Water out alarm

Abstract

The present invention provides for an improved method of determining a water out condition in a humidified gases supply apparatus. The method includes a two step process including a primary determination of a water out condition and a secondary determination of a water out condition. This primary determination is made during observation of the normal operation of the apparatus. During the secondary determination the method takes temporary control over the humidifying part of the apparatus. The secondary determination confirms or contradicts the primary determination.

Claims

1. An apparatus for supplying gases to a patient comprising: a casing; a flow generator including a fan configured to provide a flow of gases; a humidifier with a heater plate, an unsealed nasal cannula for a patient, a controller, and a valve operable as a flow control mechanism to control an O2 fraction provided to the patient, wherein the humidifier and flow generator are integrated in the casing, and the controller is configured to: adjust an output of the flow generator by controlling a fan speed, monitor system conditions during use of the apparatus, and interrupt a normal control of the apparatus in response to a control interruption condition having been detected so as to determine a possible absence of water in a humidifier chamber based on confirmation or contradiction of the monitored system conditions during the interruption of the normal control of the apparatus.

2. The apparatus of claim 1, wherein the controller is configured to determine a possible absence of water by: monitoring a temperature of gases leaving the humidifier chamber, monitoring power applied to the heater plate, and determining a possible absence of water based on a function of the temperature of gases leaving the humidifier and the power applied to the heater plate.

3. The apparatus of claim 1, wherein the controller is configured to determine a possible absence of water by: making a primary determination of an absence of water, and making a secondary determination of an absence of water.

4. The apparatus of claim 3, wherein in making the secondary determination the controller is configured to interrupt the normal control of the apparatus and take over control of a power input of the humidified heater.

5. The apparatus of claim 1, wherein the controller is configured to output the absence of water determination to activate a user alert.

6. The apparatus of claim 1, further comprising a delivery conduit.

7. The apparatus of claim 6, wherein the delivery conduit comprises a heater wire.

8. The apparatus of claim 1, wherein the controller is configured to operate the apparatus to provide O2 or an O2 fraction to the patient.

9. The apparatus of claim 1, further comprising an ambient temperature sensor located within, near or on the casing.

10. The apparatus of claim 1, further comprising a humidifier exit port sensor at or proximal to an exit of the humidifier chamber.

11. The apparatus of claim 10, wherein the humidifier exit port sensor is at the exit of the humidified chamber.

12. The apparatus of claim 6, further comprising a humidifier exit port sensor at an apparatus end of the delivery conduit.

13. The apparatus of claim 10, wherein the humidifier exit port sensor is a temperature sensor.

14. The apparatus of claim 1, further comprising a heater plate temperature sensor.

15. The apparatus of claim 1, further comprising a flow probe configured to measure a gases flow rate.

16. The apparatus of claim 6, further comprising a patient end temperature sensor at or close to a patient end of the delivery conduit.

17. The apparatus of claim 1, further comprising a patient end temperature sensor in or on the cannula.

18. The apparatus of claim 1, further comprising the humidifier chamber for water.

19. The apparatus of claim 16, wherein the controller is configured to reduce or remove power to the heater plate on determination of the absence of water.

20. The apparatus of claim 1, wherein the control interruption condition is a water-out interruption condition.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Preferred embodiments of the present invention will be described with reference to the accompanying drawings.

(2) FIG. 1 is a flow diagram illustrating the overall process for determining an absence of water in a humidifier chamber in accordance with the present invention.

(3) FIG. 2a shows a schematic view of a user receiving humidified air, with the user wearing a nasal mask and receiving air from a modular blower/humidifier breathing assistance system.

(4) FIG. 2b shows a schematic view of a user receiving humidified air, where the user is wearing a nasal cannula and receiving air from a modular blower/humidifier breathing assistance system.

(5) FIG. 3 shows a schematic view of a user receiving humidified air, where the user is wearing a nasal mask and receiving air from an integrated blower/humidifier breathing assistance system.

