Abstract:
Apparatus for treatment of obstructive sleep apnea includes a CPAP device and a detachable humidifier. The humidifier includes a tank that is removably insertable into a humidifier base. A sealed air flow path from the CPAP device and through the water tank is established when the water tank is placed in the humidifier base.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. application Ser. No. 10/533,940, filed Dec. 29, 2006, pending, which is a national phase application of PCT/AU2004/000810, filed Jun. 21, 2004 in English, which claims the benefit of Australian Application No. 2003903139, filed Jun. 20, 2003, Australian Application No. 2003905136, filed Sep. 22, 2003, and Australian Application No. 2004901008, filed Feb. 24, 2004, each incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to breathable gas supply apparatus, and particularly but not exclusively to such apparatus for use in Continuous Positive Airway Pressure (CPAP) treatment of conditions such as Obstructive Sleep Apnea (OSA) and other respiratory disorders and diseases such as emphysema. It will be described herein in its application to CPAP treatment apparatus, but it is to be understood that the features of the invention will have application to other fields of application, such as mechanical ventilation and assisted respiration. 
     2. Description of Related Art 
     CPAP treatment of OSA, a form of Noninvasive Positive Pressure Ventilation (NIPPY), involves the delivery of a pressurised breathable gas, usually air, to a patient&#39;s airways using a conduit and mask. Gas pressures employed for CPAP typically range from 4 cm H 2 O to 28 cm H 2 O, at flow rates of up to 180 L/min (measured at the mask), depending on patient requirements. The pressurised gas acts as a pneumatic splint for the patient&#39;s airway, preventing airway collapse, especially during the inspiratory phase of respiration. 
     CPAP machines comprising an air flow generator for supplying pressurised air to the patient are known, and over recent years there has been commercial imperative for more compact CPAP machines. However, in seeking to reduce the size of the CPAP machines there has been a trade-off between reduced size on the one hand and reduced performance and/or increased noise on the other, for example Malinckrodt/Tyco/Puritan Bennett ‘Goodnight’ Series. 
     The advantages of incorporating humidification of the air supply to a patient are known, and CPAP machines are known which incorporate humidifying devices, either separately from the flow generator or integrated therewith. An example of an integrated flow generator/humidifier unit is the ResMed® S7 sold by the present Applicant. 
     Another problem with some flow generators is extensive use of foam in the air path for sound absorption. The foam can degrade with time. 
     SUMMARY OF THE INVENTION 
     One of the objects of the invention is to provide a simple and compact breathable gas supply apparatus incorporating a humidifier which is simple and economic in its construction, compact, and easy to use. Other objects and advantages of the invention will be described throughout the specification. 
     It is to be understood that apparatus described herein contains a number of advances on the prior art, many of which are independent inventions, although they contribute together to the realisation of the general object expressed above. 
     The apparatus described herein incorporates novel aspects of architecture of both the flow generator and the humidifier, and of their integration, which contribute to a reduction in size compared with known units having similar performance. Techniques for noise reduction and damping are described which enable such a smaller machine to have noise performance which is at least as good as known larger machines. 
     The apparatus described herein achieves full integration of the humidifier with the flow generator, in the sense that air flow, electrical and, if required, data connection between the flow generator and the humidifier are provided automatically upon the physical engagement of the two devices, without the need for any other process of interconnection. 
     In such an integrated device, provisions to guard against flowback of water from the humidifier tank to the flow generator are important, and novel sealing arrangements, and novel arrangements for minimising the occurrence of flowback while at the same time improving the uptake of water vapour in the humidifier are also described. The humidifier is readily detached and replaced on the machine, and has very few parts to be disassembled during cleaning. 
     Also described herein are improved, modular, devices for enabling data connection with the apparatus, including the connection of data storage devices such as memory cards, smart cards, communication ports and the like to be selectively attached by the user or by medical personnel. 
     Another aspect of the invention is to reduce or eliminate the use of foam in the air path. 
     In one form, the invention provides a flow generator unit for delivering breathable gas to a patient, including:
         a flow generator case;   a powered gas flow generator within the case;   a power supply unit adapted for drop-in assembly in said case, said power supply unit including a printed circuit board, a power input connector rigidly attached to said printed circuit board and a power output connector, and   a power supply unit mounting for mounting said power supply unit in said case such that said power input connector aligns with a power input port of said case.       

     A further form of the invention provides a blower enclosure for a flow generator used in delivery of breathable gas to a patient, said blower enclosure including a metal container overmoulded with an acoustically damping polymer lining. 
     A further form of the invention provides a blower enclosure for a flow generator used in delivery of breathable gas to a patient, said blower being adapted to reduce noise from the enclosed blower, said enclosure comprising:
         a cavity within a chassis of the flow generator, the cavity defined by side walls and base, the enclosure being adapted to receive and mount a blower in said cavity and   a lid adapted to be mounted on said chassis so as to form a top surface of the cavity,
 
wherein at least one of the chassis and lid is moulded from a composite comprising a metal and a plastic.
       

     A further form of the invention provides a blower for a flow generator used in delivery of breathable gas to a patient, said blower comprising an electric motor with a shaft, an impeller adapted to be mounted on the shaft, and a volute having an air-inlet and an air-outlet, the volute defining a chamber in which a flow of air at pressure is developed, the volute being moulded from a composite comprising a first plastic material and a second plastic material, the first plastic material being generally rigid and the second material being generally elastomeric. 
     Preferably, wherein the first plastic material is overmoulded with the second plastic material. 
     Preferably, the volute comprises an upper volute and a lower volute, the lower volute incorporating the air-inlet. Preferably also, the lower volute includes feet moulded from the second plastics material. 
     Preferably, the upper volute incorporates the air-outlet. Preferably also, the upper volute includes a seal constructed from the second plastic material and which in use is adapted to provide a seal between the upper and lower volutes. 
     In one embodiment the upper and lower volutes are adapted to be snap-fitted together. 
     A further form of the invention provides a flow generator case for a flow generator used in delivery of breathable gas to a patient, said flow generator case comprising a shell of rigid plastics overmoulded with an elastomeric lining. 
     Preferably, said elastomeric lining forms external feet of said flow generator case. 
     A further form of the invention provides a fan support arrangement for a flow generator used in delivery of breathable gas to a patient, including a fan housing containing a motor and fan, said support arrangement including a plurality of mounting springs, wherein said springs, fan housing, motor and fan form a spring system having a natural resonant frequency less than one tenth of the frequency of a lowest operating speed of said fan. 
     A further form of the invention provides a flow generator unit for delivering breathable gas to a patient, including a flow generator ease having an air outlet, a fan volute contained within said case, further including a flexible tube connecting an outlet of said fan volute to said air outlet, said flexible tube having two or more corrugations therein. 
     A further form of the invention provides a flow generator and humidifier combination for continuous positive airway pressure treatment of a patient, including a flow generator and a humidifier removably attached to the flow generator, wherein said flow generator includes a humidifier attachment detector including an optical transmitter and an optical sensor and wherein said humidifier includes an optical path connector which, when said flow generator and humidifier are attached, completes an optical path between said optical transmitter and said optical sensor. 
     A further form of the invention provides a muffler arrangement in an air flow path of a flow generator used in delivery of breathable gas to a patient, including a first muffler volume, a second muffler volume and a connecting portion linking said first and second muffler portions, wherein said connecting portion is narrow relative to said muffler portions and includes a lead-in portion which narrows in a direction away from said first muffler portion. 
     Preferably said connecting portion includes a venturi. 
     A further form of the invention provides a handle assembly for a flow generator used in delivery of breathable gas to a patient, including a flow generator case, a handle including a pair of attachment arms, each attachment arm having a projection received in a respective track of said case, and a handle retention member which attaches to said case to retain said handle projections against travel along said track. 
     A further form of the invention provides a method of attachment of a handle to a flow generator case, said handle including a pair of attachment arms, each attachment arm having a projection received in a respective track of said case, including the steps of sliding said handle projections along respective of said tracks and attaching a handle retention member to said case to retain said projections against travel along said respective tracks. 
     Preferably, said sliding of said handle projections along said track occurs without substantial distortion of said attachment arms. 
     A further form of the invention provides a humidifier for delivering humidified breathable gas to a patient, including a humidifier case,
         a water container,   a heater located in heat transfer communication with said water container,   a gas flow path including a gas inlet, a humidified gas outlet and an intermediate gas flow path which contacts the gas with water vapour from said container,
 
