Patent Publication Number: US-2023149655-A1

Title: Methods and apparatus for providing concentrated therapy gas for a respiratory disorder

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present disclosure claims priority from Australia Provisional Patent Application Serial No. 2020901121, filed on 8 Apr. 2020, the entire disclosure of which is hereby incorporated herein by reference. 
     FIELD OF THE TECHNOLOGY 
     The present technology relates generally to methods and apparatus for treating respiratory disorders, such as those involving gas adsorption or controlled pressure swing adsorption. Such methodologies may be implemented in an oxygen concentrator. In some examples, the technology more specifically concerns such methods and apparatus for generating an oxygen therapy from a portable oxygen concentrator with multiple flow paths for implementing a hybrid mode where a flow of therapy gas has characteristic(s) (e.g., purity and/or flow rate) that may differ during inspiration (or part of inspiration) relative to non-inspiration times or expiration. 
     DESCRIPTION OF THE RELATED ART 
     Human Respiratory System and its Disorders 
     The respiratory system of the body facilitates gas exchange. The nose and mouth form the entrance to the airways of a patient. 
     The airways include a series of branching tubes, which become narrower, shorter and more numerous as they penetrate deeper into the lung. The prime function of the lung is gas exchange, allowing oxygen to move from the inhaled air into the venous blood and carbon dioxide to move in the opposite direction. The trachea divides into right and left main bronchi, which further divide eventually into terminal bronchioles. The bronchi make up the conducting airways, and do not take part in gas exchange. Further divisions of the airways lead to the respiratory bronchioles, and eventually to the alveoli. The alveolated region of the lung is where the gas exchange takes place, and is referred to as the respiratory zone. See “Respiratory Physiology”, by John B. West, Lippincott Williams &amp; Wilkins, 9th edition published 2012. 
     A range of respiratory disorders exist. Examples of respiratory disorders include respiratory failure, Obesity Hyperventilation Syndrome (OHS), Chronic Obstructive Pulmonary Disease (COPD), Neuromuscular Disease (NMD) and Chest wall disorders. 
     Respiratory failure is an umbrella term for respiratory disorders in which the lungs are unable to inspire sufficient oxygen or exhale sufficient CO 2  to meet the patient&#39;s needs. Respiratory failure may encompass some or all of the following disorders. 
     A patient with respiratory insufficiency (a form of respiratory failure) may experience abnormal shortness of breath on exercise. 
     Obesity Hyperventilation Syndrome (OHS) is defined as the combination of severe obesity and awake chronic hypercapnia, in the absence of other known causes for hypoventilation. Symptoms include dyspnea, morning headache and excessive daytime sleepiness. 
     Chronic Obstructive Pulmonary Disease (COPD) encompasses any of a group of lower airway diseases that have certain characteristics in common. These include increased resistance to air movement, extended expiratory phase of respiration, and loss of the normal elasticity of the lung. Examples of COPD are emphysema and chronic bronchitis. COPD is caused by chronic tobacco smoking (primary risk factor), occupational exposures, air pollution and genetic factors. Symptoms include: dyspnea on exertion, chronic cough and sputum production. 
     Neuromuscular Disease (NMD) is a broad term that encompasses many diseases and ailments that impair the functioning of the muscles either directly via intrinsic muscle pathology, or indirectly via nerve pathology. Some NMD patients are characterised by progressive muscular impairment leading to loss of ambulation, being wheelchair-bound, swallowing difficulties, respiratory muscle weakness and, eventually, death from respiratory failure. Neuromuscular disorders can be divided into rapidly progressive and slowly progressive. Rapidly progressive disorders are characterised by muscle impairment that worsens over months and results in death within a few years (e.g. Amyotrophic lateral sclerosis (ALS) and Duchenne muscular dystrophy (DMD) in teenagers). Variable or slowly progressive disorders are characterised by muscle impairment that worsens over years and only mildly reduces life expectancy (e.g. Limb girdle, Facioscapulohumeral and Myotonic muscular dystrophy). Symptoms of respiratory failure in NMD include: increasing generalised weakness, dysphagia, dyspnea on exertion and at rest, fatigue, sleepiness, morning headache, and difficulties with concentration and mood changes. 
     Chest wall disorders are a group of thoracic deformities that result in inefficient coupling between the respiratory muscles and the thoracic cage. The disorders are usually characterised by a restrictive defect and share the potential of long term hypercapnic respiratory failure. Scoliosis and/or kyphoscoliosis may cause severe respiratory failure. Symptoms of respiratory failure include: dyspnea on exertion, peripheral oedema, orthopnea, repeated chest infections, morning headaches, fatigue, poor sleep quality and loss of appetite. 
     Respiratory Therapies 
     In respiratory therapies known as “flow” therapies, the interface to the patient&#39;s airways is ‘open’ (unsealed) and the respiratory therapy may supplement the patient&#39;s own spontaneous breathing with a flow of conditioned or enriched air. In one example, High Flow therapy (HFT) is the provision of a continuous, heated, humidified flow of air to an entrance to the airway through an unsealed or open patient interface at a “treatment flow rate” that is held approximately constant throughout the respiratory cycle. The treatment flow rate is nominally set to exceed the patient&#39;s peak inspiratory flow rate. 
     Another form of flow therapy is long-term oxygen therapy (LTOT) or supplemental oxygen therapy. Doctors may prescribe a continuous flow of oxygen enriched air at a specified oxygen purity (from 21%, the oxygen fraction in ambient air, to 100%) at a specified flow rate (e.g., 1 litre per minute (LPM), 2 LPM, 3 LPM, etc.) to be delivered to the patient&#39;s airway. 
     Respiratory Therapy Systems 
     Respiratory flow therapies may be provided by a respiratory therapy system or device. A respiratory therapy system as described herein may comprise an oxygen source, an air circuit, and a patient interface. 
     Air Circuit 
     An air circuit is a conduit or a tube constructed and arranged to allow, in use, a flow of conditioned or enriched air to travel between two components of a respiratory therapy system such as the oxygen source and the patient interface. 
     Patient Interface 
     A patient interface may be used to interface respiratory equipment to its wearer, for example by providing a flow of air to an entrance to the airways. The flow of air may be provided via a mask to the nose and/or mouth, a tube to the mouth or a tracheostomy tube to the trachea of a patient. For flow therapies such as nasal HFT or LTOT, the patient interface is configured to insufflate the nares but specifically to avoid a complete seal. One example of such a patient interface is a nasal cannula. 
     Oxygen Source 
     Experts in this field have recognized that exercise for respiratory failure patients provides long term benefits that slow the progression of the disease, improve quality of life and extend patient longevity. Most stationary forms of exercise like tread mills and stationary bicycles, however, are too strenuous for these patients. As a result, the need for mobility has long been recognized. Until recently, this mobility has been facilitated by the use of small compressed oxygen tanks or cylinders mounted on a cart with dolly wheels. The disadvantage of these tanks is that they contain a finite amount of oxygen and are heavy, weighing about 50 pounds when mounted. 
     Oxygen concentrators have been in use for about 50 years to supply oxygen for respiratory therapy. Oxygen concentrators may implement processes such as vacuum swing adsorption (VSA), pressure swing adsorption (PSA), or vacuum pressure swing adsorption (VPSA). For example, oxygen concentrators, e.g., POCs, may work based on depressurization (e.g., vacuum operation) and/or pressurization (e.g., compressor operation) in a swing adsorption process (e.g., Vacuum Swing Adsorption, Pressure Swing Adsorption or Vacuum Pressure Swing Adsorption, each of which are referred to herein as a “swing adsorption process”). Pressure swing adsorption may involve using one or more compressors to increase gas pressure inside one or more canisters that contains particles of a gas separation adsorbent. Such a canister when containing a mass of gas separation adsorbent such as a layer of gas separation adsorbent may serve as a sieve bed. As the pressure increases, certain molecules in the gas may become adsorbed onto the gas separation adsorbent. Removal of a portion of the gas in the canister under the pressurized conditions allows separation of the non-adsorbed molecules from the adsorbed molecules. The adsorbed molecules may then be desorbed by venting the sieve beds. Further details regarding oxygen concentrators may be found, for example, in U.S. Published Patent Application No. 2009-0065007, published Mar. 12, 2009, and entitled “Oxygen Concentrator Apparatus and Method”, which is incorporated herein by reference. 
     Ambient air usually includes approximately 78% nitrogen and 21% oxygen with the balance comprised of argon, carbon dioxide, water vapor and other trace gases. If a gas mixture such as air, for example, is passed under pressure through a canister containing a gas separation adsorbent that attracts nitrogen more strongly than it does oxygen, part or all of the nitrogen will stay in the canister, and the gas coming out of the canister will be enriched in oxygen. When the sieve bed reaches the end of its capacity to adsorb nitrogen, the adsorbed nitrogen may be desorbed by venting. The sieve bed is then ready for another cycle of producing oxygen enriched air. By alternating pressurization cycles of the canisters in a two-canister system, one canister can be separating oxygen while the other canister is being vented (resulting in a near-continuous separation of oxygen from the air). In this manner, oxygen enriched air can be accumulated, such as in a storage container or other pressurizable vessel or conduit coupled to the canisters, for a variety of uses including providing supplemental oxygen to users. 
     Vacuum swing adsorption (VSA) provides an alternative gas separation technique. VSA typically draws the gas through the separation process of the sieve beds using a vacuum such as a compressor configured to create a vacuum within the sieve beds. Vacuum Pressure Swing Adsorption (VPSA) may be understood to be a hybrid system using a combined vacuum and pressurization technique. For example, a VPSA system may pressurize the sieve beds for the separation process and also apply a vacuum for depressurizing the sieve beds. 
     Traditional oxygen concentrators have been bulky and heavy making ordinary ambulatory activities with them difficult and impractical. Recently, companies that manufacture large stationary oxygen concentrators began developing portable oxygen concentrators (POCs). The advantage of POCs is that they can produce a theoretically endless supply of oxygen and provide mobility for patients (users) during use. In order to make these devices small for mobility, the various systems necessary for the production of oxygen enriched air are condensed. POCs seek to utilize their produced oxygen as efficiently as possible, in order to minimise weight, size, and power consumption. In some implementations, this may be achieved by delivering the oxygen as series of pulses, each pulse or “bolus” timed to coincide with the onset of inhalation. This therapy mode is known as pulsed oxygen delivery (POD) or demand mode. 
     Continuous flow mode long-term oxygen therapy is advantageous for clinical reasons, e.g. reassuring patients they are receiving therapy, and relieving anxiety. However, continuous flow mode is draining of battery life and thus is more suitable for stationary devices. A need therefore exists for a portable oxygen concentrator capable of emulating the benefits of continuous flow mode with reasonable battery life. 
     SUMMARY OF THE TECHNOLOGY 
     Examples of the present technology may provide methods and apparatus for controlled operations of an oxygen concentrator, such as a portable oxygen concentrator. In particular, the technology provides methods and apparatus for a portable oxygen concentrator configured to deliver long-term oxygen therapy in a mode of delivery referred to herein as hybrid mode, while maintaining acceptable battery life. Hybrid mode therapy is a breath-synchronised therapy in which there is a non-zero inter-bolus flow of gas to the patient as well as boluses delivered in synchrony with inhalation as in POD mode. Hybrid mode therapy may be delivered according to a bilevel purity species, a bilevel flow rate species, or species intermediate between those two species. 
     All species of hybrid mode therapy present a challenge to traditional methods of detecting the onset of user inhalation. Examples of the present technology may therefore also include a sensor configuration that allows accurate detection of the onset of inhalation in the various sub-modes of hybrid mode therapy. 
     Some implementations of the present technology may include an oxygen concentrator for providing a therapy gas to a delivery conduit for patient inhalation. The oxygen concentrator may include a compressor configured to generate a pressurised air stream. The oxygen concentrator may include one or more sieve beds. The one or more sieve beds may include adsorbent material configured to preferentially adsorb a component gas from the pressurised air stream, thereby producing oxygen enriched air from the pressurised air stream. The oxygen concentrator may include a valve set. The valve set may be configured to selectively pneumatically couple the compressor to the one or more sieve beds so as to selectively convey the pressurised air stream to the one or more sieve beds. The valve set may be configured to selectively vent exhaust gas to atmosphere from an exhaust outlet of the one or more sieve beds. The oxygen concentrator may include an accumulator pneumatically coupled to the one or more sieve beds so as to receive the oxygen enriched air produced from a product outlet of the one or more sieve beds. The oxygen concentrator may include a supply valve configured to selectively release oxygen enriched air from the accumulator via a primary flow path and then to the delivery conduit. The oxygen concentrator may include a secondary flow path configured to pass a portion of the exhaust gas from the exhaust outlet to the delivery conduit. The oxygen concentrator may include a controller operably coupled to the valve set and the supply valve. The controller may be configured to selectively actuate the valve set in a periodic pattern so as to produce oxygen enriched air for receiving by the accumulator and vent exhaust gas from the one or more sieve beds. The controller may be configured to selectively actuate the supply valve to release oxygen enriched air from the accumulator to the delivery conduit in synchrony with inhalation of the patient. The therapy gas may include the released oxygen enriched air and the portion of the exhaust gas. 
     In some implementations, the therapy gas may be provided to the delivery conduit in a hybrid mode wherein the therapy gas flows to the delivery conduit at least during patient inspiration and patient expiration. The hybrid mode may vary a characteristic of the therapy gas. The varied characteristic may be oxygen purity. The varied oxygen purity may include a first oxygen purity during at least a portion of patient inspiration and a second oxygen purity after the portion of patient inspiration. The first oxygen purity may be a purity in a range of about 50 percent to about 99 percent. The second oxygen purity may be a purity in a range of about 4 percent to 35 percent. The primary flow path may be configured to provide the therapy gas with the first oxygen purity. The secondary flow path may be configured to provide the therapy gas with the second oxygen purity. The secondary flow path may include a secondary valve configured to selectively release the portion of the exhaust gas to the delivery conduit. The controller may be further configured to selectively actuate the secondary valve in anti-sync with actuation of the supply valve to release the portion of the exhaust gas to the delivery conduit. The supply valve and the secondary valve may be implemented as a three-way valve configured to release either the oxygen enriched air or the portion of the exhaust gas to the delivery conduit. 
     In some implementations, the oxygen concentrator may further include a pressure sensor configured to generate a signal representative of a difference in pressure between a sense port and a reference port thereof, the sense port being connected to the delivery conduit, and the reference port being coupled to a flow path of the oxygen concentrator that may be downstream of the supply valve. The controller may be further configured to detect onset of inhalation from the generated pressure difference signal and to actuate the supply valve based on the detected onset of inhalation. The controller may be configured to detect onset of inhalation by detecting a drop in the generated pressure difference signal. The reference port of the pressure sensor may be connected to a downstream side of the supply valve via a flow restrictor. The controller may be configured to actuate the secondary valve in anti-sync with actuation of the supply valve in response to user activation of a control on an interface of the oxygen concentrator. 
     In some implementations, the oxygen concentrator may further include a flow restrictor within the secondary flow path and in line with the secondary valve. The flow restrictor may be configured such that a flow rate of exhaust gas when released to the delivery conduit may be approximately equal to a flow rate of the oxygen enriched air when released to the delivery conduit. The oxygen concentrator may include a further secondary valve configured to selectively release oxygen enriched air from the accumulator to the delivery conduit via a flow restrictor. The controller may be further configured to selectively actuate the further secondary valve in anti-sync with actuation of the supply valve to release oxygen enriched air to the delivery conduit. The hybrid mode may vary a further characteristic of the therapy gas. The varied further characteristic may be flow rate of the therapy gas. 
     Some implementations of the present technology may include apparatus for providing a therapy gas. The apparatus may include means for generating a pressurised air stream such as a motor operated compressor as described in more detail herein. The apparatus may include means for preferentially adsorbing a component gas from the pressurised air stream, thereby producing oxygen enriched air from the pressurised air stream such as one or more sieve beds as described in more detail herein. The apparatus may include means for selectively pneumatically coupling, in a periodic pattern, the means for preferentially adsorbing with (a) the means for generating so as to selectively convey the pressurised air stream to the means for preferentially adsorbing, and (b) an exhaust outlet to atmosphere for selectively venting exhaust gas to atmosphere from the means for preferentially adsorbing, so as to produce oxygen enriched air within the means for preferentially adsorbing, such as a controller and a set of valves described in more detail herein. The apparatus may include means for accumulating the oxygen enriched air, such as an accumulator as described in more detail herein, produced from a product outlet of the means for preferentially adsorbing. The apparatus may include means for selectively releasing oxygen enriched air from the means for accumulating to a delivery conduit for a patient in synchrony with inhalation of the patient, such as a supply valve and a controller described in more detail herein. The apparatus may include means for passing a portion of the exhaust gas to the delivery conduit, such as a secondary flow path as described in more detail herein. The therapy gas may include the released oxygen enriched air from the means for accumulating and the portion of the exhaust gas. 
     Some implementations of the present technology may include an oxygen concentrator for producing a therapy gas for a patient. The oxygen concentrator may include a compressor configured to generate a pressurised air stream. The oxygen concentrator may include one or more sieve beds. The one or more sieve beds may include adsorbent material configured to preferentially adsorb a component gas from the pressurised air stream, thereby producing oxygen enriched air from the pressurised air stream. The oxygen concentrator may include a valve set configured to selectively pneumatically couple the compressor to the one or more sieve beds so as to selectively convey the pressurised air stream to the one or more sieve beds. The oxygen concentrator may include an accumulator pneumatically coupled to the one or more sieve beds so as to receive the oxygen enriched air produced by one or more sieve beds. The oxygen concentrator may include a supply valve configured to selectively release oxygen enriched air from the accumulator, via a primary path, to a delivery conduit for the patient. The oxygen concentrator may include a secondary valve configured to selectively release oxygen enriched air from the accumulator, via a secondary path, to the delivery conduit for the patient. The oxygen concentrator may include a controller operably coupled to the valve, the supply valve, and the secondary valve. The controller may be configured to selectively actuate the valve set in a periodic pattern so as to produce oxygen enriched air in the accumulator. The controller may be configured to selectively actuate the supply valve to release oxygen enriched air to the delivery conduit in synchrony with inhalation of the patient. The controller may be configured to selectively actuate the secondary valve in anti-sync with actuation of the supply valve to release oxygen enriched air to the delivery conduit. 