(6) FIG. 4 shows a schematic view of a user receiving humidified air, where the user is wearing a nasal cannula, the breathing assistance system receiving gases from a central source via a wall inlet and providing these to a control unit, which provides the gases to a humidifier chamber in line with and downstream of the control unit.

(7) FIG. 5 is a plot against time of a function of chamber outlet temperature and heater plate power and end of heater plate temperature, according to an experiment conducted using the present invention.

(8) FIG. 6 is a flow diagram illustrating a preferred method to initially determine a possible water out condition.

(9) FIG. 7 shows a schematic representation of some of the connections between a controller suitable for use with the breathing assistance system of FIG. 2, 3 or 4, and other components of the preferred form of breathing assistance system as shown in FIG. 2, 3, or 4.

(10) FIG. 8 is a flow diagram illustrating a confirmation method according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(11) The present invention provides for an improved method of determining a water out condition in a humidified gases supply apparatus. This method has been found suitable for determining water out conditions in widely varying ambient conditions. As illustrated in FIG. 1, the method includes a two step process including a primary determination of a water out condition at step 1010. This primary determination is made during observation (at step 1050) of the normal operation of the apparatus. Following a primary determination of a water out condition the method takes temporary control over the humidifying part of the apparatus at step 1030 to make a secondary determination at step 1100 which will confirm or contradict the primary determination.

(12) For the primary determination, the controller of the apparatus may monitor a range of characteristics of the system, as sensed by one or more of the sensors included in the system. For example, the primary determination may be made by any one of a number of prior art methods described discussed in the background discussion. However, a preferred method for the primary determination monitors a function of the chamber outlet temperature and the heater plate temperature and determines the possibility of a water out condition when this function moves away from a base line value.

(13) According to a preferred aspect of the present invention, the secondary determination, interrupts the normal control of the apparatus and takes over control of the humidifier heater power input. According to the method for the secondary determination the heater power is increased to a maximum available value for a limited period of time. The secondary determination is then made based on the monitoring heater plate temperature (at step 1070). This determination may be on the basis of the heater plate temperature rising steeply during this time or staying at an elevated level during this time, or reaching an elevated level during this time indicative of a water out condition.

(14) The controller can then provide output of the water out condition (step 1120) to activate a user alert. The controller also reduces or removes power to the heater plate (step 1110).

(15) The water out determination method according to the present invention has been developed with particular application to humidified gases delivery apparatus used outside the hospital environment. However, the method may also find beneficial in controllers for apparatus intended for the hospital environment.

(16) The method can be used in the controller in a number of broad system configurations. By way of example, three typical system configurations are illustrated in FIGS. 2 to 4 and described below. These system configurations are illustrative but not an exhaustive account of the system configurations in which this method may be used.

(17) A schematic view of a user 2 receiving air from a modular assisted breathing unit and humidifier system 1 according to a first example system configuration is shown in FIGS. 2a and 2b. The system 1 provides a pressurised stream of heated, humidified gases to the user 2 for therapeutic purposes (e.g. to reduce the incidence of obstructive sleep apnea, to provide CPAP therapy, to provide humidification for therapeutic purposes, or similar). The system 1 is described in detail below.

(18) The assisted breathing unit or blower unit 3 has an internal compressor unit, flow generator or fan unit 13—generally this could be referred to as a flow control mechanism. Air from atmosphere enters the housing of the blower unit 3 via an atmospheric inlet 40, and is drawn through the fan unit 13. The output of the fan unit 13 is adjustable—the fan speed is variable. The pressurised gases stream exits the fan unit 13 and the blower unit 3 and travels via a connection conduit 4 to a humidifier chamber 5, entering the humidifier chamber 5 via an entry port or inlet port 23.

