Abstract:
A variable volume ratio compound counterlung is disclosed for use with a semi-closed circuit breathing apparatus. The compound counterlung generally comprises a flexible bag member disposed within an outer counterlung member. The flexible bag member and the outer counterlung member are in communication with an exhaled gas area of a breathing loop. The flexible bag member having first and second ends which are attached to said outer counterlung. A pair of depth sensors operatively associated with the flexible bag member are provided to vary the volume of said flexible bag member with changes in depth. The flexible bag member is driven by the outer counterlung to discharge gas stored within the flexible bag member depending on the diver&#39;s respiratory minute volume. The collapsing of said outer counterlung member also returns gas stored within the outer counterlung back into the breathing loop. The volumetric capacity of the inner counterlung is controlled by one or more ambient pressure sensing devices with an outer bellows-type counterlung that drives the inner counterlung&#39;s contents overboard with each breathing cycle. The invention provides semi-closed cycle passive gas addition for recirculating diver breathing systems that is keyed to both respiratory minute volume and depth by making each discharge mass constant relative to the volumetric relationship of the inner and outer counterlungs at the surface, thus making the system far more gas efficient than previous designs.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/034,644, filed Jan. 7, 1997. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a semi-closed circuit passive gas addition breathing apparatus and more particularly to a variable volume ratio compound counterlung used in a rebreathing apparatus. 
     2. Description of Related Art 
     Conventional semi-closed rebreathers operate by delivering a premixed gas from a scuba cylinder through a constant flow regulating device, usually by supplying a regulated gas supply to a changeable orifice. Gas is delivered at a preset rate regardless of depth. The gas being breathed is recirculated, and as the oxygen within the mixture is metabolically consumed, it is hopefully being adequately replaced on a continuous basis with a predetermined continuous flow of oxygen enriched gas. 
     Rebreathers consist of a breathing loop from which the diver inhales and into which the diver exhales. As most of the exhaled gas stays in the breathing loop, rebreathers allow for much greater gas efficiency than open circuit systems. This greater gas efficiency allows for longer duration dives as compared to open circuit systems, or, conversely, requires less gas supply for a dive of equal duration. 
     The breathing loop generally includes a relief valve, scrubber, counterlung, depth equalization regulator, continuous injection system, hoses and a mouthpiece. The relief valve is utilized for dumping or venting excess gas in the breathing loop created by the rebreather on ascent and excess gas which is produced with the use of constant (active) addition systems. The scrubber cleanses the exhaled gas of carbon dioxide. The counterlung or breathing bag allows for the retention of the diver&#39;s exhalation gas. The injection system adds fresh gas to the carbon dioxide cleansed gas in the breathing loop. The depth equalization regulator adds supply mix to the loop to keep pace with depth increases. The hoses are utilized to connect the counterlung and scrubber with the mouthpiece. The mouthpiece is connected to the two hoses and is the point on the breathing loop where the diver exhales and inhales. Typically, two conventional one-way valves are incorporated into the mouthpiece. 
     Rebreathers normally include a harness to strap the unit to the diver, with some units also including a protective case for the various above described components. 
     As stated above, rebreathers generally work by recycling most of a diver&#39;s exhaled breath, which travels through the breathing loop through the scrubber, and is returned to the diver during inhalation. The use of a rebreather allows a diver to remain underwater for a relatively long time as compared to the use of open circuit equipment. 
     Accordingly, rebreathers allow exhaled gas to be cleansed of carbon dioxide and replenished with fresh oxygen for further consumption. A traditional fixed flow (active addition) semi-closed rebreather recycles the gas the diver is breathing, removing excess carbon dioxide from the exhaled gas and replacing it with a measured amount of premixed gas to maintain an oxygen partial pressure in the inspired gas that will continue to support metabolism. 
     There are several previously known types of operating systems for semi-closed circuit rebreathers, including fixed discharge ratio, continuous injection and mechanically pulsed. In the 1970&#39;s, as electronically controlled rebreathers were coming into their own, a fixed discharge ratio counterlung (an inner bellows within an outer bellows) was developed for semi-closed use in Europe. This type of rebreather was coined the first “passive” addition or counter mass ratio system. “Passive” means gas is only added as required to replace gas that has been discharged from the breathing loop by the control mechanism. 
     The “passive” addition system discharged a fixed percentage of each exhalation overboard, thus responding to respiratory minute volume (“RMV”) or work rate. As such, reasonably tight decompression schedules could be computed for semi-closed equipment, eliminating the need for complex electronic oxygen monitoring. 