further including a drainage opening adjacent said heater allowing drainage of water past the heater to exit said humidifier case.
       

     A further form of the invention provides a humidifier for delivering humidified breathable gas to a patient, including
         a humidifier case,   a water container,   a heater pad located in heat transfer communication with said water container,   a gas flow path including a gas inlet, a humidified gas outlet and an intermediate gas flow path which contacts the gas with water vapour from said container,
 
wherein said heater pad has an upper heating surface and a peripheral heating surface which includes a side wall of said heater pad, and wherein a heat transfer surface of said water container is shaped to correspond to said heater pad so as maintain close heat transfer communication with said upper heating surface and peripheral heating surface of said heater pad.
       

     Preferably, said water container defines a water volume which extends both above and below a level of said heating pad upper heating surface, 
     A further form of the invention provides a humidifier for delivering humidified breathable gas to a patient, including
         a humidifier case having a hinged lid,   a water container adapted for drop-in assembly in said case,   a heater in heat transfer communication with said water container,   a gas flow path including a gas inlet, a humidified gas outlet and an intermediate gas flow path which contacts the gas with water vapour from said container,
 
wherein said water container has a gas passage inlet communicating with said gas flow path,
 
said humidifier further including a gas passage inlet seal for sealing connection said gas passage inlet to said gas flow path, wherein said sealing connection is actuated by drop-in assembly of said water container and hinged closing of said lid.
       

     Preferably, said gas passage inlet is located on a rear face of said water container and aligns with a gas passage aperture on an opposed face of said case. 
     A further form of the invention provides, in a humidifier assembly for a flow generator used in delivery of a supply of breathable gas to a patient for treatment of sleep disordered breathing, the humidifier assembly comprising a water tub having an inlet, a base having a blower outlet and a water-tub-receiving-portion, and a hinged lid with an engagable locking mechanism, a method of forming a seal between the water tub inlet and the blower outlet of the base comprising the steps of:
         placing the water tub in the tub-receiving-portion of the base so as to position the inlet and the outlet adjacent one another;   closing the hinged lid; and   engaging the locking mechanism.       

     Preferably, the blower outlet includes front-facing seal forming surface, and the step of placing the water tub in the water-tub-receiving portion of the base further includes the step of placing the water tub against the seal forming surface of the blower outlet. 
     A further form of the invention provides, in a humidifier assembly for a flow generator used in delivery of a supply of breathable gas to a patient for treatment of sleep disordered breathing, the humidifier assembly comprising a water tub having an air outlet and an hinged lid with an engagable locking mechanism and an air delivery portion adapted to mate with an air delivery conduit so that the supply of breathable gas can be provided to a patient interface, a method of forming a seal between the water tub air outlet and the air delivery portion comprising the steps of:
         closing the hinged lid; and   engaging the locking mechanism.       

     Preferably, the hinged lid has an underside, and the underside includes a seal forming surface comprising a removably attachable gasket. 
     A further form of the invention provides a humidifier assembly for a flow generator used in delivery of a supply of breathable gas to a patient for treatment of sleep disordered breathing, the humidifier assembly comprising a water tub having an air inlet and an air outlet, a humidifier base having a blower outlet and a water-tub-receiving portion, and a lid having an air delivery portion adapted to mate with an air delivery conduit so that the supply of breathable gas can be provided to a patient interface, wherein said water-tub-receiving portion and water tub have complementary formations adapted to guide drop-in positioning of said water tub to align said air inlet with said blower outlet. 
     Preferably, said complementary formations further guide positioning of said water tub to align said air outlet with a position of said air delivery portion of said lid when said lid is closed. 
     A further form of the invention provides a humidifier for delivering humidified breathable gas to a patient, including
         a humidifier case having a lid,   a water container within said case,   a heater in heat transfer communication with said water container,   a gas flow path including a gas inlet, a humidified gas outlet in said lid and an intermediate gas flow path which contacts the gas with water vapour from said container, and   a gas outlet seal operatively associated with said lid whereby closing said lid creates a sealed communication between said humidified gas outlet the seal and a gas space of said water container.       

     Preferably, the humidifier further includes a gas passage seal attached to the underside of said lid cooperating with a surface of said water container to form a sealed gas passage between a gas passage inlet and a gas inlet to said gas space. 
     Preferably also, said gas outlet seal and said gas passage seal are integrally formed. 
     A further form of the invention provides a humidifier for delivering humidified breathable gas to a patient, including
         a water container,   a heater in heat transfer communication with said water container,   a gas flow path including a gas inlet, a humidified gas outlet in said lid and an intermediate gas flow path which contacts the gas with water vapour from said container,   wherein said intermediate gas flow path includes a gas passage between a gas passage inlet and a gas inlet to said gas space, said gas passage having a floor sloping downwards from said gas passage inlet to said gas inlet.       

     Preferably, said gas passage includes a drainage portion below a level of the gas passage inlet, being a forwardmost portion of said gas passage having a front wall below the level of the gas passage inlet. 
     A further form of the invention provides a humidifier for delivering humidified breathable gas to a patient, including
         a water container,   a gas flow path including a gas inlet, a humidified gas outlet in said lid and an intermediate gas flow path which contacts the gas with water vapour from said container,
 
wherein said gas flow path is adapted to introduce said gas into a headspace of said water container with a swirling motion.
       

