Patent Description:
Aerosol generators generally comprise a vibratable member ("aperture plate", "AP") having a plurality of apertures extending between a first surface which is adapted to receive a liquid to be aerosolised and a second surface at which an aerosol is generated. The aperture plate may be vibrated using a piezoelectric element which receives power and mechanically vibrates in order to convert the liquid on the first surface of the aperture plate to aerosol at the second surface.

One such device is described in our <CIT>, which describes an aerosol generator with an aperture plate or membrane which is supported by a washer in a housing. The aperture plate is driven by a piezo element actuator. The aperture plate is vibrated at high frequency, typically <NUM> to <NUM>, and this action breaks the surface tension of the liquid and creates an aerosol plume of droplets. Such systems are commonly called vibrating mesh technology.

The aperture plate is operated until the medication has been fully aerosolized. Arrangements to detect end of dose are described in <CIT>), and the following published patent specifications of the present applicant also describe arrangements to detect end of the dose: <CIT>, <CIT>, <CIT>, and <CIT>. <CIT>) describes methods and apparatus for controlling piezoelectric vibration. <CIT>) an ultrasonic generator with a protector which detects changes in at least one parameter caused by transition from wet to dry.

Due to the high-frequency operation there is a risk of damage to the AP while running at a high level of actuating power, such as is required for certain uses to for example emit a dose of greater than <NUM>/min. Hence, frequent inspection is required and/or aperture plate precautionary replacement before it is necessary.

The invention provides an aerosol delivery system as set out in claim <NUM>. Various aspects of the system are set out in claims <NUM> to <NUM>. The invention also provides a method of operation of an aerosol delivery system as set out in claim <NUM>, and aspects are set out in claim <NUM>.

It has been found by the inventors that as the aerosol aperture plate transitions from a wet state to a dry state, it comes under increased mechanical stress and this may have an adverse effect on the durability of the nebulizer, significantly shortening its operational life due to cracking/fracturing of the aperture plate. This problem is particularly serious if the aerosolization output of the nebuliser is increased - the increased power applied to the nebuliser increases the risk of damage to the aperture plate.

The plate can be operated in the dry state at full power with little or no damage and similarly, it can be operated at full power in the wet state with little or no damage. However, the sudden change in displacement at the instant of the plate changing from wet to dry can cause damage. Such stress may, over time, result in cracking / fracturing of the aperture plate.

In various embodiments an aerosol delivery system has an aerosol generator with a vibrating aperture plate and an actuator, a controller. The controller in real time monitors the aerosol generator as it is driven for vibration of the aperture plate, detects a change in an electrical characteristic in response to a transition from a wet state to a dry state of the aperture plate; and automatically modifies, during the transition, operation of the aerosol generator in response to the detected change. The modification may include reducing applied power. The controller continues to monitor said change during the transition, including monitoring the aperture plate for presence of residual liquid on the aperture plate first surface. The controller monitors the electrical characteristic (such as drive current) at a number of drive frequencies and maintains data representing trends in a combination of the monitored characteristic signals. It identifies a start of a transition if a calculated value derived from the monitored signals rises above a threshold. There is one threshold for triggering a check scan for residual volume and a higher threshold to trigger an immediate shut down.

The invention effectively protects the aperture plate by performing this method in real time. Importantly, it identifies a transition of the AP to dry and hence predicts the plate becoming dry and takes appropriate action in advance. This may be reducing power or shut down, depending on the extent of residual liquid. The aspect of using a real time derived or calculated value which magnifies the changes in individual monitoring signals allows very prompt identification of a transition. By switching from the normal drive cycle to a short scan of only <NUM> measuring points it can promptly in real time identify whether there is residual liquid. The result of the invention is a major reduction in risk of damage to the AP, and hence improved operation of an aerosol generator or nebulizer during its life cycle.

In various aspects of the disclosure we describe a method of operating an aerosol delivery system comprising an aerosol generator and control means for controlling the operation of the aerosol generator wherein the aerosol generator comprises a vibratable member having a plurality of apertures extending between a first surface which is adapted to receive a liquid to be aerosolised and a second surface at which an aerosol is generated, wherein the method comprises the steps of:.