     Any system keyed to RMV is essentially using the diver as a sensor. The passive system uses a proportional discharge valve or a bellows within a bellows to discharge a fixed percentage of every exhalation overboard. The missing part of the exhalation is made up “passively” by one or two demand regulators on the following inhalation. Excess gas in the breathing loop from reduced ambient pressure is vented off by an overpressure relief valve. The fixed discharged ratio units maintain reasonably steady oxygen fractions in the breathing loop. The counterlung does not have to be purged on normal ascents to prevent hypoxia. 
     One drawback with the fixed discharged ratio semi-closed circuit is that it is not as gas efficient as electronic closed circuit rebreathers or constant flow (active) semi-closed rebreathers due to the fact that gas usage increased with depth similar to open circuit equipment. Furthermore, different diver positions often caused gas to be lost. The increased gas usage limits dive duration at depth as compared to other types of semi-closed units. Thus, despite solving decompression problems the bellow within a bellow system was ultimately abandoned due to its limited dive duration capabilities. 
     The continuous injection system is an active addition system and typically bleeds a fixed flow of single source mixed gas into the breathing loop from a variable or changeable fixed orifice. The flow rate is determined by estimating the diver&#39;s work rate for the intended dive and hopefully ensuring that enough oxygen from the mixed gas supply enters the system to meet anticipated metabolic requirements. Hypoxia is possible if the counterlung is not purged during ascent. Additionally, extended periods of higher than anticipated work loads can also produce hypoxia. 
     The mechanically pulsed semi-closed rebreather is also an active system and uses a bellows counterlung to mechanically drive a ratchet/cam that pulses gas addition valves in approximate response to respiratory minute volume. The gas addition is from a single mixed gas supply and is regulated to provide reasonably tight oxygen fractions in the breathing loop. Excess gas in the breathing loop from additions or reduced ambient pressure is vented off by an overpressure relief valve. However, with this type of unit, there are more single point addition failure possibilities. 
     Accordingly, no prior RMV controlled recirculating breathing system has incorporated a mass-constant discharge capability. Thus, there exists a need for a “passive” gas addition semi-closed circuit rebreather unit which provides for a variable discharge ratio which changes with depth to effect a mass constant discharge ratio (to reduce gas wastage) that is controlled by the diver&#39;s RMV (to make the unit responsive to actual metabolic requirements). It is therefore, to the effective resolution of the aforementioned problems and shortcomings that the present invention is directed. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a variable volume discharge ratio compound counterlung for use with a semi-closed circuit breathing apparatus. The entire breathing apparatus incorporating the compound counterlung provides for a variable discharge ratio semi-closed circuit rebreather unit which does not reduce in gas usage efficiency with depth. The compound counterlung consist of a variable volume discharge inner counterlung driven by and disposed within a weighted bellows (outer counterlung). The inner counterlung geometry is chosen such that there is always provided enough discharge capacity to exceed metabolic addition requirements, regardless of depth. The inner counterlung component arrangement takes advantage of both outer counterlung forces and exhalation pressures to ensure accurate volumetric sizing. 
     The inner counterlung reduces in volume with depth increase, allowing it to discharge exhalation gas inversely proportional to depth. As such, the same amount of mass is always discharged for any given RMV regardless of depth. The shortfall in the diver&#39;s subsequent inhalation is made by conventional redundant addition regulators, associated with the breathing loop. Addition is made when there is zero counterlung volume, thus reducing the gas in the breathing loop that is available to dilute the addition. The other components, which normally make up a rebreather, i.e. canister, scrubber, mouth piece, hoses, etc. can be conventional. 
     The system is keyed to respiratory minute volume, and makes a full and proportional oxygen correction with every breath. Accordingly, the system is reliable for holding steady inspired oxygen fractions, thus making use of standard programmable decompression computers and hard tables practical. 
     The variable discharge ratio makes the ejected portion of every breath mass constant relative to the tidal volume and breathing frequency, regardless of depth. Thus, the unit achieves an equal reclaim rate at depth as at surface. As such, the unit is five (5) times more efficient in gas use than an open circuit unit at the surface, and is twenty (20) times more efficient than an open circuit unit at 4 absolute atmospheres (99FSW). 
     Furthermore, as the present invention discharges part of every exhalation, loss of gas addition results in subsequently shorter volumes of gas available for each inhalation, making an addition failure immediately recognizable and hypoxia highly unlikely. Rational arrangement of the components in the breathing loop make other malfunctions immediately recognizable through other changes in breathing characteristics. 