     Preferably, said intermediate gas flow path includes a container air inlet adapted to introduce gas generally tangentially into said container headspace. 
     Preferably also, said intermediate gas flow path includes an arcuate gas flow path leading to said container air inlet, and further includes a container air outlet positioned generally centrally of said headspace. 
     A further form of the invention provides a control circuit for a humidifier for delivering humidified breathable gas to a patient, including a user operable control for selecting a desired gas humidity setting and a heater control circuit for determining a target heater temperature corresponding to the humidity setting and controlling a heater to attain said temperature, wherein said user operable control includes an off setting for which said heater control selects a target heater temperature less than a lowest operating temperature of said humidifier. 
     A further form of the invention provides a control circuit for a humidifier for delivering humidified breathable gas to a patient, including a user operable control for selecting a desired gas humidity setting and a heater control circuit controlling a heater current to a value corresponding to the humidity setting, said user operable control including setting a reference voltage in response to said user operable control and amplifying said voltage to control said heater current. 
     A further form of the invention provides a flow generator for delivering breathable gas to a patient, including a processor, a timer, user input means and a display, said processor being programmed to receive a reminder request input and to generate a reminder display at a time specified in said reminder request input. 
     Preferably, said processor is adapted to cancel a reminder request upon receiving a cancellation input from said user input means. 
     Also described herein are improved, modular, devices for enabling data connection with the apparatus, including the connection of data storage devices such as memory cards, smart cards, communication ports and the like to be selectively attached by the user or by medical personnel. 
     A further form of the invention provides a modular data or electrical connector arrangement for a flow generator unit for delivering breathable gas to a patient, including:
         a flow generator case including an aperture;   a gas flow generator;   a control circuit for said flow generator, said circuit including a connector positioned to be accessible through said aperture for data or electrical communication with an external device; and   a plurality of closure modules each adapted to attach to said ease to cover said aperture, at least one of said closure modules including an internal connector adapted to connect with said control circuit connector, an external data or electrical port adapted for connection to said external device and a data or electrical pathway between said internal and external connectors.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various aspects of the invention will now be described with reference to the accompanying illustrations, which show a presently proposed embodiment. 
       In the drawings: 
         FIG. 1  is a general view of breathable gas apparatus embodying the various features of the invention; 
         FIG. 2  is a general view of the flow generator of the apparatus; 
         FIG. 3  is a general view of the humidifier unit; 
         FIG. 4  is a cutaway view of the flow generator; 
         FIG. 5  is an exploded view of components of the flow generator; 
         FIG. 6  is a vertical transverse cross-section of the flow generator; 
         FIG. 7  is a more detailed illustration of the bottom case and power supply of  FIG. 5 ; 
         FIG. 8  is a more detailed illustration of the chassis, chassis lid and fan housing of  FIG. 5 ; 
         FIG. 9  is a more detailed illustration of the PCB, top case and exterior fittings of  FIG. 5 ; 
         FIG. 9A  is a schematic vertical cross-section detail of the connection of the handle to the flow generator top case; 
         FIG. 10  is an underneath view of a chassis forming part of the flow generator; 
         FIG. 11  is a vertical cross-section of the chassis through a venturi passage connecting muffler cavities of the flow generator; 
         FIG. 12  is a general view of a fan forming part of the flow generator; 
         FIG. 13  is a vertical cross-section showing the fan mounting arrangement; 
         FIG. 14  is an exploded view of a humidifier adapted for use with the flow generator of  FIG. 5 ; 
         FIG. 15  is a rear view of the humidifier assembly; 
         FIG. 16  is a perspective of a seal for the air flow path; 
         FIG. 17  is an underside perspective of the humidifier lid of  FIG. 14 ; 
         FIGS. 18 and 19  are respectively a perspective and a detail cross section of the humidifier lid seal of  FIG. 14 ; 
         FIGS. 20 and 21  are respectively a perspective and a longitudinal cross section of the humidifier tub lid of  FIG. 14 ; 
         FIG. 22  is a graph of heater target temperature against humidifier setting; 
         FIG. 23  is a schematic circuit diagram of a power control circuit to the humidifier heater; 
         FIG. 24  illustrates reminder menus of the flaw generator control; and 
         FIGS. 25 to 34  show various modular data connector arrangements; 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The illustrated apparatus comprises a flow generator  50  and a humidifier  150 , shown in their assembled condition in  FIG. 1 , and separately in  FIGS. 2 and 3  respectively. As shown in  FIG. 2 , the flow generator engages with the separable humidifier at an engagement face  52 , from which protrudes an air connector  53  for the delivery of air from the fan to the humidifier container, electrical connectors  54  for the delivery of power to the humidifier heater and an optical coupling transmitter  200  and sensor  201  described further below. 
     The face  52  also carries a pair of slots  55  which are engaged by corresponding tongues  156  provided on the humidifier engagement face  157  ( FIG. 15 ) by which the flow generator  50  and humidifier  150  are connected together, as will be described in more detail below. 
     Flow Generator 
     Externally, the flow generator  50  is also provided with an LCD screen  58  and associated keys  59  by which the user can set the operating parameters of the unit. 
     Flow Generator Case 
     The flow generator  50  has an external case of rigid plastics material moulded in two parts, a top case  60  and a bottom case  61 . The lower edge of the top case  60  is stepped and flanged at  62  ( FIG. 9 ) to mate with the periphery of the bottom case  61 . 
     With reference to  FIG. 7 , the bottom case  61  of flow generator  50  has a shell  120  of rigid plastics material, such polycarbonate/ABS blend, forming the structure of the case, integrally overmoulded with a lining  121  of an elastomer such as a synthetic rubber or thermoplastic elastomer which forms the seal  63  between the top and bottom cases and the chassis  64  and also forms the external feet of the case (see  FIG. 6 ). The lining  121  also covers the internal surface of the chassis-receiving cavity of the bottom case and the dividing wall  123  between the power supply cavity  65  and chassis-receiving cavity, the resulting composite of the rigid shell with elastomeric lining serving to reduce radiated noise levels from the flow generator by damping acoustic resonance of the walls. 
     Formed in the bottom case  61  by walls which join the outer wall of the case are the lower parts and of, respectively, a power supply cavity  65  and a first muffler cavity  134 . The upper parts of these cavities are formed by the chassis  64 , described below. 
     The first muffler cavity forms part of the air flow path from the air inlet  85  to the blower, receiving air from an air inlet path defined by the chassis  64 , as described below. 
     The chassis  64  forms the blower or fan cavity  70 , inlet and outlet air flow paths and the top of the power supply cavity  65 . The fan cavity  70  includes a metal liner tub  73  insert moulded into the chassis as described below. 
     Flow Generator Chassis 
