Abstract:
An oxygen concentrator system with altitude compensation includes at least one oxygen concentrator sub-system and a plenum subsystem. The at least one oxygen concentrator sub-system produces oxygen-enriched air which is outputted to both the oxygen concentrator system output and to a plenum chamber within the plenum subsystem. The plenum chamber is trickle charged with the oxygen-enriched air when the at least one oxygen concentrator sub-system produces an excess amount of oxygen-enriched air. Should the demand for oxygen-enriched air exceed the capability of the at least one oxygen concentrator sub-system, additional oxygen-enriched air is provided by the plenum chamber until such time that the capability of the at least one oxygen concentrator sub-system exceeds the demand for oxygen-enriched air. At that time, oxygen-enriched air is no longer provided by the plenum chamber, but rather the plenum chamber is again trickle charged. A monitor/controller having an absolute pressure transducer controls the cycle times of the oxygen concentrator subsystem in accordance with an ambient barometric pressure measured by the absolute pressure transducer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present invention is related to co-pending application entitled “OXYGEN CONCENTRATOR SYSTEM”, Ser. No. ______, filed in the U.S. Patent and Trademark Office concurrently with the present application. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to oxygen concentrator systems, and more particularly, to a patient ventilator oxygen concentrator system having altitude compensation to improve performance at higher altitudes.  
           [0004]    2. Description of the Related Art  
           [0005]    Many medical applications exist that require either oxygen-enriched air or medical grade air. Both are widely used in respiratory care treatments, for example. Furthermore, both oxygen-enriched air and medical grade air are used to power various pneumatically driven medical devices.  
           [0006]    Hospitals and other medical care facilities have a need for both oxygen-enriched air and medical grade air. In military hospitals and in hospitals in Europe, for example, these needs may be met by using oxygen concentration systems to provide oxygen-enriched air and by using a filtration system for providing medical grade air. On the other hand, hospitals and other medical care facilities in the United States often use high-pressure gas systems or liquid oxygen to gaseous conversion systems to provide oxygen-enriched air.  
           [0007]    Commonly used oxygen concentration systems often employ a pressure swing adsorption (PSA) process to remove nitrogen from a given volume of air to produce oxygen-enriched air. Such a process is disclosed in U.S. Pat. No. 4,948,391 to Noguchi and this patent is incorporated herein by reference in its entirety.  
           [0008]    In such oxygen concentration systems, as the plenum pressure is increased, the product flow, that is, the oxygen-enriched air, is decreased and the oxygen concentration increased. Accordingly, at low plenum pressures, the oxygen concentration of the oxygen-enriched air may be insufficient and at high plenum pressures the product flow output may be insufficient.  
           [0009]    The co-pending related application discloses an oxygen concentration system which obviates many of the disadvantages noted above. However, the oxygen concentration system of the co-pending related application suffers from performance degradation at higher altitudes.  
         SUMMARY OF THE INVENTION  
         [0010]    It is an object of the present invention to provide an oxygen concentrator system which utilizes at least one oxygen concentrator subsystem having altitude compensation and a plenum to provide an oxygen-enriched air output.  
           [0011]    It is a further object of the present invention to provide an oxygen concentrator system as above and including a plenum charging system to meter and to control the flow of oxygen enriched air between the at least one oxygen concentrator subsystem and the plenum and to allow the flow of oxygen enriched air only from the at least one oxygen concentrator subsystem to the plenum.  
           [0012]    It is another object of the present invention to provide an oxygen concentrator system as above and further including a discharging check valve to selectively allow the plenum reserve capacity to flow out only during a high demand oxygen flow.  
           [0013]    It is yet another object of the present invention to provide an oxygen concentrator system as above and further including a plenum bypass valve to make the transient response faster and to avoid overdrawing the at least one oxygen concentrator subsystem so as to keep the oxygen concentration of the oxygen-enriched air above a predetermined minimum value.  
