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----- Original Message -----
From: "Sean T. Stevenson" <cast55@telus.net>
Subject: ambient pressure submersible life support - was Re:
[PSUBS-MAILIST] DeepWorker's scrubbers
Hi, Sean - thanks for all this feedback. Before I go on,
so everyone has a reference point, my intentions are to build a dry, ambient
sub.
There will be no rising water levels (the cabin will be sealed
u/w except for compensating regs and the over-pressure valve) and the attendant
changes in buoyancy or humidity levels (from free surface effects) will be
eliminated. Needless to say, uncontrolled descents or ascents, and
everything that goes with them, will be minimised .
My motivation, especially since my experience with DW, is that
not having a mask glued to my face is a nice feeling. The openness was
just like being in an airplane.
All that having been said, anyone using the ambient concept,
dry or semi-dry, is stuck with all of the constraints you mentioned:
physiological mainly. There will be no decom dives or dive angles much
past horizontal (as in, depth keeping). As a result, there will be no
mixed gas diving of any kind. Anyone used to diving shouldn't have a
problem operating within these windows.
My goal is simply to have an incredible view, be dry and
comfortable, not have to obsess about the technology and cover some real bottom
and get back safely. Depth at this point is not a priority - less than one
or two atm's is fine, maybe three atm's if I feel adventurous. Emergency
egress should be straightforward since I'd likely stay over a bottom limited to
scuba depths. Any transiting would be done on the surface with reserve
tanks full.
It's the life-support aspects that I'm solely concerned with
at this point: if I wear an oral/nasal mask, or clench a reg between my teeth,
problem solved. Running a de-humidifier may be all I need. I could
flush the cabin prior to the dive with bone-dry scuba air and dump my breath
(external to the cockpit) over the side. That alone may eliminate any need
to dehumidify at all since replenishment is from
the dry scuba air.
My question is, of course, how do I achieve DW's sense of
(breathing) freedom in a dry ambient boat? Do I run free-flowing air with
sufficient O2 to keep the cabin fresh - and lose huge volumes as a result?
I think not. The constant hissing and gurgling would drive me nuts;
technology in my face. I'm hoping there's some way I can use the same or
similar system as DW.
If the O2 sensors function properly (running a humidifier
would help) maybe DW's O2 reg could be used after all IF the cabin is built well
enough to withstand at least some depth. If the cabin pressure isn't
constantly shifting second by second, the O2 reg may have a shot at being
accurate. I need to look closer at the DW
O2 reg's concept.
But, from where I sit, a possible
solution to my dilemma is to build the cockpit structure robustly enough to
withstand a pressure change of, say, (arbitrarily for now) half an atm.
That should be enough to allow the DW style O2 reg to activate. Any cabin
pressure AIR compensation should simply be an adjunct to the O2
compensation.
I'd have to zero the O2 reg once cruising depth was reached
and re-zero with occasional depth changes.
In theory.
Rick
===============================================
> Your diluent,
> which is simply a breathing mixture appropriate for the
depth and
> duration of the planned dive, is bled into the cabin at a
constant
> rate which, by design, replaces metabolized oxygen at the
anticipated
> rate of consumption. This avoids the possibility of
"shallow water
> blackout", provided your diluent gas is appropriate for
the entire
> operating depth range, and provided your metabolic oxygen
requirement
> does not exceed the oxygen provided by the passive
diluent addition.
> Since gas is continually being added to the "loop", it
must also
> continually be vented. In a rebreather, this occurs
when you exhale -
> the counterlung expands to reach its maximum volume, and
then any
> excess gas escapes to sea through an ambient-referenced
overpressure
> valve. The constant flow is always operating, but
should you deplete
> the loop volume on your next inhalation, a diluent
addition valve is
> typically actuated to meet the demand - over and above
the passive
> diluent flow. Typically, gas addition in a
semi-closed system is
> manually actuated, with oxygen
> sensor(s) providing PPO2 information to the operator, but
not actually
> requiring actuation in normal operation unless the
diver's workload,
> and thus metabolic oxygen consumption, exceeds the design
oxygen
> addition of the constant flow system. Determining
the optimum design
> flow rate in such a system is thus a tradeoff between
going too low
> and necessitating manual diluent addition to achieve the
necessary
> oxygen content, or going too high and wasting gas due to
the
> semi-closed venting behaviour. With increased
depth, the inspired
> PPO2 will be higher, but will still be limited to that of
the selected
> diluent gas, which should always be appropriate for the
depth. If the
> flow rate is too high, the excess gas simply vents to
sea, rather than
> increasing the inspired PPO2 to a dangerous level.