     The chassis  64  is formed with a peripheral wall  69  tinged around its lower edge to engage with the inner periphery of the overmoulded sealing flange  63 . The chassis  64  includes a downwardly extending fan cavity  70  in which is mounted the fan  90  described below. This cavity  70  is formed by moulded side walls  71  and base  72 , which are formed by moulding inner and outer layers of thermoplastic around an inserted steel liner tub  73 . The tub may be stainless steel, nickel plated mild steel or other suitable corrosion resistant metal. The fan cavity  70  opens to the upper surface of the chassis  64  to enable insertion of the fan  90 , this opening being closed by a lid  74 . 
     The density and stiffness of the steel tub creates a highly effective barrier to transmission of the motor and fan noise, while formation of the cavity  70  by insert moulding from differing materials provides very effective acoustic damping, as does the combination by co-moulding of the hard and soft plastics described already and further described below. In this aspect of the present invention, the use of co-moulding or overmoulding in the combination of materials of different, preferably widely different, stiffness and different, preferably widely different, density has been found to be particularly advantageous in providing acoustic damping. 
     Preferred materials for the chassis and liner tub are polypropylene thermoplastic for the chassis and metal, preferably steel (optionally stainless steel), for the liner tub. The applicant has found that by forming the fan cavity as a composite of metal and polymer—having a differential in density of greater than 5 times, preferably about 7-8 times, and also significantly different stiffness and damping properties—the resonance peaks of the composite structure are well damped so that noise generated by the fan is well-suppressed by the fan cavity construction. 
     It is especially preferred that the polymer for the chassis  64  be a glass fibre-filled polymer containing from 10-40%, and more preferably about 30%, glass fibre. The Applicant has found that the use of this material as a composite with a steel liner tub  73  gives both effective damping of fan noise and a good match in thermal expansion characteristics so that the composite material chassis performs well over a wide range of operating temperatures. Further, the Applicant has found that the use of glass fibres outperformed talc, bronze, glass bead filler materials for this purpose. 
     The top of the fan cavity is formed by the chassis lid  74 , which is formed of an embedded steel insert overmoulded with elastomer to provide acoustic damping and sealing of the top of the fan cavity  70  A preferred polymer lining for the lid is an elastomer, for example of the same type used for the lining  121  of the bottom case. 
     Again, the use of a steel and polymer composite creates an effective and well-damped barrier to transmission of fan and motor noise. 
     Drop-In Power Supply 
     The upper part of the power supply cavity  65  is formed by a side wall  75  extending downwardly from the roof of the chassis  64 , which sealingly engages the opposed wall of the lower portion of this cavity. Preferably, the lower wall is provided for this purpose with a co-moulded or overmoulded rubber sealing flange  76 . The power supply compartment is thus sealed against the ingress of moisture from the interior of the unit in the case of backflow from the humidifier. Similarly, the air path is sealed from the power supply compartment. The interior is at the same time acoustically sealed from the power supply cavity, which may not be completely sealed from the exterior, due to the necessity of providing mains power input and low voltage power output to the humidifier, via connectors  77  and  79  mounted in apertures  78  and  80  respectively in the rear and front walls of the cavity, and if necessary the venting of the compartment to outside air for cooling. This reduces assembly time and allows the overall device to be smaller. 
     With reference to  FIG. 7 , a power supply unit  124  is received in the power supply cavity  65 , for providing electrical power for operation of the fan, control functions and the humidifier heater pad. The power supply comprises a printed circuit board  133 , to which are directly attached by soldering or other suitable means a power inlet connector  77 , a fan power outlet connector  126  for the fan motor and a humidifier power outlet  79 . Each end of the power supply cavity  65  has mounting guides  136  for supporting the PCB of the power supply in an upright position so that installation of the power supply is achieved by drop-in assembly. By rigid attachment of the connectors by soldering direct to the PCB, the need for connection of wiring looms to the PCB is eliminated and the connectors align with respective ports in the bottom case  61  when the power supply is inserted. 
     PCB 
     With reference to  FIG. 8 , the fan  90  and fan housing  93 ,  94  fit into the fan cavity  70  of the chassis and connect to electrical connector  26  at the top of the power supply PCB. Elastomer overmoulding of the base  94  of the fan housing seals the housing, provides acoustic damping of the fan housing base and forms feet on the bottom of base to act as bump stops protecting the fan in case the unit is bumped or dropped. 
     As shown in  FIG. 9 , supported on the top of the chassis  64 , in the space formed between the chassis and the top of the top case  60 , is a printed circuit board  81  carrying the electronic control components of the unit. The printed circuit board  81  preferably includes an LCD display  58 . Optionally, at the rear of the board an edge connector  1082  and a sliding connector  1082 A may be accessible from a connector aperture in the rear of the case  60 , providing for modular connector arrangements to be described in more detail below with reference to  FIGS. 25 to 34 . 
     Air Inlet Path and Mufflers 
     Also provided in the rear wall of the top case is an air inlet  84 , and this communicates with an air inlet passage  85  formed in the chassis above the roof of the upper portion of the power supply cavity  65 , this passage in turn opening to first muffler cavity  134  surrounding the bottom of the fan cavity of the chassis. 
     The top case further defines an air inlet to the flow generator, and has a replaceable filter  129  of any suitable material, such as foam or fibre, and filter cover  130  fitted to the top case  60 . An inlet wedge  131  serves as an airflow guide. A blank cover  132  clips in place over apertures in the case which align with connectors  1082 ,  1082 A to provide ports on the PCB for communications, etc. Further details of the communications and/other electrical ports in the flow generator case will be described later with reference to  FIGS. 25 to 34 . 
     From the first muffler volume  134  under the fan cavity  70 , inlet air passes through a connection passage  137  ( FIG. 11 ) into a second muffler volume formed by the space between the fan cavity  70  and the fan. 
     The fan cavity and the space between the bottom case and the chassis thus form a pair of serially connected volume mufflers, with a restricted diameter passage therebetween. Noise attenuation produced by a muffler system is generally proportional to the ratio of a representative diameter of the muffler volume to that of the constriction, and thus an optimal muffler design must balance optimal noise attenuation against the constraints of available muffler volume—especially in a compact machine—and avoiding unacceptable air flow restriction through the constriction. 
     The Applicant has found that a favourable adjustment of this balance may be achieved by forming the intermediate connecting passage  137  between the muffler volumes as a venturi, as shown in  FIGS. 10 and 11 , with a relatively short, smoothly varying diameter lead in portion  137 a at the end adjacent the first muffler, an intermediate constriction  137 b and a gradually expanding lead out portion  137 c at the downstream end. In this way, the muffler system can achieve the noise attenuation according to the representative diameter of the smallest diameter portion, with better pressure drop characteristics. 
     Fan 
     It will now be convenient to describe the features of the fan, which are shown in  FIGS. 12 and 13 . 
