Abstract:
An oxygen concentrator may rely on a pressure swing adsorption process to produce an oxygen enriched gas stream from canisters filled with granules capable of separation of oxygen from an air stream. The adsorption process uses a cyclical pressurization and venting of the canisters to generate an oxygen enriched gas stream. Coupling an oxygen concentration sensor to the generated oxygen enriched gas stream may allow monitoring of the purity of the produced gas.

Description:
PRIORITY 
       [0001]    This application is a Continuation of U.S. patent application Ser. No. 12/163,549 entitled “OXYGEN CONCENTRATOR APPARATUS AND METHOD”, filed on Jun. 27, 2008, which claims the benefit of priority of U.S. Provisional Patent Application Ser. No. 60/970,371 titled “Oxygen Concentrator Apparatus and Method”, filed on Sep. 6, 2007, both of which are hereby incorporated by reference in its entirety as though fully and completely set forth herein. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to health equipment and, more specifically, to oxygen concentrators. 
         [0004]    2. Description of the Related Art 
         [0005]    Patients (e.g., those suffering with diseases such as emphysema, congestive heart failure, acute or chronic pulmonary insufficiency, etc.) may require supplemental oxygen. Other people (e.g., obese individuals) may also require supplemental oxygen, for example, to maintain elevated activity levels. Doctors may prescribe oxygen concentrators or portable tanks of medical oxygen for these patients. Usually a specific oxygen flow rate is prescribed (e.g., 1 liter per minute (LPM), 2 LPM, 3 LPM, etc.) Oxygen concentrators used to provide these flow rates may be bulky and heavy making ordinary ambulatory activities with them difficult and impractical. Portable tanks of medical oxygen may also be heavy and contain limited amounts of oxygen. 
         [0006]    Oxygen concentrators may take advantage of pressure swing absorption. Pressure swing absorption may involve using a compressor to increase air pressure inside a canister that contains granules of a micro-porous mineral. As the pressure increases, certain air molecules may become smaller and may be absorbed into the micro-pores of the granules. An example of such a granule is found in certain volcanic ash. Synthetic granules (e.g., zeolite) may also be available in various granule and pore sizes. These granules may thus be used to separate gases of different molecular size (e.g., zeolite may be used to separate nitrogen and oxygen). Ambient air usually includes approximately 78% nitrogen and 21% oxygen with the balance comprised of argon, carbon dioxide, water vapor and other trace elements. When pressurized air is applied to the granules, nitrogen in the air may be absorbed in the micro-pores of the granules because of the smaller size of the nitrogen molecule. As the granules are saturated, the remaining oxygen may be allowed to flow through the canister and into a holding tank. The pressure in the canister may then be vented from the canister resulting in the previously absorbed nitrogen being released from the pores in the granules. A small portion of the bled oxygen may be used to further purge the nitrogen from the canister. The process may then be repeated using additional ambient air. By alternating canisters in a two-canister system, one canister can be collecting oxygen while the other canister is being purged (resulting in a continuous separation of the oxygen from the nitrogen). In this manner, oxygen can be accumulated out of the air for a variety of uses include providing supplemental oxygen to patients. 
         [0007]    Prior art oxygen concentrators may have several limitations. For example, the compressor on the oxygen concentrator may be operated at a level required to meet the demands of the user regardless of the breathing rate of the user. In addition, the length of the supply tubing to the nasal cannula or mask from the oxygen concentrator may be limited to 6 to 8 feet. This limitation may be a problem for users using the device in their sleep. Prior art oxygen concentrators may also include a limited sensor and alarm to notify a user if the oxygen supplied by the oxygen concentrator is too low. Currently oxygen sensors in oxygen concentrators use a heated filament as a component. In addition, time, pressure and orifice size are used to determine a volume of air delivered to a user of an oxygen concentrator (however, this measurement technique may not account for pressure fluctuations). 
       SUMMARY 
       [0008]    In various embodiments, an oxygen concentrator for concentrating oxygen may include canisters (e.g., to hold zeolite) integrated into a molded body. The oxygen concentrator may be made of one or more plastic molded parts (i.e., housing components) and may further include valves, flow restrictors (e.g., press fit flow restrictors), air pathways, and other components coupled to or integrated into the one or more housing components. In some embodiments, the canisters may be injection molded (e.g., using plastic). The injection molded housing components may include air pathways for air flowing to and from the canisters. In some embodiments, valves may be coupled to the one or more housing components to direct air through the air pathways. In some embodiments, one or more compressors (e.g., a dual-pump diaphragm compressor) may compress air through the canisters. Zeolite (or another granule) in the canisters may separate nitrogen and oxygen in the air as the air is compressed through the canisters. Some of the separated oxygen may also be used to vent nitrogen from the canisters. In some embodiments, a spring baffle may be used to bias the granules in the canister to avoid damage to the granules when the oxygen concentrator is moved. The spring baffle may be a single molded part (e.g., injection molded part). In some embodiments, the oxygen concentrator may include two-step actuation valves. Two step actuation valve may be operable to be opened by application of a first voltage and further operable to be held open by a second voltage (the second voltage may be less than the first voltage to conserve energy). In some embodiments, a solar panel may be coupled to a battery of the oxygen concentrator to charge the battery using solar energy. 
         [0009]    In some embodiments, a pressure transducer coupled to the oxygen concentrator may detect a change in pressure corresponding to a start of a user&#39;s breath. A processor coupled to the pressure transducer may execute program instructions to implement a first mode in which a sensitivity of the pressure transducer is attenuated. In a second mode, the sensitivity of the pressure transducer may not be attenuated. For example, the sensitivity of the pressure transducer may be attenuated in windy environments or while the user is active. The sensitivity may not be attenuated, for example, while the user is asleep or otherwise sedentary. 
         [0010]    In various embodiments, the pressure transducer, coupled to the oxygen concentrator, may be used to detect a breathing rate of the user of the oxygen concentrator. The processor coupled to the pressure transducer may execute program instructions to adjust power to the one or more compressors based on the breathing rate of the user of the oxygen concentrator. In some embodiments, the compressors may switch between a first phase of operation in which only a subset of the compressors operate and a second phase of operation in which additional compressors (e.g., all available compressors) operate. For example, fewer compressors may be used during lower user breathing rates. 
         [0011]    In some embodiments, the oxygen concentrator may use a dual lumen (including a first tube and a second tube). The first tube may be used to deliver oxygen to the user&#39;s nose and the second tube may extend to the entrance of the user&#39;s nose to communicate a change in pressure (e.g., from the start of a breath through the user&#39;s nose) from the entry of the user&#39;s nose to the oxygen concentrator. In some embodiments, the second tube may have a smaller radius than the first tube to allow for increased sensitivity to pressure changes in the second tube. 
         [0012]    In some embodiments, a transducer may be coupled to the prongs of the nasal cannula to detect a change in pressure resulting from a start of a breath taken by the user. In some embodiments, a Hall-effect sensor may be used at the nasal cannula or at the oxygen concentrator to detect air movement (e.g., due to a user&#39;s breath). The Hall-effect sensor may use a magnet coupled to a vane (inserted into the nasal cannula) to detect movement of air in the nasal cannula. 
         [0013]    In some embodiments, an ultrasonic sensor may be used to detect the presence of a gas (e.g., to detect the concentration of oxygen in air delivered to a user). In some embodiments, the ultrasonic sensor may be placed on a chamber of the oxygen concentrator that receives air to be delivered to the user. An ultrasonic emitter of the ultrasonic sensor may provide an ultrasonic sound wave through the chamber and an ultrasonic receiver may detect the ultrasonic sound wave that has traveled through the air of the chamber. A processor coupled to the ultrasonic emitter and the ultrasonic receiver may execute program instructions to determine a speed of the sound wave through the chamber (the speed of the sound wave may indicate a relative concentration of a constituent of the gas (e.g., the concentration of oxygen)). 
         [0014]    In some embodiments, an audio device (e.g., an MP3 (Moving Picture Experts Group Layer-3 Audio) player, mobile phone, etc.) may be integrated into the oxygen concentrator (e.g., integrated into an outer housing of the oxygen concentrator). A microphone and headphone may be coupled to the audio device through a wire or may be wirelessly connected. In some embodiments, the microphone may be coupled to a nasal cannula or other oxygen delivery mechanism coupled to the oxygen concentrator. Other configurations are also contemplated. The headset/microphone combination may also be used with the oxygen concentrator for hands-free cellular phone use. Other uses are also contemplated. 
         [0015]    In some embodiments, various components of the oxygen concentrator may be arranged in one or more housings (e.g., a foam housing inside of a light-weight plastic enclosure). In some embodiments, the foam housing may include passages for air flow and/or electrical connections between components of the oxygen concentrator. Other configurations are also contemplated. In some embodiments, additional housings may be used. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    A better understanding of the present invention may be obtained when the following detailed description is considered in conjunction with the following drawings, in which: 
           [0017]      FIGS. 1   a - b  illustrate two molded oxygen concentrator housing components, according to an embodiment. 
           [0018]      FIGS. 2   a - b  illustrates the second housing component of the oxygen concentrator, according to an embodiment. 
           [0019]      FIG. 3  illustrates a diagram of the components of the oxygen concentrator, according to an embodiment. 
           [0020]      FIG. 4  illustrates a vented lid for the oxygen concentrator, according to an embodiment. 
           [0021]      FIGS. 5   a - h  illustrate various views of the first housing component of the oxygen concentrator, according to an embodiment. 
           [0022]      FIGS. 6   a - h  illustrate additional views of the internal structure of the first housing component of the oxygen concentrator, according to an embodiment. 
           [0023]      FIG. 7  illustrates a spring baffle, according to an embodiment. 
           [0024]      FIG. 8  illustrates a butterfly valve seat, according to an embodiment. 
           [0025]      FIGS. 9   a - f  illustrate different hose/pressure transducer configurations, according to an embodiment. 
           [0026]      FIG. 10  illustrates a hall effect pressure transducer and associated hose configuration, according to an embodiment. 
           [0027]      FIG. 11  illustrates a circuit diagram of an ultrasonic sensor assembly, according to an embodiment. 
           [0028]      FIG. 12  illustrates a shifted wave pulse as detected by the ultrasonic sensor assembly, according to an embodiment. 
           [0029]      FIG. 13  illustrates the components of the shift for the oxygen concentrator, according to an embodiment. 
           [0030]      FIG. 14  illustrates various gates for the ultrasonic sensor, according to an embodiment. 
           [0031]      FIG. 15  illustrates a solar panel coupled to the oxygen concentrator, according to an embodiment. 
           [0032]      FIG. 16  illustrates a flowchart of an embodiment for oxygen concentrator operation, according to an embodiment. 