     The volumetric capacity of the variable volume inner counterlung is controlled by one or more ambient pressure sensing devices. The depth sensing devices allow for pressure preloading to change the rate of inner counterlung volumetric changes relative to ambient pressures. Sensing device pressure envelopes are also provided that act as indicators for surface pressure registration, inner counterlung floods and bacterial growth. The depth sensing envelopes also allow for leak testing of the inner counterlung. 
     The outer bellows-type counterlung drives the inner counterlung&#39;s contents overboard with each breathing cycle. The inner bag control arrangement works in a plane perpendicular to the discharge driving motion, thus allowing for volumetric changes that do not affect the range of collapsing motion during the discharge cycle. 
     The present invention compound counterlung provides for semi-closed cycle passive gas addition for recirculating diver breathing systems that is keyed to both respiratory minute volume and depth by making each discharge mass constant relative to the volumetric relationship of the inner and outer counterlungs at the surface, thus, allowing for superior gas efficiency as compared to prior designs. 
     Some of the features of the present invention include, but are not limited to, the following: 
     (1) Depth sensing devices that allow for pressure preloading to change the rate of inner counterlung volumetric changes relative to ambient pressures; 
     (2) An inner bag control arrangement that works in a plane perpendicular to the discharge driving motion, thus allowing for volumetric changes that do not affect the range of collapsing motion during the discharge cycle; 
     (3) Depth sensing device pressure envelopes that act as indicators for surface pressure registration, counterlung floods and bacterial growth; 
     (4) Inner counterlung geometry that always provides enough discharge capacity to exceed metabolic addition requirements, regardless of depth; 
     (5) Inner counterlung component arrangement that takes advantage of both outer counterlung forces and exhalation pressures to insure accurate volumetric sizing; and 
     (6) A discharge control valve that prevents any discharge during the fill (exhalation) cycle to insure accurate volumetric sizing of the inner counterlung under pressure. 
     Some of the benefits of the present invention include, but are not limited to, the following: 
     (1) Provides the most efficient use of gas possible in a system that is keyed to RMV while still maintaining the tight inspired oxygen fractions associated with passive addition semi-closed breathing systems; 
     (2) Provides for equalization with diving bell environments to extend depth range capabilities; 
     (3) Provides the ability to change inner counterlung volumetric change rates relative to ambient pressures by applying a pressure or vacuum bias to the pressure sensing devices prior to the dive. This allows for the use of mixed gases in specialized diving applications that would not be usable otherwise; 
     (4) Provides for easy identification of pressure sensor leaks or miscalibrations; 
     (5) Provides for easy identification of counterlung leaks; 
     (6) Provides for easy identification of counterlung contaminants; and 
     (7) Provides for safe inspired oxygen fraction levels even if the counterlung proportioning mechanism or depth sensor(s) fail. 
     Accordingly, it is an object of the present invention to provide a variable volume ratio compound counterlung as part of a passive addition semi-closed circuit rebreather which is more efficient in gas usage as compared to prior art counterlungs. 
     It is another object of the present invention to provide a variable volume ratio compound counterlung as part of a passive addition semi-closed circuit rebreather which provides the most efficient use of gas possible in a system that is keyed to respiratory minute volume while still maintaining tight inspired oxygen fractions associated with prior art passive addition semi-closed breathing systems. 
     It is yet another object of the present invention to provide a variable volume ratio compound counterlung as part of a passive addition semi-closed circuit rebreather which provides for equalization with diving bell environments to extend depth range capabilities. 
     It is still another object of the present invention to provide a variable volume ratio compound counterlung as part of a passive addition semi-closed circuit rebreather which provides the ability to change inner counterlung volumetric change rates relative to ambient pressures by applying a pressure or vacuum bias to pressure sensing devices prior to the dive, allowing for the use of mixed gases in specialized diving applications not otherwise usable with prior art devices. 
     It is even still another object of the present invention to provide a variable volume ratio compound counterlung as part of a passive addition semi-closed circuit rebreather which provides for easy identification of pressure sensor leaks or miscalibrations. 
     It is a further object of the present invention to provide a variable volume ratio compound counterlung as part of a passive addition semi-closed circuit rebreather which provides for easy identification of inner counterlung leaks. 
     It is still a further object of the present invention to provide a variable volume ratio compound counterlung as part of a passive addition semi-closed circuit rebreather which provides for easy identification of counterlung contaminants. 