     In some implementations, the therapy gas may be provided to the delivery conduit in a hybrid mode wherein the therapy gas flows to the delivery conduit at least during patient inspiration and patient expiration; and wherein the hybrid mode varies a characteristic of the therapy gas. The varied characteristic may be a flow rate of the therapy gas. The flow characteristic of the primary path may be different from a flow characteristic of the secondary path. The oxygen concentrator may further include a flow restrictor within the secondary path and in line with the secondary valve. The flow restrictor may be configured such that a flow rate of oxygen enriched air when released to the delivery conduit via the secondary valve may be substantially lower than a flow rate of the oxygen enriched air when released to the delivery conduit via the supply valve. The supply valve and the secondary valve may be implemented as a three-way valve configured to release oxygen enriched air to the delivery conduit. 
     In some implementations, the oxygen concentrator may further include a pressure sensor configured to generate a signal representative of a difference in pressure between a sense port and a reference port thereof. The sense port may be connected to the delivery conduit and the reference port may be coupled to a flow path of the oxygen concentrator that may be downstream of the supply valve. The controller may be further configured to detect onset of inhalation from the generated pressure difference signal and to actuate the supply valve based on the detected onset of inhalation. The controller may be configured to detect onset of inhalation by detecting a drop in the generated pressure difference signal. The reference port of the pressure sensor may be connected to a downstream side of the supply valve via a flow restrictor. The controller may be configured to actuate the secondary valve in anti-sync with actuation of the supply valve in response to user activation of a control on an interface of the oxygen concentrator. The oxygen concentrator may further include a further secondary valve configured to selectively release a portion of exhaust gas from the one or more sieve beds to the delivery conduit, wherein the controller may be further configured to selectively actuate the further secondary valve in anti-sync with actuation of the supply valve to release the portion of the exhaust gas to the delivery conduit. The hybrid mode may vary a further characteristic of the therapy gas. The varied further characteristic may be oxygen purity of the therapy gas. 
     Some implementations of the present technology may include apparatus. The apparatus may include means for generating a pressurised air stream. The apparatus may include means for preferentially adsorbing a component gas from the pressurised air stream, thereby producing oxygen enriched air from the pressurised air stream. The apparatus may include means for selectively pneumatically coupling, in a periodic pattern, the means for preferentially adsorbing with the means for generating to selectively convey the pressurised air stream to the means for preferentially adsorbing so as to produce oxygen enriched air in the means for preferentially absorbing. The apparatus may include means for accumulating the oxygen enriched air produced by the means for preferentially adsorbing. The apparatus may include primary means for selectively releasing, in synchrony with inhalation of a patient, oxygen enriched air from the means for accumulating to a delivery conduit for the patient. The apparatus may include secondary means for selectively releasing, in anti-sync with actuation of the primary means for selectively releasing, oxygen enriched air from the means for accumulating to the delivery conduit for the patient. 
     Some implementations of the present technology may include an oxygen concentrator. The oxygen concentrator may include a compressor configured to generate a pressurised air stream. The apparatus may include one or more sieve beds. The one or more sieve beds may include adsorbent material configured to preferentially adsorb a component gas from the pressurised air stream, thereby producing oxygen enriched air from the pressurised air stream. The apparatus may include a valve set configured to selectively pneumatically couple the compressor to the one or more sieve beds so as to selectively convey the pressurised air stream to the one or more sieve beds. The apparatus may include an accumulator pneumatically coupled to the one or more sieve beds so as to receive the oxygen enriched air produced by the one or more sieve beds. The apparatus may include a supply valve configured to selectively release oxygen enriched air from the accumulator to a delivery conduit for a patient. The apparatus may include a secondary path configured to convey a flow of gas to the delivery conduit for the patient. The apparatus may include a pressure sensor configured to generate a signal representative of a difference in pressure between a sense port and a reference port thereof. The sense port may be connected to the delivery conduit. The reference port may be coupled to a flow path of the oxygen concentrator that may be downstream of the supply valve. The apparatus may include a controller operably coupled to the valve set and the supply valve. The controller may be configured to selectively actuate the valve set in a periodic pattern so as to produce oxygen enriched air for the accumulator. The controller may be configured to detect onset of inhalation of the patient from the generated pressure difference signal. The controller may be configured to selectively actuate the supply valve to release oxygen enriched air to the delivery conduit in synchrony with inhalation of the patient. 
     In some implementations, the controller may be further configured to actuate the supply valve based on the detected onset of inhalation. The controller may be configured to detect onset of inhalation by detecting a drop in the generated pressure difference signal. The reference port of the pressure sensor may be connected to a downstream side of the supply valve via a flow restrictor. The secondary path may include a secondary valve configured to selectively release exhaust gas from the one or more sieve beds to the delivery conduit. The secondary path may further include a further secondary valve configured to selectively release oxygen enriched air from the accumulator to the delivery conduit via a flow restrictor. The secondary path may include a secondary valve configured to selectively release oxygen enriched air from the accumulator to the delivery conduit via a flow restrictor. 
     Some implementations of the present technology may include apparatus. The apparatus may include means for generating a pressurised air stream. The apparatus may include means for preferentially adsorbing a component gas from the pressurised air stream, thereby producing oxygen enriched air from the pressurised air stream. The apparatus may include means for selectively pneumatically coupling, in a periodic pattern, the means for preferentially adsorbing with the means for generating so as to selectively convey the pressurised air stream to the means for preferentially adsorbing so as to produce oxygen enriched air in the means for preferentially absorbing. The apparatus may include means for accumulating the oxygen enriched air produced by the means for preferentially adsorbing. The apparatus may include means for selectively releasing oxygen enriched air from the means for accumulating to a delivery conduit for a patient. The apparatus may include secondary means for conveying a flow of gas to the delivery conduit for the patient. The apparatus may include means for generating a signal representative of a difference in pressure between a sense port and a reference port thereof. The sense port may be connected to the delivery conduit. The apparatus may include means for detecting onset of inhalation of the patient from the generated pressure difference signal and for selectively actuating the means for selectively releasing oxygen enriched air to release oxygen enriched air to the delivery conduit in synchrony with inhalation of the patient. 
     Another general aspect includes an oxygen concentrator. The oxygen concentrator includes a compressor configured to generate a pressurised air stream. The oxygen concentrator also includes at least one sieve bed, the or each each sieve bed including adsorbent material configured to preferentially adsorb a component gas from the pressurised air stream, thereby producing oxygen enriched air from the pressurised air stream. The oxygen concentrator also includes a valve configured to: selectively pneumatically couple the compressor to the or each sieve bed so as to selectively convey the pressurised air stream to the sieve bed, and selectively vent exhaust gas from the or each sieve bed. The oxygen concentrator also includes an accumulator pneumatically coupled to the or each sieve bed so as to receive the oxygen enriched air produced by the or each sieve bed. The oxygen concentrator also includes a supply valve configured to selectively release oxygen enriched air from the accumulator to a patient via a delivery conduit. The oxygen concentrator also includes a secondary valve configured to selectively release a portion of the exhaust gas to the delivery conduit. The oxygen concentrator also includes a controller operably coupled to the valve, the supply valve, and the secondary valve, the controller configured to: selectively actuate the valve in a periodic pattern so as to produce oxygen enriched air in the accumulator, selectively actuate the supply valve to release oxygen enriched air to the delivery conduit in synchrony with inhalation of the patient, and selectively actuate the secondary valve in anti-sync with actuation of the supply valve to release the portion of the exhaust gas to the delivery conduit. 
     One general aspect includes apparatus. The apparatus also includes means for generating a pressurised air stream. The apparatus also includes means for preferentially adsorbing a component gas from the pressurised air stream, thereby producing oxygen enriched air from the pressurised air stream. The apparatus also includes means for selectively pneumatically coupling the means for generating to the means for preferentially adsorbing so as to selectively convey the pressurised air stream to the means for preferentially adsorbing. The apparatus also includes means for selectively venting exhaust gas from the means for preferentially adsorbing. The apparatus also includes means for receiving the oxygen enriched air produced by the means for preferentially adsorbing. The apparatus also includes means for selectively releasing oxygen enriched air from the means for receiving to a patient via a delivery conduit. The apparatus also includes means for selectively releasing a portion of the exhaust gas to the delivery conduit. The apparatus also includes means for selectively actuating the means for selectively pneumatically coupling in a periodic pattern so as to produce oxygen enriched air in the means for receiving. The apparatus also includes means for selectively actuating the means for selectively releasing oxygen enriched air to release oxygen enriched air to the delivery conduit in synchrony with inhalation of the patient. The apparatus also includes means for selectively actuating the means for selectively releasing exhaust gas in anti-sync with actuation of the means for selectively releasing oxygen enriched air to release the portion of the exhaust gas to the delivery conduit. 
     One general aspect includes an oxygen concentrator. The oxygen concentrator also includes a compressor configured to generate a pressurised air stream. The oxygen concentrator also includes at least one sieve bed, the or each each sieve bed including adsorbent material configured to preferentially adsorb a component gas from the pressurised air stream, thereby producing oxygen enriched air from the pressurised air stream. The oxygen concentrator also includes a valve configured to selectively pneumatically couple the compressor to the or each sieve bed so as to selectively convey the pressurised air stream to the sieve bed. The oxygen concentrator also includes an accumulator pneumatically coupled to the or each sieve bed so as to receive the oxygen enriched air produced by the or each sieve bed. The oxygen concentrator also includes a supply valve configured to selectively release oxygen enriched air from the accumulator to a patient via a delivery conduit. The oxygen concentrator also includes a secondary valve configured to selectively release oxygen enriched air from the accumulator to the patient via the delivery conduit. The oxygen concentrator also includes a controller operably coupled to the valve, the supply valve, and the secondary valve, the controller configured to: selectively actuate the valve in a periodic pattern so as to produce oxygen enriched air in the accumulator, selectively actuate the supply valve to release oxygen enriched air to the delivery conduit in synchrony with inhalation of the patient, and selectively actuate the secondary valve in anti-sync with actuation of the supply valve to release oxygen enriched air to the delivery conduit. 
     One general aspect includes apparatus. The apparatus also includes means for generating a pressurised air stream. The apparatus also includes means for preferentially adsorbing a component gas from the pressurised air stream, thereby producing oxygen enriched air from the pressurised air stream. The apparatus also includes means for selectively pneumatically coupling the means for generating to the means for preferentially adsorbing so as to selectively convey the pressurised air stream to the means for preferentially adsorbing. The apparatus also includes means for receiving the oxygen enriched air produced by the means for preferentially adsorbing. The apparatus also includes means for selectively releasing oxygen enriched air from the means for receiving to a patient via a delivery conduit. The apparatus also includes secondary means for selectively releasing oxygen enriched air from the means for receiving to a patient via a delivery conduit. The apparatus also includes means for selectively actuating the means for selectively pneumatically coupling in a periodic pattern so as to produce oxygen enriched air in the means for receiving. The apparatus also includes means for selectively actuating the means for selectively releasing oxygen enriched air to release oxygen enriched air to the delivery conduit in synchrony with inhalation of the patient. The apparatus also includes means for selectively actuating the secondary means for selectively releasing oxygen enriched air in anti-sync with actuation of the means for selectively releasing oxygen enriched air to release oxygen enriched air to the delivery conduit. 
     One general aspect includes an oxygen concentrator. The oxygen concentrator also includes a compressor configured to generate a pressurised air stream. The oxygen concentrator also includes at least one sieve bed, the or each each sieve bed including adsorbent material configured to preferentially adsorb a component gas from the pressurised air stream, thereby producing oxygen enriched air from the pressurised air stream. The oxygen concentrator also includes a valve configured to selectively pneumatically couple the compressor to the or each sieve bed so as to selectively convey the pressurised air stream to the sieve bed. The oxygen concentrator also includes an accumulator pneumatically coupled to the or each sieve bed so as to receive the oxygen enriched air produced by the or each sieve bed. The oxygen concentrator also includes a supply valve configured to selectively release oxygen enriched air from the accumulator to a patient via a delivery conduit. The oxygen concentrator also includes a secondary path configured to convey a flow of gas to the patient via the delivery conduit. The oxygen concentrator also includes a pressure sensor configured to generate a signal representative of a difference in pressure between a sense port and a reference port thereof, the sense port being connected to the delivery conduit. The oxygen concentrator also includes a controller operably coupled to the valve and the supply valve, the controller configured to: selectively actuate the valve in a periodic pattern so as to produce oxygen enriched air in the accumulator, detect onset of inhalation of the patient from the generated pressure difference signal, and selectively actuate the supply valve to release oxygen enriched air to the delivery conduit in synchrony with inhalation of the patient. 
     One general aspect includes apparatus. The apparatus also includes means for generating a pressurised air stream. The apparatus also includes means for preferentially adsorbing a component gas from the pressurised air stream, thereby producing oxygen enriched air from the pressurised air stream. The apparatus also includes means for selectively pneumatically coupling the means for generating to the means for preferentially adsorbing so as to selectively convey the pressurised air stream to the means for preferentially adsorbing. The apparatus also includes means for receiving the oxygen enriched air produced by the means for preferentially adsorbing. The apparatus also includes means for selectively releasing oxygen enriched air from the means for receiving to a patient via a delivery conduit. The apparatus also includes means for conveying a flow of gas to the patient via the delivery conduit. The apparatus also includes means for generating a signal representative of a difference in pressure between a sense port and a reference port thereof, the sense port being connected to the delivery conduit. The apparatus also includes means for selectively actuating the means for selectively pneumatically coupling in a periodic pattern so as to produce oxygen enriched air in the means for receiving. The apparatus also includes means for detecting onset of inhalation of the patient from the generated pressure difference signal. The apparatus also includes means for selectively actuating the means for selectively releasing oxygen enriched air to release oxygen enriched air to the delivery conduit in synchrony with inhalation of the patient. 
     Of course, portions of the aspects may form sub-aspects of the present technology. Also, various ones of the sub-aspects and/or aspects may be combined in various manners and also constitute additional aspects or sub-aspects of the present technology. 
     Other features of the technology will be apparent from consideration of the information contained in the following detailed description, abstract, drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Advantages of the present technology will become apparent to those skilled in the art with the benefit of the following detailed description of implementations and upon reference to the accompanying drawings in which similar reference numerals indicate similar components: 
         FIG.  1 A  depicts an oxygen concentrator in accordance with one form of the present technology. 
         FIG.  1 B  is a schematic diagram of the components of the oxygen concentrator of  FIG.  1 A . 
         FIG.  1 C  is a side view of the main components of the oxygen concentrator of  FIG.  1 A . 
         FIG.  1 D  is a perspective side view of a compression system of the oxygen concentrator of  FIG.  1 A . 
         FIG.  1 E  is a side view of a compression system that includes a heat exchange conduit. 
         FIG.  1 F  is a schematic diagram of example outlet components of the oxygen concentrator of  FIG.  1 A . 
         FIG.  1 G  depicts an outlet conduit for the oxygen concentrator of  FIG.  1 A . 
         FIG.  1 H  depicts an alternate outlet conduit for the oxygen concentrator of  FIG.  1 A . 
         FIG.  1 I  is a perspective view of a disassembled canister system for the oxygen concentrator of  FIG.  1 A . 
         FIG.  1 J  is an end view of the canister system of  FIG.  1 I . 
         FIG.  1 K  is an assembled view of the canister system end depicted in  FIG.  1 J . 
         FIG.  1 L  is a view of an opposing end of the canister system of  FIG.  1 I  to that depicted in  FIGS.  1 J and  8 K . 
         FIG.  1 M  is an assembled view of the canister system end depicted in  FIG.  1 L . 
         FIG.  1 N  depicts an example control panel for the oxygen concentrator of  FIG.  1 A . 
         FIG.  1 O  depicts a connected POC therapy system that includes the oxygen concentrator of  FIG.  1 A . 
         FIG.  2    contains a graph illustrating the bilevel purity implementation of hybrid delivery mode according to one aspect of the present technology. 
         FIG.  3    is a schematic diagram of a modification to the outlet system of  FIG.  1 F  according to one implementation of the present technology. 
         FIG.  4    contains a graph illustrating the bilevel flow rate implementation of hybrid delivery mode according to one aspect of the present technology. 
         FIG.  5    is a schematic diagram of a modification to the outlet system of  FIG.  1 F  according to one implementation of the present technology. 
         FIG.  6    contains a graph illustrating various modes of delivery of oxygen enriched air by an oxygen concentrator. 
         FIG.  7    is a schematic diagram of a modification to the outlet system of  FIG.  1 F  according to one implementation of a combination of the outlet systems of  FIGS.  3  and  5    of the present technology. 
     