           [0033]      FIG. 17  illustrates a flowchart of an embodiment for oxygen concentrator assembly, according to an embodiment. 
           [0034]      FIG. 18  illustrates a flowchart of an embodiment for compressor control, according to an embodiment. 
           [0035]      FIG. 19  illustrates a flowchart of an embodiment for ultrasonic sensor operation, according to an embodiment. 
           [0036]      FIG. 20  illustrates a headset/microphone boom, according to an embodiment. 
           [0037]      FIGS. 21   a - c  illustrate outer housings, according to two embodiments. 
           [0038]      FIG. 22  illustrates an embodiment of an enclosure housing. 
           [0039]      FIG. 23  illustrates an embodiment of two half sections of the enclosure housing. 
           [0040]      FIG. 24  illustrates an embodiment of a first foam housing. 
           [0041]      FIG. 25  illustrates an embodiment of a complimentary second foam housing. 
           [0042]      FIG. 26  illustrates a side and front profile of a component arrangement in the foam housings, according to an embodiment. 
           [0043]      FIG. 27  illustrates three embodiments of gas mixture delivery profiles for the oxygen concentrator. 
           [0044]      FIGS. 28   a - d  illustrate an attachable external battery pack for the oxygen concentrator, according to an embodiment. 
       
    
    
       [0045]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. Note, the headings are for organizational purposes only and are not meant to be used to limit or interpret the description or claims. Furthermore, note that the word “may” is used throughout this application in a permissive sense (i.e., having the potential to, being able to), not a mandatory sense (i.e., must). The term “include”, and derivations thereof, mean “including, but not limited to”. The term “coupled” means “directly or indirectly connected”. 
       DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0046]      FIGS. 1   a - 2   b  illustrate various views of housing components  111   a - b  for an oxygen concentrator  100 , according to an embodiment. In some embodiments, the oxygen concentrator  100  may concentrate oxygen out of the air to provide supplemental oxygen to a user. The oxygen may be collected from ambient air by pressurizing the ambient air in a canister (e.g., canisters  101   a - b ) with granules  139  (e.g., molecular sieve granules) such as zeolite  391  (see  FIG. 3 ). Other materials (used instead of or in addition to zeolite  391 ) may be used. In some embodiments, the air may be pressurized in the canister  101  using one or more compressors  301 . In some embodiments, the ambient air may be pressurized in the canisters  101  to a pressure approximately in a range of 13-20 pounds per square inch (psi). Other pressures may also be used (e.g., if a different granule type is used). Under pressure, the nitrogen molecules in the pressurized ambient air may enter the pores of the granules  139  in the canister  101  which may hold the nitrogen molecules as oxygen molecules flow through the canister  101  and out of a respective exit aperture  601  (see  FIG. 6 ). While examples provided herein describe separating nitrogen and oxygen, it is to be understood that other embodiments may include separating other atom/molecules types. In some embodiments, the oxygen molecules leaving aperture  601  may be collected in an oxygen accumulator  103  prior to being provided to a user through outlet  107 . In some embodiments, a tube (e.g., tube  907  in  FIGS. 9   a - b ) may be coupled to the outlet  107  to deliver the oxygen to the user through a nasal cannula  903 . In some embodiments, tube  907  may be coupled to an exit nozzle  2111   a,b  (see  FIGS. 21   a - b ) that is coupled to outlet  107  through a silicone rubber tube  197  (other materials for the tube  197  are also contemplated). Other delivery mechanisms and routes are also contemplated. For example, the outlet may include a tube that directs the oxygen toward a user&#39;s nose and/or mouth that may not be directly coupled to the user&#39;s nose. In some embodiments, the oxygen provided to the user may be of 90 percent or greater purity (e.g., 97 percent purity). Other oxygen concentrations are also contemplated (e.g., lower purity levels may be desired). 
         [0047]    In some embodiments, after applying the initial pressurized air to a canister  101  (e.g., canister  101   a ), the pressure in the canister  101  may be released, and the nitrogen molecules in the canister  101  may be expelled from the oxygen concentrator  100  (e.g., through respective valve  305   c  or  305   d  and then through muffled vent  327 ). Other exit mechanisms may also be used. In some embodiments, the canister  101  may be further purged of nitrogen using concentrated oxygen that is introduced into the canister  101  through respective aperture  601  (e.g., from oxygen being concentrated from the other canister  101 ). In some embodiments, the oxygen concentrator  100  may include two or more canisters  101 . For example, while canister  101   a  is being purged of nitrogen, canister  101   b  may be collecting oxygen. Other configurations are also contemplated (e.g., one canister, four canisters, etc.). 
         [0048]    In some embodiments, pressurized air from the compressors  301  may enter air inlets  109   a - b  and then may be directed using various valves  305  (attached to valve seats  105 ) and internal air pathways. As shown in  FIGS. 1   a - 3 , valve seats  105   a - g  may correspond to respective valves  305   a - g  (e.g., valve  305   a  is seated in valve seat  105   a , etc.). As seen in the example valve  305  in  FIG. 2   a , valves  305  may include high pressure stems (e.g., stem  211   a ) and low pressure stems (e.g., stem  211   b ). The valves  305  may also include gaskets around the stems (e.g., gasket  209 ). The valves  305  may be actuated/powered through electrical connection  213 . In some embodiments, the valves  305  may be coupled to and controlled by processor  399 . The valves  305  may be coupled to their respective valve seats  105  (e.g., through size 256 screws  299  through slots  215  on either side of the valve  305  and into their respective fastening apertures (e.g., screw apertures  135   a,b )). The valves  305  may also be coupled to the valve seats  105  through other techniques (e.g., using adhesive, rivets, etc.). Other valve and valve seat configurations are also contemplated. 
         [0049]    In some embodiments, air may be pulled into the oxygen concentrator  100  through compressors  301   a - b  (which may be dual-pump diaphragm compressors). In some embodiments, air may flow into the air inlets  109   a - b  from compressors  301   a - b  (e.g., one inlet per respective compressor). In some embodiments, one of valves  305   a  or  305   b  may be closed (e.g., as signaled by processor  399 ) resulting in the combined output of both compressors  301  flowing through the other respective valve seat  105 /valve  305  into a respective canister  101  (e.g., either canister  101   a  or canister  101   b ). For example, if valve  305   b  (seated in valve seat  105   b ) is closed, the air from both compressors  301  may flow through valve  305   a  (seated in valve seat  105   a ). If valve  305   a  is closed, the air from both compressors  301  may flow through valve  305   b . In some embodiments, valve  305   a  and valve  305   b  may alternate to alternately direct the air from the compressors  301  into respective canisters  101   a  or  101   b . In some embodiments, if one of the two compressors  301  fails, the working compressor&#39;s output may be alternately directed between canisters  101   a,b . This may allow the oxygen concentrator  100  to at least partially work (e.g., on half output) until the user can arrange another oxygen source. 
         [0050]    In some embodiments, as air flows through respective canister  101   a  or  101   b , oxygen may pass through the granules  139  in the canister  101  while the nitrogen is retained in the granules  139 . As seen in  FIG. 6G , the oxygen may pass through opening  601   a  at the end of canister  101   a , through side tube  121   a , through check valve  123   a , and into oxygen accumulator  103 . Alternately, the oxygen may pass through opening  601   b  at the end of canister  101   b , through side tube  121   b , through check valve  123   b , and into oxygen accumulator  103 . From oxygen accumulator  103 , the air may flow through valve  305   g  (which may be a high pressure F-valve) seated in valve seat  105   g . In some embodiments, the air may flow through a flow restrictor  311  (e.g., a 0.025 R flow restrictor). Other flow restrictor types and sizes are also contemplated. In some embodiments, a separate restrictor may not be used (e.g., the diameter of the air pathway in the housing may be restricted). The air may then flow through an oxygen sensor (e.g., ultrasonic sensor  307  comprised of an ultrasonic emitter  201  and receiver  203 ), a filter  385  (e.g., to filter bacteria, dust, granule particles, etc), through silicone rubber tube  197 , and then out of the oxygen concentrator  100  and to the user (e.g., through a tube  907  and nasal cannula  903  coupled to outlet  107 ). 
         [0051]    In some embodiments, ultrasonic emitter  201  may include multiple ultrasonic emitters (e.g., emitters  201   a,b ) and ultrasonic receiver  203  may include multiple ultrasonic receivers (e.g., receivers  203   a,b ). In some embodiments, the multiple ultrasonic emitters and multiple ultrasonic receivers may be axially aligned (e.g., across the gas mixture flow path which may be perpendicular to the axial alignment). Other emitter/receiver configurations are also contemplated. In some embodiments, the ultrasonic sensor  307  and, for example, a gas flow meter  1143  (as seen in  FIG. 11 ) may provide a measurement of flow delivery (or actual amount of oxygen being delivered). For example, the gas flow meter  1143  may use the Doppler effect to measure a volume of gas provided and the ultrasonic sensor  307  may provide the concentration of oxygen of the gas provided. These two measurements together may be used by the processor to determine an approximation of the actual amount of oxygen provided to the user. Other sensors may also be used in flow delivery measurement. 
         [0052]    In some embodiments, valve  305   a  may be closed and valve  305   c  (seated in valve seat  105   c ) may be opened to direct nitrogen (under pressure) out of canister  101   a  and through the muffled vent out  327 . Similarly, valve  305   b  may be closed and valve  305   d  (seated in valve seat  105   d ) may be opened to direct nitrogen (under pressure) out of canister  101   b  and through the muffled vent out  327 . 
         [0053]    In some embodiments, a portion of the collected oxygen may be transferred from one canister  101  (e.g., the canister  101  currently producing oxygen) to the back of the other canister  101  (e.g., the canister  101  currently venting nitrogen) in order to further purge the nitrogen. The oxygen may travel through flow restrictors  321 ,  323 , and  325  between the two canisters  101 . Flow restrictor  321  may be a trickle flow restrictor. Flow restrictor  321  may be a 0.011 R flow restrictor (e.g., with a radius 0.011*the radius of the tube it is inside) and flow restrictor  323  and flow restrictor  325  may be a 0.013 R flow restrictors. Other flow restrictor types and sizes are also contemplated. For example, flow restrictor  321  may be a 0.009 R flow restrictor. In some embodiments, the flow restrictors may be press fit flow restrictors that restrict air flow by introducing a narrower radius in their respective tube. In some embodiments, the press fit flow restrictors may be made of sapphire, metal or plastic (other materials are also contemplated). 