     It is still a further object of the present invention to provide a variable volume ratio compound counterlung as part of a passive addition semi-closed circuit rebreather which provides for safe oxygen fraction levels even if the counterlung proportioning mechanism and/or depth sensor(s) fail. 
     In accordance with these and other objects which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The invention may be better understood by reference to the drawings in which: 
     FIG. 1 is a perspective view of the variable volume ratio compound counterlung in accordance with the present invention; 
     FIG. 2 is a perspective view of the inner bag member and associated depth sensors of the variable volume ratio compound counterlung; 
     FIG. 3 is a perspective view of the inner bag member and associated depth sensors of the variable volume ratio compound counterlung with the mounting plates removed; 
     FIG. 4 is a top view of the inner bag member and associated depth sensors illustrated in FIG.  2  and illustrating the shape of the bag member when the control bellow within the depth sensor is fully compressed; 
     FIG. 5 is a perspective view of the inner bag member and associated depth sensor illustrated in FIG.  2  and illustrating the shape of the bag member when the control bellow within the depth sensor is fully compressed; 
     FIG. 6 a  is a front view of a depth sensor in accordance with the present invention illustrating the sensor bellow in a fully compressed position; 
     FIG. 6 b  is a front view of a depth sensor in accordance with the present invention illustrating the sensor bellow in a fully expanded position; 
     FIG. 7 is a breathing loop schematic for a passive addition semi-closed circuit breathing apparatus incorporating the variable volume ration compound counterlung of the present invention; 
     FIG. 8 is an exploded perspective view of one embodiment of the rebreathing unit incorporating the variable volume ratio compound counterlung also illustrating a protective case which can be utilized to protect the various components of the rebreathing unit; 
     FIG. 9 perspective view of the variable volume ratio compound counterlung in accordance with the present invention; 
     FIG. 10 is a side view of the variable volume ratio compound counterlung in accordance with the present invention; 
     FIG. 11 is a perspective view of the inner bag member and associated depth sensors of the variable volume ratio compound counterlung; and 
     FIG. 12 is a perspective view of the inner bag member and associated depth sensor illustrated in FIG.  11  and illustrating the shape of the bag member when the control bellow within the depth sensor is fully, therefore compressed. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As seen in the drawings a compound counterlung is provided and is generally designated as reference numeral  20 . The compound counterlung consists of an inner counterlung  30  and an outer counterlung  60 . Inner counterlung  30  includes a flexible bag  32  and comprises a depth controlled variable volume inner bag system which is enclosed within, attached to and driven by outer counterlung  60 . Outer counterlung includes an accordion-like shaped bellow body member  62 . 
     Exhaled gases enter a manifold inlet  80  through a tube  81  (primary gas path), which is in communication with an exhaled gas hose or conduit of a breathing loop, such as breathing loop  200  (FIG.  7 ), and pass through into outer counterlung  60  through outer tube  82  (first auxiliary gas path) and into inner counterlung  30  through non-return valve  84  and inner tube  86  (second auxiliary gas path). The components of manifold inlet  80  (tube  81 , tube  82  and tube  86 ) can be transparent. 
     A flexible bag member  32  which is capable of retaining gas within its walls is preferably provided for the inner counterlung system  30 . A pair of inner counterlung plates  36  are attached to the outer surface of respective opposite ends of flexible bag member  32  by conventional means. Likewise a pair of outer counterlung plates  66  are attached to the inner surface of respective opposite ends of outer bellow members  62 . Plates  36  and  66  are provided for attaching inner counterlung  30  to outer counterlung  60 , as respective plates  36  and  66  mate with each other. 
     As the inner counterlung  30  is attached to outer counterlung  60  it is filled both by exhalation pressure and suction created by the expanding outer counterlung  60 . Ambient gas or water is prevented, by a non-return valve  88 , from entering inner counterlung  30  through discharge outlet  90 . 
     At the surface, regardless of its volume, the exhalation gas is distributed between inner and outer counterlungs  30  and  60 , respectively, in the ratio determined by the physical volumetric maximum capacities of counterlungs  30  and  60 , in relation to each other. This ratio is typically from 20%/80% to 25%/75%. As an example, with a ratio of 25%/75%, a two (2) liter exhalation by the user would enter compound counterlung  20  with 1.5 liters passing into outer counterlung  60  and 0.5 liter passing into inner counterlung  30 . 