    
    
     DETAILED DESCRIPTION OF THE IMPLEMENTATIONS 
     Examples implementations of the present disclosure are described in detail with reference to the drawing figures wherein like reference numerals identify similar or identical elements. It is to be understood that the disclosed embodiments are merely examples of the disclosure, which may be embodied in various forms. Well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present disclosure in virtually any appropriately detailed structure. 
       FIGS.  1 A- 8 N  illustrate an implementation of an oxygen concentrator  100 . As described herein, oxygen concentrator  100  uses pressure swing adsorption (PSA) processes to produce oxygen enriched air. However, in other embodiments, oxygen concentrator  100  may be modified such that it uses vacuum swing adsorption (VSA) processes or vacuum pressure swing adsorption (VPSA) processes to produce oxygen enriched air. 
     Outer Housing 
       FIG.  1 A  depicts an implementation of an outer housing  170  of an oxygen concentrator  100 . In some implementations, outer housing  170  may be comprised of a light-weight plastic. Outer housing  170  includes compression system inlets  105 , cooling system passive inlet  101  and outlet  173  at each end of outer housing  170 , outlet port  174 , and control panel  600 . Inlet  101  and outlet  173  allow cooling air to enter the housing, flow through the housing, and exit the interior of housing  170  to aid in cooling of the oxygen concentrator  100 . Compression system inlets  105  allow air to enter the compression system. Outlet port  174  is used to attach a conduit to provide oxygen enriched air produced by the oxygen concentrator  100  to a user. 
     Components 
       FIG.  1 B  illustrates a schematic diagram of components of an oxygen concentrator  100 , according to an implementation. Oxygen concentrator  100  may concentrate oxygen within an air stream to provide oxygen enriched air to a user. 
     Oxygen concentrator  100  may be a portable oxygen concentrator. For example, oxygen concentrator  100  may have a weight and size that allows the oxygen concentrator to be carried by hand and/or in a carrying case during use. As discussed further herein, such a device typically operates with an included power supply that provides power to the oxygen concentrator using one or more batteries, such as Lithium ion batteries, which are typically rechargeable. In one implementation, oxygen concentrator  100  has a weight of less than about 20 pounds, less than about 15 pounds, less than about 10 pounds, or less than about 5 pounds. In an implementation, oxygen concentrator  100  has a volume of less than about 1000 cubic inches, less than about 750 cubic inches, less than about 500 cubic inches, less than about 250 cubic inches, or less than about 200 cubic inches. 
     Oxygen enriched air may be produced from ambient air by pressurising ambient air in canisters  302  and  304 , which contain a gas separation adsorbent and are therefore referred to as sieve beds. Gas separation adsorbents useful in an oxygen concentrator are capable of separating at least nitrogen from an air stream to produce oxygen enriched air. Examples of gas separation adsorbents include molecular sieves that are capable of separating nitrogen from an air stream. Examples of adsorbents that may be used in an oxygen concentrator include, but are not limited to, zeolites (natural) or synthetic crystalline aluminosilicates that separate nitrogen from an air stream under elevated pressure. Examples of synthetic crystalline aluminosilicates that may be used include, but are not limited to: OXYSIV adsorbents available from UOP LLC, Des Plaines, IW; SYLOBEAD adsorbents available from W. R. Grace &amp; Co, Columbia, Md.; SILIPORITE adsorbents available from CECA S.A. of Paris, France; ZEOCHEM adsorbents available from Zeochem AG, Uetikon, Switzerland; and AgLiLSX adsorbent available from Air Products and Chemicals, Inc., Allentown, Pa. 
     As shown in  FIG.  1 B , air may enter the oxygen concentrator through air inlet  105 . Air may be drawn into air inlet  105  by compression system  200 . Compression system  200  may draw in air from the surroundings of the oxygen concentrator and compress the air, forcing the compressed air into one or both canisters  302  and  304 . In an implementation, an inlet muffler  108  may be coupled to air inlet  105  to reduce sound produced by air being pulled into the oxygen concentrator by compression system  200 . In an implementation, inlet muffler  108  may reduce moisture and sound. For example, a water adsorbent material (such as a polymer water adsorbent material or a zeolite material) may be used to both adsorb water from the incoming air and to reduce the sound of the air passing into the air inlet  105 . 
     Compression system  200  may include one or more compressors configured to compress air. Pressurized air, produced by compression system  200 , may be forced into one or both of the canisters  302  and  304 . In some implementations, the ambient air may be pressurized in the canisters to a pressure approximately in a range of 13-20 pounds per square inch gauge pressure (psig). Other pressures may also be used, depending on the type of gas separation adsorbent disposed in the canisters. 
     The oxygen concentrator may typically include a valve set of one more valves for directing the pressurized air for the processes of the oxygen concentrator so as to produce the oxygen enriched air. For example, coupled to each canister  302 / 304  are inlet valves  122 / 124  and outlet valves  132 / 134 . As shown in  FIG.  1 B , inlet valve  122  is coupled to canister  302  and inlet valve  124  is coupled to canister  304 . Outlet valve  132  is coupled to canister  302  and outlet valve  134  is coupled to canister  304 . Inlet valves  122 / 124  are used to control the passage of air from compression system  200  to the respective canisters. Outlet valves  132 / 134  are used to release gas from the respective canisters during a venting process. In some implementations, inlet valves  122 / 124  and outlet valves  132 / 134  may be silicon plunger solenoid valves. Other types of valves, however, may be used. Plunger valves offer advantages over other kinds of valves by being quiet and having low slippage. 
     In some implementations, a two-step valve actuation voltage may be generated to control inlet valves  122 / 124  and outlet valves  132 / 134 . For example, a high voltage (e.g., 24 V) may be applied to an inlet valve to open the inlet valve. The voltage may then be reduced (e.g., to 7 V) to keep the inlet valve open. Using less voltage to keep a valve open may use less power (Power=Voltage*Current). This reduction in voltage minimizes heat buildup and power consumption to extend run time from the power supply  180  (described below). When the power is cut off to the valve, it closes by spring action. In some implementations, 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 a final 7 V). 
     In an implementation, pressurized air is sent into one of canisters  302  or  304  while the other canister is being vented. For example, during use, inlet valve  122  is opened while inlet valve  124  is closed. Pressurized air from compression system  200  is forced into canister  302 , while being inhibited from entering canister  304  by inlet valve  124 . In an implementation, a controller  400  is electrically coupled to valves  122 ,  124 ,  132 , and  134 . Controller  400  includes one or more processors  410  operable to execute program instructions stored in memory  420 . The program instructions configure the controller to perform various predefined methods that are used to operate the oxygen concentrator, such as the methods described in more detail herein. The program instructions may include program instructions for operating inlet valves  122  and  124  out of phase with each other, i.e., when one of inlet valves  122  or  124  is opened, the other valve is closed. During pressurization of canister  302 , outlet valve  132  is closed and outlet valve  134  is opened. Similar to the inlet valves, outlet valves  132  and  134  are operated out of phase with each other. In some implementations, the voltages and the durations of the voltages used to open the input and output valves may be controlled by controller  400 . The controller  400  may include a transceiver  430  that may communicate with external devices to transmit data collected by the processor  410  or receive instructions from an external device for the processor  410 . 
     Check valves  142  and  144  are coupled to canisters  302  and  304 , respectively. Check valves  142  and  144  may be one-way valves that are passively operated by the pressure differentials that occur as the canisters are pressurized and vented, or may be active valves. Check valves  142  and  144  are coupled to the canisters to allow oxygen enriched air produced during pressurization of each canister to flow out of the canister, and to inhibit back flow of oxygen enriched air or any other gases into the canister. In this manner, check valves  142  and  144  act as one-way valves allowing oxygen enriched air to exit the respective canisters during pressurization. 
     The term “check valve”, as used herein, refers to a valve that allows flow of a fluid (gas or liquid) in one direction and inhibits back flow of the fluid. Examples of check valves that are suitable for use include, but are not limited to: a ball check valve; a diaphragm check valve; a butterfly check valve; a swing check valve; a duckbill valve; an umbrella valve; and a lift check valve. Under pressure, nitrogen molecules in the pressurized ambient air are adsorbed by the gas separation adsorbent in the pressurized canister. As the pressure increases, more nitrogen is adsorbed until the gas in the canister is enriched in oxygen. The nonadsorbed gas molecules (mainly oxygen) flow out of the pressurized canister when the pressure reaches a point sufficient to overcome the resistance of the check valve coupled to the canister. In one implementation, the pressure drop of the check valve in the forward direction is less than 1 psi. The break pressure in the reverse direction is greater than 100 psi. It should be understood, however, that modification of one or more components would alter the operating parameters of these valves. If the forward flow pressure is increased, there is, generally, a reduction in oxygen enriched air production. If the break pressure for reverse flow is reduced or set too low, there is, generally, a reduction in oxygen enriched air pressure. 
     In an exemplary implementation, canister  302  is pressurized by compressed air produced in compression system  200  and passed into canister  302 . During pressurization of canister  302  inlet valve  122  is open, outlet valve  132  is closed, inlet valve  124  is closed and outlet valve  134  is open. Outlet valve  134  is opened when outlet valve  132  is closed to allow substantially simultaneous venting of canister  304  to atmosphere via the canister&#39;s exhaust outlet while canister  302  is being pressurized. Canister  302  is pressurized until the pressure in canister is sufficient to open check valve  142 . Oxygen enriched air produced in canister  302  exits from the canister&#39;s product outlet and passes through a check valve and, in one implementation, is collected in accumulator  106 . 
     After some time, the gas separation adsorbent will become saturated with nitrogen and will be unable to separate significant amounts of nitrogen from incoming air. This point is usually reached after a predetermined time of oxygen enriched air production. In the implementation described above, when the gas separation adsorbent in canister  302  reaches this saturation point, the inflow of compressed air is stopped and canister  302  is vented to desorb nitrogen. During venting, inlet valve  122  is closed, and outlet valve  132  is opened. While canister  302  is being vented, canister  304  is pressurized to produce oxygen enriched air in the same manner described above. Pressurization of canister  304  is achieved by closing outlet valve  134  and opening inlet valve  124 . The oxygen enriched air exits canister  304  through check valve  144 . 
     During venting of canister  302  from its exhaust outlet, outlet valve  132  may be opened allowing exhaust gas to exit the canister to atmosphere through concentrator outlet  130 . In an implementation, the vented exhaust gas may be directed through muffler  133  to reduce the noise produced by releasing the pressurized gas from the canister. As gas is released from canister  302 , the pressure in the canister  302  drops, allowing nitrogen to become desorbed from the gas separation adsorbent. The vented exhaust gas exits the oxygen concentrator through outlet  130 , resetting the canister to a state that allows renewed separation of nitrogen from an air stream. Muffler  133  may include open cell foam (or another material) to muffle the sound of the gas leaving the oxygen concentrator. In some implementations, the combined muffling components/techniques for the input of air and the output of oxygen enriched air may provide for oxygen concentrator operation at a sound level below 50 decibels. 
     During venting of the canisters, it is advantageous that at least a majority of the nitrogen is removed. In an implementation, at least about 50%, at least about 60%, at least about 70%, at least about 80%, at least about 90%, at least about 95%, at least about 98%, or substantially all of the nitrogen in a canister is removed before the canister is re-used to separate nitrogen from air. In some implementations, nitrogen removal may be assisted using an oxygen enriched air stream that is introduced into the canister from the other canister or stored oxygen enriched air. 
     In an exemplary implementation, a portion of the oxygen enriched air may be transferred from canister  302  to canister  304  when canister  304  is being vented of exhaust gas. Transfer of oxygen enriched air from canister  302  to canister  304  during venting of canister  304  helps to desorb nitrogen from the adsorbent by lowering the partial pressure of nitrogen adjacent the adsorbent. The flow of oxygen enriched air also helps to purge desorbed nitrogen (and other gases) from the canister. In an implementation, oxygen enriched air may travel through flow restrictors  151 ,  153 , and  155  between the two canisters. Flow restrictor  151  may be a trickle flow restrictor. Flow restrictor  151 , for example, may be a 0.009D flow restrictor (e.g., the flow restrictor has a radius 0.009″ which is less than the diameter of the tube it is inside). Flow restrictors  153  and  155  may be 0.013D flow restrictors. Other flow restrictor types and sizes are also contemplated and may be used depending on the specific configuration and tubing used to couple the canisters. In some implementations, the flow restrictors may be press fit flow restrictors that restrict air flow by introducing a narrower diameter in their respective tube. In some implementations, the press fit flow restrictors may be made of sapphire, metal or plastic (other materials are also contemplated). 
     Flow of oxygen enriched air between the canisters is also controlled by use of valve  152  and valve  154 . Valves  152  and  154  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. Other durations are also contemplated. In an exemplary implementation, canister  302  is being vented and it is desirable to purge canister  302  by passing a portion of the oxygen enriched air being produced in canister  304  into canister  302 . A portion of oxygen enriched air, upon pressurization of canister  304 , will pass through flow restrictor  151  into canister  302  during venting of canister  302 . Additional oxygen enriched air is passed into canister  302 , from canister  304 , through valve  154  and flow restrictor  155 . Valve  152  may remain closed during the transfer process, or may be opened if additional oxygen enriched air is needed. The selection of appropriate flow restrictors  151  and  155 , coupled with controlled opening of valve  154  allows a controlled amount of oxygen enriched air to be sent from canister  304  to canister  302 . In an implementation, the controlled amount of oxygen enriched air is an amount sufficient to purge canister  302  and minimize the loss of oxygen enriched air through venting valve  132  of canister  302 . While this implementation describes venting of canister  302 , it should be understood that the same process can be used to vent canister  304  using flow restrictor  151 , valve  152  and flow restrictor  153 . 
     The pair of equalization/vent valves  152 / 154  work with flow restrictors  153  and  155  to optimize the gas flow balance between the two canisters. This may allow for better flow control for venting one of the canisters with oxygen enriched air from the other of the canisters. It may also provide better flow direction between the two canisters. It has been found that, while flow valves  152 / 154  may be operated as bi-directional valves, the flow rate through such valves varies depending on the direction of fluid flowing through the valve. For example, oxygen enriched air flowing from canister  304  toward canister  302  has a flow rate faster through valve  152  than the flow rate of oxygen enriched air flowing from canister  302  toward canister  304  through valve  152 . If a single valve was to be used, eventually either too much or too little oxygen enriched air would be sent between the canisters and the canisters would, over time, begin to produce different amounts of oxygen enriched air. Use of opposing valves and flow restrictors on parallel air pathways may equalize the flow pattern of the oxygen enriched air between the two canisters. Equalising the flow may allow for a steady amount of oxygen enriched air to be available to the user over multiple cycles and also may allow a predictable volume of oxygen enriched air to purge the other of the canisters. In some implementations, 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. 
     At times, oxygen concentrator may be shut down for a period of time. When an oxygen concentrator is shut down, the temperature inside the canisters may drop as a result of the loss of adiabatic heat from the compression system. As the temperature drops, the volume occupied by the gases inside the canisters will drop. Cooling of the canisters may lead to a negative pressure in the canisters. Valves (e.g., valves  122 ,  124 ,  132 , and  134 ) leading to and from the canisters are dynamically sealed rather than hermetically sealed. Thus, outside air may enter the canisters after shutdown to accommodate the pressure differential. When outside air enters the canisters, moisture from the outside air may be adsorbed by the gas separation adsorbent. Adsorption of water inside the canisters may lead to gradual degradation of the gas separation adsorbents, steadily reducing ability of the gas separation adsorbents to produce oxygen enriched air. 
     In an implementation, outside air may be inhibited from entering canisters after the oxygen concentrator is shut down by pressurising both canisters prior to shutdown. By storing the canisters under a positive pressure, the valves may be forced into a hermetically closed position by the internal pressure of the air in the canisters. In an implementation, the pressure in the canisters, at shutdown, should be at least greater than ambient pressure. As used herein the term “ambient pressure” refers to the pressure of the surroundings in which the oxygen concentrator is located (e.g. the pressure inside a room, outside, in a plane, etc.). In an implementation, the pressure in the canisters, at shutdown, is at least greater than standard atmospheric pressure (i.e., greater than 760 mmHg (Torr), 1 atm, 101,325 Pa). In an implementation, the pressure in the canisters, at shutdown, is at least about 1.1 times greater than ambient pressure; is at least about 1.5 times greater than ambient pressure; or is at least about 2 times greater than ambient pressure. 