In one embodiment the method comprises monitoring the electrical characteristic at an operating frequency and at one or more one other frequency adjacent to the operating frequency, and preferably the aperture plate is driven at these frequencies in time division multiplexing cycles. A frequency within <NUM> of the operating frequency may be used. In one case the method comprises monitoring the electrical characteristic at a first frequency less than <NUM> below the operating frequency and at a second frequency less than <NUM> above the operating frequency to determine if a change has occurred. A first frequency about <NUM> below and a second frequency about <NUM> above the operating frequency may be used, in one case.

The duration of at least some, and in one case all, of the monitoring steps may be small in the order of ms, such as <NUM>, and preferably in the range of <NUM> to <NUM>.

If there is even a small deviation from this trend it may immediately detect the start of a transition. The output of the algorithm is typically steady, but has an exaggerated jump when the transition starts. If the electrical characteristic is monitored at each of a number of drive frequencies in a time division multiplexed manner there may be two electrical characteristic changes simultaneously, thereby allowing immediate detection. This may, for example, be an increase in drive current at one drive frequency and a decrease in drive current at a different frequency.

The method may comprise monitoring the vibratable member for the presence of residual liquid on the first surface. The method may comprise activating power at a low level in response to detection of residual liquid on the first surface. The method may comprise activating the power incrementally until the residual liquid is aerosolised. The method may comprise monitoring the vibratable member in order to detect when the first surface is in a dry state and indicating confirmation that the remaining liquid has been aerosolised. The method may comprise shutting off power to the aerosol generator upon detection of the dry state.

The controller may be configured to continually reduce the applied power to the aperture plate in response to the change in electrical characteristics.

In one case the controller is configured for monitoring the electrical characteristic at a frequency within <NUM> of the operating frequency.

In one embodiment the controller is configured for monitoring the electrical characteristic at a first frequency less than <NUM> below the operating frequency and at a second frequency less than <NUM> above the operating frequency to determine if a change has occurred. In one case the controller is configured for monitoring at a first frequency about <NUM> below and at a second frequency about <NUM> above the operating frequency. The duration of at least some, and in one case all, of the monitoring steps is small in the order of ms, such as <NUM>.

In one case the controller is configured to indicate end of dose upon detection of the dry aperture plate. The controller is configured to shut off power to the aperture plate at end of dose. In one case the controller has a processor programmed to execute an algorithm as set out above in any embodiment.

Referring to <FIG>, an aerosol delivery system <NUM> comprises an aerosol generator with an aperture plate or membrane <NUM> which is supported in a housing <NUM> by a washer <NUM> also supporting an annular piezo element actuator <NUM>. A drive pin <NUM> is in contact with the top surface of the actuator <NUM> and a drive pin <NUM> is in contact with the washer <NUM>, so that power is applied across the actuator <NUM> directly to the top surface by the pin <NUM> and via the washer <NUM> to the bottom surface from the pin <NUM>. A controller <NUM> includes a digital processor which is programmed to control a drive circuit <NUM> which delivers power to the pins <NUM> and <NUM>. The drive circuit includes shunt voltage sensors to monitor current supplied to the pins <NUM> and <NUM>. The processor is programmed to control the drive signals so that the aperture plate is driven at each of a number of drive frequencies in cycles, as described in more detail below.

The hardware arrangement is illustrated in more detail in <FIG>. The controller <NUM> comprises an <NUM>-bit (or in other embodiments a <NUM>-bit for example) microprocessor. A controllable voltage source is provided to allow adjustment of power to the AP. A drive stage applies an AC signal to the piezo actuator <NUM> via the pins <NUM> and <NUM>. A pulse width modulator (PWM) output from the microprocessor controls frequency of operation, a current sense module provides power feedback, and an enable/disable pin of the microprocessor switches on and off the power.

The aperture plate is vibrated at high frequency of typically <NUM> to <NUM> and this action breaks the surface tension of the liquid and creates an aerosol plume as droplets pass through the aperture plate.

In general, the aperture plate continues vibrating throughout the delivery of the aerosolised liquid. However, we have found that as the first surface of the aperture plate transitions from a wet state to a dry state, the aperture plate experiences increased mechanical stress. If allowed to continue this would have an adverse effect on the durability of the aerosol generator, significantly shortening its operational life, due to cracking or fracturing of the aperture plate. This is the case even if power is shut off before the plate becomes dry.