     After exhalation, on the following inhalation by the user (diver), the contents (gas) is drawn out of outer counterlung  60  through outer tube  82  and manifold inlet  80  where the contents re-enters the breathing loop. The drawing out process causes the collapse of bellow member  62  which in turn squeezes the attached bag member  30  to drive the contents of bag member  30  out through inner tube  86 . Non-return valve  84  prevents the drawn out contents of bag member  30  from re-entering the breathing loop. As such, the drawn out contents (gas) is discharged through non-return valve  88  and discharge outlet  90  into the ambient air or water. 
     At the end of the inhalation, approximately twenty-five (25%) percent of the gas volume needed to fill the diver&#39;s lungs is missing. This gas is made up from a supply source by a conventional demand regulator, an additional valve  63  tripped by the collapsed outer counterlung  60  (FIG.  7 ), or both. There is enough oxygen present in the new gas to meet metabolic demands regardless of the diver&#39;s exercise level, as respiratory minute volume (lung ventilation, more breaths per minute or more tidal volume per breath or both) will change in direct response to oxygen needs. Thus, the present invention uses the diver him or herself as an oxygen sensor and makes a full correction in the inspired oxygen fraction with every breath. 
     At the surface, inner counterlung  30  expands to full capacity, as it has not been subjected to any control of its capacity by depth sensors  100   a  and  100   b . Depth sensors  100   a  and  110   b  are each provided with volume control components. Inner counterlung  30  and the pressure envelope provided by sensor housing  102 , first and second housing caps  104  and  106 , and a plurality of flexible tubes  120  are all sealed as a common pressure enclosure  101 . Sensor housing  102  can be transparent (FIG. 6) to allow monitoring of the bellow member  110 /control rod  112  disposed within. Sensor housing  102  being transparent also allows for detection of water and/or organic growth in housing  102  which normally indicates water and/or organic growth in inner counterlung  30 . 
     As the diving depth (ambient pressure) increases, the pressure in common pressure enclosure  101  and compound counterlung  20  increases proportionally, with gas additions from an underpressure or demand valve being made periodically when counterlung  20  collapses fully to provide an adequate volume of gas to maintain pressure equalization between counterlung  20  and the ambient. 
     A control bellow  110  is disposed within sensor housing  102  and is affixed at one end to first housing cap  124  and to control rod  112  at its outer end  111 . Control bellow  110  constitutes an independent pressure enclosure that can be adjusted with pressure or vacuum preloads at the surface through adjustable valve  130  to retard or advance inner counterlung  30  control as needed for specialized diving conditions, such as deep bell operations. 
     During normal operation, control bellow  110  is equalized with the air at the surface by opening and closing valve  130 . This opening and closing of valve  130  calibrates the interior pressure of sensor housing  102  to ambient pressure at the surface and provides a zero reference point where the traveling end  111  of bellow  110  aligns with a corresponding point on transparent sensor housing  102 . When pressure outside control bellows  110  increases to equalize with a greater ambient pressure (depth), control bellow  110  shortens proportionally through range  103 , shown on sensor housing  102  (FIG. 6 a ), to equalize its interior pressure with the pressure around it. 
     As one end of bellow  110  is attached to first housing cap  104 , only end  111  moves to provide for equalization. As stated above a first end of control rod  112  is affixed to control bellow  110  at movable end  111 . Thus, the movement of end  111  also proportionally draws inward control rod  112  to produce a reduction of control rod  112 &#39;s extension beyond flexible tube  120  through a range  105  (FIG. 6 a ). Thus, the amount of reduction of the extension of control rod  112  beyond tube  120  is in direct proportion to the motion of control bellow  110  through range  103 . 
     The opposite ends of each control rod  112  is attached to a respective control arm  150 . Control arms  150  are affixed to flexible sides of bag member  32  at their narrowest point  31  (adjacent the point on each side wall  43  where end walls  45  meet) by conventional means. Control arms  150  run along and adjacent to their respective side walls  43 . The narrowest portion of inner counterlung  30  is attached to outer counterlung  60  at a flap  40 . This attachment at flap  40  prevents control arms  150   a  and  150   b  from being drawn toward one another at the flap  40  attachment point. However, control arms  150   a  and  150   b  are allowed to move with the rest of the mechanism at the wide portion  39  of inner counterlung, where control rods  112   a  and  112   b  are attached to control arms  150   a  and  150   b , respectively. 