(19) The humidifier chamber 5 in use contains a volume of water 20. In the preferred embodiment, in use the humidifier chamber 5 is located on top of a humidifier base unit 21 which has a heater plate 12. The heater plate 12 is powered to heat the base of the chamber 5 and thus heat the contents of the chamber 5. As the water in the chamber 5 is heated it evaporates, and the gases within the humidifier chamber 5 (above the surface of the water 20) become heated and humidified. The gases stream entering the humidifier chamber 5 via inlet port 23 passes over the heated water (or through these heated, humidified gases—applicable for large chamber and flow rates) and becomes heated and humidified as it does so. The gases stream then exits the humidifier chamber 5 via an exit port or outlet port 9 and enters a delivery conduit 6.

(20) When a ‘humidifier unit’ is referred to in this specification with reference to the invention, this should be taken to mean at least the chamber 5, and if appropriate, the base unit 21 and heater plate 12.

(21) The heated, humidified gases pass along the length of the delivery conduit 6 and are provided to the patient or user 2 via a user interface 7. The conduit 6 may include a heater wire 75 or similar to help prevent rain-out.

(22) The user interface 7 shown in FIG. 2a is a nasal mask which surrounds and covers the nose of the user 2. However, it should be noted that a nasal cannula (as shown in FIG. 2b), full face mask, tracheostomy fitting, or any other suitable user interface could be substituted for the nasal mask shown. A central controller or control system 8 is located in either the blower casing (controller 8a) or the humidifier base unit (controller 8b). In modular systems of this type, it is preferred that a separate blower controller 8a and humidifier controller 8b are used, and it is most preferred that the controllers 8a, 8b are connected (e.g. by cables or similar) so they can communicate with one another in use.

(23) The control system 8 receives user input signals via user controls 11 located on either the humidifier base unit 21, or on the blower unit 3, or both. In the preferred embodiments the controller 8 also receives input from sensors located at various points throughout the system 1.

(24) FIG. 7 shows a schematic representation of some of the inputs and outputs to and from the controller 8. It should be noted that not all the possible connections and inputs and outputs are shown—FIG. 7 is representative of some of the connections and is a representative example.

(25) The sensors and their locations will be described in more detail below. In response to the user input from controls 11, and the signals received from the sensors, the control system 8 determines a control output which in the preferred embodiment sends signals to adjust the power to the humidifier chamber heater plate 12 and the speed of the fan 13. The programming which determines how the controller determines the control output will be described in more detail below.

(26) A schematic view of the user 2 receiving air from an integrated blower/humidifier system 100 according to a second form of the invention is shown in FIG. 3. The system operates in a very similar manner to the modular system 1 shown in FIG. 2 and described above, except that the humidifier chamber 105 has been integrated with the blower unit 103 to form an integrated unit 110. A pressurised gases stream is provided by fan unit 113 located inside the casing of the integrated unit 110. The water 120 in the humidifier chamber 105 is heated by heater plate 112 (which is an integral part of the structure of the blower unit 103 in this embodiment). Air enters the humidifier chamber 105 via an entry port 123, and exits the humidifier chamber 105 via exit port 109. The gases stream is provided to the user 2 via a delivery conduit 106 and an interface 107. The controller 108 is contained within the outer shell of the integrated unit 100. User controls 111 are located on the outer surface of the unit 100.

(27) A schematic view of the user 2 receiving air from a further form of breathing assistance system 200 is shown in FIG. 4. The system 200 can be generally characterised as a remote source system, and receives air from a remote source via a wall inlet 1000.

(28) The wall inlet 1000 is connected via an inlet conduit 201 to a control unit 202, which receives the gases from the inlet 1000. The control unit 202 has sensors 250, 260, 280, 290 which measure the humidity, temperature and pressure and flow respectively of the incoming gases stream.

(29) The gases flow is then provided to a humidifier chamber 205, with the gases stream heated and humidified and provided to a user in a similar manner to that outlined above. It should be noted that when ‘humidifier unit’ is referred to for a remote source system such as the system 200, this should be taken to mean as incorporating the control unit 202—the gases from the remote source can either be connected directly to an inlet, or via the control unit 202 (in order to reduce pressure or similar), but the control unit and the humidifier chamber should be interpreted as belonging to an overall ‘humidifier unit’.

