the illustrated apparatus comprises a flow generator 50 and a humidifier 150 , shown in their assembled condition in fig1 , and separately in fig2 and 3 respectively . as shown in fig2 , 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 ( fig1 ) by which the flow generator 50 and humidifier 150 are connected together , as will be described in more detail below . 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 . 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 ( fig9 ) to mate with the periphery of the bottom case 61 . with reference to fig7 , 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 fig6 ). 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 . 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 . 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 fig7 , 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 . with reference to fig8 , 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 fig9 , 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 fig2 to 34 . 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 fig2 to 34 . from the first muffler volume 134 under the fan cavity 70 , inlet air passes through a connection passage 137 ( fig1 ) 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 fig1 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 . it will now be convenient to describe the features of the fan , which are shown in fig1 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 ( fig1 ), 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 . a keypad 59 , facia 127 and transport handle 128 attach to the top case 60 . with reference to fig9 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 . as shown in fig1 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 . 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 ( fig1 ) 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 fig1 ) 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 . 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 . 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 fig2 , 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 fig2 , 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 fig2 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 microswitch ( 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 . fig1 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 fig1 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 fig1 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 . 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 . fig2 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 . fig2 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 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 . fig2 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 ; 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 fig2 . fig2 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 . 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 . fig2 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 fig2 , 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 fig2 , 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 fig2 . 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 fig2 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 fig2 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 fig2 a , 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 . fig3 to 32 are a series , of rear perspective views of the flow generator , illustrating one embodiment of the modular data connector arrangement . fig3 shows the front , inner surface of the usb closure element module , and fig3 is a vertical cross - section of the flow generator . fig3 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 . fig3 shows the arrangement of fig3 where no data connection is required , with the slot covered by a blank closure element 132 generally as described above with reference to fig2 to 27 . fig3 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 ( fig3 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 .
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 .