     Ends  122  of each flexible tube  120  are affixed to respective plates  36 . Plates  36  are attached to outer counterlung bellow plates  66 , allowing flexible bag member  32  to move as a compound unit  20  with expansion and contraction of outer counterlung  60 . The static sides  35  of inner counterlung  30  (bag member  32 ) are cemented to their respective plate  36  at area  37  (FIG.  3 ). This allows redefinition of inner counterlung  30  (bag member  32 ) side geometry solely by control arms  150 , while allowing the plate attachment sides of bag member  32  to follow the motion of counterlung plates  36 , which are controlled by outer counterlung  60 . 
     In use, as depth increases, collapsing control bellows  110  of each depth sensor  100  pulls its corresponding control rod  112  inward. This inward movement causes control arms  150 , which are attached to respective control rods  112 , to move toward the opposite side of inner counterlung  30  in conversely parallel motions to avoid interference with one another, which changes the shape of flexible bag member  32 , and simultaneously varies the volume of flexible bag member  32 . FIGS. 4 and 5 illustrate the shape of inner counterlung  30  with a maximum range of travel of bellows  110  (fully collapsed—FIG. 6 a ) . The shape of inner counterlung  30  in FIGS. 4 and 5 allows for substantial volume control without restricting the travel of inner and outer counterlung plates, which must be unimpaired to respond to varying tidal volumes. The discharged volume proportion determined by controlling inner counterlung  30  is inversely proportional to depth changes, thus making the discharge mass constant relative to respiratory minute volume. 
     When the maximum travel of the volume proportioning control mechanism (control bellows  110 ) has been reached (i.e. approximately 13 atmospheres absolute), control arms  112  will remain fully drawn in. The shape of bag member  32  remains the same due to the fact that the position of control bellows  110 , control rods  112  and control arms  150  remains constant after maximum bellow  110  travel has occurred. Thus, inner counterlung  30  (flexible bag member  32 ) will continue to eject the amount of gas that was being ejected when mechanism (bellow  110 ) travel ceased. At this point, gas use efficiency is reduced with further increases in depth, unless control bellow  110  is preloaded with pressure through valve  130  prior to the dive or equalized in a bell or chamber at depth through valve  130 , thus shifting the range of mechanism travel. Mechanism (control bellow) travel can be reduced for use with supply gases containing high fractions of oxygen in shallower water by applying a vacuum bias to control bellows  110  through valve  130  prior to the dive. 
     The present invention works in reverse for depth decreases as that described above for depth increases. Thus, inner counterlung  30  is restored to volumetric capacities that automatically assure enough passive gas addition to meet metabolic requirements regardless of depth. 
     After the dive, gas tight integrity of control bellow  110  on each depth sensor  100  can be performed by determining if traveling end  111  of control bellow  110  aligns with a pre-dive registration mark  107  on transparent housing  102 . Water or organic growth in inner counterlung  30  can be detected by the presence of either or both in housing  102 . Loss of pressure integrity in inner counterlung  30  or non-return valve  84  can be detected by blocking discharge outlet  90  and applying a small amount of gas pressure to the system through valve  132 . Loss of vacuum integrity in inner counterlung  30  or non-return valve  88  can be detected by blocking manifold inlet  80  and applying a small vacuum to the system through valve  132 . 
     FIG. 7 illustrates one embodiment for a breathing loop  200  incorporating variable volume ratio compound counterlung  20 . Breathing loop  200  generally consist of a conventional mouthpiece  201  incorporating conventional one-way valves  202  and  203 , a conventional exhaled breath path (hose)  204 , compound counterlung  20  in accordance with the present invention, a conventional scrubber (canister)  206 , one or more conventional regulator(s)  208  and a conventional inhaled breath path  210 . The configuration of breathing loop  200  is shown by way of example and should not be considered limiting. 
     Accordingly, other breathing loop configurations incorporating compound counterlung  20  can be utilized and are considered within the scope of the invention. Furthermore, compound counterlung  20  can be utilized with other types of rebreathing apparatuses. 
     Additionally, a conventional harness can be provided to strap the rebreathing unit to the diver. A protective case  300   a  and  300   b  (FIG.  8 ), with attachment straps  304  affixed to the outer surface of the case, can also be provided for the rebreathing unit. The case provides protection to the various components of the rebreathing unit. 