     In an implementation, pressurization of the canisters may be achieved by directing pressurized air into each canister from the compression system and closing all valves to trap the pressurized air in the canisters. In an exemplary implementation, when a shutdown sequence is initiated, inlet valves  122  and  124  are opened and outlet valves  132  and  134  are closed. Because inlet valves  122  and  124  are joined together by a common conduit, both canisters  302  and  304  may become pressurized as air and/or oxygen enriched air from one canister may be transferred to the other canister. This situation may occur when the pathway between the compression system and the two inlet valves allows such transfer. Because the oxygen concentrator operates in an alternating pressurize/venting mode, at least one of the canisters should be in a pressurized state at any given time. In an alternate implementation, the pressure may be increased in each canister by operation of compression system  200 . When inlet valves  122  and  124  are opened, pressure between canisters  302  and  304  will equalize, however, the equalized pressure in either canister may not be sufficient to inhibit air from entering the canisters during shutdown. In order to ensure that air is inhibited from entering the canisters, compression system  200  may be operated for a time sufficient to increase the pressure inside both canisters to a level at least greater than ambient pressure. Regardless of the method of pressurization of the canisters, once the canisters are pressurized, inlet valves  122  and  124  are closed, trapping the pressurized air inside the canisters, which inhibits air from entering the canisters during the shutdown period. 
     Referring to  FIG.  1 C , an implementation of an oxygen concentrator  100  is depicted. Oxygen concentrator  100  includes a compression system  200 , a canister system  300 , and a power supply  180  disposed within an outer housing  170 . Inlets  101  are located in outer housing  170  to allow air from the environment to enter oxygen concentrator  100 . Inlets  101  may allow air to flow into the compartment to assist with cooling of the components in the compartment. Power supply  180  provides a source of power for the oxygen concentrator  100 . Compression system  200  draws air in through the inlet  105  and muffler  108 . Muffler  108  may reduce noise of air being drawn in by the compression system and also may include a desiccant material to remove water from the incoming air. Oxygen concentrator  100  may further include fan  172  used to vent air and other gases from the oxygen concentrator via outlet  173 . 
     Compression System 
     In some implementations, compression system  200  includes one or more compressors. In another implementation, compression system  200  includes a single compressor, coupled to all of the canisters of canister system  300 . Turning to  FIGS.  1 D and  8 E , a compression system  200  is depicted that includes compressor  210  and motor  220 . Motor  220  is coupled to compressor  210  and provides an operating force to the compressor to operate the compression mechanism. For example, motor  220  may be a motor providing a rotating component that causes cyclical motion of a component of the compressor that compresses air. When compressor  210  is a piston type compressor, motor  220  provides an operating force which causes the piston of compressor  210  to be reciprocated. Reciprocation of the piston causes compressed air to be produced by compressor  210 . The pressure of the compressed air is, in part, estimated by the speed that the compressor is operated at, (e.g., how fast the piston is reciprocated). Motor  220 , therefore, may be a variable speed motor that is operable at various speeds to dynamically control the pressure of air produced by compressor  210 . 
     In one implementation, compressor  210  includes a single head wobble type compressor having a piston. Other types of compressors may be used such as diaphragm compressors and other types of piston compressors. Motor  220  may be a DC or AC motor and provides the operating power to the compressing component of compressor  210 . Motor  220 , in an implementation, may be a brushless DC motor. Motor  220  may be a variable speed motor configured to operate the compressing component of compressor  210  at variable speeds. Motor  220  may be coupled to controller  400 , as depicted in  FIG.  1 B , which sends operating signals to the motor to control the operation of the motor. For example, controller  400  may send signals to motor  220  to: turn the motor on, turn motor the off, and set the operating speed of motor. Thus, as illustrated in  FIG.  1 B , the compression system may include a speed sensor  201 . The speed sensor may be a motor speed transducer used to determine a rotational velocity of the motor  220  and/or other reciprocating operation of the compression system  200 . For example, a motor speed signal from the motor speed transducer may be provided to the controller  400 . The speed sensor or motor speed transducer may, for example, be a Hall effect sensor. The controller  400  may operate the compression system via the motor  220  based on the speed signal and/or any other sensor signal of the oxygen concentrator, such as a pressure sensor (e.g., accumulator pressure sensor  107 ). Thus, as illustrated in  FIG.  1 B , the controller  400  receives sensor signals, such as a speed signal from the speed sensor  201  and accumulator pressure signal from the accumulator pressure sensor  107 . With such signal(s), the controller may implement one or more control loops (e.g., feedback control) for operation of the compression system based on sensor signals such as accumulator pressure and/or motor speed as described in more detail herein. 
     Compression system  200  inherently creates substantial heat. Heat is caused by the consumption of power by motor  220  and the conversion of power into mechanical motion. Compressor  210  generates heat due to the increased resistance to movement of the compressor components by the air being compressed. Heat is also inherently generated due to adiabatic compression of the air by compressor  210 . Thus, the continual pressurization of air produces heat in the enclosure. Additionally, power supply  180  may produce heat as power is supplied to compression system  200 . Furthermore, users of the oxygen concentrator may operate the device in unconditioned environments (e.g., outdoors) at potentially higher ambient temperatures than indoors, thus the incoming air will already be in a heated state. 
     Heat produced inside oxygen concentrator  100  can be problematic. Lithium ion batteries are generally employed as power supplies for oxygen concentrators due to their long life and light weight. Lithium ion battery packs, however, are dangerous at elevated temperatures and safety controls are employed in oxygen concentrator  100  to shutdown the system if dangerously high power supply temperatures are detected. Additionally, as the internal temperature of oxygen concentrator  100  increases, the amount of oxygen generated by the concentrator may decrease. This is due, in part, to the decreasing amount of oxygen in a given volume of air at higher temperatures. If the amount of produced oxygen drops below a predetermined amount, the oxygen concentrator  100  may automatically shut down. 
     Because of the compact nature of oxygen concentrators, dissipation of heat can be difficult. Solutions typically involve the use of one or more fans to create a flow of cooling air through the enclosure. Such solutions, however, require additional power from the power supply  180  and thus shorten the portable usage time of the oxygen concentrator. In an implementation, a passive cooling system may be used that takes advantage of the mechanical power produced by motor  220 . Referring to  FIGS.  1 D and  8 E , compression system  200  includes motor  220  having an external rotating armature  230 . Specifically, armature  230  of motor  220  (e.g., a DC motor) is wrapped around the stationary field that is driving the armature. Since motor  220  is a large contributor of heat to the overall system it is helpful to transfer heat off the motor and sweep it out of the enclosure. With the external high speed rotation, the relative velocity of the major component of the motor and the air in which it exists is very high. The surface area of the armature is larger if externally mounted than if it is internally mounted. Since the rate of heat exchange is proportional to the surface area and the square of the velocity, using a larger surface area armature mounted externally increases the ability of heat to be dissipated from motor  220 . The gain in cooling efficiency by mounting the armature externally, allows the elimination of one or more cooling fans, thus reducing the weight and power consumption while maintaining the interior of the oxygen concentrator within the appropriate temperature range. Additionally, the rotation of the externally mounted armature creates movement of air proximate to the motor to create additional cooling. 
     Moreover, an external rotating armature may help the efficiency of the motor, allowing less heat to be generated. A motor having an external armature operates similar to the way a flywheel works in an internal combustion engine. When the motor is driving the compressor, the resistance to rotation is low at low pressures. When the pressure of the compressed air is higher, the resistance to rotation of the motor is higher. As a result, the motor does not maintain consistent ideal rotational stability, but instead surges and slows down depending on the pressure demands of the compressor. This tendency of the motor to surge and then slow down is inefficient and therefore generates heat. Use of an external armature adds greater angular momentum to the motor which helps to compensate for the variable resistance experienced by the motor. Since the motor does not have to work as hard, the heat produced by the motor may be reduced. 
     In an implementation, cooling efficiency may be further increased by coupling an air transfer device  240  to external rotating armature  230 . In an implementation, air transfer device  240  is coupled to the external armature  230  such that rotation of the external armature  230  causes the air transfer device  240  to create an air flow that passes over at least a portion of the motor. In an implementation, air transfer device  240  includes one or more fan blades coupled to the external armature  230 . In an implementation, a plurality of fan blades may be arranged in an annular ring such that the air transfer device  240  acts as an impeller that is rotated by movement of the external rotating armature  230 . As depicted in  FIGS.  1 D and  8 E , air transfer device  240  may be mounted to an outer surface of the external armature  230 , in alignment with the motor  220 . The mounting of the air transfer device  240  to the armature  230  allows air flow to be directed toward the main portion of the external rotating armature  230 , providing a cooling effect during use. In an implementation, the air transfer device  240  directs air flow such that a majority of the external rotating armature  230  is in the air flow path. 
     Further, referring to  FIGS.  1 D and  8 E , air pressurized by compressor  210  exits compressor  210  at compressor outlet  212 . A compressor outlet conduit  250  is coupled to compressor outlet  212  to transfer the compressed air to canister system  300 . As noted previously, compression of air causes an increase in the temperature of the air. This increase in temperature can be detrimental to the efficiency of the oxygen concentrator. In order to reduce the temperature of the pressurized air, compressor outlet conduit  250  is placed in the air flow path produced by air transfer device  240 . At least a portion of compressor outlet conduit  250  may be positioned proximate to motor  220 . Thus, air flow, created by air transfer device  240 , may contact both motor  220  and compressor outlet conduit  250 . In one implementation, a majority of compressor outlet conduit  250  is positioned proximate to motor  220 . In an implementation, the compressor outlet conduit  250  is coiled around motor  220 , as depicted in  FIG.  1 E . 
     In an implementation, the compressor outlet conduit  250  is composed of a heat exchange metal. Heat exchange metals include, but are not limited to, aluminum, carbon steel, stainless steel, titanium, copper, copper-nickel alloys or other alloys formed from combinations of these metals. Thus, compressor outlet conduit  250  can act as a heat exchanger to remove heat that is inherently caused by compression of the air. By removing heat from the compressed air, the number of molecules in a given volume at a given pressure is increased. As a result, the amount of oxygen enriched air that can be generated by each canister during each pressure swing cycle may be increased. 
     The heat dissipation mechanisms described herein are either passive or make use of elements required for the oxygen concentrator  100 . Thus, for example, dissipation of heat may be increased without using systems that require additional power. By not requiring additional power, the run-time of the battery packs may be increased and the size and weight of the oxygen concentrator may be minimized. Likewise, use of an additional box fan or cooling unit may be eliminated. Eliminating such additional features reduces the weight and power consumption of the oxygen concentrator. 
     As discussed above, adiabatic compression of air causes the air temperature to increase. During venting of a canister in canister system  300 , the pressure of the gas being released from the canisters decreases. The adiabatic decompression of the gas in the canister causes the temperature of the gas to drop as it is vented. In an implementation, the cooled exhaust gases  327  vented from canister system  300  are directed toward power supply  180  and toward compression system  200 . In an implementation, base  315  of canister system  300  receives the exhaust gases from the canisters. The exhaust gases  327  are directed through base  315  toward outlet  325  of the base and toward power supply  180 . The exhaust gases, as noted, are cooled due to decompression of the gases and therefore passively provide cooling to the power supply  180 . When the compression system is operated, the air transfer device  240  will gather the cooled exhaust gas and direct the exhaust gas toward the motor of compression system  200 . Fan  172  may also assist in directing the exhaust gas across compression system  200  and out of the housing  170 . In this manner, additional cooling may be obtained without requiring any further power from the battery. 
     Canister System 
     Oxygen concentrator  100  may include at least two canisters, each canister including a gas separation adsorbent. The canisters of oxygen concentrator  100  may be disposed formed from a molded housing. In an implementation, canister system  300  includes two housing components  310  and  510 , as depicted in  FIG.  1 I . In various implementations, the housing components  310  and  510  of the oxygen concentrator  100  may form a two-part molded plastic frame that defines two canisters  302  and  304  and accumulator  106 . The housing components  310  and  510  may be formed separately and then coupled together. In some implementations, housing components  310  and  510  may be injection molded or compression molded. Housing components  310  and  510  may be made from a thermoplastic polymer such as polycarbonate, methylene carbide, polystyrene, acrylonitrile butadiene styrene (ABS), polypropylene, polyethylene, or polyvinyl chloride. In another implementation, housing components  310  and  510  may be made of a thermoset plastic or metal (such as stainless steel or a lightweight aluminum alloy). Lightweight materials may be used to reduce the weight of the oxygen concentrator  100 . In some implementations, the two housings  310  and  510  may be fastened together using screws or bolts. Alternatively, housing components  310  and  510  may be solvent welded together. 
     As shown, valve seats  322 ,  324 ,  332 , and  334  and air pathways  330  and  346  may be integrated into the housing component  310  to reduce the number of sealed connections needed throughout the air flow of the oxygen concentrator  100 . 
     Air pathways/tubing between different sections in housing components  310  and  510  may take the form of molded conduits. Conduits in the form of molded channels for air pathways may occupy multiple planes in housing components  310  and  510 . For example, the molded air conduits may be formed at different depths and at different x,y,z positions in housing components  310  and  510 . In some implementations, a majority or substantially all of the conduits may be integrated into the housing components  310  and  510  to reduce potential leak points. 
     In some implementations, prior to coupling housing components  310  and  510  together, O-rings may be placed between various points of housing components  310  and  510  to ensure that the housing components are properly sealed. In some implementations, components may be integrated and/or coupled separately to housing components  310  and  510 . For example, tubing, flow restrictors (e.g., press fit flow restrictors), oxygen sensors, gas separation adsorbents, check valves, plugs, processors, power supplies, etc. may be coupled to housing components  310  and  510  before and/or after the housing components are coupled together. 
     In some implementations, apertures  337  leading to the exterior of housing components  310  and  510  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). In some implementations, flow restrictors may be inserted into passages prior to inserting plugs to seal the passages. 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 implementations, 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 implementations, 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). In some implementations, 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. Other features are also contemplated (e.g., a bump in the side of the tubing, threads, etc.). In some implementations, press fit flow restrictors may be molded into the housing components (e.g., as narrow tube segments). 
     In some implementations, spring baffle  139  may be placed into respective canister receiving portions of housing components  310  and  510  with the spring side of the baffle  139  facing the exit of the canister. Spring baffle  139  may apply force to gas separation adsorbent in the canister while also assisting in preventing gas separation adsorbent from entering the exit apertures. Use of a spring baffle  139  may keep the gas separation adsorbent compact while also allowing for expansion (e.g., thermal expansion). Keeping the gas separation adsorbent compact may prevent the gas separation adsorbent from breaking during movement of the oxygen concentrator  100 . 
     In some implementations, filter  129  may be placed into respective canister receiving portions of housing components  310  and  510  facing the inlet of the respective canisters. The filter  129  removes particles from the feed gas stream entering the canisters. 
     In some implementations, pressurized air from the compression system  200  may enter air inlet  306 . Air inlet  306  is coupled to inlet conduit  330 . Air enters housing component  310  through inlet  306  and travels through inlet conduit  330 , and then to valve seats  322  and  324 .  FIG.  1 J  and  FIG.  1 K  depict an end view of housing component  310 .  FIG.  1 J  depicts an end view of housing component  310  prior to fitting valves to housing component  310 .  FIG.  1 K  depicts an end view of housing component  310  with the valves fitted to the housing component  310 . Valve seats  322  and  324  are configured to receive inlet valves  122  and  124  respectively. Inlet valve  122  is coupled to canister  302  and inlet valve  124  is coupled to canister  304 . Housing component  310  also includes valve seats  332  and  334  configured to receive outlet valves  132  and  134  respectively. Outlet valve  132  is coupled to canister  302  and outlet valve  134  is coupled to canister  304 . Inlet valves  122 / 124  are used to control the passage of air from inlet conduit  330  to the respective canisters. 
     In an implementation, pressurized air is sent into one of canisters  302  or  304  while the other canister is being vented. For example, during use, inlet valve  122  is opened while inlet valve  124  is closed. Pressurized air from compression system  200  is forced into canister  302 , while being inhibited from entering canister  304  by inlet valve  124 . During pressurization of canister  302 , outlet valve  132  is closed and outlet valve  134  is opened. Similar to the inlet valves, outlet valves  132  and  134  are operated out of phase with each other. Valve seat  322  includes an opening  323  that passes through housing component  310  into canister  302 . Similarly valve seat  324  includes an opening  375  that passes through housing component  310  into canister  302 . Air from inlet conduit  330  passes through openings  323  or  375  if the respective valves  122  and  124  are open, and enters a canister. 