         [0054]    Valve  305   e  and valve  305   f  may be opened to direct oxygen from the producing canister  101  to the venting canister  101 . The valves may be opened for a short duration during the venting process (and may be closed otherwise) to prevent excessive oxygen loss out of the purging canister  101 . Other durations are also contemplated. The pair of equalization/vent valves  305   e,f  may work with flow restrictors  323  and  325  to optimize the air flow balance between the two canisters  101   a,b . This may allow for better flow control for venting the canisters  101   a,b  with oxygen from the other of canisters  101   a,b . It may also provide better flow direction between the two canisters  101   a,b . For example, when directing oxygen from canister  101   b  to canister  101   a  to vent the nitrogen out of canister  101   a , oxygen may flow through flow restrictor  323  and then open valve  305   f  on a first air pathway, and through open valve  305   e  and then flow restrictor  325  on the second air pathway (one air pathway ideal and one air pathway less ideal). Similarly, when directing oxygen from canister  101   a  to canister  101   b  to vent the nitrogen out of canister  101   b , oxygen may flow through open valve  305   f  and then flow restrictor  323  on one air pathway and through flow restrictor  325  then open valve  305   e  on the second air pathway (one air pathway ideal and one air pathway less ideal). Therefore, a similar volume of oxygen may be used from each canister  101  when purging the other canister  101 . The opposite arrangement of the valve and flow restrictor on parallel air pathways may equalize the flow pattern of the oxygen between the two canisters  101 . If not equalized, more oxygen may be used in venting one of the canisters  101  than the other of the canisters  101  (resulting in less oxygen available to the user on every other cycle). Equalizing the flow may allow for a steady amount of oxygen available to the user over multiple cycles and also may allow a predictable volume of oxygen to purge the other of the canisters  101 . Other numbers of valves and/or flow resistors are also contemplated. Other arrangements are also contemplated. For example, one air pathway may be provided with a balanced flow pattern in either direction. In some embodiments, the air pathway may include a first flow restrictor, a valve, and a second flow restrictor (of similar size as the first flow restrictor) such that when the valve is open, air flows through the restrictors and valve in a similar pattern (restrictor, valve, restrictor) regardless of direction. In some embodiments, the air pathway may not have restrictors but may instead have a valve with a built in resistance (or the air pathway itself may have a narrow radius to provide resistance) such that air flow through the valve has the same resistance regardless of direction through the valve. 
         [0055]    Air being vented out of the canisters  101  may travel through canister exit aperture  297   a  or  297   b , through respective valve  305   c  or  305   d , through the muffled vent out  137 , and then through the vent  401  (e.g., see  FIG. 4 ). The muffled vent out  137  may include open cell foam (or another material) between the nitrogen exit aperture  217   a  of the housing component  111   a  and the vent  401  to muffle the air leaving the oxygen concentrator  100 . Other muffling techniques are also contemplated. In some embodiments, the combined muffling components/techniques may provide for oxygen concentrator operation at a sound level below 50 decibels. The oxygen concentrator may also operate at lower or higher sound levels. In some embodiments, the vent  401  may include apertures  403  that may be smaller in cross section than the open cell foam in the muffled vent out  137 . This may allow air to exit while keeping the open cell foam in the muffled vent out  137 . In some embodiments, the vent  401  may be made of a molded plastic (e.g., injection molded). Other materials are also contemplated. In some embodiments, the vent  401  may be coupled to the muffled vent out  137  of housing component  111   a  through an adhesive or solvent weld. Other coupling techniques are also contemplated (e.g., the vent  401  may snap in place). 
         [0056]    In some embodiments, the valves  305  may be silicon plunger solenoid valves (other valves are also contemplated). Plunger valves may be quiet and have low slippage. In some embodiments, a two-step valve actuation voltage may be used to control the valves  305 . For example, 24 volts (V) may be applied to the valve to open the valve  305 , and then the voltage may be reduced to 7 V to keep the valve  305  open. In some embodiments, the voltages and the duration of the voltages may be controlled by processor  399 . The valves  305  may require more voltage to overcome static friction, but once open, less voltage may be required to keep the valve  305  open (the sliding friction may be less than the static friction on the valve  305 ). Using less voltage to keep the valve  305  open may use less power (Power=Voltage*Current). Lower power requirements may lead to a longer battery life. In some embodiments, the voltage may be applied as a function of time that is not necessarily a stepped response (e.g., a curved downward voltage between an initial 24 V and 7 V). Other response patterns are also contemplated. Other voltages are also contemplated (e.g., voltages larger or smaller than 24V, 7V). For example, different voltages may be used for different valves. 
         [0057]    In some embodiments, the housing for the oxygen concentrator  100  may include two housing components  111   a - b . The housing components  111   a - b  may be formed separately and then coupled together (other numbers of housing components are also contemplated). In some embodiments, the housing components  111   a - b  may be injection molded (e.g., from an injection die molded plastic). Other manufacturing techniques are also contemplated (e.g., compression molding). The housing components  111   a - b  may be made of a thermoplastic such as polycarbonate, methylene carbide, polystyrene, acrylonitrile butadiene styrene (ABS), polypropylene, polyethylene, or polyvinyl chloride. Other materials are also contemplated (e.g., the housing components  111   a - b  may be made of a thermoset plastic or metal (such as stainless steel or a light-weight aluminum alloy)). Lightweight materials may be used to reduce the weight of the oxygen concentrator  100 . In some embodiments, the two housings  111   a  and  111   b  may be fastened together using screws or bolts. For example, screws may be placed through apertures  131   a - g  (e.g., one screw through aperture  131   a  and  131   e , etc.). Other fastening techniques are also contemplated (e.g., rivets). As another example, the housing components  111   a,b  may be solvent welded together. 
         [0058]    As shown, valve seats  105   a - f  and air pathways may be integrated into the housing components  111   a - b  to reduce a number of seal connections needed throughout the air flow of the oxygen concentrator  100  (this may reduce leaks and potential failure points). In various embodiments, the housing components  111   a - b  of the oxygen concentrator  100  may form a two-part molded plastic frame that includes, for example, two canisters  101  coupled to two compressors and an air delivery mechanism through multiple air pathways and valve seats  105   a - f  integrated into the frame. In some embodiments, the oxygen concentrator  100  may be formed out of a different number of molded components (e.g., one unitary component or using three or more components). Other techniques for forming the oxygen concentrator are also contemplated (e.g., laser sintering, machining, etc.). 
         [0059]    In some embodiments, air pathways/tubing between different sections in the housing components  111   a,b  (e.g., between the canisters  101   a,b  and the oxygen accumulator  103 ) may take the form of molded channels. The tubing in the form of molded channels for air pathways may occupy multiple planes in the housing components  111   a,b  (e.g., may be formed at different depths and at different x,y,z positions in the housing components  111   a,b ). In some embodiments, a majority or substantially all of the tubing may be integrated into the molded housing (e.g., housing components  111   a,b ) to reduce potential leak points. 
         [0060]    In some embodiments, prior to coupling the housing components  111   a,b  together, O-rings may be placed between various points of the housing components  111   a,b  (e.g., O-rings  135   a,b  between housing components  111   a  and  111   b  at tubes  121   a,b ). O-rings may also be placed between the ends of canisters  101   a,b  and the housing component  111   b  (which may function as a manifold) and between the end of the oxygen accumulator  103  and the housing component  111   b . Other O-rings are also contemplated. In some embodiments, filters  207   a,b  may also be fastened (e.g., welded or using an adhesive) to the inside of the housing component  111   a  and/or  111   b  to prevent granules  139  from getting into the tubing/valves coupled to the canisters  101   a,b . The filters  207  may also be welded onto either side of the spring baffles  701  to keep the granules  139  out of the tubing, etc. of housing component  111   b . For example, the filter  207  may be welded onto the non-spring side of the spring baffle  701 . The filters  207  may be spunbond filters made of one or more layers of textile cloth. Other filters are also contemplated. In some embodiments, the granules  139  may be added prior to coupling the housing components  111   a,b  together. 
         [0061]    In some embodiments, components may be integrated and/or coupled separately to the housing components  111   a - b . For example, tubing, flow restrictors (e.g., press fit flow restrictors), oxygen sensors (e.g., comprising an emitter  201  and receiver  203 ), granules  139  (e.g., zeolite), check valves  123 , plugs, processors and other circuitry, battery  395 , etc. may be coupled to the housing components  111   a - b  before and/or after the housing components  111   a - b  are coupled together. As disclosed, the oxygen concentrator  100  and components together may weigh less than 5 pounds and be smaller than 200 cubic inches. Other dimensions are also contemplated. 
         [0062]    In some embodiments, apertures leading to the exterior of the housing components  111   a - b  may be used to insert devices such as flow restrictors. Apertures may also be used for increased moldability. One or more of the apertures may be plugged after molding (e.g., with a plastic plug). Plugs such as plug  125   a  and  125   b  may be used to plug apertures formed in housing component  111  to facilitate the injection molding process. In some embodiments, flow restrictors may be inserted into passages prior to inserting plug to seal the passage. For example, as seen in  FIG. 6   g , flow restrictor  321  may be a press-fit flow restrictor that is inserted into aperture  603   a  followed by a plug  127   a . Flow restrictor  323  may be inserted into aperture  603   b  followed by plug  127   b . Flow restrictor  325  may be inserted into aperture  603   e  followed by plug  127   e . Other plugs may also be used (e.g., plug  127   c  (for aperture  603   c ), plug  127   d  (for aperture  603   d ), plug  127   f  (for aperture  603   f ), and plug  127   g  (for aperture  603   g )). Press fit flow restrictors may have diameters that may allow a friction fit between the press fit flow restrictors and their respective apertures. In some embodiments, an adhesive may be added to the exterior of the press fit flow restrictors to hold the press fit flow restrictors in place once inserted. In some embodiments, the plugs may have a friction fit with their respective tubes (or may have an adhesive applied to their outer surface). The press fit flow restrictors and/or other components may be inserted and pressed into their respective apertures using a narrow tip tool or rod (e.g., with a diameter less than the diameter of the respective aperture). Other insertion mechanisms are also contemplated. In some embodiments, the press fit flow restrictors may be inserted into their respective tubes until they abut a feature in the tube to halt their insertion. For example, the feature may include a reduction in radius (e.g., see reduction  605  in  FIG. 6G ). Other features are also contemplated (e.g., a bump in the side of the tubing, threads, etc.). In some embodiments, press fit flow restrictors may be molded into the housing components  111   a,b  (e.g., as narrow tube segments). 