     The fan  90  comprises a motor  91 , preferably brushless DC motor, provided with a coaxial impeller  92 , mounted vertically within a fan housing consisting of a cover  93  and a base  94 . An air inlet  95  is provided in the floor of the base  94  on the impeller axis, and cavities in the cover and base form a volute  96  which leads from the impeller to an air outlet  97 . The cover and base  93  and  94  are joined by means of slotted tabs  98  which extend upwardly from the base to snap over stepped ribs  99 , the tabs  98  being further located by fitting between parallel ribs on the cover  93 . The joint between the cover  93  and the base  94  is sealed by an elastomeric over- or co-moulded sealing ring  101 . 
     The bottom surface of the fan housing base  94  is provided with radial stiffening ribs, and overmoulded to the base  94  is an elastomer damping member  103  which covers that bottom surface between the ribs, and extends around the edge of the base by a flange portion and peripherally spaced tabs. By overmoulding to the rigid plastics base  94  an elastomer of much lower stiffness substantial acoustical damping is provided to the fan housing. 
     Moulded integrally with the rigid plastics portion of the fan housing base are feet  106  which extend proud of overmoulded elastomer member  103  to receive helical mounting springs  102  ( FIG. 13 ), preferably of metal, by which the fan is mounted on the base  72  of the fan cavity. 
     The degree of size reduction which is an objective of the present invention requires great care to be taken to minimise the transmission of noise and vibration, particularly from the motor and the impeller of the fan  90 . The mounting springs are therefore chosen to ensure minimal transmission of the vibration frequencies encountered during operation. This is achieved by choosing the springs with reference to the mass of the fan  90 , such that the natural frequency of the system comprising the springs and the fan is less than approximately one tenth of the shaft speed of the motor when running at its lowest operating speed. 
     The air outlet  97 , upon the introduction of the fan into the fan cavity, is connected by means of a thermoplastic elastomer or silicone rubber coupling member  108  with an air passage which extends from the side wall of the fan cavity to a connecting nozzle  110  extending through an aperture provided for this purpose in the front face of the flow generator. It is preferred that the coupling member  108  includes at least two corrugations which provide flexibility to the connection and improved resistance against transfer of vibration from the fan to the flow generator case. 
     The fan  90  therefore floats within its cavity  70  in the chassis  64  with minimum acoustic coupling to the remainder of the flow generator. The characteristics of the mounting springs and the coupling member  108  are chosen to minimise the transmission of characteristic vibration frequencies of the fan. 
     Further details of the fan construction and fan mounting are described in US20030168064 and WO99/64747, the contents of which are incorporated herein by reference. 
     The illustrated flow generator construction and materials combinations are adapted to result in a compact CPAP flow generator unit of similar performance and noise characteristics to larger units—eg. capable of generating from 4-20 cmH 2 O pressure and a flow rate of 120 L/min and a total radiated noise volume of less than 33 dbA, more preferably less than about 30 dbA, when operating at 10 cmH 2 O—in a flow generator unit having a total volume of about 2 litres or less. 
     Handle Attachment 
     A keypad  59 , facia  127  and transport handle  128  attach to the top case  60 . 
     With reference to  FIGS. 9 and 9A , a novel and easily assembled handle attachment assembly is described and shown. The handle  128  has opposed arms with inwardly projecting pins  140  at their distal ends. The top case  60  includes a pair of channel-shaped tracks  141  with one open and one closed end, for receiving respective of the pins. To assemble the handle to the top case, the pins are inserted from the open ends of their respective channels and slid toward the closed ends. The facia  127  clips onto the top ease  60 , and includes projections  142  which trap the pins  140  in the end of their tracks  141 . 
     The handle attachment configuration thus provides a quick and simple means of assembly without requiring flexing of the handle arms to locate the pins into small recesses as in the prior art. 
     Humidifier 
     As shown in  FIGS. 14 to 21 , the humidifier  150  comprises a base unit designed for simple attachment to and detachment from the flow generator  50 , which forms a cradle for a water container which is in turn attachable to and detachable from the base unit. 
     The general arrangement of the humidifier components includes a base (rear cover  803  and front cover  602 ) onto which is fitted a heater comprising a heater plate (plate  632  with ceramic heater pad  800 ) which supports a water tub (tub base  698 , seal  699  and tub lid  700 ) and a hinged humidifier lid  648  which seals against the tub lid  700  to form an air path into the tub through the tub lid. 
     The rear face of the base has a peripheral flange  153  which seats in a corresponding peripheral recess  113  surrounding the front face of the flow generator  50  when the two units are brought together by linear movement towards each other. A latch  404  is held in place by latch retainer  404 a to be moveable vertically and resiliently urged downwardly by spring  404 b, so that the tongues  156  engage in the slots  55  and snap home to engage the two units by means of the downwardly extending fingers  158  at the ends of the tongues. 
     Coupling of Flow Generator and Humidifier 
     The PCB of the flow generator is provided at the end adjacent the humidifier with an optical transmitter  200  which emits a periodic flash of light from the end face of the flow generator case, and an optical sensor  201  to detect the presence or absence of the humidifier. The rear face of the humidifier contains a curved reflector  202  which, when the humidifier is attached to the flow generator, completes an optical path from the transmitter to the sensor so that the flow generator PCB detects the presence of the humidifier and may adjust the control algorithms accordingly. 
     The rear face of the base unit also carries a connector  162 , in this embodiment a pair of flat male blade connectors, for engagement with a mating connector  114  on the front face of the flow generator, to provide power to the humidifier heater from the power supply in the power supply cavity  65 . Although not shown in the illustrated embodiment, the respective faces may also carry further interconnecting devices, where other electrical or data connections are required to be established between the flow generator and the humidifier or downstream devices including the air conduit or the mask. Such devices may take the form of optically coupled devices, or connectors of other suitable kinds. 
     The use of such an opto-coupling connector enables the implementation of a simple protocol for communications between the flow generator and the humidifier. For example, the current flow levels of the flow generator can be sent to the humidifier controller which then adjusts the operation of the humidifier according to a predetermined algorithm. 
     In the humidifier construction, the back cover  803  which its to the real of the front cover  602  provides the air, electrical and communications connections to the flow generator and provide support for a control PCB  804  and the catch assembly. The catch assembly includes a latch  404  which is retained by a latch retainer  404 a and spring  404 b, and operates to attach the humidifier to the flow generator generally as described for the earlier embodiments. A control knob  805  on the top of the front cover  602  is connected to the PCB  804  to allow patient control over the degree of humidification. 
     There is also provided an aperture  264  ( FIG. 15 ) for electrical connections between the humidifier and the flow generator, or for electrical and signal connections to the humidifier. 
     The air port  807  in the humidifier rear face mates with the outlet  110  of the flow generator. 
     An elastomer airway seal  722  fits between the front and back covers to connect the air port  807  in the back cover  803  to the aperture  626  of the front cover  602 . The seal (shown in more detail in  FIG. 16 ) has an inlet connector portion  722 a which connects to the flow generator output via the air port  807  formed in the back cover  803 , and a peripheral seal portion  722 b which extends about the aperture  626  periphery at the front face of the cover  602 . A wall portion  722 c of the seal closes off a lower part of the aperture  626 , leaving a smaller aperture  722 d defined by the seal. 