     Check valves  142  and  144  (See  FIG.  1 I ) are coupled to canisters  302  and  304 , respectively. Check valves  142  and  144  are one way valves that are passively operated by the pressure differentials that occur as the canisters are pressurized and vented. Oxygen enriched air produced in canisters  302  and  304  passes from the canisters into openings  542  and  544  of housing component  510 . A passage (not shown) links openings  542  and  544  to conduits  342  and  344 , respectively. Oxygen enriched air produced in canister  302  passes from the canister though opening  542  and into conduit  342  when the pressure in the canister is sufficient to open check valve  142 . When check valve  142  is open, oxygen enriched air flows through conduit  342  toward the end of housing component  310 . Similarly, oxygen enriched air produced in canister  304  passes from the canister through opening  544  and into conduit  344  when the pressure in the canister is sufficient to open check valve  144 . When check valve  144  is open, oxygen enriched air flows through conduit  344  toward the end of housing component  310 . 
     Oxygen enriched air from either canister travels through conduit  342  or  344  and enters conduit  346  formed in housing component  310 . Conduit  346  includes openings that couple the conduit to conduit  342 , conduit  344  and accumulator  106 . Thus, oxygen enriched air, produced in canister  302  or  304 , travels to conduit  346  and passes into accumulator  106 . As illustrated in  FIG.  1 B , gas pressure within the accumulator  106  may be measured by a sensor, such as with an accumulator pressure sensor  107 . (See also  FIG.  1 F .) Thus, the accumulator pressure sensor provides a signal representing the pressure of the accumulated oxygen enriched air. An example of a suitable pressure transducer is a sensor from the HONEYWELL ASDX series. An alternative suitable pressure transducer is a sensor from the NPA Series from GENERAL ELECTRIC. In some versions, the pressure sensor may alternatively measure pressure of the gas outside of the accumulator  106 , such as in an output path between the accumulator  106  and a valve (e.g., supply valve  160 ) that controls the release of the oxygen enriched air for delivery to a user in a bolus. 
     After some time, the gas separation adsorbent will become saturated with nitrogen and will be unable to separate significant amounts of nitrogen from incoming air. When the gas separation adsorbent in a canister reaches this saturation point, the inflow of compressed air is stopped and the canister is vented to desorb nitrogen from the adsorbent. Canister  302  is vented by closing inlet valve  122  and opening outlet valve  132 . Outlet valve  132  releases the exhaust gas from canister  302  into the volume defined by the end of housing component  310 . Foam material may cover the end of housing component  310  to reduce the sound made by release of gases from the canisters. Similarly, canister  304  is vented by closing inlet valve  124  and opening outlet valve  134 . Outlet valve  134  releases the exhaust gas from canister  304  into the volume defined by the end of housing component  310 . 
     While canister  302  is being vented, canister  304  is pressurized to produce oxygen enriched air in the same manner described above. Pressurization of canister  304  is achieved by closing outlet valve  134  and opening inlet valve  124 . The oxygen enriched air exits canister  304  through check valve  144 . 
     In an exemplary implementation, a portion of the oxygen enriched air may be transferred from canister  302  to canister  304  when canister  304  is being vented of nitrogen. Transfer of oxygen enriched air from canister  302  to canister  304  during venting of canister  304  helps to desorb nitrogen from the adsorbent by lowering the partial pressure of nitrogen adjacent the adsorbent. The flow of oxygen enriched air also helps to purge desorbed nitrogen (and other gases) from the canister. Flow of oxygen enriched air between the canisters is controlled using flow restrictors and valves, as depicted in  FIG.  1 B . Three conduits are formed in housing component  510  for use in transferring oxygen enriched air between canisters. As shown in  FIG.  1 L , conduit  530  couples canister  302  to canister  304 . Flow restrictor  151  (not shown) is disposed in conduit  530 , between canister  302  and canister  304  to restrict flow of oxygen enriched air during use. Conduit  532  also couples canister  302  to  304 . Conduit  532  is coupled to valve seat  552  which receives valve  152 , as shown in  FIG.  1 M . Flow restrictor  153  (not shown) is disposed in conduit  532 , between canister  302  and  304 . Conduit  534  also couples canister  302  to  304 . Conduit  534  is coupled to valve seat  554  which receives valve  154 , as shown in  FIG.  1 M . Flow restrictor  155  (not shown) is disposed in conduit  534 , between canister  302  and  304 . The pair of equalization/vent valves  152 / 154  work with flow restrictors  153  and  155  to optimize the air flow balance between the two canisters. 
     Oxygen enriched air in accumulator  106  passes through supply valve  160  into expansion chamber  162  which is formed in housing component  510 . An opening (not shown) in housing component  510  couples accumulator  106  to supply valve  160 . In an implementation, expansion chamber  162  may include one or more devices configured to estimate an oxygen purity (fractional oxygen concentration, typically expressed as a percentage) of the gas passing through the chamber. 
     Outlet System 
     An outlet system, coupled to one or more of the canisters, includes one or more conduits for providing oxygen enriched air to a user. In an implementation, oxygen enriched air produced in either of canisters  302  and  304  is collected in accumulator  106  through check valves  142  and  144 , respectively, as depicted schematically in  FIG.  1 B . The oxygen enriched air leaving the canisters may be collected in an oxygen accumulator  106  prior to being provided to a user. In some implementations, a tube may be coupled to the accumulator  106  to provide the oxygen enriched air to the user. Oxygen enriched air may be provided to the user through an airway delivery device that transfers the oxygen enriched air to the user&#39;s mouth and/or nose. In an implementation, an 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. 
     Turning to  FIG.  1 F , a schematic diagram of an implementation of an outlet system for an oxygen concentrator is shown. A supply valve  160  may be coupled to an outlet tube to control the release of the oxygen enriched air from accumulator  106  to the user. In an implementation, supply valve  160  is an electromagnetically actuated plunger valve. Supply valve  160  is actuated by controller  400  to control the delivery of oxygen enriched air to a user. Actuation of supply valve  160  is not timed or synchronized to the pressure swing adsorption process. Instead, actuation is synchronized to the user&#39;s breathing as described below. In some implementations, supply valve  160  may have continuously-valued actuation to establish a clinically effective amplitude profile for providing oxygen enriched air. 
     Oxygen enriched air in accumulator  106  passes through supply valve  160  into expansion chamber  162  as depicted in  FIG.  1 F . In an implementation, expansion chamber  162  may include one or more devices configured to estimate an oxygen purity of gas passing through the expansion chamber  162 . Oxygen enriched air in expansion chamber  162  builds briefly, through release of gas from accumulator  106  by supply valve  160 , and then is bled through a small orifice flow restrictor  175  to a flow rate sensor  185  and then to particulate filter  187 . Flow restrictor  175  may be a 0.25 D flow restrictor. Other flow restrictor types and sizes may be used. In some implementations, the diameter of the air pathway in the housing may be restricted to create restricted gas flow. Flow rate sensor  185  may be any sensor configured to generate a signal representing the rate of gas flowing through the conduit. Particulate filter  187  may be used to filter bacteria, dust, granule particles, etc., prior to delivery of the oxygen enriched air to the user. The oxygen enriched air passes through filter  187  to connector  190  which sends the oxygen enriched air to the user via delivery conduit  192  and to pressure sensor  194 . 
     The fluid dynamics of the outlet pathway, coupled with the programmed actuations of supply valve  160 , may result in a bolus of oxygen being provided at the correct time and with an amplitude profile that assures rapid delivery into the user&#39;s lungs without excessive waste. 
     Expansion chamber  162  may include one or more oxygen sensors adapted to determine an oxygen purity of gas passing through the chamber. In an implementation, the oxygen purity of gas passing through expansion chamber  162  is estimated using an oxygen sensor  165 . An oxygen sensor is a device configured to measure oxygen purity of a gas. Examples of oxygen sensors include, but are not limited to, ultrasonic oxygen sensors, electrical oxygen sensors, chemical oxygen sensors, and optical oxygen sensors. In one implementation, oxygen sensor  165  is an ultrasonic oxygen sensor that includes an ultrasonic emitter  166  and an ultrasonic receiver  168 . In some implementations, ultrasonic emitter  166  may include multiple ultrasonic emitters and ultrasonic receiver  168  may include multiple ultrasonic receivers. In implementations having multiple emitters/receivers, the multiple ultrasonic emitters and multiple ultrasonic receivers may be axially aligned (e.g., across the gas flow path which may be perpendicular to the axial alignment). 
     In use, an ultrasonic sound wave from emitter  166  may be directed through oxygen enriched air disposed in chamber  162  to receiver  168 . The ultrasonic oxygen sensor  165  may be configured to detect the speed of sound through the oxygen enriched air to determine the composition of the oxygen enriched air. The speed of sound is different in nitrogen and oxygen, and 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 gas in the mixture. In use, the sound at the receiver  168  is slightly out of phase with the sound sent from emitter  166 . This phase shift is due to the relatively slow velocity of sound through a gas medium as compared with the relatively fast speed of the electronic pulse through wire. The phase shift, then, is proportional to the distance between the emitter and the receiver and inversely proportional to the speed of sound through the expansion chamber  162 . The density of the gas in the chamber affects the speed of sound through the expansion chamber and the density is proportional to the ratio of oxygen to nitrogen in the expansion chamber. Therefore, the phase shift can be used to measure the concentration of oxygen in the expansion chamber. In this manner the relative concentration of oxygen in the accumulator may be estimated as a function of one or more properties of a detected sound wave traveling through the accumulator. 
     In some implementations, multiple emitters  166  and receivers  168  may be used. The readings from the emitters  166  and receivers  168  may be averaged to reduce errors that may be inherent in turbulent flow systems. In some implementations, 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. 
     The sensitivity of the ultrasonic oxygen sensor system may be increased by increasing the distance between the emitter  166  and receiver  168 , for example to allow several sound wave cycles to occur between emitter  166  and the receiver  168 . In some implementations, 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 expansion chamber  162  may be reduced or cancelled. The shift caused by a change of the distance between the emitter  166  and receiver  168  may be approximately the same at the measuring intervals, whereas a change owing to a change in oxygen purity may be cumulative. In some implementations, 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. Further details regarding sensing of oxygen in the expansion chamber may be found, for example, in U.S. patent application Ser. No. 12/163,549, entitled “Oxygen Concentrator Apparatus and Method”, which published as U.S. Publication No. 2009/0065007 A1 on Mar. 12, 2009 and is incorporated herein by reference. 
     Flow rate sensor  185  may be used to determine the flow rate of gas flowing through the outlet system. Flow rate sensors that may be used include, but are not limited to: diaphragm/bellows flow meters; rotary flow meters (e.g. Hall effect flow meters); turbine flow meters; orifice flow meters; and ultrasonic flow meters. Flow rate sensor  185  may be coupled to controller  400 . The rate of gas flowing through the outlet system may be an indication of the breathing volume of the user. Changes in the flow rate of gas flowing through the outlet system may also be used to determine a breathing rate of the user. Controller  400  may generate a control signal or trigger signal to control actuation of supply valve  160 . Such control of actuation of the supply valve may be based based on the breathing rate and/or breathing volume of the user, as estimated by flow rate sensor  185 . 
     In some implementations, ultrasonic oxygen sensor  165  and, for example, flow rate sensor  185  may provide a measurement of an actual amount of oxygen being provided. For example, flow rate sensor  185  may measure a volume of gas (based on flow rate) provided and ultrasonic oxygen sensor  165  may provide the concentration of oxygen of the gas provided. These two measurements together may be used by controller  400  to determine an approximation of the actual amount of oxygen provided to the user. 
     Oxygen enriched air passes through flow rate sensor  185  to filter  187 . Filter  187  removes bacteria, dust, granule particles, etc. prior to providing the oxygen enriched air to the user. The filtered oxygen enriched air passes through filter  187  to connector  190 . Connector  190  may be a “Y” connector coupling the outlet of filter  187  to pressure sensor  194  and delivery conduit  192 . Pressure sensor  194  may be used to monitor the pressure of the gas passing through delivery conduit  192  to the user. In some implementations, pressure sensor  194  is configured to generate a signal that is proportional to the amount of positive or negative pressure applied to a sensing surface. Changes in pressure, sensed by pressure sensor  194 , may be used to determine a breathing rate of a user, as well as to detect the onset of inhalation (also referred to as the trigger instant) as described below. Controller  400  may control actuation of supply valve  160  based on the breathing rate and/or onset of inhalation of the user. In an implementation, controller  400  may control actuation of supply valve  160  based on information provided by either or both of the flow rate sensor  185  and the pressure sensor  194 . 
     Oxygen enriched air may be provided to a user through delivery conduit  192 . In an implementation, delivery conduit  192  may be a silicone tube. Delivery conduit  192  may be coupled to a user using an airway delivery device  196 , as depicted in  FIGS.  1 G and  8 H . An airway delivery device  196  may be any device capable of providing the oxygen enriched air to nasal cavities or oral cavities. Examples of airway delivery devices include, but are not limited to: nasal masks, nasal pillows, nasal prongs, nasal cannulas, and mouthpieces. A nasal cannula airway delivery device  196  is depicted in  FIG.  1 G . Nasal cannula airway delivery device  196  is positioned proximate to a user&#39;s airway (e.g., proximate to the user&#39;s mouth and or nose) to allow delivery of the oxygen enriched air to the user while allowing the user to breathe air from the surroundings. 
     In an alternate implementation, a mouthpiece may be used to provide oxygen enriched air to the user. As shown in  FIG.  1 H , a mouthpiece  198  may be coupled to oxygen concentrator  100 . Mouthpiece  198  may be the only device used to provide oxygen enriched air to the user, or a mouthpiece may be used in combination with a nasal delivery device (e.g., a nasal cannula). As depicted in  FIG.  1 H , oxygen enriched air may be provided to a user through both nasal cannula airway delivery device  196  and mouthpiece  198 . 
     Mouthpiece  198  is removably positionable in a user&#39;s mouth. In one implementation, mouthpiece  198  is removably couplable to one or more teeth in a user&#39;s mouth. During use, oxygen enriched air is directed into the user&#39;s mouth via the mouthpiece. Mouthpiece  198  may be a night guard mouthpiece which is molded to conform to the user&#39;s teeth. Alternatively, mouthpiece may be a mandibular repositioning device. In an implementation, at least a majority of the mouthpiece is positioned in a user&#39;s mouth during use. 
     During use, oxygen enriched air may be directed to mouthpiece  198  when a change in pressure is detected proximate to the mouthpiece. In one implementation, mouthpiece  198  may be coupled to a pressure sensor  194 . When a user inhales air through the user&#39;s mouth, pressure sensor  194  may detect a drop in pressure proximate to the mouthpiece. Controller  400  of oxygen concentrator  100  may control release of a bolus of oxygen enriched air to the user at the onset of inhalation. 
     During typical breathing of an individual, inhalation may occur through the nose, through the mouth or through both the nose and the mouth. Furthermore, breathing may change from one passageway to another depending on a variety of factors. For example, during more active activities, a user may switch from breathing through their nose to breathing through their mouth, or breathing through their mouth and nose. A system that relies on a single mode of delivery (either nasal or oral), may not function properly if breathing through the monitored pathway is stopped. For example, if a nasal cannula is used to provide oxygen enriched air to the user, an inhalation sensor (e.g., a pressure sensor or flow rate sensor) is coupled to the nasal cannula to determine the onset of inhalation. If the user stops breathing through their nose, and switches to breathing through their mouth, the oxygen concentrator  100  may not know when to provide the oxygen enriched air since there is no feedback from the nasal cannula. Under such circumstances, oxygen concentrator  100  may increase the flow rate and/or increase the frequency of providing oxygen enriched air until the inhalation sensor detects an inhalation by the user. If the user switches between breathing modes often, the default mode of providing oxygen enriched air may cause the oxygen concentrator  100  to work harder, limiting the portable usage time of the system. 
     In an implementation, mouthpiece  198  is used in combination with nasal cannula airway delivery device  196  to provide oxygen enriched air to a user, as depicted in  FIG.  1 H . Both mouthpiece  198  and nasal cannula airway delivery device  196  are coupled to an inhalation sensor. In one implementation, mouthpiece  198  and nasal cannula airway delivery device  196  are coupled to the same inhalation sensor. In an alternate implementation, mouthpiece  198  and nasal cannula airway delivery device  196  are coupled to different inhalation sensors. In either implementation, the inhalation sensor(s) may detect the onset of inhalation from either the mouth or the nose. Oxygen concentrator  100  may be configured to provide oxygen enriched air to the delivery device (i.e. mouthpiece  198  or nasal cannula airway delivery device  196 ) proximate to which the onset of inhalation was detected. Alternatively, oxygen enriched air may be provided to both mouthpiece  198  and nasal cannula airway delivery device  196  if onset of inhalation is detected proximate either delivery device. The use of a dual delivery system, such as depicted in  FIG.  1 H  may be particularly useful for users when they are sleeping and may switch between nose breathing and mouth breathing without conscious effort. 
     Controller System 