           [0014]    These and other objects of the present invention may be achieved by providing an oxygen concentrator system with altitude compensation, the system comprising: a system air inlet to receive supply air; at least one system outlet to output oxygen-enriched air; at least one oxygen concentrator subsystem comprising a pair of oxygen PSA (Pressure Swing Adsorption) beds and including an input to receive supply air from the system air inlet and an output to output oxygen-enriched air to the at least one system outlet; a plenum and a plenum charging system located between the output of the at least one oxygen concentrator subsystem and the at least one system outlet, the plenum charging system selectively enabling oxygen-enriched air to flow from the at least one oxygen concentrator subsystem to the plenum; an optional plenum bypass valve to selectively bypass the plenum so as to enable oxygen-enriched air to flow from the at least one oxygen concentrator subsystem to the at least one system outlet; an absolute pressure transducer to provide an electrical signal indicative of a measured ambient barometric pressure; and a monitor/controller to receive the electrical signal from the absolute pressure transducer and to control cycle times of the pair of oxygen PSA beds based on the measured ambient barometric pressure.  
           [0015]    The foregoing and other objects may be achieved by providing a method of increasing oxygen concentration, the method comprising: receiving supply air from a system air inlet at an input of at least one oxygen concentrator subsystem comprising a pair of PSA (Pressure Swing Adsorption) oxygen beds and outputting oxygen-enriched air to at least one system outlet; selectively enabling oxygen-enriched air to flow from the at least one oxygen concentrator subsystem to the plenum; optionally selectively bypassing the plenum to enable oxygen-enriched air to flow from the at least one oxygen concentrator system to the at least one system outlet; measuring ambient barometric pressure and providing an electrical signal indicative of the measured ambient barometric pressure; and controlling cycle times of the pair of PSA oxygen beds with a monitor/controller based on the signal representative of the ambient barometric pressure.  
           [0016]    The foregoing and a better understanding of the present invention will become apparent from the following detailed description of an example embodiment and the claims when read in connection with the accompanying drawings, all forming a part of the disclosure of this invention. While the foregoing and following written and illustrated disclosure focuses on disclosing an example embodiment of the invention, it should be clearly understood that the same is by way of illustration and example only and that the invention is not limited thereto. This spirit and scope of the present invention are limited only by the terms of the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The following represents brief descriptions of the drawings, wherein:  
         [0018]    [0018]FIG. 1 is a pneumatic diagram of a patient ventilator oxygen concentrator system in accordance with an example embodiment of the invention disclosed in the co-pending related application.  
         [0019]    [0019]FIG. 2 is a simplified pneumatic diagram of the patient ventilator oxygen concentrator system of FIG. 1.  
         [0020]    [0020]FIG. 3 is a timing diagram illustrating the timing cycles for the oxygen beds of FIG. 1.  
         [0021]    [0021]FIG. 4 is a timing diagram illustrating the synchronization between the oxygen beds and air beds of FIG. 1.  
         [0022]    [0022]FIG. 5 is a simplified pneumatic diagram of the patient ventilator oxygen concentrator system in accordance with an example embodiment of the present invention.  
         [0023]    [0023]FIGS. 6 and 7 respectively illustrate oxygen bed timing diagrams for the low altitude and high altitude cases.  
         [0024]    [0024]FIG. 8 is a graph illustrating a comparison in the flow performance of the oxygen concentrator system in accordance with an embodiment of the co-pending related application and the oxygen concentrator system in accordance with an example embodiment of the present invention versus altitude. 
     
    
     DETAILED DESCRIPTION  
       [0025]    Before beginning a detailed description of the subject invention, mention of the following is in order. When appropriate, like reference numerals and characters may be used to designate identical, corresponding, or similar components in differing drawing figures. Furthermore, in the detailed description to follow, example sizes/models/value/ranges may be given, although the present invention is not limited thereto. Still furthermore, any clock or timing signals in the drawing figures are not drawn to scale but rather, exemplary and critical time values are mentioned when appropriate. When specific details are set forth in order to describe example embodiment of the invention, it should be apparent to one skilled in the art that the invention can be practiced without, or with variations of, these specific details. Lastly, it should be apparent that differing combinations of hard-wired control circuitry and software instructions may be used to implement embodiments of the present invention, that is, the present invention is not limited to any specific combination of hardware and software.  
         [0026]    As noted above, while the patient ventilator oxygen concentrator system disclosed in the co-pending related application offers numerous advantages over prior art concentrator systems, it nevertheless has a problem in that its performance is degraded at higher altitudes, particularly above 6000 feet. By the addition of an absolute pressure transducer, the present invention enables the oxygen concentrator system to maintain its performance at higher altitudes, namely, between 6000 and 13,000 feet.  