     As a result, the airway seal  722  defines a closed passage from the circular air port  807  to the rectangular aperture  722 d in the vertical wall of the front cover. 
     Heater Pad 
     The heater pad comprises lower and upper parts  806 ,  800  and a heater pad cover  632 . 
     The heater pad cover  632  has an upper heating surface  634 , a downwardly extending peripheral wall  636  acting as a further heating surface and a rear flange with a pair of attachment portions  640  for attachment of the heater pad to tubular protrusions  628  on the rear of the front cover  602 . 
     The heater pad cover  632  is configured to accommodate, below the upper wall  634  and within bounds of the wall  636 , a heater pad or other heating means such as an induction heater, for causing heating of the water in the humidifier water container. 
     The front of the heater pad cover  632  has a forwardly extending tab  646  of dog-legged shape, which extends to the front of the humidifier cradle front cover  632  to support the heater and also provide a catch for the humidifier lid  648 . 
     Water Tub 
     The water container consists of a water tub  698 , seal  699  and tub lid  700 . 
     The floor of the tub  698  is of complementary shape to the heater pad, and is formed of metal or other material suitable to conduct heat from the heater pad to the water in the tub. The floor has a generally horizontal portion  900  corresponding to the upper heating surface  634  of the heater pad and a U-shaped portion below the level of the heater pad upper surface, including a generally vertical heat transfer portion  902  below the horizontal portion corresponding to the peripheral heating surface. When the water container is placed in the humidifier cradle and the hinged lid  648  closed, the water tub base is held in close contact with the heater pad to transfer heat into the water in the tub. 
     By providing a part of the water tub volume and heat transfer surface about the periphery of the heater pad, a similar water volume and heating area to those in prior art humidifiers can be obtained in a more compact assembly. 
     As shown in  FIG. 20 , the rear surface of the tub lid has an air inlet aperture  801  leading to an inlet end of the U-shaped air passage  718 . When the humidifier lid  648  is closed, the tub  698  and tub lid  700  are pressed rearwards so that the peripheral seal  722 b abuts the rear surface of the tub lid in a locus surrounding the rear opening of the inlet aperture  801 , creating a sealed air path from the flow generator outlet to air passage  718  and into the headspace of the humidifier tub. This allows the humidifier tub to be removed for refilling and replaced without the need for a separate operation to connect the air flow. 
     With reference to  FIG. 21 , the inside wall of the tub lid  700  has projections  802 a,  802 b which serve to limit the press fitting of the tub lid onto the tub base  698 . One projection  802 a is provided at the front of the tub, and further projections  802 b, or sets of projections, are provided on opposed side walls of the tub lid, forward of the rear of the tub. This positioning of the projections  802 b allows one-handed disengagement of the tub base and tub lid by squeezing together of the base and lid at their rear end, causing the connection to pivot about the side projections  802 b and the tub and lid to separate at the front. The ability to separate these components one-handed is a feature of considerable utility, especially for stroke patients or other users with limited dexterity. 
     As best seen in  FIGS. 20 and 21 , the water container lid  700  has an air passage  718  formed as a U-shaped channel, leading to the humidified air entry aperture  720  into the headspace of the water container. The channel floor slopes down in the direction of air flow from the air inlet end to the end at which the air enters the water container. The water container lid also has an elliptical humidified air exit aperture  722 . These air passages and apertures co-operate with the humidifier lid  648  when closed to define the air flow paths within the humidifier, as will be described below. 
     Water may be added to the water container via the air exit aperture  722  while the tub lid is in place, or by removing the tub lid. 
     The tank is intended to be filled via the air outlet  722 , and the apparatus may be provided with a filling bottle with a spout dimensioned for a convenient fit with that outlet. Such a bottle may be provided with a spout of the kind incorporating an air bleed passage which will allow the tank to fill to the correct predetermined height. 
     In alternative embodiments, other filling arrangements may be employed, for example by removing the tub lid. The correct filling height may also indicated by filling level graduations scribed or otherwise marked on the wall of the water tub. 
     A micro switch (not shown) or other sensing means may be provided to turn off power to the heater pad when the lid is opened, and/or when the water container is removed. 
     Humidifier Lid and Air Flow Paths 
       FIGS. 17 to 19  show the underside of the humidifier lid  648  and the seal  676  which provides a seal to the tub lid  700  about the U-shaped passage  718  and the humidified air exit aperture  716 . The seal  676  comprises an edge seal portion  676 a and membrane portion  676 b, as shown in  FIGS. 18 and 19 . 
     The lid  648  has an upper wall  650  and a front wall  652  which extends downwards, and outwardly, from the upper wall. The upper wall  650  has a recess at its rear side, such that the part of the upper wall and front wall  652  on each side of the recess constitutes a rearwardly projecting arm  656 . At the rearmost extremity of each arm  656  there is an inwardly projecting hub  658 . The hubs  658  are configured to be received in the sockets  622  of the humidifier front cover  602  such that each hub and its corresponding socket constitute a hinge connection, for attaching the lid  648  to the front cover. 
     During opening of the lid  648 , it may be freely rotated about the hubs through greater than 90° until it reaches a maximum extent of normal travel. The lid and front cover are configured such that, if the lid is then rotated further, the hubs pop out of the sockets  622 . This may be achieved, as would be understood by a person skilled in the art, by providing suitable chamfers on the hubs and/or sockets, or other suitable formations on the lid or cover, so that the lid flexes to release the hubs from the sockets. 
     The lower edge of each arm  656  is shaped complementarily to the shape of the upper portion of the face of the front cover to accommodate that part of the arm when the lid  648  is in a closed position. 
     The lid  648  includes a humidified air outlet pipe  662  which passes through the upper wall  650  and extends upwards and forwards at an acute angle from the top of the upper wall, for attachment of a hose to supply humidified air to a patient. The pipe  662  continues below the lower surface of the upper wall  650  to define an elliptical rim  664 . 
     Extending downwards from the lower surface of the upper wall  650  is a wall  666  which is configured to define a closed path and hence a U-shaped enclosed region  668  within the confines of the wall. 
     At the front extremity of the front wall  652 , that is, adjacent the lower edge of that wall, there is provided a recessed notch  674  on the rear (inner) surface of that wall, for snap-fit engagement with the tab  646  of the heater pad cover to act as the catch for the lid. The lid may be opened by flexing the assembly to release the tab from the notch. 
     Attached to the lid  648  is an elastomer lid seal  676 , which is illustrated in  FIGS. 18 and 20 . The edge seal portion  676 a of the lid seal includes a channel  676 c which fits over the wall  664  and rim  666  on the bottom of the lid  648 , and a curved sealing flange  676 d which seals against the top surface of the tub lid, so that the space between the U-channel  718  on the tub lid and the seal membrane forms an inlet air passage of the tub, and the air outlet aperture  722  of the tub lid communicates via the elliptical opening  676 e in the lid seal to the air outlet pipe  662  of the humidifier lid  648 . This is achieved without the need to connect and disconnect air tubes to remove the water container. 