(30) If required, the system 200 can provide O.sub.2 or an O.sub.2 fraction to the user, by having the central source as an O.sub.2 source, or by blending atmospheric air with incoming O.sub.2 from the central source via a venturi 90 or similar located in the control unit 202. It is preferred that the control unit 202 also has a valve or a similar mechanism to act as a flow control mechanism to adjust the flow rate of gases through the system 200.

(31) Sensors

(32) The modular and integrated systems 1, 100 and 200 shown in FIGS. 2, 3 and 4 have sensors located at points throughout the system. These will be described below in relation to the breathing assistance system 1.

(33) The preferred form of modular system 1 as shown in FIG. 2 has at least the following sensors in the following preferred locations:

(34) 1) An ambient temperature sensor 60 located within, near, or on the blower casing, configured or adapted to measure the temperature of the incoming air from atmosphere. It is most preferred that temperature sensor 60 is located in the gases stream after (downstream of) the fan unit 13, and as close to the inlet or entry to the humidifier chamber as possible.

(35) 2) A humidifier unit exit port temperature sensor 63 located either at the chamber exit port 9, or located at the apparatus end (opposite to the patient end) of the delivery conduit 6. Outlet temperature sensor 63 is configured or adapted to measure the temperature of the gases stream as it exits chamber 5 (in either configuration the exit port temperature sensor 63 can be considered to be proximal to the chamber exit port 9).

(36) Similarly, sensors are arranged in substantially the same locations in the integrated system 100 shown in FIG. 3 and the system 200 of FIG. 4. For example, for the integrated system of FIG. 3, an ambient temperature sensor 160 is located within the blower casing in the gases stream, just before (upstream of) the humidifier chamber entry port 123. A chamber exit port temperature sensor 163 is located either at the chamber exit port 109 and is configured to measure the temperature of the gases stream as it exits chamber 105 (in either configuration the exit port temperature sensor 163 can be considered to be proximal to the chamber exit port 109). Alternatively, this sensor can be located at the apparatus end (opposite to the patient end) of the delivery conduit 106, for either embodiment. A similar numbering system is used for the breathing assistance system shown in FIG. 4—ambient temperature sensor 260, fan unit 213, chamber exit port temperature sensor 263 located at the chamber exit port 209, etc.

(37) It is also preferred that the breathing assistance system 1 (and 100, 200) has a heater plate temperature sensor 62 located adjacent to the heater plate 12, configured to measure the temperature of the heater plate. The breathing assistance system(s) having a heater plate temperature sensor is preferred as it gives an immediate indication of the state of the heater plate. This sensor should be in the heat path between the source of the heat and the reservoir. So, for example, a sensor on a conductive plate that contacts the water chamber on one side and has a heater on the other side is preferred.

(38) It is also most preferred that the systems have a flow probe—flow probe 61 in system 1—located upstream of the fan unit 13 and configured to measure the gases flow. The preferred location for the flow probe is upstream of the fan unit, although the flow probe can be located downstream of the fan, or anywhere else appropriate. Again, it is preferred that a flow probe forms part of the system, but it is not absolutely necessary for a flow probe to be part of the system.

(39) The layout and operation of the breathing assistance system 1 will now be described below in detail. The operation and layout of the systems 100 and 200 is substantially the same, and will not be described in detail except where necessary.

(40) For the breathing assistance system 1, the readings from all of the sensors are fed back to the control system 8. The control system 8 also receives input from the user controls 11.

(41) Further Alternative Sensor Layouts

(42) In a variant of the apparatus and method outlined above, the system (system 1 or system 100 or system 200) also has additional sensors as outlined below.