     Accordingly, the compound counterlung of the present invention provides many advantages including the following (1) utilizing a variable volume control device to automatically achieve mass constant passive gas addition at varying depths; (2) utilizing a pressure differential control mechanism to change the volumetric relationship between the two counterlung elements; (3) changing the volumetric capacity of one or both counterlung elements by reducing its ability to expand in one axis while retaining full movement in another axis; (4) utilizing a variable volume control device that provides for external verification of gas tight integrity in the inner counterlung and/or related non-return valves during positive and/or negative pressure loads; (5) providing for indication of interior conditions by using a transparent housing element that is part of an externally mounted variable volume control device; (6) utilizing an external proportioning control device that is atmospherically common to any part of the interior of the counterlung to prevent loss of breathing loop integrity if the pressure sensing element fails; (7) providing for external equalization or pressure/vacuum bias of the depth sensing element of the proportioning control system; (8) providing for external verification of gas tight integrity and/or pressure/vacuum preload condition of the pressure sensing elements; (9) utilizing a remote pressure sensing element that transfers proportioning control to the interior of either counterlung through a flexible control rod moving in a flexible guide; (10) providing a compound counterlung which utilizes both a bellow and a variable volume bag element; (11) linking the discharge of the variable volume inner bag to the motion of an external bellow; (12) providing an inner bag which is controlled by creating overlapping folds in the bag material to achieve greater volumetric capacity reduction; (13) providing a compound counterlung which prevents total inner counterlung volumetric capacity reduction by limiting the travel of the control mechanism at one end of the bag; and (14) providing a compound counterlung that uses both a physical link between the inner and outer counterlungs and exhalation pressure to help expand the inner counterlung to the limits dictated by a proportioning control mechanism. 
     The operation of the present invention will be discussed below. As stated above, the compound counterlung consists of a depth controlled variable volume inner bag  30  enclosed within, attached to and driven by an outer bellows  60 , which in addition to the above figures is also illustrated in FIGS. 9,  10 , and  11 . Exhaled gas enters the manifold inlet  190  and passes into the outer counterlung  60  through tube  193  and the inner counterlung  30  through tube  191  and non-return valve  195 . No discharge to ambient through discharge control valve  240  can occur because the positive pressure of the exhalation expanding the bellows  60  is transmitted to an elastomeric discharge control diaphragm  241  through tube  242  and sealing chamber  243 . 
     This pressure forces the diaphragm against discharge outlet  244  with considerable hydraulic advantage because the diaphragm is a much larger diameter than the discharge outlet. The inner counterlung  30  is filled both by exhalation pressure and suction created by the expanding outer counterlung  60  because it is attached at outer counterlung plates  205  and  206 . Ambient gas or water is prevented from entering inner counterlung  30  through discharge outlet  244  by non-return valve  245 . 
     At the surface, regardless of its volume, the exhalation gas is distributed between the two counterlungs in the ratio determined by the physical volumetric maximum capacities of the counterlungs in relation to one another, typically 20%/80% of 25%/75%. Using the latter ratio, a 2 liter exhalation would enter the compound counterlung with 1.5 liters going to the outer counterlung  2  and 0.5 liter going to the inner counterlung  30 . 
     On the following inhalation, the contents are drawn out of the outer counterlung  60  through tube  193  and manifold inlet  190 . The contents of the inner counterlung are prevented from reentering the breathing loop by non-return valve  195 . The negative pressure created by the inhalation within the bellows  60  is transmitted to the discharge control diaphragm  241  through tube  242  and sealing chamber  243 , lifting it away from the discharge outlet  244  and allowing the contents of the inner counterlung to be discharged to the ambient environment through diffuser  246 . The collapsing outer counterlung  60  squeezes the inner counterlung  30  and drives its contents overboard through the discharge control valve  240 . 
     At the end of the inhalation, 25% of the gas volume needed to fill the diver&#39;s lungs will be missing. This gas is made up from a supply source by an addition valve tripped by the collapsed outer counterlung. There is enough oxygen present in the new gas to meet metabolic demands regardless of the diver&#39;s exercise level, because respiratory minute volume (more breaths per minute or more tidal volume per breath or both) will change in direct response to metabolic oxygen needs. This type of system is using the diver himself as an oxygen sensor and makes a full correction in the inspired oxygen fraction with every breath. 
     At the surface, the inner counterlung  30  is able to expand to its full capacity (see FIG. 11) because it has not been subjected to any control of its capacity by the depth sensors  102  and their related volume control components. The outer counterlung (bellows) and the pressure envelope provided by the sensor housings, housing caps, and flexible tubes are all sealed as a common pressure enclosure. As the diving depth (ambient pressure) increases, the pressure in the aforementioned enclosure and the entire compound counterlung increases proportionately (see FIG.  6 ). 