     As the air supplied from the flow generator is under pressure, this pressure assists the sealing flange  676 d of the sealing member  676  to create a firm seal around the recess  718  by forcing the extension portion outwards and downwards. A similar effect is created on the seal surrounding the elliptical aperture  716  in the tub lid due to the pressure of the air exiting the water receptacle. 
     Once the air from the flow generator passes into the water container, the air then travels across the surface of the water so that the air becomes humidified. The heating of the water by the heating pad enhances this humidification. The air then exits the water container through the outlet opening  716  to the air outlet pipe  662 , which is in turn attached to a suitable hose (not shown) for supplying the humidified air to a patient. 
     By providing the air inlet to the water tub headspace via an arcuate path, the air mass within the container is caused to swirl and thus enhance the uptake of water vapour from the water contained in the tub. 
     The enhanced uptake of water vapour achieved by inducing the swirling of air as it passes through the tank enables, in an alternative embodiment of the invention, the elimination of the heating of the water in the tub. In such an embodiment the heating element and its controls, and the heat transfer components including the heating plate and the metal tank base are eliminated, and the humidifier becomes a simpler, passive, device. 
     A humidifier assembly in accordance with the present invention has a number of advantages over the prior art. One advantage relates to convenience of use. Convenience of use is important for all patients, especially those who have poor dexterity. 
     The base of the humidifier assembly includes a generally “negative” U-shaped channel. The bottom portion of the water tub has a corresponding “positive” U-shape. The outer wall of the U-shape is sloping, whereas the inner wall is generally vertical. Because the base and water tubs have complementary configurations, placing the water tub generally in the correct position means that it will to some extent self-align into the correct position, which as described below, is a sealing position. 
     A water tub according to the present design can be easily placed in a sealing position without requiring a patient to connect small fiddly tubes such as used in the prior art. An aspect of this is that a seal is provided by placing a generally flat surface such as the rear of the water tub, or the top surface of the water tub, against respective silicone gaskets that present a corresponding flat surface. The respective seals are formed when the two flat surfaces contact. Thus the humidifier assembly has a very convenient “drop-in” configuration. 
     The water tub is held in position by the simple motion of swinging the pivoting lid through approximately 90° from fully open to closed. The lid is locked in position via a robust mechanism which provides and audible and reassuring “click”-sound when engaged. Whilst in the preferred embodiment, a pivoting movement is used for the lid, other movements are contemplated including sliding and translation. 
     The lid of the humidifier assembly includes an air delivery tube connector, which in a preferred form is generally cylindrical. Connection of the air delivery tube to the lid can be achieved regardless of whether the water tub is in position. This arrangement means that the water tub can be removed and refilled with water if necessary without requiring disengagement of the air delivery tube from the humidifier assembly. 
     The illustrated humidifier construction provides a compact humidifier adapted for ease of manufacture and use, and further provides protection against backflow of water into the flow generator when the humidifier and flow generator units are assembled together. Backflow protection is provided by the sloping floor of the air passage and the location of the air inlet aperture  801  and the aperture  722 d in the seal  722  relative to the air inlet  720  from the air passage  718  into the headspace of the humidifier tub  698 . In particular, if the tub is overfilled while in its horizontal position, the water will flow back along the U-shaped passage  718  only as far as its forwardmost portion, which has a front wall  717  lower than the air inlet aperture  801 , and will drain towards the front of the machine. If the machine is tipped up onto its rear, the water will be prevented from flowing back along the air passage from the tub to the air inlet  801  as the intermediate portion of the air passage  718  will be above the level of the aperture  720 . The water will then flow back into the tub once the machine is righted. 
     If the machine is tipped onto its side, either the air inlet aperture  720  or the air inlet  801  will be above the water level and thus water should not flow back into the low generator. Again, any water which escapes the tub will flow back into the tub once the machine is righted. 
     If desired, further security against backflow can be provided by locating a non-return valve at an appropriate point, for example a flexible membrane supported in the mouth of the humidifier air inlet. 
     In addition to those features and advantages already described, the components and features of the humidifier according to the present embodiment have various advantages. 
     By providing the top seal to the water receptacle as part of the humidifier lid, improved simplicity of use is achieved while minimising the risk of spillage of water. In addition, the contour of the lid seal is adapted to collect condensation which may form in the lid cavity and the headspace of the water receptacle, preventing backflow of this condensation to the flow generator when the lid is opened. 
     Furthermore, the configuration of the front and back covers of the humidifier and of the heater pad is adapted to allow fitting together in a vertical orientation, to minimise the need for reorientation during assembly of the humidifier unit on the production line. 
     In addition, the resilience of the connection between the lid and the water receptacle, provided by the lid seal, is adapted to maintain downwards pressure on the water receptacle when the lid is closed, to maintain good heat-transfer contact between the base of the water receptacle and the heater pad without the added complexity and expense of spring-loaded mounting of the heater pad. 
     Humidifier Power Supply 
     The humidifier is provided with a control knob allowing adjustment of the humidity of the air supply to the patient. With increasing humidity setting, the temperature of the water container is increased by providing increased power to the heater, to raise the humidity of the air leaving the humidifier. The control knob may have a smoothly variable control, or a series of discrete humidity settings, and will have an ‘off’ setting where no power is supplied to the heating pad. The correlation between the humidity setting and the power to the heater is controlled by a circuit on the PCB  804 . 
       FIG. 22  is a sketch of a preferred calibration curve of target water container temperature (y axis) against humidity setting (x axis), including upper and lower tolerances. 
     At the left hand end of the correlation curve, corresponding to the low humidity settings and the off position of the control knob, the heater control selects a very low target heater temperature—less than ambient temperature, and preferably lower than the lowest operating temperature of the humidifier. In this way, the heating is turned off when the control knob is in its off position, while allowing use of a less expensive potentiometer without an integral off switch or a separate on/off switch. The mounting of the control knob mechanism may provide a tactile ‘click’ at the off position of the control knob, to confirm to the user that the heater is turned off. 
       FIG. 23  is a circuit diagram of the humidifier control circuit for controlling the water temperature, including a potentiometer POT 1  actuated by the control knob  805  and an operational amplifier OA 1  providing power to the heater  800 . 
     A potentiometer may be used in series with the heating element to set the operating temperature. However, this may result in large heat losses through the potentiometer as in the following equation
 