An ambient
> pressure submersible works the same way, but as the
"counterlung" is
> of constant volume in the submersible, the addition
system needs to
> actuate not on counterlung collapse, but rather on a
reduction of
> pressure within the passenger compartment as referenced
to the ambient
> pressure. If your passenger compartment is
separated from the sea by
> an overpressure valve, you can monitor this pressure
differential
> directly. If your passenger compartment is open to
the sea, then what
> would otherwise be a drop in pressure will instead raise
the water
> level in your sub. This can be monitored and used
to actuate gas
> addition - be aware, however, that a water level change
will affect
> your vessel's buoyancy, so all of these issues need be
considered in concert.
>
> Going a step further to fully closed operation, the fully
closed
> circuit rebreather does not vent gas to sea (except upon
ascent when
> the gas within the loop expands beyond the loop
volume). Instead of
> the constant flow diluent addition, loop volume and
oxygen content are
> controlled separately with two separate gas sources - the
diluent,
> similar to the single gas used in the semi-closed system,
and an
> additional pure oxygen source intended to replace
metabolized oxygen.
> Since the loop gas in a closed-circuit system is not
continually being
> replenished through constant-flow passive addition, the
oxygen content
> in the loop will decrease as the diver breathes, and this
needs to be
> replenished by oxygen addition. Typically, an array
of oxygen sensors
> will monitor the inspired PPO2 for this purpose, and
electronically
> activated valves will control oxygen addition to maintain
the PPO2 at
> a desired setpoint. Closed circuit systems are thus
not as inherently
> safe as their semi-closed circuit counterparts, since
they rely both
> on control electronics, and on the robustness of the
chosen oxygen
> sensors for accurate PPO2 monitoring and control.
They are, however,
> immensely efficient with regard to gas consumption.
The high moisture
> problem that Jay alluded to is just one of a number of
considerations
> that have prompted CCR designers to incorporate
hydrophobic membranes,
> multiple oxygen sensor arrays with voting logic, and
other means of
> increasing reliability of PPO2 monitoring and control
systems. As a
> submersible homebuilder, this is an area that demands
diligence in
> design effort should you choose to implement a fully
closed-circuit
> life support system in an ambient pressure
submersible. The "shallow water blackout"
> problem you mentioned, is the result of ascending with a
loop PPO2
> that is sufficient at the initial depth, but which
decreases
> (typically to
> 0.16 ATA or less) on ascent to a point at which
consciousness cannot
> be sustained. Note that this only occurs when the
oxygen addition
> system either fails or cannot deliver oxygen at a great
enough rate to
> meet the setpoint, or if the inspired PPO2 of the diluent
gas is so
> low as to be inappropriate for use throughout the
operating depth
> range - only encountered when mixed gas diving beyond
typical sport diving limits.
> This is analogous to an open circuit diver ascending on
bottom gas
> instead of switching to a more appropriate gas for the
depth - while
> CCR divers can use the rebreather to their advantage in
optimizing
> inspired
> PPO2 for a given exposure, and to drastically increase it
for
> accelerated decompression on ascent, just as switching to
high oxygen
> mixtures on an open circuit dive would accomplish the
same objective,
> to do this in an ambient pressure submersible requires
sufficient
> diluent gas to flush the "loop" (in fact, the entire
passenger
> compartment
> volume) upon ascent. The required quantity of gas
to accomplish this
> somewhat negates the advantage of using a closed-circuit
life support
> system at all, if you are intent on performing deep
mixed-gas dives in
> your ambient submersible. Consequently, the
applicability of an
> ambient pressure submersible to dives much in excess of
common sport
> diving limits needs to be assessed. As before, the
difference between
> the CCR and the ambient pressure submersible
implementation of this
> life support system is predominately the inflexible
counterlung
> volume, so gas addition would again need to be controlled
on the basis
> of cabin pressure in a closed cabin or waterline in an
open cabin,
> only this time the control scheme must add oxygen as
demanded by the
> PPO2 monitoring / control scheme, or diluent in the event
of only a
> change in commanded
depth.
>
> In either life support system implementation, (and indeed
with
> free-flow systems as well, if the constant gas flow is
sufficient to
> meet oxygen requirements but not for keeping inspired CO2
below
> acceptable levels), a CO2 scrubber must be
implemented. The obvious
> difference between the ambient pressure submersible and
the rebreather
> in this respect is the gas path through the system, as in
a rebreather
> the gas is 100% constrained to pass through the scrubber
on each
> inhalation / exhalation cycle, while in the submersible
consideration
> must be given not only to adequate gas flow through the
scrubber, but
> also to gas flow throughout the cabin, such that gas is
effectively
> circulated and the objective of keeping inspired CO2
levels below acceptable limits is achieved.