     Control bellows  110  is affixed to cap  124  at one end and flexible control rod  122  at the other end, and constitutes a completely separate pressure enclosure that can be adjusted with pressure or vacuum preloads at the surface through valve  130  to retard or advance inner counterlung control as needed to specialized diving conditions, such as deep bell operations. For normal operation, the control bellows is equalized with the air at the surface by opening and closing valve  130 . This calibrates the interior pressure to ambient pressure at the surface and provides a zero reference point where the traveling end of the bellows aligns with a corresponding place on transparent sensor housing  102 . 
     When the pressure outside control bellows  110  increases to equalize with a greater ambient pressure (depth), the control bellows shortens proportionately through range  103  to equalize its interior pressure with the pressure around it. Because one end of the control bellows is fixed to cap  124 , only the other (traveling) end can move to allow equalization, drawing control rod  112  with it and producing a reduction of the control rod&#39;s extension beyond the end of flexible tube  120  through range  105  in direct proportion to the motion of the control bellows through range  103 . 
     The ends of control rods  250  and  251  are affixed to control arms  150  which are in turn affixed to the flexible sides of inner counterlung  30  at their narrowest point  31 . The narrowest portion of the inner counterlung is in turn affixed to the outer counterlung at flap  40 , preventing the control arms to move with the rest of the mechanism at the other end. The inboard ends of the flexible tubes  120  are affixed to the outer counterlung lung bellows plates so that they all move as a unit with counterlung expansion and contraction. The static sides of the inner counterlung are sealed at their plates by port fittings at  260  and  261  to allow redefinition of the inner counterlung side geometry with control rods  250  and  251 , while allowing the other (static) sides to follow the motion of the counterlung plates. 
     As depth increases, the collapsing control bellows  110  (see FIG. 6) of each sensor unit draws its corresponding control rod  112 , at  250  and  251 , and control arm  150  (see FIG. 12) toward the opposite side of the inner counterlung in conversely parallel motions to avoid interference with one another, with the maximum range of travel producing the inner counterlung shape shown in FIG.  12 . This shape allows for substantial volume control without restricting counterlung plate travel, which must be unimpaired to respond to varying tidal volumes. The discharged volume proportion determined by the controlling of the inner counterlung is inversely proportional to depth changes, thus making the discharge mass constant relative to RMV. 
     After maximum travel of the volume proportioning control mechanism has been reached (typically at around 13 atmospheres absolute), the inner counterlung will continue to eject the amount of gas that was being ejected when mechanism travel ceased, thus reducing gas use efficiency with further depth increase, unless the control bellows  110  (see FIG. 6) has been preloaded with pressure through valve  130  prior to the dive or equalized in a bell or chamber at depth through the same valve, thus shifting the range of mechanism travel. Mechanism travel can be reduced for use with supply gases containing high fractions of oxygen in shallower water by applying a vacuum bias to the control bellows  110  through valve  130  prior to the dive. 
     The proportioning mechanism and compound counterlung will work in reverse during depth decreases, restoring the inner counterlung to volumetric capacities that automatically assure enough passive gas addition to meet metabolic requirements regardless of depth. 
     After the dive, gas tight integrity of the control bellows  110  (see FIG. 6) is verified on each depth sensor by seeing if the traveling end of the bellows aligns with the pre-dive registration mark  107  on the transparent housing  102 . Water or organic growth in the inner counterlung can be detected by the presence of either or both in the same housing. 
     Loss of pressure integrity in the inner counterlung  30  or non-return valve  195  (see FIG. 9) can be detected by blocking the discharge outlet and applying a small amount of gas pressure to the system through valve  132  (see FIG.  6 ). Loss of vacuum integrity in the inner counterlung  30  or non-return valve can be detected by blocking manifold inlet  190  and applying a small vacuum to the system through valve  132 . 
     Applicant also incorporates by reference the disclosure of its co-pending application entitled Balanced Breathing Loop Compensating Resistive Alarm System and Lung Indexed Biased Gas Addition for any Semi-Closed Circuit Breathing Apparatus and Components and Accessories Therefor which was filed on Jan. 6, 1998. 
     The instant invention has been shown and described herein in what is considered to be the most practical and preferred embodiment. It is recognized, however, that departures may be made therefrom within the scope of the invention and that obvious modifications will occur to a person skilled in the art.