         [0027]    [0027]FIG. 1 is a pneumatic diagram of a patient ventilator oxygen concentrator system in accordance with an example embodiment of the invention disclosed in the co-pending related application and FIG. 2 is a simplified pneumatic diagram of the patient ventilator oxygen concentrator system of FIG. 1. The following discussion refers both to FIG. 1 and FIG. 2.  
         [0028]    As illustrated in FIG. 1, the oxygen concentrator system  100  includes three main elements, namely, a plenum system  30 , a front panel assembly  40 , and a bed module  50 . A fourth element of the oxygen concentrator system  100  includes a monitor/controller  200  and input/output electrical panel  210  having switches and indicators and a display. For simplicity, the fourth element of the oxygen concentrator system has been omitted from FIG. 1 but is illustrated in FIG. 2.  
         [0029]    As illustrated in FIG. 1, supply air is input into the plenum system  30 . Relief valve RV 1  is provided to protect the system from overpressures. Similarly, relief valves RV 2 -RV 4  are also included in the system to protect against overpressures. After passing through filters FLTR 1  and FLTR 2 , and pressure regulator REG 1 , the supply air is fed to solenoid valves SV 1 , SV 2 , and SV 7 .  
         [0030]    The three two-way solenoid valves SV 1 , SV 7 , and SV 2  respectively control the inputting of the supply air to the medical air modules AIR- 1  and AIR- 2  and to the oxygen PSA modules O 2 - 1  and O 2 - 2 , O 2 - 3  and  02 - 4  of the bed module  50 . Each of the medical air modules AIR- 1  and AIR- 2  includes its own two-way solenoid valve SV 12  and SV 13  which allows the supply air to selectively enter and exit respective air beds  1  and  2 .  
         [0031]    Similarly, each of the oxygen and PSA modules O 2 - 1  to O 2 - 4  includes its own three-way solenoid valve SV 8 -SV 11  which allows the supply air to selectively enter and exit oxygen beds  1 - 4 . The other connection of all of the two-way solenoid valves SV 8 -SV 13  are connected together to a muffler MUF whose output is connected to an exhaust output of the plenum system  30 . Orifices ORF 5 - 0 RF 7  are respectively disposed between oxygen beds  1  and  2  and between oxygen beds  3  and  4  and between air beds  1  and  2 . Check valves CV 1 -CV 6  are respectively connected to the air beds  1  and  2  and the oxygen beds  1 - 4 .  
         [0032]    The output of air beds  1  and  2  are connected via check valves CV 1  and CV 2  to serially connected filters FLTR 3  and FLTR 4  whose output is in turn connected via solenoid valve SV 6  and regulator REG 4  to a medical air line which is connected to the front panel assembly  40 . A source of backup medical air, for example, a compressed air tank, is connected to the solenoid valve SV 6  so as to provide a continuous source of medical air should the oxygen concentrator system fail.  
         [0033]    Various monitoring devices, such as: a carbon monoxide monitor  120  connected to the medical air line via the orifice ORF 4  and having an output connected to a vent, a dewpoint monitor  130  connected to the medical air line, the relief valve RV 2  connected to the monitor air line, a pressure switch PSW 2  for detecting a low-pressure in the medical air line, and a gauge G 3  located on the front panel assembly  40  to indicate the actual medical air line pressure, have been provided.  
         [0034]    The medical air line is connected to a solenoid valve SV 5  so as to be selectively connected to an oxygen sensor  140  which includes a regulator REG 5  to control the pressure therethrough. The medical air line is also connected to a manifold having 4 valves V 5 -V 8  whose outputs are respectively connected to AIR OUT  1 - 4 .  
         [0035]    The outputs of oxygen enriched air beds  1  and  2  are connected together to orifice  0 RF 1  while the outputs of oxygen enriched air beds  3  and  4  are connected together to orifice  0 RF 2 . The outputs of orifice ORF 1  and orifice  0 RF 2  are connected together to the plenum  110  via back pressure regulator REG 2  and filter FLTR 5 . The output of the plenum  110  is connected via solenoid valve SV 4  and regulator REG 3  to an oxygen line on the front panel assembly  40  and via a filter FLTR 6  and regulator REG 5  to a low-pressure oxygen line on the front panel assembly  40 .  