>
> As you can surmise from the preceeding discussion, the
implementation
> of a life support system, other than a free-flow or open
circuit
> demand (SCUBA facemask or mouthpiece) system in an
ambient pressure
> submersible is not trivial. Indeed, the challenges
presented in
> implementing such a system in the presence of pressure
variations due
> not only to oxygen metabolism by the occupants but also
due to changes
> in commanded depth of the vessel, necessitate systems
which may be
> much more complex in design and operation than those used
in one
> atmosphere submersibles. As you mentioned, the
simplicity of the
> bellows addition system used by Nuytco Research is
unfortunately not
> applicable to ambient pressure vessels. The
complexity inherent to
> such a life support system, added to the complexity
inherent to
> buoyancy control schemes in the presence of varying cabin
pressure and
> volume (varying internal waterline), added to the risks
associated
> with ambient pressure diving - decompression obligations,
narcosis,
> CNS and pulmonary oxygen toxicity, gas supply
considerations,
> entanglement or entrapment scenarios, unintended depth
excursions or
> runaway ascents, etc., provide a body of reasons why many
participants
> on this list choose to incur additional expense in
construction and
> materials to develop one atmosphere vehicles - apart from
the expense
> and attention in design to achieve the required hull
integrity, they
> are arguably simpler in design and operation than their
ambient pressure counterparts, with significantly greater dive
endurance.
>
> In consideration of the above, my advice to anyone intent
on building
> an ambient pressure submersible, is to use an open
circuit constant
> gas flow life support system which meets or exceeds
actual oxygen
> demand, augmenting this with a CO2 scrubber if necessary,
and limiting
> the vessel in operation to depths and exposure times
which can be
> reasonably achieved through the use of a single breathing
gas suitable
> for the entire operational depth range of the
vessel. Should your
> operational requirements demand the use of mandatory
in-water
> decompression stops, multiple breathing gas switches or
complicated
> control systems to manage
> PPO2 and PPN2, I respectfully suggest that either a one
atmosphere
> submersible, or the employ of surface supplied or
self-contained
> diving techniques where life support is provided
independently of
> systems on a conveyance might be a more appropriate means
of achieving
> your objective than a dry ambient pressure
submersible. YMMV.
>
> -Sean
>
>
> Alan James wrote:
> > Hi Rick,
> > I havn't quite got my head around what happens with
the life support
> > in an ambient, but will comment in case no-one else
does.
> > If you scrub out the CO2 in an ambient you take away
a physiological
> > tool that tells your brain theres too
much
> > CO2 & hence not enough O2. With lack of O2 you
pass out with no
> > warning. You also don't have the added safety
feature of a barometer
> > monitering pressure to indicate a drop in O2 levels.
You can't use
> > the bellows add method of Phil Nuytten wich relies
on changes of
> > cabin pressure to add O2 in an ambient.
> > Jay commented that O2 monitors don't work well in
high moisture
> > environments wich you get in ambients. ( ie at 100ft
you have 4x the
> > moisture as you have 4x the air.) Then you have to
do calculations
> > for your depth re the
> > PPO2 (partial pressure of O2). You may have enough
O2 at depth, but
> > as you approach the surface the O2% can change
dramatically & you
> > can suffer a shallow water black out.
> > You'd probably need to read up on rebreathers &
diving with them to
> > perfect the system.
> > It also depends on how big your cabin is. You might
find that if you
> > have a certain flow of O2 in you'll never run out
for your expected
> > dive duration.
> > Apparently you're at a greater fire risk with higher
levels of O2 in
> > the cabin. So watch that you're wiring insulation is
not of a
> > material that will combust easily.
> > There is a discription of how to build a scrubber on
the psub site
> > http://www.psubs.org/design/ Regards
Alan
> >
> >
> > ----- Original Message
-----
> > *From:* Rick & Marcia
<mailto:empiricus@telus.net>
> > *To:*
personal_submersibles@psubs.org
> > *Sent:* Thursday, October
22, 2009 9:42 PM
> > *Subject:* Re:
[PSUBS-MAILIST] DeepWorker's scrubbers
> >
> > For years I'd been avoiding
the whole idea of scrubbers: too
> > noisy, maintenance,
etc.
> >
> > After my experience with
DeepWorker I do believe I've become a
> > convert. Those fans
were so quiet. And no spider mask on my head
> > or oral/nasal mask glommed
onto my face. I could breathe and talk
> > normally. Sigh.
There was even a nice breeze. It takes some of
> > us a while.
> >
> > To wit . . .
> >
> > [a] Is the DW scrubber
design open to discussion - is it so simple
> > you can share what makes it
work for DW? I'd love to have a
> > design handed to me or be
steered in that direction. I did find
> > something on the Net but it
requires machining. Built originally
> > for a
rebreather.
> >
> > [b] Next: would the
scrubbers be any trouble in a dry-ambient?
> >
> > [c] How's breathing moisture
handled in DW?
> >
> >
> > Rick
> >
>
>
>
>
>
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