         [0063]    In some embodiments, spring baffle  129  may be placed into respective canister receiving portions of the housing component  111   b  with the spring side of the baffle  129  facing the exit of the canister  101 . In some embodiments, the spider legs  701  of the spring baffle  129  may engage the ridges  133  on the back of the canisters  101 .  FIG. 7  also illustrates an embodiment of the spring baffle  129 . The spring baffle  129  may apply force to granules  139  in the canister  101  while also assisting in preventing granules  139  from entering the exit apertures  601   a,b . The spring baffle  129  may keep the granules  139  compact while also allowing for expansion (e.g., thermal expansion). For example, during thermal expansion (or, for example, during a physical shock), spider legs  701  may compress. Keeping the granules  139  compact may prevent the granules  139  from breaking (e.g., during movement of the oxygen concentrator  100 ). The spring baffle  129  may be made of one piece molded plastic. Other materials and manufacturing techniques are also contemplated (e.g., stainless steel). 
         [0064]    In some embodiments, check valves  123  may prevent oxygen from tube  121   a  or the oxygen accumulator  103  from entering tube  121   b  and may prevent oxygen from tube  121   b  and the oxygen accumulator  103  from entering tube  121   a . In some embodiments, a butterfly check valve  123  may be used (other check valve types are also contemplated).  FIG. 8  illustrates an embodiment of a butterfly check valve  123  (e.g., see butterfly valves  123   a,b  in  FIG. 1 ) with a butterfly component  801 . In some embodiments, the butterfly component  801  may be pulled into the valve seat  813  until the ball  803  of the butterfly component  801  snaps through the aperture  805  to hold the butterfly component  801  in place. As air flows in direction  807  through the check valve  123 , (e.g., through apertures  811   a - d ) the butterfly component  801  may bend to allow air through the valve  123  (see configuration  809 ). If air tries to flow in the opposite direction (or if air flow is at rest), the butterfly component  801  may take configuration  813  to prevent air flow through the check valve  123 . 
         [0065]    In some embodiments, one or more compressors (e.g., two compressors  301   a,b ) may provide compressed air in a parallel arrangement. In some embodiments, dual-pump diaphragm compressors may be used for longer life (e.g., &gt;20000 operating hours). Dual-pump diaphragm compressors may also work without needing additional oil. Dual-pump diaphragm compressors may also require less volume than larger single compressors used to compress a similar amount of air. Other compressors may also be used (e.g., a two stage compressor may be used). 
         [0066]    In some embodiments, both compressors  301   a,b  may be used during normal operation (e.g., during normal user breathing rates/normal required oxygen flow rates). Air from the compressors  301  may enter the oxygen concentrator  100  through both inlets  109   a  and  109   b  and may be directed to a canister (e.g., canister  101   a  or  101   b ) through valves  305   a  and  305   b  (through respective valve seats  105   a ,  105   b ). At lower user breathing rates/lower required oxygen flow rates, a subset of the compressors  301  may be used. For example, only one compressor ( 301   a  or  301   b ) may be used and the air from the compressor  301   a  or  301   b  may enter through inlet  109   a  or  109   b . The air may be similarly directed into a canister  101   a  or  101   b  through valves  305   a  and  305   b  (through respective valve seats  105   a ,  105   b ). In some embodiments, when a subset of the compressors  301  are operating, the subset that is operating may alternate operating time with the inactive compressors. For example, during single compressor operation, the two compressors  301  may alternate (e.g., to keep wear evenly distributed between the two compressors  301 ). In some embodiments, other numbers of compressors  301  may be used. For example, four compressors may be used during normal operation (e.g., with two compressors placing air into inlet  109   a  and two compressor placing air into inlet  109   b ). With four compressors, a subset of the compressors may include two operating compressors (e.g., either the two compressors placing air into inlet  109   a  or the two compressors placing air into inlet  109   b  or one compressor placing air into inlet  109   a  and one compressor placing air into inlet  109   b ). Other configurations are also contemplated. Using a subset of the compressors  301  may reduce power consumption during low activity times for the user (e.g., while the user is sitting). The reduced power consumption may allow for a smaller battery  395  to be used in the oxygen concentrator  100 . 
         [0067]    In some embodiments, a single compressor may be used (e.g., in different power modes). For example, during normal operation the compressor may be operated at full power, while, during lower breathing rates, the compressor may be operated at a lower power setting. In some embodiments, the compressors in multiple compressor operation may also be operated at different power levels (e.g., at lower power settings during lower breathing rates). 
         [0068]    In some embodiments, if one or more of the compressors fails, the other compressors may provide at least a subset of the required oxygen to the user. This may provide oxygen to the user until the user can locate other oxygen arrangements. In some embodiments, one or more of the compressors may be redundant compressors such that if a compressor fails, the user may still receive the prescribed oxygen rate. In some embodiments, the redundant compressor may be activated when one of the active compressors fails. In some embodiments, the redundant compressor may have already been active (e.g., additional power may be supplied to the active compressors when one of the compressors fails). 
         [0069]    In some embodiments, the compressors  301  may be controlled through a compressor control system implemented by processor  399  (which may include, for example, one or more field programmable gate arrays (FPGAs), a microcontroller, etc. comprised on circuit board  2607  as seen in  FIG. 26 ) executing programming instructions stored on memory  397 . In some embodiments, the programming instructions may be built into processor  399  such that a memory  397  external to the processor  399  may not be separately accessed (i.e., the memory  397  may be internal to the processor  399 ). In some embodiments, the processor  399  may be coupled to the compressors  301 . The processor  399  may also be coupled to other components of the oxygen concentrator (e.g., valves  305 , oxygen sensor  307 , demand transducer  331 , etc.). In some embodiments, a separate processor (and/or memory) may be coupled to the other components of the oxygen concentrator  100 . In some embodiments, the demand transducer  331  may be a pressure transducer  901  detecting inhalations to detect the breathing rate (and, for example, the volume). In some embodiments, the demand transducer  331  may be a separate transducer than the pressure transducer  901 . The information from the demand transducer  331  may assist the processor  399  in making a determination as to how many compressors  301  should be operating. For example, if the user has a low breathing rate (e.g., less than an average breathing rate), the processor  399  may activate only a subset of the compressors  301  (e.g., one compressor). The user may have a low breathing rate if relatively inactive (e.g., asleep, sitting, etc.) as determined by comparing the detected breathing rate to a threshold. In some embodiments, the available compressors may be alternately used during low activity cycles to even out wear over the available compressors (instead of concentrating wear on one compressor). If the user has a relatively high breathing rate (e.g., at or more than an average breathing rate), the processor  399  may implement a greater number of compressors (e.g., both compressors  301   a - b ). The user may have a high breathing rate if relatively active (e.g., walking, exercising, etc.). The active/sleep mode may be determined automatically and/or the user may manually indicate a respective active or sleep mode (e.g., the user may press a button  2113  (active)/ 2115  (sleep) to indicate active or sleep mode (e.g., see  FIG. 21   b )). Other numbers of activity settings are also possible (e.g., low, moderate, active, and very active). Additional activity settings may use different numbers of subsets of compressors  301  (or different power levels for the operating compressors). 
         [0070]    A user breathing at a rate of 30 breaths per minute (BPM) may consume two and one-half times as much oxygen as user who is breathing at 12 BPM. As noted above, if the breathing rate of the user is calculated and used to adjust the number of and/or power input to the compressors  301 , less power may be used. For example, a user who is more active (e.g., walking) may consume more oxygen and require more power than the user who is less active (e.g., sitting or sleeping). In some embodiments, the breathing rate of the user may thus be detected and the bolus may be adjusted (e.g., by adjusting the power to or the operating number of the compressors  301 ) to provide more or less oxygen to allow the oxygen concentrator  100  to perform more efficiently by meeting the user&#39;s changing oxygen demands without operating at full power continuously. Using less power may reduce power consumption and increase battery life and/or decrease battery size requirements. 
         [0071]    In some embodiments, if the user&#39;s current activity level (e.g., as determined using the detected user&#39;s breathing rate or some other factor such as airflow near the nasal cannula  903 ) exceeds a threshold (e.g., a predetermined threshold), the processor  399  may implement an alarm (e.g., visual and/or audio) to warn the user that the current breathing rate exceeds a safe operating threshold (and therefore, for example, the user may not be receiving a prescribed amount of oxygen). For example, the threshold may be set at 20 breaths per minute (other breathing thresholds are also contemplated). In some embodiments, the oxygen sensor  307  coupled to the oxygen concentrator  100  may measure an oxygen level (e.g., as percent oxygen) in the gas being delivered to the user and an alarm may be activated if the percent oxygen drops below a threshold. In addition, a gas flow meter  1143  may measure a volume of gas flowing to the user. The volume measurement and percent oxygen measurement may provide the volume of oxygen being delivered to the user and an alarm may be activated if the volume drops below a threshold. In some embodiments, an alarm may be activated if the percent and/or volume of oxygen exceeds a threshold (e.g., too much oxygen is being delivered to the user). In some embodiments, the processor  399  may implement several levels of alarms (e.g., colored lights to indicate the current demand on the oxygen concentrator  100 ). Alarms may also include auditory alarms and/or messages provided on LED (Light Emitting Diode) display  2105 . In some embodiments, if the user&#39;s breathing rate exceeds the threshold and/or one or more compressors is inoperable, the operable compressors may be driven at a higher power setting (which may be only temporarily sustainable over an emergency period). Other compensation techniques are also contemplated. 
         [0072]    In some embodiments, oxygen from the canisters  101  may be stored in an oxygen accumulator  103  in the oxygen concentrator  100  and released to the user as the user inhales. For example, the oxygen may be provided in a bolus in the first few milliseconds of a user&#39;s inhalation. The user&#39;s inhalation may be detected using a demand transducer (e.g., pressure transducer  901 ). In some embodiments, the size of the bolus may be reduced if the response time is decreased and, therefore, the oxygen needed to provide a prescribed flow rate for the user may also be reduced as response time is reduced. Releasing the oxygen to the user as the user inhales may prevent unnecessary oxygen generation (further reducing power requirements) by not releasing oxygen, for example, when the user is exhaling. Reducing the amount of oxygen required may effectively reduce the amount of air compressing needed for the oxygen concentrator  100  (and subsequently may reduce the power demand from the compressors). In some embodiments, the bolus may be 8 cubic centimeters (cc) to provide the equivalent of a prescribed  1  LPM (or 16 ccs for 2 LPM or 24 ccs for 3 LPM). Slower responses may require a larger bolus (e.g., 15 or 16 cc for a 1 LPM prescribed rate). 