     Operation of oxygen concentrator  100  may be performed automatically using an internal controller  400  coupled to various components of the oxygen concentrator  100 , as described herein. Controller  400  includes one or more processors  410  and internal memory  420 , as depicted in  FIG.  1 B . Methods used to operate and monitor oxygen concentrator  100  may be implemented by program instructions stored in internal memory  420  or an external memory medium coupled to controller  400 , and executed by one or more processors  410 . 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), Random Access Memory (RAM), etc.; or a non-volatile memory such as a magnetic medium, 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 proximate to the controller  400  by which the programs are executed, or may be located in an external computing device that connects to the controller  400  over a network, as described below. In the latter instance, the external computing device may provide program instructions to the controller  400  for execution. The term “memory medium” may include two or more memory media that may reside in different locations, e.g., in different computing devices that are connected over a network. 
     In some implementations, controller  400  includes processor  410  that includes, for example, one or more field programmable gate arrays (FPGAs), microcontrollers, etc. included on a circuit board disposed in oxygen concentrator  100 . Processor  410  is configured to execute programming instructions stored in memory  420 . In some implementations, programming instructions may be built into processor  410  such that a memory external to the processor  410  may not be separately accessed (i.e., the memory  420  may be internal to the processor  410 ). 
     Processor  410  may be coupled to various components of oxygen concentrator  100 , including, but not limited to compression system  200 , one or more of the valves used to control fluid flow through the system (e.g., valves  122 ,  124 ,  132 ,  134 ,  152 ,  154 ,  160 ), oxygen sensor  165 , pressure sensor  194 , flow rate sensor  185 , temperature sensors (not shown), fan  172 , and any other component that may be electrically controlled. In some implementations, a separate processor (and/or memory) may be coupled to one or more of the components. 
     Controller  400  is configured (e.g. programmed by program instructions) to operate oxygen concentrator  100  and is further configured to monitor the oxygen concentrator  100  such as for malfunction states or other process information. For example, in one implementation, controller  400  is programmed to trigger an alarm if the system is operating and no breathing is detected by the user for a predetermined amount of time. For example, if controller  400  does not detect a breath for a period of 75 seconds, an alarm LED may be lit and/or an audible alarm may be sounded. If the user has truly stopped breathing, for example, during a sleep apnea episode, the alarm may be sufficient to awaken the user, causing the user to resume breathing. The action of breathing may be sufficient for controller  400  to reset this alarm function. Alternatively, if the system is accidentally left on when delivery conduit  192  is removed from the user, the alarm may serve as a reminder for the user to turn oxygen concentrator  100  off. 
     Controller  400  is further coupled to oxygen sensor  165 , and may be programmed for continuous or periodic monitoring of the oxygen purity of the oxygen enriched air passing through expansion chamber  162 . A minimum oxygen purity threshold may be programmed into controller  400 , such that the controller lights an LED visual alarm and/or an audible alarm to warn the user of the low concentration of oxygen. 
     Controller  400  is also coupled to internal power supply  180  and may be configured to monitor the level of charge of the internal power supply. A minimum voltage and/or current threshold may be programmed into controller  400 , such that the controller lights an LED visual alarm and/or an audible alarm to warn the user of low power condition. The alarms may be activated intermittently and at an increasing frequency as the battery approaches zero usable charge. 
       FIG.  1 O  illustrates one implementation of a connected POC therapy system  450  including the POC  100 . Controller  400  of the POC  100  includes the transceiver  430  configured to allow the controller  400  to communicate, using a wireless communication protocol such as the Global System for Mobile Telephony (GSM) or other protocol (e.g., WiFi), with a remote computing device such as a cloud-based server  460  such as over a network  470 . The network  470  may be a wide-area network such as the Internet, or a local-area network such as an Ethernet. The controller  400  may also include a short range wireless module in the transceiver  430  configured to enable the controller  400  to communicate, using a short range wireless communication protocol such as Bluetooth™, with a portable computing device  480  such as a smartphone. The portable computing device, e.g. smartphone,  480  may be associated with a user  1000  of the POC  100 . 
     The server  460  may also be in wireless communication with the portable computing device  480  using a wireless communication protocol such as GSM. A processor of the smartphone  480  may execute a program  482  known as an “app” to control the interaction of the smartphone  480  with the user  1000 , the POC  100 , and/or the server  460 . The server  460  may have access to a database  466  that stores operational data about the POC  100  and user  1000 . 
     The server  460  includes an analysis engine  462  that may execute methods of operating and monitoring the POC  100 . The server  460  may also be in communication via the network  470  with other devices such as a personal computing device  464  via a wired or wireless connection. A processor of the personal computing device  464  may execute a “client” program to control the interaction of the personal computing device  464  with the server  460 . One example of a client program is a browser. 
     Further functions that may be implemented with or by the controller  400  are described in detail in other sections of this disclosure. 
     Control Panel 
     Control panel  600  serves as an interface between a user and controller  400  to allow the user to initiate predetermined operation modes of the oxygen concentrator  100  and to monitor the status of the system.  FIG.  1 N  depicts an implementation of control panel  600 . Charging input port  605 , for charging the internal power supply  180 , may be disposed in control panel  600 . 
     In some implementations, control panel  600  may include buttons to activate various operation modes for the oxygen concentrator  100 . For example, control panel may include power button  610 , flow rate setting buttons  620  to  626 , active mode button  630 , sleep mode button  635 , altitude button  640 , and a battery check button  650 . In some implementations, 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. Power button  610  may power the system on or off. If the power button is activated to turn the system off, controller  400  may initiate a shutdown sequence to place the system in a shutdown state (e.g., a state in which both canisters are pressurized). 
     Flow rate setting buttons  620 ,  622 ,  624 , and  626  allow a flow rate of oxygen enriched air to be selected (e.g., 0.2 LPM by button  620 , 0.4 LPM by button  622 , 0.6 LPM by button  624 , and 0.8 LPM by button  626 ). In other implementations, the number of flow rate settings may be increased or decreased. After a flow rate setting is selected, oxygen concentrator  100  will then control operations to achieve production of the oxygen enriched air according to the selected flow rate setting. Optionally, the control panel may include one or more hybrid button(s) to activate any of the hybrid modes described herein. Optionally, the control panel may include a POD button to activate a POD mode. Thus, the device may be set to operate in a traditional bolus mode (POD) where the device merely releases bolus for patient inspiration such as in accordance with the set flow rate, a continuous flow mode (CFM) where the device provides gas flow with gas characteristics that generally remain constant for inspiration and expiration such as in accordance with the set flow rate, and/or a hybrid mode where the gas characteristics generally change for inspiration and expiration as discussed herein. In some cases, the controller may automatically change from a mode of higher power consumption to a mode of lower power consumption based on remaining battery life. For example, on detection of a low battery condition such as when the controller is configured with a voltage detection circuit (e.g., an undervoltage detector) to sample battery voltage, the controller may switch from a continuous mode to a hybrid mode or a POD mode. Similarly, on detection of a low or lower battery condition, the controller may switch from hybrid mode to a POD mode. 
     Altitude button  640  may be activated when a user is going to be in a location at a higher elevation than the oxygen concentrator  100  is regularly used by the user. 
     Battery check button  650  initiates a battery check routine in the oxygen concentrator  100  which results in a relative battery power remaining LED  655  being illuminated on control panel  600 . 
     The user may manually indicate active mode or sleep mode by pressing button  630  for active mode or button  635  for sleep mode. 
     Triggering the POC 
     The methods of operating and monitoring the POC  100  described below may be executed by the one or more processors, such as the one or more processors  410  of the controller  400 , configured by program instructions, such as including, as previously described, the one or more functions and/or associated data corresponding thereto, stored in a memory such as the memory  420  of the POC  100 . Alternatively, some or all of the steps of the described methods may be similarly executed by one or more processors of an external computing device, such as the server  460 , forming part of the connected POC therapy system  450 , as described above. In this latter implementation, the processors  410  may be configured by program instructions stored in the memory  420  of the POC  100  to transmit to the external computing device the measurements and parameters necessary for the performance of those steps that are to be carried out at the external computing device. 
     The main use of an oxygen concentrator  100  is to provide supplemental oxygen to a user. One or more flow rate settings may be selected on a control panel  600  of the oxygen concentrator  100 , which then will control operations to achieve production of the oxygen enriched air according to the selected flow rate setting. In some versions, a plurality of flow rate settings may be implemented (e.g., five flow rate settings). The controller  400  may implement a POD (pulsed oxygen delivery) or demand mode of operation. Controller  400  may regulate the volume of the one or more released pulses or boluses to achieve delivery of the oxygen enriched air according to the selected flow rate setting. The flow rate settings on the control panel  600  may correspond to minute volumes (bolus volume multiplied by breathing rate per minute) of delivered oxygen, e.g. 0.2 LPM, 0.4 LPM, 0.6 LPM, 0.8 LPM, 1.1 LPM. 
     Oxygen enriched air produced by oxygen concentrator  100  is stored in an oxygen accumulator  106  and, in a POD mode of operation, released to the user as the user inhales. The amount of oxygen enriched air provided by oxygen concentrator  100  is controlled, in part, by supply valve  160 . In an implementation, supply valve  160  is opened for a sufficient amount of time to provide the appropriate amount of oxygen enriched air, as estimated by controller  400 , to the user. In order to minimize the wastage of oxygen, controller  400  may be programmed to open the supply valve  160  to release a bolus of oxygen enriched air soon after the onset of a user&#39;s inhalation is detected. For example, the bolus of oxygen enriched air may be provided in the first few milliseconds of a user&#39;s inhalation. Releasing a bolus of oxygen enriched air to the user as the user inhales may prevent wastage of oxygen by not releasing oxygen, for example, when the user is exhaling. 
     In an implementation, a sensor such as a pressure sensor  194  may be used to detect the onset of inhalation by the user and thereby trigger the release of a bolus. For example, the onset of inhalation may be detected by using pressure sensor  194 . In use, delivery conduit  192  for providing oxygen enriched air is coupled to the user&#39;s nose and/or mouth through the nasal airway delivery device  196  and/or mouthpiece  198 . The pressure in delivery conduit  192  is therefore representative of the user&#39;s airway pressure and hence indicative of user respiration. At the onset of inhalation, the user begins to draw air into their body through the nose and/or mouth. As the air is drawn in, a negative pressure is generated at the end of delivery conduit  192 , due, in part, to the venturi action of the air being drawn across the end of delivery conduit  192 . Controller  400  analyses the pressure signal from the pressure sensor  194  to detect a drop in pressure indicating the onset of inhalation. 
     A positive change or rise in the pressure in delivery conduit  192  indicates an exhalation by the user. Controller  400  may analyze the pressure signal from pressure sensor  194  to detect a rise in pressure indicating the onset of exhalation. In one implementation, when a positive pressure change is sensed, supply valve  160  is closed until the next onset of inhalation is detected. Alternatively, supply valve  160  may be closed after a predetermined interval known as the bolus duration. 
     By measuring the intervals between adjacent onsets of inhalation, the user&#39;s breathing rate may be estimated. By measuring the intervals between onsets of inhalation and the subsequent onsets of exhalation, the user&#39;s inspiratory time may be estimated. 
     In other implementations, the pressure sensor  194  may be located in a sensing conduit that is in pneumatic communication with the user&#39;s airway, but separate from the delivery conduit  192 . In such implementations the pressure signal from the pressure sensor  194  is therefore also representative of the user&#39;s airway pressure. 
     Hybrid Mode Oxygen Delivery 
     Hybrid mode therapy is a breath-synchronised therapy in which there is a non-zero inter-bolus flow of gas to the patient as well as boluses delivered in synchrony with inhalation as in POD mode. In such a mode, the controller may control operations of the device to activate delivery of the bolus at such synchrony times and otherwise control or operate the device to deliver the non-zero inter-bolus flow of gas. Thus, the device may provide a generally continuous flow of therapy gas during each respiratory cycle (i.e., inspiration and expiration) but the characteristic(s) of the gas flow (e.g., purity and/or flow rate) may differ during inspiration (or part of inspiration) relative to non-inspiration times or expiration. Examples of such modes are described in more detail herein. Such gas characteristic delivery differences for the hybrid mode(s) may be implemented with multiple flow paths within the oxygen concentrator that employ different configurations. For example, such gas characteristic delivery differences may be implemented with a primary flow path (or primary path) and one or more secondary flow paths (or secondary paths). In this regard, the primary path generally concerns the typical path for flow of therapy gas from the accumulator through the supply valve that releases inspiratory triggered boluses to the delivery conduit. The primary path may provide the therapy gas to the delivery conduit with a first gas characteristic. Moreover, a secondary path generally concerns a path for flow of therapy gas to the delivery conduit that is separate from the primary path. Such a secondary path may provide the therapy gas to the delivery conduit with a second gas characteristic that is different from the first gas characteristic. In some example implementations, therapy gas provision via the primary path may generally involve therapy gas provided for inspiration times, whereas therapy gas provision via the secondary path(s) may generally involve therapy gas provided for expiration times or non-inspiratory times. However, in other examples, the secondary path(s) may also provide therapy gas for inspiratory times. Examples of such different paths for different hybrid modes are discussed in more detail herein. 
     Bilevel Purity 
       FIG.  2    contains a graph  260  illustrating an example of a hybrid mode, referred to as bilevel purity. In bilevel purity hybrid mode, each bolus of oxygen enriched air is released in synchrony with inhalation, as in POD mode, at a flow rate referred to as the bolus flow rate and at an oxygen purity referred to as the bolus purity. As such, the bolus purity may be equivalent to the oxygen purity of oxygen enriched air. This is illustrated by the period  270  in the graph  260 , wherein the period  270  represents a period of time at which the device operates to produce gas flow so that the patient is provided a flow of gas at the bolus flow rate at the bolus purity. However, in between periods of bolus release referred to as inter-bolus periods, such as the period  280 , the device operates to produce gas flow so that the patient is provided a flow of gas at the bolus flow rate, except at lower oxygen purity. 
     The lower oxygen purity of the inter-bolus flow means less oxygen is wasted than during conventional continuous flow, in which the oxygen purity and flow rate are generally constant. This in turn helps to extend battery life, since the device, including the compressor, does not need to work as hard as during conventional continuous flow to maintain system pressure at the desired value for the current flow rate setting. In addition, portable oxygen concentrators are limited in the volume of oxygen they can produce in a given time due to the design constraints (size, weight, power consumption, adsorbent mass). By conserving oxygen delivery, bilevel purity hybrid mode allows the other design constraints more room for optimisation. 
       FIG.  3    is a schematic of a modification of the outlet system of  FIG.  1 F , according to one implementation of bilevel purity hybrid mode. The modified outlet system  350  of  FIG.  3    is the same as that illustrated in  FIG.  1 F , except with new elements: a flow source  700 , secondary valve  710 , such as a two-way or two-port valve, a flow restrictor  720 , and a restrictor  730 . The flow rate sensor  185  may be omitted from the modified outlet system  350  as illustrated in  FIG.  3   , or optionally may be included after the flow restrictor  175  as illustrated in  FIG.  1 F . 
     The flow source  700  may be coupled to the downstream side of the flow restrictor  175  via a secondary flow path (SFP) comprising the secondary valve  710  and the flow restrictor  720 . The secondary flow path is a different path from the primary flow path and may be operated to provide therapy gas with a different gas characteristic than the primary path. Thus, the flow in the secondary flow path is at a lower purity than the oxygen enriched air released by the supply valve  160  to the patient via the primary flow path (PFP). The controller  400  controls the secondary valve  710  to allow flow along the lower-purity path when a bolus is not being released by the supply valve  160 . The controller  400  may also control the secondary valve  710  to prevent flow along the lower-purity path during bolus release. In other words, the secondary valve  710  may be actuated in anti-sync with the supply valve  160 . As such, the controller  400  generates a control signal to control the secondary valve  710  to be open when the supply valve  160  is closed and closed when the supply valve  160  is open. In an alternative to the modified outlet system  350  for implementing bilevel purity hybrid mode, the two valves  160  and  710  may be replaced by a three-way valve (or a three-port valve) that is configured to couple the accumulator  106  to either the primary flow path (when triggered by the onset of inhalation) or the secondary, lower-purity path at all other times. The three-way valve may be either downstream of the flow restrictors  175  and  720  or upstream of a single flow restrictor which replaces and combines the effects of the flow restrictors  175  and  720 . 