P=V 2 /R
 
where V=the supply voltage and is normally fixed and R=RH+RP
 
where RH is the resistance of the heater and is fixed and RP is the resistance of the potentiometer which is variable and provides the temperature control. The current is: I=V/R, and the proportion of heat through the potentiometer is I 2 *RP=RP*V 2 /(RP+RH) 2 . The remainder of the heat is used by the heater element to heat the water.
 
     These heat losses in the potentiometer require large heat dissipation surfaces to prevent overheating. 
     In the present embodiment, the potentiometer is used in the control path of a semiconductor arrangement to set the operating temperature. This substantially reduces the current through the potentiometer because the potentiometer now only carries a semiconductor control current rather than the load current required to drive the heater element. 
     In a preferred embodiment, the potentiometer is used in conjunction with a temperature sensing element to control an operational amplifier which drives the heater directly or through a high current semiconductor switch. 
       FIG. 23  shows an arrangement for controlling temperature via an operational amplifier OA 1 . 
     The operational amplifier n 1  has a pair of inputs, V+ being an adding input and V− being a subtracting input. The output of the amplifier is proportional to the difference between the voltages on the inputs V+ and V−. 
     Input V− is connected to a reference voltage determined by the ratio of resistors R 21  and R 17 ;
 
Vref=Vs*R12/(R12+R17)
 
     The temperature of the water is sensed by temperature sensitive resistive element, thermistor TH 1 , and the operating point is set by potentiometer POT 1 . The operational amplifier input V+ is connected to the junction of R 106  and thermistor TH 1 . The operational amplifier switching threshold is determined by the ratio of the resistance of the potentiometer POT 1  plus resistor R 106  to the resistance of the resistance network formed by thermistor TH 1  plus resistor R 11  in parallel with resistor R 10  plus resistor R 10  equals the ratio of resistor R 17  to resistor R 21 . That is, the operational amplifier switches when the junction between the thermistor TH 1  and resistor R 106  crosses over the potential at V−. 
     The operational amplifier is powered from supply points Vss and Vo, so the drive current does not pass through the potentiometer. Vss may be the same as Vs, and Vo may be the same as 0 v. The operational amplifier may drive the heater element directly or it may control a power transistor which drives the heater element, 
     This arrangement significantly reduces the dissipation through the potentiometer, allowing a smaller potentiometer, with smaller cooling needs, to be used. The arrangement is also well adapted for use in implementing the ‘soft’ off setting arrangement described above with reference to  FIG. 22 . 
     Reminder Menu 
       FIG. 24  is a flowchart of a Reminder menu to set a number of reminders to alert the patient to specific events; for example, when to replace their mask, when to insert a Data Card (if their device is Data Card enabled) and so on. It can also be used to set special customised reminders. 
     When a reminder is due, a message is displayed on the LCD and remains whenever the device is not delivering therapy. The backlight on the LCD flashes when a message is displayed. If more than one reminder for a patient is scheduled for the same date, all scheduled reminders are displayed during that day. A patient can clear a message by pressing the LEFT key (or inserting a Data Card, in the case of the Data Card reminder). 
     The default setting for all reminders is that they are disabled. To use the reminder menu, the patient enters the Reminder Menu from the standby screen by pressing LEFT and DOWN for at least three seconds. 
       FIG. 24  summarises the Reminder Menu screens: 
     REPLACE MASK—to set a timed reminder to remind a patient when they need to replace their mask. The patient can press the LEFT (clear) key to remove the message from the LCD. 
     CALL PROVIDER—to set a reminder for the patient to phone the therapist at a certain time; for example, to discuss how their therapy is going. The patient can press the LEFT (clear) key to remove the message from the LCD. 
     INSERT CARD—if a patient&#39;s flow generator is Data Card enabled, the therapist can set a timed reminder on the flow generator to remind them that they need to insert a Data Card to transfer patient data. This enables the therapist to establish compliance. The patient should actually insert the Data Card in order to clear the message from the LCD. (They can also press the LEFT (clear) key to remove the message.) 
     REPLACE FILTER—to set a timed reminder to remind the patient when to replace the air filter. The patient can press the LEFT (clear) key to remove the message from the LCD. 
       FIGS. 25 to 32  are rear views of the flow generator, showing various forms of modular data connections foreshadowed earlier, utilising the slot  83  in the rear of the flow generator housing. 
     With reference to  FIG. 25 , the slot  83  is provided in the wall of a rectangular recess  1115 . An arcuate depression  1123  is provided in the upper surface of the unit above the recess  1115  to facilitate removal of closure elements from the depression, as described below. 
     At the rear of the printed circuit board  81 , an edge connector  1082  and a sliding connector  1082 A are aligned with and accessible through the connector slot  83  in the rear of the case  60 , providing for the modular connector arrangements to be described in more detail below. 
     Where, as shown in  FIG. 26 , the flow generator in question is not intended to be employed with any data connection, the slot  83  is closed off by a blank closure element  132 , shaped to fit into the recess  1115 . The closure element is shown in more detail in  FIG. 27 . This element snaps into the recess by means of lower tabs  1118  and an upper tab  1119  which fit corresponding depressions such as  1122  in the walls of the recess  1115 , to close the slot  83  and conform to the contours of the surrounding surface of the unit. 
     Complementarily shaped closure elements can be provided for the reception of different kinds of data devices. Shown in  FIG. 28  is an element  1116 a provided with a slot for the reception of a smart card  1120 . The element  1116 a or the printed circuit board itself may carry the necessary smart card socket. 
     Shown in  FIG. 29  is a closure element  1116 b provided with a DB type data socket. In this case the element  1116 b is contoured to provide a lower front recess  1121  to facilitate gripping of the associated plug. A cross-section of a modified form of this arrangement is shown in  FIG. 29A , illustrating the connection between the internal connector  1086  of the element  1116 b and the edge connector  1082  of the PCB, and the external DB9 connector  1088 . 
     Other forms of element  1116  can be provided to enable the connection of devices such as memory cards and pre-programmed devices as required. This facility furthermore enables a wide range of devices to be integrated with the apparatus in modular fashion, for example a clock display which may utilise the system clock contained in the flow generator controller, a voice activation unit, oximetry, ECG and other diagnostic aids, a sound recorder, a light. 
       FIGS. 30 to 32  are a series, of rear perspective views of the flow generator, illustrating one embodiment of the modular data connector arrangement.  FIG. 33  shows the front, inner surface of the USB closure element module, and  FIG. 34  is a vertical cross-section of the flow generator. 
       FIG. 30  shows the slot  83  open, exposing the edge connector  1082  and sliding connector (not visible in this view) at the rear of the flow generator PCB  81 . The connectors  1082 ,  1082 A comprise a plurality of electrical contacts for carrying data and/or power between the PCB and an external device. 
       FIG. 31  shows the arrangement of  FIG. 31  where no data connection is required, with the slot covered by a blank closure element  132  generally as described above with reference to  FIGS. 25 to 27 . 
       FIG. 32  shows a removable closure element module  1116 c carrying a standard universal serial bus (USB) port  1084  on its rear surface. The element  1116 c incorporates an electrical/data pathway to an electrical connector  1090  at its forward, inner surface ( FIGS. 33 and 34 ) adapted to connect with all or selected ones of the contacts of the PCB connector  1082  for electrical and/or data transmission. The closure module  1116 c has internal electrical components completing a data and/or electrical pathway between its internal and external connectors so that the module acts as an adaptor between die PCB connector and a standard USB port. 
     By providing the modular data connection arrangements as described above, in which a plurality of interchangeable connection modules fit to one or more fixed, standard connectors on the PCB, the cost and size of the flow generator unit may be reduced as the unit may be provided with only those connectors which are needed by that patient, and additional connector modules supplied only if the need arises. Furthermore, the arrangement facilitates upgrade of the data connection arrangement of the flow generator to keep up with technological advances or changes in global data connection standards. 
     In this specification, the word “comprising” is to be understood in its “open” sense, that is, in the sense of “including”, and thus not limited to its “closed” sense, that is the sense of “consisting only of”. A corresponding meaning is to be attributed to the corresponding words “comprise, comprised and comprises where they appear. 
     While particular embodiments of this invention have been described, it will be evident to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. It will further be understood that any reference herein to known prior art does not, unless the contrary indication appears, constitute an admission that such prior art is commonly known by those skilled in the art to which the invention relates.