<FIG> is a pair of plots of aperture plate displacement vs. drive frequency, one for a wet plate and one for a dry plate. The vertical line indicates the difference for a drive frequency of <NUM>, about <NUM> when wet and about <NUM> when dry. Hence if the plate transitions from wet to dry when being driven at this frequency the displacement will increase until there is a four-fold increase in displacement. We have found that the transition to dry can cause considerable damage to the plate as it changes to the much higher displacement. Operation in a constant wet or dry environment does not cause the same effect on life of the aperture plate.

This may be a particular problem in delivery of aerosolized medication where it is necessary to increase the aerosolization output from for example <NUM>/min, by increasing the power applied to the actuator from for example <NUM> Watts to <NUM> Watts. The damaging stress on the aperture plate may be particularly high during transition from a wet state to a dry state in these circumstances.

The aerosol delivery system (or "nebulizer") reduces mechanical stress on the aperture plate during transition from wet to dry, i.e. in advance of it becoming dry, as set out in more detail below.

The digital processor is programmed to execute an algorithm which monitors AP applied power consumption to detect start of transition of the aperture plate from wet to dry, in effect giving an early prediction of the AP dry state. Upon detection of the start wet to dry transition the controller is configured to automatically reduce the power applied to the aperture plate. The reduction may be partial or complete, either immediately upon detection of the start of the transition or during the transition. The controller <NUM> performs this monitoring and control in real time, by automatically detecting small changes in the electrical characteristics of the aerosol generator. The controller in one example predicts an imminent change of state from wet to dry and removes power from the device prior to the plate reaching a complete dry state.

In some cases, the method comprises monitoring of the electrical parameters in combination with utilizing a software method to perform calculations to determine a derived calculated value in real time for determining the status of the plate. The derived calculated value is referred to also in this specification as the "numerical output".

<FIG> shows the power consumed by the aerosol generator (based on drive current because voltage is static) over time for a drive frequency of <NUM> (top line), <NUM> (middle line), and <NUM> (bottom line). In one example the plate is driven at each of these three frequencies in turn in a cyclic manner, with the drive at each frequency being for about <NUM>, hence the drive at each frequency is for about <NUM> vibration cycles of the plate. The centre frequency may be termed the operating frequency and the upper and lower as first and second frequencies. In other embodiments the drive scheme may be different. For example, the differences between the drive frequencies may be more than <NUM>, possibly up to <NUM>. Also, the duration at each frequency may be different from <NUM>, for example a value in the range from about <NUM> up to about <NUM>. The applied power is provided at a fixed DC voltage, such as <NUM> V. The power consumed varies according to the AP status, and this is in many embodiments detected by monitoring current drawn using a shunt resistor. The power consumption values are used as inputs to an algorithm which provides in real time the numerical output. The algorithm, as described in more detail below, provides the numerical output with an exaggerated change in comparison to the sensed values. As shown in <FIG> the power drawn rises for one frequency but reduces for the other two, and the extents of change are different. The algorithm uses these seemingly contradictory signals to provide a single numerical output which rapidly changes, thereby allowing real time dry state prediction and hence real time control of the AP drive to avoid damage to the AP as it transitions to the dry state. In <FIG> the left hand vertical axis is power as represented by units derived from the drive circuits. This may be measured for example by measuring voltage across a shunt resistor. The right hand vertical axis is the numerical output referred to above.

The vertical line near the centre of <FIG> shows the point at which the processor detects the start of transition and hence predicts the dry state. According to the settings the controller may reduce power or it may remove power at this time (about <NUM> from the start of this test). The processor algorithm numerical output begins to increase and once a predetermined value (in this case <NUM>) is reached, the controller will take an action as determined by either a profile setting or a scan to verify that there is a transition to dry. The action is to prevent the AP from reaching its final dry state, or allow it to become dry with incremental reductions in power so that damage is prevented. It is expected that the residual volume remaining at the transition detection will be only in the range of <NUM>% to <NUM>% of the original volume (dose).

The digital processor of the controller is programmed to dynamically, in real time, monitor all three traces according to the algorithm, so that it can immediately detect the start of the transition to dry.