         [0036]    The oxygen line on the front panel assembly  40  is connected to a manifold having four valves V 1 -V 4  whose outputs are respectively connected to O 2  OUT  1 - 4 . A gauge G 2  is located on the front panel assembly  40  and is connected to the oxygen line so as to indicate the actual oxygen line pressure. A plenum pressure gauge G 1  and a pressure switch PSW 4  as well as orifice  0 RF 3  are also connected to the output of the plenum  110 .  
         [0037]    The output of the orifice  0 RF 3  is connected via solenoid valve SV 3  and valve V 9  to the exhaust of the system so as to allow the purging of the contents of the plenum  110 . A source of backup oxygen, such as a tank of compressed oxygen, is connected to the solenoid valve SV 4  to provide a continuous source of oxygen should be oxygen concentrator system fail. Pressure switch PSW 1  and relief valves RV 3  and RV 4  are also provided.  
         [0038]    Lastly, the low-pressure oxygen line is respectively connected via check valves CV 1  and CV 2  to flow meters FLM 1  and FLM 2  whose outputs are respectively connected to LOW P O 2  OUT  1 - 2 .  
         [0039]    Referring to FIG. 2, which is a simplified pneumatic diagram of the patient ventilator oxygen concentrator system of FIG. 1, some elements have been consolidated for simplicity and other elements, such as the relief valves, have not been shown so as not to obscure the features of the system. Similarly, other elements, such as the monitor/controller  200 , were not shown in FIG. 1 but are shown in FIG. 2.  
         [0040]    The operation of the concentrator system illustrated in FIGS. 1 and 2 is as follows. Air is supplied to the supply air inlet where it is received by the inlet pressure regulator and filter assembly REG 1 , FLTR 1  and FLTR 2 . The pressure regulator REG 1  regulates the air pressure of the air supplied to the air inlet so as to be at a constant value, for example, 80 PSIG. The filters FLTR 1  and FLTR 2  remove particulate matter and water which may be present in the air supplied to the air inlet. A line labeled DRAIN is used to convey the remove water to the EXHAUST via an element labeled EXHAUST SUM which may be a manifold, for example.  
         [0041]    The oxygen PSA sub-systems  1  and  2  respectively include oxygen beds  1  and  2  and oxygen beds  3  and  4 . Each bed comprises a molecular sieve bed which generates an oxygen product gas by the pressure-swing-adsorption method. Quantitatively, each subsystem may be designed to generate up to 10 liters per minute of oxygen product at an oxygen concentration of 93+/−3%.  
         [0042]    The medical air sub-system consists of air beds  1  and  2  which may each include an activated alumina air dryer bed which operates in the pressure-swing-adsorption mode, a micron filter to remove particulates and an odor removal filter, such as activated charcoal. Quantitatively, the medical grade air sub-system may be designed to generate up to 150 liters per minute of medical air, for example.  
         [0043]    As illustrated in FIG. 3, oxygen beds  1 - 4  are each cycled between a charging cycle and a flushing cycle. PSA beds typically have a charging cycle equal to 55% of the total cycle time and a flushing cycle equal to 45% of the total cycle time. As illustrated in FIG. 3, beds  1  and  2  have an overlap and beds  3  and  4  also have an overlap. As an example, the total cycle time may be on the order of 12 seconds with the overlap time being on the order of 0.5 seconds. By having two sets of oxygen PSA sub-systems, it is possible to operate one oxygen PSA sub-system when the demand for oxygen is below a preset amount and to operate both PSA sub-systems when the demand for oxygen exceeds the preset amount.  
         [0044]    In a similar fashion, air beds  1  and  2  also cycle between a charging cycle and a flushing cycle. As an example, the total cycle time for the air beds may be four times that of the oxygen beds. Accordingly, the total cycle time may be on the order of 48 seconds and the default overlap time may be on the order of 3 seconds with the PSA time being 21 seconds.  
         [0045]    [0045]FIG. 4 is a timing diagram illustrating the synchronization between the air beds and the oxygen beds. While it is not absolutely necessary for the sets of air beds and oxygen beds to be in synchronization with each other, the synchronization therebetween can simplify the monitor controller/ 200 .  
         [0046]    The monitor/controller  200 , in conjunction with the input/output panel  210 , is used to activate and switch the various valves utilized in the system. Furthermore, in conjunction with the carbon monoxide sensor  120 , dew point sensor  130  and oxygen sensor  140  and self-test valve SV 5 , the monitor/controller monitors the oxygen concentration in the oxygen product gas, as well as monitoring the dewpoint level and carbon monoxide level and the oxygen concentration in the medical grade air. Based on the status of the system, as a monitored by the monitor/controller  200 , status indications may be displayed on the input/output panel  210  utilizing a digital display or LED indicators, for example.  
