just be in the length of the canisters (constant diameter)
when designing for various capacities?
3. What size canister(s) are you contemplating for your
next design?
I like the complete redundancy Alan referenced on the
DW2000, but accessible space can get short in a hurry in very small subs.
Thanks for your comments.
Jim
Emile
When I build my boat, I to did some investigation into
commercially available scrubber systems that would be applicable for
a one man psub. I did not find any that were economically
viable. While I agree with Leander that fundamentally the scrubber is
not complicated, to make an efficient and practical scrubber
takes some work. I went through several iterations on
mine. One of the key items is, as you mention, the fan. ABS
requires 72 hours of emergency life support after your primary mission
time and this has to be able to run off the emergency battery
bank. If you under size the fan to minimize the power requirement to
the fan, then the fan can not generate sufficient differential
pressure across the fan to circulate the contaminated air. This is
particularly true if you use an axial flow fan rather than a radial
centrifugal fan. Axial fans are commonly installed on most PC
computers. They are designed for high flow rate but low head.
I made the mistake of using an axial flow fan on my first iteration of my
scrubber. While it would last 80 hours ( the current load is
very small), the scrubber effectiveness was not great probably because the
blades very stalling. My initial CO2 level would start at about
350 ppm but would build to 5000 ppm (0.5 mole %) over 30
minutes. My second iteration replaced the axial fan with a small
radial centrifugal fan (squirrel cage) which developed four
times the head at about 1/4 of the flow rate. The
current demand was only slightly higher. This worked great while
still meeting the low current requirment.
The scrubber absorbent choice to me is a no brainier. Sodasorb
HP is what is used most often these days for psubs
and re-breathers for divers. It is easy to get and not very
expensive. From a design perspective, after sorting out the
fan, it comes down to how do want to handle the storage of the emergency
Sodasorb. There are three options, 1) cartridges for the primary and
emergency absorbent, 2) scrubber sized to hold the primary and emergency
absorbent 3) scrubber sized for the primary absorbent and spare
Sodasorb stored in bulk requiring refill when primary absorbent becomes
saturated. There are advantages and disadvantages to each
technique. While cartridges are easy to replace in an
emergency situation, the spare cartridges don't pack as
efficiently as bulk absorbent. For a small psub like mine,
this rules them out; I just did not have the room. I note that the
Deepworker uses the option 2) . For small one man psubs, I
think option 2 is probably the best way to go. The
disadvantage to this approach is that for non emergency conditions, you
end of tossing a lot of partially used Sodasorb after each
dive. For a one person sub, this is not an issue but for more
than one, this makes this option not as attractive. Also, because it
is inexpensive, this is not a big deal for a one person sub. This
option provides the least stress in a stressful emergency as the operator
does not have to worry about changing out the absorbent when it becomes
saturated. On my boat, I use option 3 primarily because of
the storage issue. On my next one person boat, I would go with
option 2. Replacing 30 lbs of Sodasorb (80 hours of
life support for one man) is not a big deal given that it simplifies
life.
While the manufactures of these absorbents add a chemical that
turns the absorbent a different color than white when it becomes
saturated, this is not very practical to use as lighting in subs make
it hard to detect the color change. A better detection of a
saturated absorbent is a good CO2 sensor with an audible
alarm.
So to me, it comes down to correctly specifying the fan and how
to package the spare absorbent.
Cliff
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