Referring to <FIG>, the benefit of the cyclic multi-frequency drive is illustrated. A single frequency point scan would run the risk of having little change when the plate transitions between wet and dry, because the actuator power consumption signals for wet and dry cross over. In the specific example of <FIG> this crossover frequency is about <NUM>, however it may be different for not only different APs but also for different liquid viscosities. The fact that the algorithm uses a <NUM>-point scan method avoids risk of such an occurrence, the numerical output being an exaggerated value based on multiple signals each changing in their own way. <FIG> shows diagrammatically the differences in power consumption for different frequencies (X-axis is frequency, kHz; and Y-axis is power, no units). In general, the Y-axis may be measured as shunt voltage, current or power (depending on measurement technique). These are related and will show a similar change when the AP starts to transition from wet to dry. The X-axis is the frequency range in kHz. By using the algorithm when one point is on the crossover (showing little or no change between wet and dry), the other two points will receive a larger signal change at the start of transition. The algorithm takes the baseline value and integrates the area under change from all three resulting in excellent sensitivity to the transition.

The algorithm output signal line shows a low background level while the plate is wet. However, as it starts to become dry there is a spike in the line signal, and the ramp time is in this example from about <NUM> to <NUM>. For the <NUM> line there is step change to an increased electrical power consumption (indicated by an increase in drive current drawn). However, for the <NUM> line there is little change in drive current, merely a deviation which appears like low-level noise. However, at some frequencies this is a reduction in power at the change, and this is the case for the <NUM> plot. The change in electrical power is an indication of the plate transitioning to running dry which results in a large jump in displacement. This stepped change in input power and hence aperture plate mechanical power shows how the transition from wet to dry results in more mechanical exertion and damage on the aperture plate.

The algorithm predicts (based for example on monitoring a number of drive current previous readings such as <NUM> previous readings) an imminent change of state from wet to dry and once this has been determined, the controller reduces or removes power from the actuator prior to the plate reaching a completely dry state. The algorithm exaggerates the small changes in drive current and by monitoring trends deviations from the current trends it detects very small deviations at one, two, or all three frequencies. In this example it will only detect significant changes for the <NUM> and <NUM> frequencies. This illustrates an important advantage arising from having multiple frequencies; if there is a late response to start of transition at one of them, there is still likely to be a good discernible change at one or more of the others. In any event, the algorithm uses all of the changes irrespective of their direction or extent. It is not possible to know in advance which is the most useful one, as this depends on the nature of the liquid (typically including a drug) being aerosolised and other conditions which are not known in advance.

The processor algorithm may operate even for only a single drive frequency because of the numerical generation of a trend and consequent early detection of a change above a threshold.

In use, when an imminent change of state of the aperture plate from wet to dry has been determined by the controller, the power applied to the aperture plate by the actuator is reduced prior to powering off the aperture plate completely. This stage is implemented at the start of the transition of the plate from wet to dry. This results in a reduction in the step change and significantly reduces the mechanical stress experienced by the aperture plate when in a dry state. Furthermore, this results in an increase in the life of the aperture plate.

The wet to dry transition may be identified by utilizing the data from the plot of <FIG>; the algorithm uses the electrical characteristics during normal delivery to generate an "operating" profile. Any event that is uncharacteristic of this operating performance indicates a change of state. For example, a number of previous readings, such as eight readings, are used to predict an imminent change from wet to dry.

As noted above, it is particularly advantageous that even if a minimal change in electrical characteristics exists at the operating frequency, one or both of the side frequencies will record a change sufficient to identify the wet-dry transition as illustrated in <FIG>.

In the illustrated example, the drive is at <NUM> for a short time such as <NUM>, it then changes frequency to <NUM> for a short time such as <NUM> to determine if a change has occurred. It then reverts back to <NUM> for a short time such as <NUM> before checking at <NUM> for a short time such as <NUM>. It continually follows this short sequence. However, the frequency values, the durations at each, and indeed the number of different frequencies used may be different.

The time spent at the two side frequencies has no discernible effect on flow rate, because this time is very short. A response will be detected at one or more frequencies when a change from wet to dry occurs. The wet to dry transition occurs over a period of about <NUM> to <NUM>. The controller checks the drive current status at the three frequencies multiple times within the transition duration, ensuring that the change is identified and acted upon prior to the device going completely dry.