         [0047]    Since the oxygen sensor  140  output varies with altitude, the absolute pressure regulator REG 5  is provided to keep the pressure of the oxygen sensor&#39;s chamber at a relatively constant value, for example, 16 PSIA so as to allow the system to operate at various altitudes without requiring the recalibration of the oxygen sensor  140 .  
         [0048]    The muffler MUF has been provided so has to reduce the noise caused by the exhausts from the oxygen PSA sub-systems  1  and  2  and the medical air sub-system since it is common to utilize oxygen concentrator systems in hospital environments requiring low noise levels.  
         [0049]    Initially, during startup of the system, and particularly when there is no pressure in the plenum  110 , the monitor/controller  200  activates, that is, allows gas to flow therethrough, the dump valve SV 3  and deactivates, that is, prevents gas from flowing therethrough, the plenum bypass valve BPV so as to flush the plenum  110  of any residual gas contained therein.  
         [0050]    Alternatively, on start-up, we flow gas through SV 3  until the oxygen is above 90%. Then SV 3  closes to the vent line and the plenum pressure will increase to normal operating pressure.  
         [0051]    The oxygen PSA sub-systems  1  and  2  are then operated so as to produce the output oxygen product which flows through the charging check valves CV 1 - 4  and charging control orifices ORF 1  and ORF 2  and the flow control regulator REG 2  into the plenum  110 . The oxygen concentration of the oxygen product leaving the plenum  110  is measured by the oxygen sensor  140 .  
         [0052]    When the oxygen concentration exceeds a predetermined amount, for example, 90%, as measured by the oxygen sensor  140 , the dump valve SV 3  is opened so as to allow the oxygen product from the oxygen PSA sub-systems  1  and  2  to charge the plenum  110  via a charging control circuit including the charging check valves CV 1 - 4 , the charging control orifices  0 RF 1  and  0 RF 2 , and the flow control regulator REG 2 . The charging control circuit limits the charging rate to a level which is less than a maximum output from the oxygen PSA sub-systems  1  and  2  when the plenum pressure is below the switch point of the plenum pressure switch PSW 4 , for example, 65 PSIG so as not to overdraw the oxygen PSA sub-systems  1  and  2 .  
         [0053]    When the plenum pressure switch PSW 4  changes state to indicate to the monitor/controller  200  that the pressure at the output of the plenum  110  is above its setpoint, the monitor/controller  200  opens the plenum bypass valve BPV to allow the oxygen product to flow directly to the various oxygen outlets. The direct flow of the oxygen product to the oxygen outlets rather than flowing through the plenum  110  enables the system to respond faster to transients such as line pressure changes or output flow changes.  
         [0054]    When the system is in a high oxygen flow mode, for example, a 65 liters per minute purge flow, the discharging check valve DCV opens to the pressure drop downstream of the check valve DCV to discharge the plenum  10  and thereby allow the high-pressure purge. The reserve capacity of the plenum  110  is mainly used for purging for short periods of time, such as 18 seconds, for example. Upon the completion of the purging, the charging control circuit trickle charges the plenum  110  when the output pressure of PSA sub-systems  1  and  2  is higher than the plenum pressure. That is, excess capacity of the PSA sub-systems  1  and  2  are used to recharge the plenum to maintain its reserve capacity.  
         [0055]    Unfortunately, as illustrated in FIG. 8 by the points labeled with triangles, the oxygen concentrator system noted above suffers performance degradation above a certain altitude, for example, at altitudes above 6000 feet.  
         [0056]    [0056]FIG. 5 is a simplified pneumatic diagram of the patient ventilator oxygen concentrator system in accordance with an example embodiment of the present invention. The system of FIG. 5 differs from that of FIG. 2 in that an absolute pressure transducer  666  has been added.  
         [0057]    The absolute pressure transducer  666  has an electrical output signal which is inputted to the monitor/controller  200 , the electrical output signal being indicative of the measured absolute pressure, that is, the measured barometric pressure. During the startup of the oxygen concentrator system, the monitor/controller  200  utilizes the electrical output signal to determine the suitable cycle times for the oxygen beds at the measured barometric pressure. If the system is at a fixed location, the cycle times can remain fixed after startup of the oxygen concentrator system. On the other hand, if the oxygen concentrator system is located in a moving vehicle or aircraft which can change altitudes, then the monitor/controller  200  can be programmed to again determine the suitable cycle times for the oxygen beds at either periodic time intervals or if the measured barometric pressure changes by more than a predetermined amount.  