         [0073]    In some embodiments, as seen in  FIG. 27 , the bolus may include two or more pulses. For example, with a one liter per minute (LPM) delivery rate, the bolus may include two pulses: a first pulse  2701   a  at approximately 7 cubic centimeters and a second pulse  2701   b  at approximately 3 cubic centimeters. Other delivery rates, pulse sizes, and number of pulses are also contemplated. For example, at 2 LPMs, the first pulse may be approximately 14 cubic centimeters and a second pulse may be approximately 6 cubic centimeters and at 3 LPMs, the first pulse may be approximately 21 cubic centimeters and a second pulse may be approximately 9 cubic centimeters. In some embodiments, the larger pulse  2701   a  may be delivered when the onset of inhalation is detected (e.g., detected by demand transducer  331 ). In some embodiments, the pulses  2701  may be delivered when the onset of inhalation is detected and/or may be spread time-wise evenly through the breath. In some embodiments, the pulses  2701  may be stair-stepped through the duration of the breath. In some embodiments, the pulses  2701  may be distributed in a different pattern. Additional pulses may also be used (e.g., 3, 4, 5, etc. pulses per breath). While the first pulse  2701   a  is shown to be approximately twice the second pulse  2701   b , in some embodiments, the second pulse  2701   b  may be larger than the first pulse  2701   a . In some embodiments, pulse size and length may be controlled by, for example, valve F  305   g  which may open and close in a timed sequence to deliver the pulses  2701 . A bolus with multiple pulses  2701  may have a smaller impact on a user than a bolus with a single pulse. The multiple pulses  2701  may also result in less drying of a user&#39;s nasal passages and less blood oxygen desaturation. The multiple pulses  2701  may also result in less oxygen waste. 
         [0074]    In some embodiments, silicone rubber tube  197  ( FIG. 2   a ) may be compliant such that the diameter of the silicone rubber tube  197  may expand as the pulses  2701  travel through the silicone rubber tube  197  (and then return to a normal diameter between pulses  2701 ). The expansion may smooth out the pulses  2701  such that the pulses  2701  may be received by the user with a smoother peak. The smoother pulses may also be received by the user over a greater time period than the time period for the release of the boluses from valve  305   g.    
         [0075]    In various embodiments, the user&#39;s inhalation may be detected by using pressure transducer  901  on nasal cannula  903  detecting a negative pressure generated by venturi action at the start of a user&#39;s inhalation. The pressure transducer  901  may be operable to create a signal when the inhalation is detected to open a supply valve (e.g., valve  305   g ) to release an oxygen bolus from the oxygen accumulator  103 . In some embodiments, the pressure transducer  901  may be located at the exit of oxygen concentrator  100  (e.g., see  FIG. 9   a ) and may detect a pressure difference of the air in the tube  907 . In some embodiments, the pressure transducer  901  may be located at the end of a tube  907  delivering oxygen to the user to detect a pressure difference at the user&#39;s nose. For example, the pressure transducer  901  may use Whetstone bridge microgauges to detect a pressure difference at the exit of the oxygen concentrator  100  or on the nasal cannula  903 . Other placements of the pressure transducer  901  are also contemplated. Other pressure transducer types are also contemplated. In some embodiments, a plurality of pressure transducers may be used. In some embodiments, the pressure transducer  901  may be disposable. 
         [0076]    In some embodiments, pressure transducers  901  may provide a signal that is proportional to the amount of positive or negative pressure applied to a sensing surface. The pressure transducers  901  may need to be sensitive enough to provide a predictable relationship between the output of the pressure transducers  901  and the signal the pressure transducers  901  deliver. In some embodiments, the processor  399  may use information from the pressure transducer  901  to control when the bolus of oxygen should be released. The processor  399  may also control other components based on information from the pressure transducer  901  (e.g., the sensitivity of the pressure transducer  901 , the number of active compressors  301  and/or the power level of the compressors  301 , etc.). 
         [0077]    In some embodiments, the sensitivity of the pressure transducer  901  may be affected by the physical distance of the pressure transducer  901  from the user, especially if the pressure transducer  901  is located on the oxygen concentrator  100  and the pressure difference is detected through the tubing  907  to the nasal cannula  903 . In some embodiments, the pressure transducer sensitivity may not be affected by the length of the tubing  907  because the pressure transducer  901  may be placed in the mask or nasal cannula  903  (e.g., see  FIG. 9   b ) and a signal from the pressure transducer  901  may be delivered to a processor  399  in the oxygen concentrator  100  electronically via wire  905  (which may be co-extruded with the tubing  907 ) or through telemetry such as through Bluetooth™ or other wireless technology (e.g., using a wireless transmitter at the pressure transducer  901  and a wireless receiver at the oxygen concentrator  100 ). Placing the pressure transducer  901  on the nasal cannula  903  may allow for a longer delivery tube  907 . In some embodiments, the pressure transducer  901  may be placed near a prong on the nasal cannula used to deliver oxygen into the user&#39;s nose. 
         [0078]    In some embodiments, a dual lumen tube  909  may be used. One lumen (e.g., see cross section of lumens  911   a ,  911   b , or  911   c ) may deliver the oxygen to the user and one lumen  913  (e.g., see cross section of lumens  913   a ,  913   b , or  913   c ) may have a smaller diameter than the first lumen  911  and may transfer a pressure difference to the pressure transducer  901  mounted in the pressure transducer  901  at the oxygen concentrator  100 . With a smaller diameter, the second lumen  913  may reduce the volume of air between the user and the pressure transducer  901  for a given length of tubing. As the volume of air is reduced, compliance of a pressure spike delivery medium may be reduced and the sensitivity of the pressure transducer  901  may correspondingly be increased. For example, the pressure difference in lumen  913  resulting from a user&#39;s inhalation may be easier to detect at the pressure transducer  901  at the oxygen concentrator  100  than if the pressure difference were being detected through a lumen with a greater diameter. In some embodiments, the detectable pressure difference may decrease along the length of the lumen such that at a certain length of lumen, the pressure difference may not be detectable. Reducing the diameter of the lumen may result in the pressure difference being easier to detect at farther distances (i.e., because there is less air in the lumen to transmit the pressure difference and, correspondingly, less transporting air volume to weaken the pressure difference). The pressure difference may also be detectable more quickly in a narrow diameter lumen than in a lumen with a greater diameter. In some embodiments, the dual lumen  909  may take on the configuration shown in  FIG. 9   d  or  9   e . Other configurations are also contemplated. In some embodiments, the dual lumens  909  may be co-extruded plastic. Other manufacturing techniques and materials are also contemplated. 
         [0079]    Pressure transducer  901  may detect a pressure difference and/or a quantitative measurement of the inhalation pressure drop. Detecting the user&#39;s inhalation may not require a quantitative measurement of the inhalation pressure difference, but may rely on a temporal indicator to sense the inhalation. In some embodiments, devices other than or in addition to pressure transducers  901  may be used to detect a user&#39;s inhalation. For example, in some embodiments, a Hall-effect sensor  1001  (see  FIG. 10 ) may be used to detect a user&#39;s inhalation. The Hall-effect sensor  1001  may include a vane  1003  with a magnet  1007  on the vane  1003 . The vane  1003  may be positioned in the nasal cannula  903  and a second magnet  1005  (e.g., a rare earth magnet) may be arranged to assist in detection of movement of the magnet  1007  on the vane  1003  (using the Hall-effect) relative to the Hall-effect sensor  1001 . For example, when the vane  1003  is detected moving toward the second magnet  1005  (e.g., through the effect on a current in wire  1009  to the changing magnetic field), the sensor  1001  may indicate a negative pressure (which may correspond to the beginning of a user inhalation). For example, air movement toward the user&#39;s nose as the user begins taking a breath may move the vane  1003  toward the second magnet  1005 . The Hall-effect sensor  1001  may provide a more sensitive detector of the time the inhalation begins in the users breathing cycle. In some embodiments, the signal from the Hall-Effect sensor  1001  may be sent down wire  905  (or wirelessly transmitted). Other magnet-based sensors may also be used (e.g., a small magnet moved by the user&#39;s inhalation that acts to close a circuit). Other Boolean type sensors may be used. 
         [0080]    In some embodiments, the sensitivity of the oxygen concentrator  100  may be selectively attenuated to reduce false inhalation detections due to movement of air from a different source (e.g., movement of ambient air). For example, the oxygen concentrator  100  may have two selectable modes—an active mode and an inactive mode. In some embodiments, the user may manually select a mode (e.g., through a switch or user interface). In some embodiments, the mode may be automatically selected by the oxygen concentrator  100  based on a detected breathing rate. For example, the oxygen concentrator  100  may use the pressure transducer  901  to detect a breathing rate of the user. If the breathing rate is above a threshold, the oxygen concentrator  100  may operate in an active mode (otherwise, the oxygen concentrator may operate in an inactive mode). Other modes and thresholds are also contemplated. 
         [0081]    In some embodiments, in active mode, the sensitivity of the pressure transducer  901  may be mechanically, electronically, or programmatically attenuated. For example, during active mode, the processor  399  may look for a greater pressure difference to indicate the start of a user breath (e.g., an elevated threshold may be compared to the detected pressure difference to determine if the bolus of oxygen should be released). In some embodiments, the pressure transducer  901  may be mechanically altered to be less sensitive to pressure differences. In some embodiments, an electronic signal from the pressure transducer  901  may be electronically attenuated to indicate a smaller pressure difference than detected at the pressure transducer  901  (e.g., using a transistor). In some embodiments, during the inactive mode the sensitivity of the pressure transducer  901  may not be attenuated (e.g., the sensitivity of the pressure transducer  901  may be increased during sleep periods). For example, the processor  399  may look for a smaller pressure difference to indicate the start of a user breath (e.g., a smaller threshold may be compared to the detected pressure difference to determine if the bolus of oxygen should be released). In some embodiments, with increased sensitivity, the response time for delivery of the bolus of oxygen during the user&#39;s inhalation may be reduced. The increased sensitivity and smaller response time may reduce the size of the bolus necessary for a given flow rate equivalence. The reduced bolus size may also reduce the size and power consumption of the oxygen concentrator  100  that may reduce the size of a battery  395  needed to operate the oxygen concentrator (which may make the oxygen concentrator smaller and more portable). 
         [0082]      FIG. 11  illustrates a circuit diagram of an ultrasonic sensor assembly, according to an embodiment. In some embodiments, the oxygen sensor  307  may be an ultrasonic sensor that may be used to measure an oxygen level or the percent oxygen in the gas being delivered to the user. Other uses of the ultrasonic sensor assembly are also contemplated (e.g., to detect/measure the presence of other gases for other devices). An ultrasonic sound wave (from emitter  201 ) may be directed through a chamber  1101  containing a sample of the gas mixture (e.g., from the supply line providing oxygen to the user) to receiver  203 . The sensor  307  may be based on detecting the speed of sound through the gas mixture to determine the composition of the gas mixture (e.g., the speed of sound is different in nitrogen and oxygen). In a mixture of the two gases, the speed of sound through the mixture may be an intermediate value proportional to the relative amounts of each in the mixture. In some embodiments, the concentration of oxygen may be determined by measuring the transit time between an emitter  201  and the receiver  203 . In some embodiments, multiple emitters  201  and receivers  203  may be used. Emitters  201  may be axially aligned with respective receivers  203 . Other configurations are also contemplated. The readings from the emitters  201  and receivers  203  may be averaged to cancel errors that may be inherent in turbulent flow systems. In some embodiments, the presence of other gases may also be detected by measuring the transit time and comparing the measured transit time to predetermined transit times for other gases and/or mixtures of gases. 