Referring to <FIG>, a method <NUM> of the invention may have steps <NUM> to <NUM> as follows:.

The flowchart shows a series of steps and stages involved in the completion of the dosage required to be nebulized. The aperture plate state is monitored in order to determine if the transition from wet to dry has started. If not, this loop is repeated. If the transition has started, the power to the actuator is reduced. The aperture plate is then monitored to check if residual liquid is still present on the plate. If there is liquid present, a low level of power increment is activated. The aim of this stage is to nebulize the remaining droplet of liquid at the minimum power level required, and therefore minimal stress for the aperture plate during the wet to dry transition prior to confirming the status by dry detect indication. This loop is repeated until there is no residual liquid detected, whereby end of dose is detected and power to the plate is shut off. In other embodiments the step <NUM> increases the power back to the full level for faster aerosolization of the residual liquid, subject to the checks of steps <NUM> and <NUM>.

The flow may also include an initial determination at power-up if the AP is wet, upon which the actuator continues with the step <NUM> with cycling across the three frequencies of <NUM>, <NUM>, and129 kHz until the transition is detected. Hence, the controller will immediately shut off when powered up as a protection measure if the plate is dry to begin with. This initial check is preferably made with minimum power applied to the AP actuator. This "wet detect" step makes a, low voltage (10V), wet/dry determination based on the plate response at a frequency close to the natural frequency of the aerosol generator, within a band of +/- <NUM> around the natural frequency. Upon detection of liquid at startup, the generator is activated at 15V. The drive frequency changes every <NUM> alternating from +/- <NUM> around the center frequency of <NUM> in a cyclic manner (frequency hopping). After an initial settling time of <NUM>, the controller begins to monitor the plate response at the three operating frequencies (<NUM>,<NUM> & <NUM>).

<FIG> and <FIG> provide more detail about the steps <NUM> and <NUM>. <FIG> shows two events as follows.

The numerical output ("ASO") value exceeds a trigger level of <NUM> at about <NUM> in this example. This causes the step <NUM> detection and the step <NUM> reduction of power to 10V. The controller then performs a scan in a manner such as described in <CIT>. This is a scan with multiple measuring points, at each of which there is a different drive frequency and drawn current is measured. The drive current readings for the scan allow the processor to determine if there is residual liquid on the AP, for performance of step <NUM>. In this case the number of measuring points is low, as shown in <FIG> only five. The scan in the example of <FIG> shows typical plots for wet (higher drive current, but is sloping downwards), and dry (sloping upwards).

After the scan is performed the controller reverts to the three-point cyclic drive as represented by the steps <NUM> and <NUM>. In another embodiment, the controller may alternatively continue with one or more subsequent scans to repeat step <NUM>. In doing so it is continuing with aerosolizaton (albeit with a non-optimal drive scheme) and detecting further transition very quickly.

After reverting to steps <NUM> and <NUM>, if the numerical output value exceeds a "state change" trigger or threshold level of <NUM> (shown to the right of <FIG>) it shuts off power. Referring to the <FIG> logic flow, this is effectively step <NUM> being reduction to OFF, step <NUM> indicating no residual volume, and shut off in step <NUM>.

The start of a change of state can be seen close to <NUM> into the delivery cycle, and the controller shuts off at <NUM> i.e. <NUM> after a change began. The device would have taken another <NUM> to go completely dry i.e. it would have been fully dry at <NUM>.

The ASO removes power while there is still a very small volume of liquid, about 6µl, on the plate, therefore preventing the displacement of the plate from reaching levels in the range of <NUM> to <NUM> seen when completely dry.

The controller is preferably pre-set with values representing AP resonant frequency, a range of current for a wet state, and a range of currents for a dry state. This scan is completed in a very short period of time (<NUM>) to ensure minimal interruption to plume. This scan for step <NUM> is preferably run at reduced power, for example dropping from <NUM> V to <NUM> V, to avoid any damage to the plate.

In one example, mode of operation, after the first step <NUM> scan the controller may return to normal run mode at 15V, then stops after a period such as <NUM> and repeat the scan. It will keep on checking in this way until the ASO value returns below the lower threshold of <NUM>.

Whether there is one scan or multiple in series, by use of the two threshold levels the controller accurately determines if a step <NUM> is only noise (left peak of <FIG>) or a real transition (right peak of <FIG>).