         [0058]    The suitable cycle times for the oxygen beds, and for the air beds, versus barometric pressure are most easily determined empirically utilizing prototype oxygen and air beds. The then determined suitable cycle times for the oxygen beds and for the air beds versus barometric pressure may then be stored in a look-up table for the monitor/controller  200  and then retrieved by the monitor/controller  200  to set the most suitable cycle times for the oxygen beds and for the air beds. Table I is an example of the cycle times (in seconds) of both the oxygen bed cycles and the air beds cycles for both low flow and high flow versus barometric pressure (in mm of mercury).  
                                               Barometric   Oxygen Bed                   pressure   Cycle       Air Bed Cycle       (mmHg)   Low Flow   High Flow   Low Flow   High Flow                   &gt;620   11 s   11 s   44 s   44 s       600-620   11 s   12 s   44 s   48 s       580-600   11 s   13 s   44 s   52 s       560-580   11 s   14 s   44 s   56 s       540-560   11 s   15 s   44 s   60 s       520-540   11 s   16 s   44 s   64 s       490-520   11 s   17 s   44 s   68 s       &lt;490   11 s   18 s   44 s   78 s                  
 
         [0059]    [0059]FIGS. 6 and 7 respectively illustrate oxygen bed timing diagrams for the low altitude and high altitude cases. FIG. 6 illustrates the low altitude case, for example, at a barometric pressure greater than 620 mm of mercury. The upper waveform illustrates the timing cycle for oxygen bed  1  while the lower waveform illustrates the timing cycle for oxygen bed  2 . A “high” level indicates that the bed is charging while a “low” level indicates that the bed is flushing.  
         [0060]    For exemplary purposes, the cycle times of oxygen beds  1 - 4  are shown at a 55% charging/45% flushing duty cycle. The present invention is not limited thereto. Furthermore, a cycle time is defined to be equal to a charging cycle and a flushing cycle of a bed.  
         [0061]    Similarly, FIG. 7 illustrates the high altitude case, for example, at a barometric pressure less than 490 mm of mercury. The upper waveform illustrates the timing cycle for oxygen bed  1  while the lower waveform illustrates the timing cycle for oxygen bed  2 .  
         [0062]    As illustrated in FIGS. 6 and 7, the cycle time at a low altitude is 11 seconds whereas at a high altitude, the cycle time increases to 18 seconds. This reflects the decreased amount of available oxygen at a high altitude as compared with the amount of oxygen available at a low altitude. That is, it requires a greater period of time to increase the oxygen concentration of a supply of a air when the supply of a air initially has a lower oxygen partial pressure (which is the case at higher altitudes).  
         [0063]    [0063]FIG. 8 is a graph illustrating a comparison in the flow performance of the oxygen concentrator system of the co-pending related application and the oxygen concentrator system of the present invention versus altitude. As shown therein, in comparing the oxygen concentrator system of the co-pending related application having a fixed 12 second cycle time for all altitudes with the oxygen concentrator system of the present invention having a variable cycle time in the range of 11-18 seconds, it is clear that both systems operate effectively, that is, maintain a flow of 20 liters per minute up to an altitude of 6000 feet. Above 6000 feet, the system in accordance with the present invention maintains a flow of 20 liters per minute up to an altitude of 13,000 feet. On the other hand, the system of the co-pending related application reduces its flow as the altitude increases such that its flow is reduced to below 5 liters per minute at an altitude of 13,000 feet.  
         [0064]    This concludes the description of the example embodiment. Although the present invention has been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this invention. More particularly, reasonable variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangements within the scope of the foregoing disclosure, the drawings, and the appended claims without departing from the spirit of the invention. In additions to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.  
         [0065]    For example, the number of oxygen beds and oxygen PSA sub-systems is not limited to the number shown in the illustrative embodiment. Furthermore, the present invention is not limited to the exact arrangement of solenoid valves, check valves, relief valves, pressure switches, and pressure regulators shown in the illustrative embodiment. Still furthermore, the bypass valve and discharge check valve may be omitted in some configurations.