(43) 1) A patient end temperature sensor 15 (or 115 or 215) is located at the patient end of the delivery conduit 6 (or alternatively in or on the interface 7). That is, at or close to the patient or point of delivery. When read in this specification, ‘patient end’ or ‘user end’ should be taken to mean either close to the user end of the delivery conduit (e.g. delivery conduit 6), or in or on the patient interface 7. This applies unless a specific location is otherwise stated. In either configuration, patient end temperature sensor 15 can be considered to be at or close to the user or patient 2.

(44) Preferred Method for Primary Water Out Determination

(45) With reference to FIG. 6, in the most preferred embodiment of the present invention the controller evaluates a function of the chamber outlet temperature and the heater plate power on a continuing basis during normal use of the apparatus. The controller determines the potential for a water out condition when the result of this function varies from a base line level. The preferred function is a ratio of the chamber outlet temperature from sensor 63 (or 163 or 263) and the heater plate power, as represented by the heater plate duty cycle. This ratio can be represented as:

(46) $\Theta =\frac{\mathrm{CBO\_Temp}}{\mathrm{HP\_duty}\mathrm{\_control}\left(\mathrm{average}\right)}$ where Θ=monitoring function,

(47) CBO_Temp=Chamber outlet temperature from sensor 63,

(48) HP_duty_control(average)=Heater plate duty control value, recorded each second and averaged over 128 data points.

(49) In tests conducted by the inventors, this ratio has been found to remain substantially constant in steady state conditions where water remains in the humidification chamber, but to increase when the water runs out. This was the case across a range of ambient temperatures, including ambient temperatures from 8° C. to 32° C.

(50) In the worst case, this ratio increased by a minimum of 10% from a pre-existing base line value after the chamber ran dry.

(51) A baseline value, θ.sub.0, may be calculated in many ways. For example, θ.sub.0 could be a moving window average of θ, or a function of a moving window average of CBO_Temp and a moving window average of the heater plate duty cycle. Alternatively, the value θ.sub.0 could be periodically updated with the present value θ, the period being, for example, 10 minutes or more.

(52) An example of this monitoring function is illustrated in FIG. 5. FIG. 5 includes a plot of this preferred function against time for a Fisher & Paykel Healthcare Ltd AIRVO humidified gases delivery apparatus operating at 15 litres per minute with a Fisher & Paykel MR290 humidification chamber, and an ambient temperature of 32° C. The plot is of a normalised ratio against time and of the heater plate temperature against time. The normalised ratio is seen to remain steady until approximately t=1,300 s and then begin increasing, eventually reaching a value of approximately 1.8.

(53) Similar testing was conducted for two different humidifier chambers used with the Fisher & Paykel Ltd AIRVO humidified gases delivery apparatus across three ambient temperatures and several flow rates. The tested conditions and flow rates represent the extreme operating conditions of the apparatus.

(54) The normalised maximum of the evaluated ratio as the apparatus entered the water out condition is indicated in Table 1:

(55) TABLE-US-00001 TABLE 1 Normalised maximum T.sub.CBO/power ratio. HP Temperature/power Ratioθ/θ.sub.0 22° C. 32° C. 8° C. 15 L 45 L 15 L 45 L 15 L 45 L Chamber min.sup.−1 min.sup.−1 min.sup.−1 min.sup.−1 min.sup.−1 min.sup.−1 HC360 2.6 2.5 1.6 3.0 1.1 2.9 MR290 1.2 5.2 1.8 3.1 1.4 2.7

(56) Within this range of conditions two typical system responses to a water out conditions were observed.

(57) In a first type of response, the chamber outlet temperature decreased while the heater plate temperature increased to a maximum value. This was followed by the chamber outlet temperature increasing once more.

(58) In the second type of response, the chamber outlet temperature increased directly as the water out condition occurred, but the heater plate duty control decreased while maintaining a set chamber outlet temperature. In both typical responses, the ratio of chamber outlet temperature to heater plate power increased compared to the value at the start of the water out condition. In the case of the first type of response, a longer time is required for the ratio to increase relative to the base line value than for the second type of response.