     In one implementation, the flow source  700  may be the compressor  210  with an outlet to the secondary path. In such an implementation, the flow restrictor  720  is chosen such that the flow rate in the lower-purity path is approximately equal to the bolus flow rate in the higher-purity primary flow path (or primary path). In some implementations, the flow restrictor  720  may be omitted altogether, depending on the pressure of the flow source  700  and the pneumatic impedance of the secondary flow path. 
     In an alternative implementation, the flow source  700  may be a secondary compressor with an outlet to the secondary path. Such a compressor may be configured to generate a flow of air at flow rates approximately equal to the bolus flow rates in the higher-purity path. In such an implementation the flow restrictor  720  may be omitted. The secondary compressor may optionally be controlled by the controller  400  to achieve the specified flow rates. 
     In either such implementation, the oxygen purity in the lower-purity path is approximately that of ambient air (21%). 
     In yet a further implementation of bilevel purity hybrid mode, the flow source  700  is a portion of the vented exhaust gas that has been re-routed from the outlet  130  (e.g., from the exhaust outlet of the canister(s)) to the lower-purity path. Such vented exhaust gas may be of oxygen purity typically around the ambient purity of 21%, but may be as high as 35% and as low as 4% depending on the amount of the purge flow. In one such implementation, the flow restrictor  720  is chosen such that the flow rate in the lower-purity path is approximately equal to the bolus flow rates in the higher-purity primary path. Thus, the therapy gas provided to the delivery conduit in such a hybrid mode may use both accumulated enriched gas (e.g., a bolus) and exhaust gas that may flow to the delivery conduit at least during patient inspiration and patient expiration. The hybrid mode may then vary a characteristic of the therapy gas, such as where the varied characteristic is oxygen purity. The varied oxygen purity may have a first oxygen purity during at least a portion of patient inspiration and a second oxygen purity after the portion of patient inspiration (e.g., during remaining inspiration and/or expiration). The first oxygen purity may be a purity in a range of about 50 percent to about 99 percent, which may be attributable to the bolus release gas and may be provided via the primary path to the delivery conduit. Moreover, the second oxygen purity may be a purity in a range of about 4 percent to 35 percent, which may be attributable to the vented exhaust gas and may be provided via the secondary path to the delivery conduit. Thus, the primary path, which generally concerns the path for flow of therapy gas from the accumulator via the supply valve that releases inspiratory triggered boluses to the delivery conduit, may provide the therapy gas with the first oxygen purity. Moreover, the secondary path, which is a flow path to the delivery conduit that is separate from the primary path, may provide the therapy gas with the second oxygen purity. 
     In some such implementations, the sensor configuration of the outlet system of  FIG.  1 F  may be modified in the modified outlet system  350 . For example, the pressure sensor  194  in the outlet system of  FIG.  1 F  typically is differentially connected, so that it has its “sense port” (SP) connected to connector  190  or elsewhere in the delivery conduit  192  and it has its “reference port” (RP) connected to ambient (not shown in  FIG.  1 F ). This sensor configuration may be modified in the modified outlet system  350  so that the reference port (RP) is instead within the system such as being coupled downstream of the supply valve. For example, it may be connected to the downstream side of a flow restrictor  730 . The upstream side of the flow restrictor  730  is connected to the downstream side of the flow restrictor  175 . With such a differential connection, the modified outlet system  350  may be able to trigger more accurately than if the pressure sensor  194  were connected as in  FIG.  1 F . The lower-purity flow through the secondary path in the inter-bolus periods causes the pressure at the connector  190 , and therefore the sense port of the pressure sensor  194 , to be elevated substantially above ambient just before the onset of inhalation. If the reference port of the pressure sensor  194  were otherwise connected to ambient, the substantially positive pressure difference between the ports of the pressure sensor  194  might saturate the pressure sensor  194  just before the onset of inhalation, making it more difficult to reliably sense the drop in pressure at the connector  190  resulting from the onset of inhalation. 
     However, with the differential connection of  FIG.  3   , the pressure difference between the ports of the pressure sensor  194  is much smaller, just before the onset of inhalation, and in fact may even be slightly negative. The pressure sensor  194  therefore remains unsaturated. Because of the flow restrictor  730 , the dynamic or adaptive reference pressure is in a sense a damped or lagged version of the pressure at the connector  190 . The onset of inhalation causes the pressure at the sense port (the connector  190 ) to drop sharply, while due to the flow restrictor  730  the pressure at the reference port stays constant for a short interval after the onset of inhalation. The pressure difference across the ports of the pressure sensor  194  is therefore pulled in the negative direction for long enough to be detected by the controller  400 . The modified reference port connection effectively acts as a dynamic or adaptive threshold against which the pressure at the connector  190  is compared to detect the onset of inhalation. 
     Optionally, the device may be controlled so that the bilevel purity hybrid mode can be deactivated. Thus, with the aforementioned secondary valve configuration(s), oxygen enriched air is not required to be produced in bilevel purity hybrid at all times using the modified outlet system  350 . In some implementations, the controller  400  may maintain the secondary valve  710  in a closed state so that the oxygen enriched air can be delivered according to a different mode without use of the secondary path. For example, with the maintained closure state, the controller can operate the device to provide gas flow in a POD mode via the primary path. Optionally, the controller may be configured to operate in the POD mode until a control (e.g., a hybrid button or a comfort button) on the control panel  600  is activated. For example, the control may be activated if the user is experiencing dyspnea or shortness of breath and is in need of reassurance or comfort. Once such a control is activated, the controller  400  may generate control signals to operate the secondary valve such as to begin to open and close the secondary valve  710  in anti-sync with the supply valve  160  as described above to implement bilevel purity hybrid mode. Optionally, pressing of the button may trigger operation in the hybrid mode for a predetermined period or for an indefinite period until the control on the control panel is de-activated. For example, the comfort button may activate the hybrid mode for such a predetermined time period. The controller  400  then reverts to control of oxygen enriched air in a POD mode after the predetermined time. Pressing the hybrid button may activate the hybrid mode in a more continuous fashion such as until the user activates another mode or the device is turned off. 
     Bilevel Flow Rate 
       FIG.  4    contains a graph  435  illustrating another example of a hybrid mode, referred to as bilevel flow rate. In bilevel flow rate hybrid mode, each bolus of oxygen enriched air is released in synchrony with inhalation, as in POD mode and bilevel purity hybrid mode, at the bolus flow rate. This is illustrated by the period  440  in the graph  435 . However, during inter-bolus periods such as the period  445 , the device operates to produce gas flow so that the patient is provided with a flow of gas at the bolus oxygen purity, except at a lower flow rate referred to as the inter-bolus flow rate. As discussed in more detail herein, such a mode may be implemented with a primary flow path and a secondary flow path. To achieve the different gas flow rate characteristics, the paths may be configured with different flow characteristics. 
     The lower flow rate of the inter-bolus flow means less oxygen is wasted than during conventional continuous flow, in which the flow rate and oxygen purity are generally constant over the breathing cycle. This in turn helps to extend battery life, since the device, including the compressor, does not need to work as hard as during conventional continuous flow to maintain system pressure at the desired value for the current flow rate setting. In addition, portable oxygen concentrators are limited in the volume of oxygen they can produce in a given time due to the design constraints (size, weight, power consumption, adsorbent mass). By conserving oxygen delivery, bilevel flow rate hybrid mode allows the other design constraints more room for optimisation. 
       FIG.  5    is a schematic of a modification of the outlet system of  FIG.  1 F , according to one implementation of bilevel purity hybrid mode. The modified outlet system  500  of  FIG.  5    is similar to the modified outlet system  350  illustrated in  FIG.  3   , except that instead of receiving flow from the flow source  700  like the secondary valve  710 , a secondary valve  810  (e.g., a two-way or two port valve) receives flow from the accumulator  106 . In other words, the secondary valve  810  and the flow restrictor  820 , which may be placed in any order, form a secondary flow path (SFP) for the oxygen enriched air from the accumulator  106 . The flow restrictor  820  is chosen so that the secondary flow path is a lower-flow path. That is, the flow of the secondary path is substantially lower than the bolus flow rate in the primary flow path (PFP). 
     The controller  400  is configured to control the secondary valve  810  to allow flow along the lower-flow path when the controller is not controlling a release of a bolus with the supply valve  160 . The controller  400  may also control the secondary valve  810  to prevent flow along the lower-flow path during that controlled bolus release. In other words, the secondary valve  810  may be actuated in anti-sync with the supply valve  160 . As such, the controller  400  generates a control signal to control the secondary valve  710  to be open when the supply valve  160  is closed and closed when the supply valve  160  is open. 
     The modified outlet system  500  may also implement the differentially connected pressure sensor  194  with the flow restrictor  730 , as in the modified outlet system  350 , to enable more accurate triggering. 
     Optionally, the device may be controlled so that the bilevel flow rate hybrid mode can be deactivated. Thus, with the aforementioned secondary valve configuration(s), oxygen enriched air is not required to be produced in bilevel flow rate hybrid mode at all times using the modified outlet system  500 . In some implementations, the controller  400  may maintain the secondary valve  810  in a closed state so that the oxygen enriched air can be delivered according to a different mode without use of the secondary path. For example, with the maintained closure state, the controller can operate the device to provide gas flow in a POD mode via the primary flow path (or primary path). Similar to the operations previously described, the controller  400 , such as in response to a user pressing a control button (e.g., a comfort button or a hybrid button), can operate in the bilevel flow rate hybrid mode by generating control signals to the aforementioned valves, either for a predetermined period of time or in a more continuous fashion as previously described. 
     In an alternative implementation, the modified outlet system  500  may be configured for providing the bilevel flow rate hybrid mode without the secondary valve  810 . With the secondary valve  810  removed, the secondary, lower-flow path through the flow restrictor  820  provides a gas flow as long as the POC  100  itself is operating. To permit a lower flow rate of the secondary path relative to the primary path, the paths may be configured with different flow characteristics such that a flow characteristic of the primary path is different from a flow characteristic of the secondary path. For example, a flow restrictor of the secondary path may be chosen to restrict flow so as to achieve a lower flow rate of gas in the secondary path when compared to the flow rate of the primary path. Similarly, the pneumatic resistance of the primary and secondary flow paths may be chosen, such as according to different conduit sizes, to achieve the flow rate differences. For example, a smaller, more restrictive conduit may be chosen for the secondary path when compared to the conduit of the primary path. 
     In a further alternative modified outlet system for implementing bilevel flow rate hybrid mode, the valves  160  and  810  may be replaced by a three-way valve (e.g., three-port valve) that pneumatically couples the accumulator  106  to the primary path and the secondary flow path. Thus, the three-way valve may be activated by the controller to selectively pneumatically couple the accumulator to one of the primary path, such as when the controller is triggered by detection of the onset of inhalation, and the secondary, lower-flow path, such as at all other times. 
     One benefit of the bilevel flow rate hybrid delivery mode is that the oxygen enriched air delivered at a low flow rate via the secondary, lower-flow path “pools” within the delivery conduit  192  and is therefore available for inhalation as soon as inhalation begins, even before the opening of the primary path for the release of the bolus. 
     Intermediate Implementations 
       FIG.  6    contains a graph  660  illustrating various modes of delivery of oxygen enriched air by an oxygen concentrator. The horizontal axis represents the inter-bolus flow rate and the vertical axis represents the inter-bolus oxygen purity. The point  665  represents continuous flow delivery, in which the inter-bolus flow rate equals the bolus flow rate and the inter-bolus purity is the same as the purity of the oxygen enriched air, i.e. the bolus purity (e.g. 93%). The point  670  represents POD mode, in which the inter-bolus flow rate is zero. The point  675  represents the bilevel purity species of hybrid delivery mode, in which the inter-bolus flow rate is equal to the bolus flow rate but the inter-bolus purity is much reduced, typically to 21% for room air. The point  680  represents the bilevel flow rate species of hybrid delivery mode, in which the inter-bolus flow rate is substantially less than the bolus flow rate but the inter-bolus purity is the same as the bolus purity. The line  685  represents a progression of intermediate versions of a hybrid delivery mode between the bilevel purity species (point  675 ) and the bilevel flow rate species (point  680 ). The point  690  represents one such intermediate version in which the inter-bolus flow rate is somewhat less than the bolus flow rate and the inter-bolus purity is somewhat less than the bolus purity, while being greater than the purity of bilevel purity species. 
     Such intermediate versions may be implemented with a controller  400  controlling a combination of components of the modified outlet systems  350  and  500  to implement both of the secondary lower-purity path (SPF from  FIG.  3   ) and the secondary lower-flow path (SPF from  FIG.  5   ). Such a combination of secondary paths (SFP- 1 , SFP- 2 ) is illustrated in  FIG.  7   . In one such example, the controller  400  may generate control signals to control the secondary valves  710 ,  810 , each of which are opened in anti-sync with the valve  160  of the primary path. The combination of the flows in the two secondary paths makes up the total inter-bolus flow. The respective sizes of the flow restrictors  720  and  820  may be chosen to set the flow rates in the two secondary paths and therefore the inter-bolus purity and flow rate to achieve the desired characteristics of the versions illustrated in the line of the graph  FIG.  6   . 
     Although the components of  FIG.  7    illustrate the secondary valves  710  and  810 , in some implementations one or both of these may be omitted. 
     The differentially connected pressure sensor  194  may be used with all examples of hybrid mode delivery in order to improve the accuracy of detection of inspiration and control of triggering of valve  160  for release of a bolus (and thereby the signals associated with the anti-sync operation of valves  710 ,  810 . 
     Glossary 
     For the purposes of the present technology disclosure, in certain forms of the present technology, one or more of the following definitions may apply. In other forms of the present technology, alternative definitions may apply. 
     General 
     Air: In certain forms of the present technology, air may be taken to mean atmospheric air, consisting of 78% nitrogen (N 2 ), 21% oxygen (O 2 ), and 1% water vapour, carbon dioxide (CO 2 ), argon (Ar), and other trace gases. 
     Oxygen enriched air: Air with a concentration of oxygen greater than that of atmospheric air (21%), for example at least about 50% oxygen, at least about 60% oxygen, at least about 70% oxygen, at least about 80% oxygen, at least about 90% oxygen, at least about 95% oxygen, at least about 98% oxygen, or at least about 99% oxygen. “Oxygen enriched air” is sometimes shortened to “oxygen”. 
     Medical Oxygen: Medical oxygen is defined as oxygen enriched air with an oxygen purity of 80% or greater. 
     Ambient: In certain forms of the present technology, the term ambient will be taken to mean (i) external of the treatment system or patient, and (ii) immediately surrounding the treatment system or patient. 
     Flow rate: The volume (or mass) of air delivered per unit time. Flow rate may refer to an instantaneous quantity. In some cases, a reference to flow rate will be a reference to a scalar quantity, namely a quantity having magnitude only. In other cases, a reference to flow rate will be a reference to a vector quantity, namely a quantity having both magnitude and direction. Flow rate may be given the symbol Q. ‘Flow rate’ is sometimes shortened to simply ‘flow’ or ‘airflow’. 
     Flow therapy: Respiratory therapy comprising the delivery of a flow of air to an entrance to the airways at a controlled flow rate referred to as the treatment flow rate that is typically positive throughout the patient&#39;s breathing cycle. 
     Patient: A person, whether or not they are suffering from a respiratory disorder. 
     Pressure: Force per unit area. Pressure may be expressed in a range of units, including cmH 2 O, g-f/cm2 and hectopascal. 1 cmH 2 O is equal to 1 g-f/cm2 and is approximately 0.98 hectopascal (1 hectopascal=100 Pa=100 N/m 2 =1 millibar˜0.001 atm). In this specification, unless otherwise stated, pressure is given in units of cmH 2 O. 
     General Remarks 
     The term “coupled” as used herein means either a direct connection or an indirect connection (e.g., one or more intervening connections) between one or more objects or components. The phrase “connected” means a direct connection between objects or components such that the objects or components are connected directly to each other. As used herein the phrase “obtaining” a device means that the device is either purchased or constructed. 
     In the present disclosure, certain U.S. patents, U.S. patent applications, and other materials (e.g., articles) have been incorporated by reference. The text of such U.S. patents, U.S. patent applications, and other materials is, however, only incorporated by reference to the extent that no conflict exists between such text and the other statements and drawings set forth herein. In the event of such conflict, then any such conflicting text in such incorporated by reference U.S. patents, U.S. patent applications, and other materials is specifically not incorporated by reference in this patent. 
     Further modifications and alternative implementations of various aspects of the present technology 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 technology. It is to be understood that the forms of the technology shown and described herein are to be taken as implementations. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the technology may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the technology. Changes may be made in the elements described herein without departing from the spirit and scope of the technology as described in the appended claims. 
     LABEL LIST 
       