         [0083]    In some embodiments, a zero-crossing point of the sound wave  1205  may be used as a reference point for these measurements (other points may also be used). The sensitivity of the sensor  307  may be increased by increasing the distance between the emitter  201  and receiver  203  (e.g., to allow several sound wave cycles to occur between the emitter  201  and the receiver  203 ). In some embodiments, if at least two sound cycles are present, the influence of structural changes of the transducer may be reduced by measuring the phase shift relative to a fixed reference at two points in time. If the earlier phase shift is subtracted from the later phase shift, the shift caused by thermal expansion of the transducer housing may be reduced or cancelled. The shift caused by a change of the distance between the emitter  201  and receiver  203  may be the approximately the same at the measuring intervals, whereas a change owing to a change in oxygen concentration may be cumulative. In some embodiments, the shift measured at a later time may be multiplied by the number of intervening cycles and compared to the shift between two adjacent cycles. 
         [0084]    In some embodiments, a pulse generator  1103  may send an enable pulse  1105  to a NAND gate U 2   1107 , which may channel a 40 kHz excitation signal to the emitter  201 , via amplifier U 1   1109 . Other excitations signals are also contemplated. After traversing the gaseous mixture in the chamber  1101 , the ultrasonic sound wave may impinge on the receiver  203 , and in the process, may undergo a phase shift, relative to the excitation signal. The gas may be introduced (prior to or during the sound wave transmission) into the chamber  1101  via ports  1131   a,b  that are perpendicular to the direction of the sound wave. The velocity-induced components of the phase shift may be reduced or cancelled. Turbulence may create a uniform gaseous mixture in the chamber  1101 . A change in the composition of the gas may affect the sound velocity of the sound wave traveling between the emitter  201  and the receiver  203 . A higher concentration of oxygen may correspond to a lower sound velocity (and, correspondingly, more phase shift). The sound wave captured by the receiver  203  may be amplified by U 3   1111  and put into a zero-crossing detector U 4   1113  (which may provide zero crossing pulse  1207  to flip-flop U 5   1117 ). The pulse generator  1103  may provide reference pulse  1   1115  to flip-flop U 5   1117 , clear the flip-flop U 5   1117  and the output  1207  of the zero-crossing detector  1113 , and create a negative-going pulse to gate pulse  1   1201 , as shown in  FIG. 12 . The length of this pulse may correspond to the phase shift occurring in the interval T 2 −T 1 . In an analogous fashion, gating pulse  2   1203  may be derived in the interval T 4 −T 3  (e.g., with reference pulse  2   1209  and zero crossing pulse  2   1211  provided to flip-flop U 6   1135 ). Phase shifts caused by structural changes in the transducer housing may be reduced or cancelled by subtracting interval T 2 −T 1  from interval T 4 −T 3 . An embodiment of the process is illustrated in  FIG. 14 . The integrator  1133  may be zeroed to reduce or eliminate drift that may have accrued since the last operation. Then, the subtraction gate  1407  may be opened by gating pulse  1   1201 . After the gate has closed, the voltage at the integrator output may be V 1  (see  1401  in  FIG. 14 ): 
         [0000]        V 1= K 1×( St+Sc )
 
         [0000]    (where St is the phase shift caused by temperature  1301 , Sc is the phase shift caused by changes in oxygen concentration  1303 , and K 1 =t/RC×(−Vref), where RC is a reflection coefficient and Vref is the reference voltage). After the gate has closed, the integrator output  1407  may remain stable until the addition gate  1409  opens. (The flat sections in the figure have been omitted for clarity.) After the termination of the addition gating pulse, the output voltage may be V 2  (see  1403  in  FIG. 14 ): 
         [0000]        V 2= K 1×[( St+Sc )−( St+ 2× Sc )]= K 1× Sc  
 
         [0000]    Termination of the addition gate may clear flip-flop U 7   1119 , which may output a gating pulse that opens the calibrating gate, U 8 C  1121 . U 7   1119  may be set by U 4   1113 , when V 3 =0 (see  1403 ): 
         [0000]        V 3= V 2− K 2× t= 0;  K 1× Sc=K 2× t; t=K 1/ K 2× Sc  
 
         [0000]    (where K 2 =t/RC×(+Vref)). The length of the negative-going pulse from U 7   1119  may be proportional to the phase shift Sc. An embodiment of the relationship between St and Sc is shown in  FIG. 13 . The pulse generator shown in  FIG. 11  may issue a concentration reference pulse  1413  whose length is set to correspond to, for example, the minimum acceptable oxygen concentration (e.g., as defined by the user&#39;s prescription or other source). As shown in  FIG. 14 , low oxygen concentration may cause the zero crossing to occur earlier and make both inputs of U 11   1123  high at the same time. The resulting pulse may be used to activate an audible alarm  1139  (through amplifier U 12   1141 ) to alert the user that the oxygen concentration may be too low. The point at which the alarm is triggered may be set by adjusting P 2   1125 , (e.g., see  FIG. 11 ). The velocity of sound may increase with temperature (which may incorrectly indicate a decrease in oxygen concentration). This effect may be reduced or cancelled by using a thermistor  1127  whose resistance increases with temperature to restore the duration of the concentration pulse to a corrected value. The amount of correction introduced may be varied by adjusting P 1   1129 .  FIG. 6   a - h  shows the sensor constructed with discrete components. In some embodiments, the processing may be performed by a processor  399  (e.g., a field programmable gate array (FPGA)). 
         [0085]    In some embodiments, the oxygen sensor  307  may include a gas flow meter  1143  that uses the Doppler effect to measure the volume of gas flow past the sensor. With the volume measurement from the gas flow meter  1143  and the percent oxygen reading from the ultrasonic sensor, the amount of oxygen delivered to the user may be measured and controlled. For example, if the concentration of oxygen is greater than a desired percentage, (e.g., as indicted by the length of the concentration reference pulse  1413 ), then the user is receiving at least a volume of oxygen equal to the volume of gas flow*the desired percentage of oxygen. In some embodiments, one or more signals from the ultrasonic sensor may be relayed to the processor  399  for a determination of an actual percentage of oxygen in the sample. For example, the processor  399  may receive an indication of gating pulse  1201 , gating pulse  1203 , and/or concentration reference pulse  1413  to determine an approximate percentage of oxygen in the gas sample. Other signals may also be used. Using a gas flow meter  1143  that uses the Doppler effect to measure the volume of gas flow may be more accurate than simply using time, pressure and orifice size to determine delivered volume. 
         [0086]    In some embodiments, the battery  395  may be a rechargeable lithium battery. Other battery types are also contemplated. Larger batteries may be used for longer battery life. Smaller batteries may have a shorter battery life, but may be lighter. In some embodiments, a battery large enough to provide a battery life of 2 hours (using the various power saving mechanisms discussed herein) may be used. Other battery lifetimes/sizes are also contemplated. As seen in  FIG. 15 , in some embodiments, additional power may be provided to the oxygen concentrator  100  through a solar powered recharging circuit including solar panel  1501  so that the battery  395  may be supplemented to increase battery life or reduce battery size (e.g., especially while the user may be consuming more oxygen (and thus more power) outdoors). In some embodiments, an alternating current power adapter may be provided to charge the battery and/or provide power to the oxygen concentrator. Other power sources are also contemplated (e.g., an adapter to allow the oxygen concentrator to be plugged into a power outlet in an automobile). 
         [0087]      FIG. 16  illustrates a flowchart of an embodiment for oxygen concentrator operation, according to an embodiment. It should be noted that in various embodiments of the methods described below, one or more of the elements described may be performed concurrently, in a different order than shown, or may be omitted entirely. Other additional elements may also be performed as desired. 
         [0088]    At  1601 , air may be pulled into the compressor  301 . The compressor may include, for example, dual-pump diaphragm compressors  301   a - b . The air may pass through a moisture and sound absorbing muffler  393  prior to entering the compressor  301 . For example, a water absorbent (such as a polymer water absorbent) may be used. Other absorbents may also be used. 
         [0089]    At  1603 , air from the compressor  301  may be delivered to a first canister  101   a  comprising zeolite  391 . The air from the compressor  301  may be directed through one or more valves  305  on the path to the first canister  101   a . The valves  305  may be coupled to and controlled by a microprocessor (e.g., processor  399 ). 
         [0090]    At  1605 , a gas mixture (which may be comprised of mainly oxygen) may be delivered out of the first canister  101   a  and into an oxygen accumulator  103 . In some embodiments, the gas mixture may pass through a check valve  123   a  (e.g., a butterfly check valve) between the first canister  101   a  and the oxygen accumulator  103 . In some embodiments, a pressure transducer  389  may detect a pressure of the oxygen accumulator  103 . The pressure of the oxygen accumulator may be used, for example, by the processor to determine if one or more of the canisters has a leak, etc. Other uses for the pressure are also contemplated. 
         [0091]    At  1607 , an inhalation may be detected by the user through a demand transducer  331  (e.g., pressure transducer  901 ). 
         [0092]    At  1609 , the gas mixture from the oxygen accumulator  103  may be passed through an oxygen sensor  307  (e.g., an ultrasonic sensor) to detect a concentration of oxygen in the gas mixture. The sensor may also include or be coupled to a gas flow meter  1143  to detect a volume of the gas passing the gas flow meter  1143 . 
         [0093]    At  1611 , the gas mixture may pass through a tube (e.g., tube  907  or tube  909 ) to be delivered to the user through a nasal cannula  903 . In some embodiments, the gas mixture may be delivered to the user in a single pulse or in two or more pulses (e.g., see  FIG. 27 ). 
         [0094]    At  1613 , air from the compressor  301  may be delivered into the second canister  101   b  comprising zeolite  391 . 
         [0095]    At  1615 , a gas mixture (which may be comprised of mainly oxygen) may be delivered out of the second canister  101   b  and into the oxygen accumulator  103 . 
         [0096]    At  1617 , nitrogen from the first canister  101   a  may be purged from the first canister  101   a  by releasing a pressure (e.g., by opening valve  305   c  or  305   d  (and closing valves  305   a  and  305   b ) to open up an air pathway between the first canister  101   a  and the output vent  327 ) from the first canister  101   a.    
         [0097]    At  1619 , oxygen from the oxygen accumulator  103  may be passed through an opposite end of the first canister  101   a  to further purge the nitrogen from the first canister  101   a.    