(59) This primary water out detection method is illustrated in FIG. 6. Step 610 represents ongoing control of the heater plate power according to the system conditions. This control aims to keep the delivered gases temperature and humidity at or close to preferred levels.

(60) As an ongoing process while this control continues, the method monitors for a possible water out condition based on a loop of steps 620 to 650.

(61) At step 620, the method reads the chamber outlet temperature and the heater plate duty cycle. At step 630, the method calculates the ratio θ. At step 640, the method calculates θ.sub.0 as a moving window average of the most recent values of the function θ.

(62) At step 650, the method compares the value calculated at step 630 with a multiple of the value calculated at step 640. Alternatively, the method compares the value calculated at step 630 with a value calculated at step 640 plus an arbitrary offset, namely the equation in step 650 of FIG. 6 can be θ>θ.sub.0+α, where a is constant for all temperatures or flow rates. In the exemplary embodiment, this multiple is 1.1, which has been found a usable threshold for an initial determination of a water out condition across a range of use conditions for the humidified gases delivery apparatus tested. If the present value sufficiently exceeds the long term value, the method proceeds to step 660 and returns a possible water out condition. If the present value is less than the long term value or not significantly greater than the long term value, the method returns to step 620 and continues to monitor the gases outlet temperature and heater plate duty cycle under the normal use conditions.

(63) The example describes an implementation using normalised values, and step 650 uses a threshold ratio. Alternatively, this step could be calculated to directly use measured values and determine a possible water out condition based on a threshold difference.

(64) Preferred Method for Secondary Water Out Determination

(65) According to the present invention a secondary determination of the water out condition is made in response to a primary water out determination. According to the preferred embodiment, the method takes control of the humidifier power input, adjusts the heater plate power input and observes the temperature response of the heater plate. This secondary determination is implemented to confirm the water out condition if the primary monitoring function returns a possible water out condition.

(66) The primary monitoring function may falsely determine a water out state under certain conditions. These conditions include disturbances in the mains power supply, the occurrence of a sudden heat impulse to the chamber temperature sensor due to blocking of the flow or vigorous shaking of the unit causing water from the chamber to enter the outlet tube. These imposed increases in chamber temperature will likely lead to a decrease in heater plate duty control and hence a change to the result of the evaluation of the ratio, which could be incorrectly interpreted by the controller as a water out state.

(67) According to the present invention, after the controller determines a primary water out condition, the control program enters a routine for performing a secondary water out determination.

(68) As illustrated in FIG. 8, according to the preferred routine, the controller increases the heater plate duty cycle to the maximum available duty cycle, applying full power to the heater plate at step 802. The control program proceeds to monitor the heater plate temperature at steps 806. and 808 until the heater plate temperature exceeds 125° C. The control program then proceeds to monitor the time for which the heater plate temperature continues to exceed 125° C. at steps 810 to 814. If the heater plate temperature continuously exceeds 125° C. for 30 s the controller determines a water out condition and sets a water out flag at step 816. The controller also reduces or removes power to the heater plate at step 818.

(69) If the control routine does not determine a water out condition, then at step 820 the control routine returns control of the humidifier to the main operating control method. This can occur either because the heater plate temperature does not exceed 125° C. for 30 seconds within 120 seconds of applying maximum heater power at step 802. Implementing this, the method starts a first timer at step 804 and checks this timer at step 822 after determining that the temperature is not above 125° C. at step 808. If the timer is still less than 90 seconds, the method proceeds to loop back to step 806, to read the updated heater plate temperature. If the timer exceeds 90 seconds, the method proceeds from step 822 to step 820, returning to normal control.

(70) For the other condition for returning to normal control, the method includes a check at step 812 whether the heater plate temperature is still above 125° C. In each loop, the method reads the heater plate temperature at step 826, and checks this temperature against the threshold at step 812. If the present heater plate temperature exceeds the threshold, the loop continues, otherwise the method proceeds to step 822 and checks whether 90 seconds has elapsed since step 802.

