In consideration of life support in an ambient pressure submersible operating in a mode other than the simple free-flow described above, such a system is functionally no different than that in a diving rebreather, except for three important considerations:
1) The loop volume is relatively immense - the counterlung(s) of the rebreather are replaced by the interior gas volume in the passenger compartment 2) The counterlung is not a variable volume as in a rebreather, but rather fixed as the interior volume of the passenger compartment, and 3) In the absence of facemask / mouthpiece breathing gas delivery, the path of exhaled gas is not constrained such that it must pass through the scrubber before re-inhalation.
Life support systems in an ambient pressure submersible are analogous to rebreathers in that they can replicate either semi-closed or fully-closed operation. In the former case, the semi-closed rebreather is essentially just a gas extension device. 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 <mailto: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 aconvert. 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 simpleyou 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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