         [0098]    At  1621 , nitrogen from the first canister  101   a  may pass through a muffled output vent  327  and out of the oxygen concentrator  100 . 
         [0099]    At  1623 , air from the compressor  301  may be delivered into the first canister  101   a  comprising zeolite  391 . 
         [0100]    At  1625 , a gas mixture (which may be comprised of mainly oxygen) may be delivered out of the first canister  101   a  and into an oxygen accumulator  103 . 
         [0101]    At  1627 , nitrogen from the second canister  101   b  may be purged from the second canister  101   b  by releasing a pressure from the second canister  101   b.    
         [0102]    At  1629 , oxygen from the oxygen accumulator  103  may be passed through an opposite end of the second canister  101   b  to further purge the nitrogen from the second canister  101   b.    
         [0103]      FIG. 17  illustrates a flowchart of an embodiment for oxygen concentrator assembly, according to an embodiment. It should be noted that in various embodiments of the methods described below, one or more of the elements described may be performed concurrently, in a different order than shown, or may be omitted entirely. Other additional elements may also be performed as desired. 
         [0104]    At  1701 , a first housing component  111   a  of the oxygen concentrator  100  may be injection molded. The first housing component  111   a  may include internal air pathways and zeolite canisters  101 . In some embodiments, an inverted mold may be formed (with solid portions corresponding to the air pathways/inner canisters of the first housing component  111   a ) and placed inside a container with an inner shape with dimensions similar to the outer dimensions of the first housing component  111   a . Spacers may be added between the solid portions and the container to hold the solid portions relative to the container. A plastic (e.g., a liquid thermoplastic) may be injected into the spaces between the outer container and the solid portions to form the injection molded first housing component  111   a . The mold (comprising the container and solid portions) may then be removed and/or broken away. In some embodiments, the mold may be melted away from the injection molded first housing component  111   a  after the injection molded first housing component  111   a  has cooled. Other methods of injection molding are also contemplated. Other molding techniques are also contemplated. 
         [0105]    At  1703 , a second housing component  111   b  of the oxygen concentrator  100  may be injection molded. The second housing component  111   b  may include internal air pathways and endcaps for the zeolite canisters  101 . 
         [0106]    At  1705 , spring baffles  129  may be placed into the endcaps for the zeolite canisters on the second housing component  111   b . In some embodiments, the spider legs  701  of the spring baffle  129  may engage the ridges  133  on the back of the canisters  101   a,b.    
         [0107]    At  1707 , filters (e.g., filters  207 ) may be fastened to the inner end of the zeolite canisters on the first housing component  111   a  and the inner end (end without the spider legs) of spring baffles  129  in the second housing component  111   b.    
         [0108]    At  1709 , O-rings  135  may be added between air pathways  121  between the first housing component  111   a  and the second housing component  111   b . For example, O-rings  135  may be placed between the endcaps for the zeolite canisters  101  on the second housing component  111   b  and the zeolite canisters  101  on the first housing component  111   a . Other O-rings may also be used. 
         [0109]    At  1711 , zeolite  391  may be added to the zeolite canisters  101  and the first housing component  111   a  and the second housing component  111   b  may be fastened together (e.g., through an adhesive, solvent weld, etc.). 
         [0110]    At  1713 , press fit flow restrictors (e.g., press fit flow restrictors  311 ,  321 ,  323 , and  325 ) may be inserted into apertures (e.g., formed during the injection molding process) of the first housing component  111   a  and/or the second housing component  111   b.    
         [0111]    At  1715 , plugs (e.g., plugs  127 ) may be inserted and fastened into the apertures to seal the apertures. For example, the plugs may be fastened through the use of an adhesive or solvent weld. Other fastening techniques are also contemplated. 
         [0112]    At  1717 , check valves  123  may be inserted into and fastened (e.g., through an adhesive) to the first housing component  111   a  and/or second housing component  111   b.    
         [0113]    At  1719 , an ultrasonic sensor emitter  201  and receiver  203  may be inserted into and fastened to the second housing component. For example, the ultrasonic sensor emitter  201  and receiver  203  may be coupled to the second housing component through an adhesive or friction fit. In some embodiments, multiple ultrasonic sensor emitters  201  and ultrasonic receivers  203  may be used. Emitters  201  may be axially aligned with respective receivers  203  such that the gas flows perpendicular to the axis of alignment. Other configurations are also contemplated. 
         [0114]    At  1721 , valves (e.g., valves  305 ) may be fastened to the first housing component  111   a  and/or the second housing component  111   b  (e.g., screwed onto the exterior). Other fastening techniques for the valves are also contemplated (e.g., adhesive). 
         [0115]    At  1723 , one or more compressors  301  may be coupled to the canisters  101  of the first housing component (e.g., through one or more tubes  199  coupled to valves  305  coupled to the first housing component). 
         [0116]    At  1725 , the ultrasonic emitter  201  and receiver  203 , valves, and one or more compressors may be wired to one or more microcontrollers (e.g., processor  399 ). Other electronic components may also be coupled to the microcontrollers. For example, an on/off button  2103   a,b  and an LED display  2105   a,b  (see  FIGS. 21   a,b ) to convey information such as low oxygen or low power warnings to the user. 
         [0117]    At  1727 , a battery  395  may be electrically coupled to the ultrasonic emitter  201  and receiver  203 , valves, one or more compressors  301 , and one or more microcontrollers. The battery  395  may also be electrically coupled to other components of the oxygen concentrator  100 . In some embodiments, the battery  395  may be electrically coupled to components of the oxygen concentrator  100  through other components (e.g., the battery  395  may be coupled to the valves  305  through the processor  395 ). 
         [0118]    At  1729 , open cell foam and the vent  401  may be coupled to the first housing component  111   a  (e.g., the foam may be inserted into the vent out  137  and vent  401  may be fastened over the vent out  137  through, for example, an adhesive). 
         [0119]    At  1731 , the oxygen concentrator components (e.g., first housing component  111   a , second housing component  111   b , battery  395 , compressors  301 , etc.) may be packaged together into an outer housing  2101   a,b  (e.g., see  FIGS. 21   a,b ). In some embodiments, the outer housing  2101  may be a durable, light-weight plastic. Other materials are also contemplated. Other outer housing configurations are also contemplated. In some embodiments, the components may be placed in an foam housings  2401  (see  FIGS. 24-25 ) and the foam housings  2401  may be placed inside an enclosure housing  2201  before being placed inside outer housing  2101 . 
         [0120]    At  1733 , a tube (e.g., tube  907  or  909 ) with a nasal cannula  903  may be coupled to the oxygen outlet  107 . If a dual lumen is used, lumen  913  may be coupled to a pressure transducer  901  coupled to the oxygen concentrator  100 . 
         [0121]      FIG. 18  illustrates a flowchart of an embodiment for compressor control, according to an embodiment. It should be noted that in various embodiments of the methods described below, one or more of the elements described may be performed concurrently, in a different order than shown, or may be omitted entirely. Other additional elements may also be performed as desired. 
         [0122]    At  1801 , a breathing rate of the user may be detected (e.g., by determining how may inhalations pressure sensor  901  detects per minute). 
         [0123]    At  1803 , a determination may be made as to whether the breathing rate is below a first threshold. The first threshold may be, for example, 15 breaths per minute (other thresholds are also contemplated). In some embodiments, the threshold may be predetermined and/or may be variable (e.g., adjusted according to an external temperature detected by a temperature sensor coupled to the oxygen concentrator  100 ). In some embodiments, the threshold may be set by the user (or, for example, by a doctor&#39;s prescription). 
         [0124]    At  1805 , if the breathing rate is below a first threshold, a subset of the compressors may be used (e.g., one of two compressors may be used). Using a subset of compressors may lower power requirements and conserve the battery. In some embodiments, the user may manually place the oxygen concentrator  100  into a lower power mode that uses a subset of the compressors  301 . 
         [0125]    At  1807 , if the breathing rate is above the first threshold, a greater number than the subset of compressors may be used (e.g., two of two compressors may be used). In some embodiments, if one or more of the available compressors malfunctions, all of the available compressors may be used (regardless of detected breathing rate) until the compressor can be repaired. In some embodiments, fewer than all of the available compressors may be used if another compressor malfunctions. 
         [0126]      FIG. 19  illustrates a flowchart of an embodiment for ultrasonic sensor operation, according to an embodiment. It should be noted that in various embodiments of the methods described below, one or more of the elements described may be performed concurrently, in a different order than shown, or may be omitted entirely. Other additional elements may also be performed as desired. 
         [0127]    At  1901 , an ultrasonic sound wave may be produced by the ultrasonic emitter  201 . 
         [0128]    At  1903 , the ultrasonic sound wave may pass through a sample of gas mixture (e.g., which may be comprised of mostly oxygen) in a chamber between the emitter  201  and receiver  203 . 
         [0129]    At  1905 , the ultrasonic sound wave may be received by the ultrasonic receiver  203 . 
         [0130]    At  1907 , the transit time for the sound wave may be determined. 
         [0131]    At  1909 , the transit time for the sound wave through the gas mixture may be compared to predetermined transit times for other gases to determine an approximate concentration of the gas constituents of the mixture. In some embodiments, a phase shift due to structural changes in the housing may be accounted for in the comparison. 
         [0132]      FIG. 20  illustrates an embodiment of a headset/microphone boom  2003 . In some embodiments, a device  387  (e.g., an MP3 player, mobile phone, etc.) may be integrated into the oxygen concentrator  100  (e.g., integrated into the outer housing  2101 ). The microphone  2005  and headphones  2007  may be coupled to the device through a wire  2001  (e.g., which may be coextruded with the tube  909 , coupled to wire  905 , or wire  2001  and wire  905  may be one wire). The oxygen concentrator may have an audio output/input jack  2109  (other locations of the audio/input jack  2109  are also contemplated). In some embodiments, the headset  2003  may be wireless (e.g., may use Bluetooth™). In some embodiments, the microphone  2005  may be coupled to the nasal cannula  903  and the headphones  2007  may be coupled to wire  905 . Other configurations are also contemplated. For example, the oxygen from the oxygen concentrator may be directed at the user&#39;s nose and/or mouth from a tube coupled to microphone  2005  (instead of or in addition to a nasal cannula). The microphone  2005  may be embedded in the tube directing the oxygen toward the user&#39;s nose and/or mouth (and, correspondingly, may be near the user&#39;s mouth). The headset/microphone boom  2003  may also be used with the oxygen concentrator  100  for hands-free cellular phone use. Other uses are also contemplated. 