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 oxygen concentrator 
                 100 
               
               
                   
                 inlet 
                 101 
               
               
                   
                 air inlet 
                 105 
               
               
                   
                 accumulator 
                 106 
               
               
                   
                 pressure sensor 
                 107 
               
               
                   
                 inlet muffler 
                 108 
               
               
                   
                 inlet valve 
                 122 
               
               
                   
                 inlet valve 
                 124 
               
               
                   
                 filter 
                 129 
               
               
                   
                 outlet 
                 130 
               
               
                   
                 outlet valve 
                 132 
               
               
                   
                 muffler 
                 133 
               
               
                   
                 outlet valve 
                 134 
               
               
                   
                 spring baffle 
                 139 
               
               
                   
                 check valve 
                 142 
               
               
                   
                 check valve 
                 144 
               
               
                   
                 flow restrictor 
                 151 
               
               
                   
                 valve 
                 152 
               
               
                   
                 flow restrictor 
                 153 
               
               
                   
                 valve 
                 154 
               
               
                   
                 flow restrictor 
                 155 
               
               
                   
                 supply valve 
                 160 
               
               
                   
                 expansion chamber 
                 162 
               
               
                   
                 oxygen sensor 
                 165 
               
               
                   
                 ultrasonic emitter 
                 166 
               
               
                   
                 ultrasonic receiver 
                 168 
               
               
                   
                 outer housing 
                 170 
               
               
                   
                 fan 
                 172 
               
               
                   
                 outlet 
                 173 
               
               
                   
                 outlet port 
                 174 
               
               
                   
                 flow restrictor 
                 175 
               
               
                   
                 power supply 
                 180 
               
               
                   
                 flow rate sensor 
                 185 
               
               
                   
                 particulate filter 
                 187 
               
               
                   
                 connector 
                 190 
               
               
                   
                 delivery conduit 
                 192 
               
               
                   
                 pressure sensor 
                 194 
               
               
                   
                 airway delivery device 
                 196 
               
               
                   
                 mouthpiece 
                 198 
               
               
                   
                 compression system 
                 200 
               
               
                   
                 speed sensor 
                 201 
               
               
                   
                 compressor 
                 210 
               
               
                   
                 compressor outlet 
                 212 
               
               
                   
                 motor 
                 220 
               
               
                   
                 external rotating armature 
                 230 
               
               
                   
                 air transfer device 
                 240 
               
               
                   
                 compressor outlet conduit 
                 250 
               
               
                   
                 graph 
                 260 
               
               
                   
                 period 
                 270 
               
               
                   
                 period 
                 280 
               
               
                   
                 canister system 
                 300 
               
               
                   
                 canister 
                 302 
               
               
                   
                 canister 
                 304 
               
               
                   
                 air inlet 
                 306 
               
               
                   
                 housing component 
                 310 
               
               
                   
                 base 
                 315 
               
               
                   
                 valve seat 
                 322 
               
               
                   
                 opening 
                 323 
               
               
                   
                 valve seat 
                 324 
               
               
                   
                 outlet 
                 325 
               
               
                   
                 exhaust gases 
                 327 
               
               
                   
                 inlet conduit 
                 330 
               
               
                   
                 valve seat 
                 332 
               
               
                   
                 valve seat 
                 334 
               
               
                   
                 apertures 
                 337 
               
               
                   
                 conduit 
                 342 
               
               
                   
                 conduit 
                 344 
               
               
                   
                 conduit 
                 346 
               
               
                   
                 outlet system 
                 350 
               
               
                   
                 opening 
                 375 
               
               
                   
                 controller 
                 400 
               
               
                   
                 processor 
                 410 
               
               
                   
                 internal memory 
                 420 
               
               
                   
                 transceiver 
                 430 
               
               
                   
                 graph 
                 435 
               
               
                   
                 period 
                 440 
               
               
                   
                 period 
                 445 
               
               
                   
                 POC therapy system 
                 450 
               
               
                   
                 server 
                 460 
               
               
                   
                 analysis engine 
                 462 
               
               
                   
                 personal computing device 
                 464 
               
               
                   
                 database 
                 466 
               
               
                   
                 network 
                 470 
               
               
                   
                 smartphone 
                 480 
               
               
                   
                 program 
                 482 
               
               
                   
                 outlet system 
                 500 
               
               
                   
                 housing component 
                 510 
               
               
                   
                 conduit 
                 530 
               
               
                   
                 conduit 
                 532 
               
               
                   
                 conduit 
                 534 
               
               
                   
                 opening 
                 542 
               
               
                   
                 opening 
                 544 
               
               
                   
                 valve seat 
                 552 
               
               
                   
                 valve seat 
                 554 
               
               
                   
                 control panel 
                 600 
               
               
                   
                 input port 
                 605 
               
               
                   
                 power button 
                 610 
               
               
                   
                 flow rate setting button 
                 620 
               
               
                   
                 flow rate setting button 
                 622 
               
               
                   
                 flow rate setting button 
                 624 
               
               
                   
                 flow rate setting button 
                 626 
               
               
                   
                 active mode button 
                 630 
               
               
                   
                 mode button 
                 635 
               
               
                   
                 altitude button 
                 640 
               
               
                   
                 battery check button 
                 650 
               
               
                   
                 relative battery power remaining LED 
                 655 
               
               
                   
                 graph 
                 660 
               
               
                   
                 point 
                 665 
               
               
                   
                 point 
                 670 
               
               
                   
                 point 
                 675 
               
               
                   
                 point 
                 680 
               
               
                   
                 line 
                 685 
               
               
                   
                 point 
                 690 
               
               
                   
                 flow source 
                 700 
               
               
                   
                 secondary valve 
                 710 
               
               
                   
                 flow restrictor 
                 720 
               
               
                   
                 flow restrictor 
                 730 
               
               
                   
                 secondary valve 
                 810 
               
               
                   
                 flow restrictor 
                 820 
               
               
                   
                 user 
                 1000