(71) This check is illustrated in the plots of FIG. 5. The start of the secondary determination method is evident at t=1850 s. The heater plate temperature has been approximately 53° C. and slowly dropping. At t=1850 s the heater plate temperature sharply rises, rapidly reaching values above 125° C. and remaining at these elevated levels until the controller removes power from the heater plate in response to the confirmed water out condition.

(72) Essentially this secondary confirmation applies a predetermined heater power and monitors the response of the heater plate temperature and determines whether this response is characteristic of the chamber being in a water out condition.

(73) In the preferred method described, the applied power is the maximum available power. In some systems the applied power could be a lesser value if it has been determined that a lower power was sufficient to achieve a response that can differentiate between a water out condition and a condition where water remains in the chamber across all of the expected operating conditions of the apparatus.

(74) According to the preferred method, the controller monitors for an increase in the heater plate temperature to a value exceeding a predetermined limit and for the heater plate temperature to remain above this limit continuously for a predetermined period of time.

(75) Alternatively the controller could monitor for a rate of increase in the heater plate temperature that might have been shown by experimentation to be characteristic of a water out condition, or for some other characteristic of the heater plate temperature that has been determined experimentally to be characteristic of a water out condition, compared to a condition in which there is water in the chamber.

(76) The predetermined temperature values and times described in this preferred embodiment of the present invention have been determined for a particular humidified gases delivery apparatus. For other humidified gases delivery apparatus, suitable heater plate temperature thresholds and periods would need to be determined by experimentation.

(77) In the case of the apparatus on which this testing and experimentation was conducted, tests have shown that in a water out condition, maximum applied heater power achieves a heater plate temperature in excess of 125° C. for a consistent and continuous time. Where water remains in the humidified chamber, the heater plate temperature, with full duty cycle and in the highest ambient condition (32° C.) the heater plate temperature did not exceed 123° C. This is the worst case condition for the secondary determination, and the condition under which the secondary examination could be most likely to provide a false positive for a water out condition. Accordingly, for this device, the described secondary determination has been shown to reliably distinguish an empty chamber from a chamber with water remaining.

(78) The invention has been described with particular reference to humidified gases delivery apparatus having a heater plate that contacts a heat conductive base of a water containing reservoir. The system of the present invention may be applied to other humidifier configurations. For example, the heater of the humidifier may reside within the water reservoir or be integrated to the base or wall of the water reservoir. In either case, the temperature sensor for the secondary determination should be influenced by both the contents of the reservoir and the heat source of the heater.

(79) In these cases, it would be expected that the threshold temperature for the secondary determination would be lower than the threshold temperature determined experimentally for the Fisher & Paykel Healthcare Ltd AIRVO appliance.

(80) In an appliance with a different heater arrangement, the heater temperature might be determined by a thermistor in contact with the heating element, or in contact with a heat conductive substrate in contact with the heating element. Typically there will be a maximum temperature that the substrate achieves for a given heater plate power (for example, maximum power) when there is water in the chamber due to the limiting temperature at this boundary defined by the boiling point of water. The heat supplied to the heater plate under these conditions is absorbed as latent heat of vaporisation, and does not yield a change in temperature. In the absence of water, the latent heat energy requirement disappears, and the excess energy over that required to maintain steady state temperatures gives rise to an increase in gas temperature rather than the production of water vapour.

(81) The preferred embodiment of the present invention has been described as providing maximum power to the heater for a sustained period. The invention could be implemented so that a first elevated power is provided to the heater to bring the heater above the threshold temperature, with the power subsequently reduced to a level that maintains the temperature above the threshold temperature, but does not further elevate of the temperature of the heater plate. This could be achieved for example, by using a closed feedback loop based on heater plate temperature and could be particularly suitable where the power available to be applied to the heater plate is much larger than the power required to maintain the temperature above the threshold with temperature of the heater applied above the threshold level with only gases in the chamber.