         [0133]      FIGS. 21   a - c  illustrate two embodiments of an outer housing  2101   a,b . In some embodiments, the outer housing  2101   a,b  may be comprised of a light-weight plastic. Other materials are also contemplated. Other outer housing configurations are also contemplated. In some embodiments, outer housing  2101   b  may include buttons to activate active mode  2113 , sleep mode  2115 , dosage buttons (e.g., 1 LPM button  2117   a,  2 LPM button  2117   b , and 3 LPM button  2117   c ), and a battery check button  2119  (which may result in a relative battery power remaining LED being illuminated in LED panel  2105   b ). In some embodiments, one or more of the buttons may have a respective LED that may illuminate when the respective button is pressed (and may power off when the respective button is pressed again). Other buttons and indicators are also contemplated. In some embodiments, outer housing  2101   b  may include inlet air slot  2121  for receiving external air. Vent  2123  may be used to vent air (e.g., nitrogen) from the oxygen concentrator. In some embodiments, a vent  2123  may also be on the opposing side of the outer housing  2101   b . Plug receptacle  2125  may plug into an external power adapter or battery pack (e.g., receive connector  2823  as seen in  FIG. 28   c ). Other power sources are also contemplated. In some embodiments, the solar panel  1501  may be coupled to an outside of the outer housing  2101   a,b . In some embodiments, the solar panel  1501  may be coupled to an exterior of a backpack that receives the oxygen concentrator. 
         [0134]      FIG. 22  illustrates an embodiment of an enclosure housing  2201 .  FIG. 23  illustrates an embodiment of two half sections  2201   a,b  of the enclosure housing  2201 . In some embodiments, a section of foam  2203  may be included between the enclosure housing  2201  and the outer housing  2101 . For example, the foam may be approximately ¼ inch thick. Other thicknesses are also contemplated. The foam may reduce vibration transferred to the outer housing  2101  and/or user. The reduction in vibration may reduce noise (e.g., reduce noise by 1 decibel) from the oxygen concentrator while operating. Other sound reduction levels are also contemplated. In some embodiments, the foam may substantially surround the enclosure housing  2201 . In some embodiments, the components of the oxygen concentrator  100  may be placed inside of foam housings (e.g.,  FIG. 24  illustrates an embodiment of a first foam housing  2401   a  and  FIG. 25  illustrates an embodiment of a complimentary second foam housing  2401   b ) and the foam housings  2401  may be placed inside the enclosure housing half sections  2201   a,b . The enclosure housing half sections  2201   a,b  may be coupled together (e.g., through an adhesive, solvent weld, rivets, etc.) to form enclosure housing  2201 . The enclosure housing  2201  may be made of a light-weight plastic. Other materials are also contemplated. The enclosure housing  2201  may then be placed in the outer housing  2101 . The foam housings  2401  may be comprised of open cell foam or closed cell foam (which may reduce more internal sound). Other materials for the foam housings  2401  are also contemplated. In some embodiments, the foam housings  2401   a,b  may be separately coupled together (e.g., sealed together through an adhesive or solvent weld). In some embodiments, the oxygen concentrator components may not be rigidly mounted to the enclosure housing  2201 , but may be held by the foam (which may also protect the components, for example, from outer forces on the oxygen concentrator). The placement of the oxygen concentrator components in the foam may be aligned for efficiency to reduce the size and weight of the oxygen concentrator. 
         [0135]      FIG. 26  illustrates a side and front profile of a component arrangement in the foam housings  2401 , according to an embodiment. The foam housings  2401  may be configured to conform to the oxygen concentrator components (e.g., compressors  301   a,b , housing components  111   a,b , batteries  395   a,b , fans  2601   a,b , etc.). For example, the foam housings  2401  may be configured with pockets to receive the oxygen concentrator components. The foam housings  2401  may also incorporate airflow passages  2603   a - d  (e.g., cutouts in the foam). Air may be pulled into (e.g., through vent  2203 ) and/or moved around in the foam housings  2401  through fans  2601   a,b . In some embodiments, vent  2203  may comprise a sonic baffle with a felt air filter. Other air filters are also contemplated. Air entering the vent  2203  may be filtered by the felt prior to entering the compressors  301 . Air may move through air pathways/channels in the foam. The channeled foam may reduce/baffle the sound of the air movement. In some embodiments, the expansion and contraction of the sound (e.g., as the sound/air passes through vent  2203 ) may reduce the sound. The fans  2601  may be, for example, 12 volt, 1-inch square fans. Other types, numbers, and placements of fans may also be used. Warm air and/or nitrogen may exit the enclosure housing  2201  through vent  2205 ,  2605  and through outer housing  2101  through a corresponding vent (e.g., vent  2107 ). 
         [0136]    In some embodiments, two compressors  301   a,b  may be used (e.g., two dual-pump diaphragm compressors). In some embodiments, the two compressors  301   a,b  may be 12 volt compressors. In some embodiments, each compressor may be attached to a fan  2601  (e.g., compressor  301   a  may be electrically coupled to fan  2601   a  and compressor  301   b  may be electrically coupled to fan  2601   b ). In some embodiments, increasing or decreasing power to a compressor (e.g., compressor  301   a ) may result in a corresponding increase or decrease in power to the compressor&#39;s corresponding fan (e.g., fan  2601   a ). This may further conserve power by decreasing power to a fan when the fan&#39;s corresponding compressor is operating under decreased power (and vice-versa). Other compressor/fan arrangements are also contemplated. 
         [0137]    In some embodiments, the airflow passages  2603   a - d  may be used to for entering cooling air, exiting warm air, nitrogen, etc. In some embodiments, the foam housings  2401  may dampen sound and insulate heat from the oxygen concentrator components (e.g., to prevent hot spots on the outer casings from the oxygen concentrator components). Other configurations of the foam housings  2401  are also contemplated. For example, foam may be applied around the oxygen concentrator components and allowed to set. In some embodiments, materials other then foam may be used. 
         [0138]    In some embodiments, passages in the foam housings  2401  may be used for electrical connections. For example, passage  2403  may be used for connections (e.g., wires) from the batteries  395  to various components of the oxygen concentrator (e.g., compressors  301 , circuit board  2607 , etc.). Passages  2405  and  2407  may also be used for electrical connections. Passages may also be provided for air tubes. For example, passages  2501   a  and  2501   b  may be provided for air tubes between the compressors  301  and the housing component  111   a . In some embodiments, the oxygen may exit through a tube through passage  2407  and through exit port or exit nozzle  2111   a,b  in the outer casing (other exit locations are also contemplated). 
         [0139]      FIGS. 28   a - d  illustrate an attachable external battery pack  2807  for the oxygen concentrator, according to an embodiment. In some embodiments, an outer covering  2801  on the oxygen concentrator may include various fasteners for coupling the oxygen concentrator to external battery pack  2807 . For example, Velcro™ receiving portions  2811   a,b  may receive Velcro™ tabs  2805   a,b , respectively. For example, Velcro™ receiving portions  2811   a,b  may include Velcro™ loops and tabs  2805   a,b  may include Velcro™ hooks. Other configurations are also contemplated. In some embodiments, straps  2803   a,b  may loop through receiving rings  2813   a,b , respectively. The straps  2803   a,b  may be pulled through their respective rings  2813   a,b , and then the strap may be folded over (with the fold aligned with the rings  2813   a,b ). Straps  2803   a,b  may also have Velcro™ portions. For example, Velcro™ portions  2831   a,b  (e.g., hook portions) may engage respective Velcro™ portions  2829   a,b  (e.g., loop portions) when the straps  2803   a,b  are folded over (after passage through their respective hooks  2813   a,b ). Other Velcro™ placements are also contemplated (e.g., between a top of external battery pack  2807  and the bottom of cover  2801 ). Other fastener types are also contemplated (e.g., adhesive, tape, buckles, etc). In some embodiments, the covering  2801  may include one or more mesh vents (e.g., vents  2819   a,b , and  2815   a,b ). Covering  2801  may also include belt loops  2821   a,b  to receive a user belt (e.g., to hold the oxygen concentrator on a user&#39;s waist). Rings  2817   a,b  may be used to attach a shoulder strap to carry the oxygen concentrator over a user&#39;s shoulder (e.g., a strap with respective Velcro™ portions may be inserted through each ring and the Velcro™ portions folded over on each other). In some embodiments, the external battery pack  2807  may include a connector  2823  to plug into a receiving connector (e.g., plug receptacle  2125  in  FIG. 21   c ) on the oxygen concentrator to deliver power from the batteries in the external battery pack  2807 . The external battery pack  2807  may include, for example, 16 cells to deliver direct current (other battery types and cell numbers are also contemplated). The battery pack  2807  may also include a battery power indicator  2809 . For example, a series of light emitting diodes (LEDs)  2827  may light up to indicate an amount of battery power remaining (e.g., 0%, 25%, 50%, 75%, 100%, etc). Other indicators are also contemplated. In some embodiments, the external battery pack  2807  may include feet  2825   a,b . In some embodiments, the covering  2801  may be made of canvas, nylon, plastic, etc. Other materials for the covering are also contemplated. In some embodiments, rings  2813   a,b  and  2817   a,b  may be made of stainless steel, plastic, etc. Rings  2813   a,b  and  2817   a,b  may be fastened to the covering  2801  through adhesive, through sewed-on patches (e.g., which overlap a portion of the respective ring), etc. Feet  2825   a,b  may be made of rubber (other materials for the feet  2825   a,b  are also contemplated). 
         [0140]    Embodiments of a subset or all (and portions or all) of the above may be implemented by program instructions stored in a memory medium (e.g., memory  397 ) or carrier medium and executed by a processor (e.g., processor  399 ). A memory medium may include any of various types of memory devices or storage devices. The term “memory medium” is intended to include an installation medium, e.g., a Compact Disc Read Only Memory (CD-ROM), floppy disks, or tape device; a computer system memory or random access memory such as Dynamic Random Access Memory (DRAM), Double Data Rate Random Access Memory (DDR RAM), Static Random Access Memory (SRAM), Extended Data Out Random Access Memory (EDO RAM), Rambus Random Access Memory (RAM), etc.; or a non-volatile memory such as a magnetic media, e.g., a hard drive, or optical storage. The memory medium may comprise other types of memory as well, or combinations thereof. In addition, the memory medium may be located in a first computer in which the programs are executed, or may be located in a second different computer that connects to the first computer over a network, such as the Internet. In the latter instance, the second computer may provide program instructions to the first computer for execution. The term “memory medium” may include two or more memory mediums that may reside in different locations, e.g., in different computers that are connected over a network. 
         [0141]    In some embodiments, a computer system at a respective participant location may include a memory medium(s) on which one or more computer programs or software components according to one embodiment of the present invention may be stored. For example, the memory medium may store one or more programs that are executable to perform the methods described herein. The memory medium may also store operating system software, as well as other software for operation of the computer system. 
         [0142]    Further modifications and alternative embodiments of various aspects of the invention may be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.