The flow of <FIG> provides a reliable system, capable of durable high-power delivery utilizing existing nebulizer technology. It advantageously determines the status of the plate prior to informing the user of the dose completion. Advantageously, the nebulizer allows for high-power to be applied to aperture plates, where higher dose levels are required, whilst mitigating against the mechanical stress that occurs on the plate when transitioning from the wet to dry state. Furthermore, by eliminating or reducing this mechanical stress, the aperture plate life is extended with minimal changes in electrical parameters.

The accuracy of the modes of the invention is extremely important as it is often very important that the patient receive a complete dose of medication. The risk of false triggering can result in a partial dose delivery, which is highly undesirable.

The invention thereby prevents cracking or fracturing of aperture plates and thereby extends the life of aperture plates.

The algorithm was tested on <NUM> samples on a bench top test. The <NUM> samples went through <NUM> cycles using <NUM> % saline. Each sample was monitored to ensure that a full dose was delivered and after the dose was completed the aerosol generator would automatically power down. All samples passed. The test checks that the aerosol generator has a minimum life expectancy of <NUM> years equating to <NUM> hours of operation, or <NUM> doses. This was achieved for all of <NUM> test devices.

Referring to <FIG> for the steps <NUM> and <NUM> the controller records the drive current (as represented by shunt voltage) over an extended period, for the three frequencies by storing the data in a number of software buffers. It first determines the trend of the current draw for each frequency. Every <NUM> the current drawn at each frequency is recorded and added to the previous average (using the formula shown below). The intention of this is to create a running baseline from which any sudden changes can be identified.

The baseline formula is calculated as: <MAT> where:.

As shown in <FIG> a differential FIFO buffer is then used to record the difference from the baseline for a total of <NUM> readings (<NUM> x <NUM> =<NUM>). Every frequency (<NUM>, <NUM> & <NUM>) will have an individual buffer containing the previous <NUM> differential readings. For each frequency the difference buffer is totalled (G_sumOfDifference127/<NUM>/<NUM>). This is illustrated in <FIG>.

The controller continues to deliver AP drive power while the following condition is met: <MAT>.

Every <NUM> the combinedSumOfDifferences is compared to the predetermined maximum numerical output value (ASO_MAX_DIFFERENTIAL = <NUM>).

If this maximum value is exceeded, an alert is triggered and power is immediately reduced or removed from the drive stage. The setting to decide on whether power is completely removed immediately or reduced progressively depends on the nature of the therapy. If the medication is small in volume then the controller may operate to a setting of gradual reduction. Otherwise, the controller setting will cause immediate removal of power upon detection of start of transition to dry.

It will be appreciated that the invention in a very effective manner avoids risk of the AP becoming dry, providing an optimum performance for protecting the AP while ensuring that there is optimum delivery of medication. It prevents significant physical plate displacement when a device changes from wet state to the dry state. For example, at <NUM> operating frequency the displacement can change from <NUM> when wet to <NUM> or higher when dry.

Avoidance of this sudden change in displacement prevents plate deformation and damage.

The invention is not limited to the embodiments hereinbefore described which may be varied in detail within the scope of the claims. According to the invention, the residual liquid check step <NUM> is performed by continuing with the cyclic drive and monitoring of the numerical output.

Claim 1:
An aerosol delivery system comprising:
an aerosol generator comprising a vibrating aperture plate (<NUM>) and an actuator (<NUM>, <NUM>, <NUM>, <NUM>), wherein the aperture plate has a plurality of apertures extending between a first surface which is adapted to receive a liquid to be aerosolised and a second surface at which an aerosol is generated, and
a controller (<NUM>, <NUM>) for controlling operation of the aerosol generator,
wherein the controller is configured to perform in real time a method comprising the steps of:-
monitoring (<NUM>) at least one electrical characteristic of the aerosol generator as it is driven for vibration of the aperture plate;
detecting (<NUM>) a change in the electrical characteristic in response to a transition from a wet state to a dry state of the aperture plate; and
automatically modifying (<NUM>, <NUM>, <NUM>), during said transition, operation of the aerosol generator in response to said detected change, said modification including reducing power (<NUM>) applied to the aerosol generator,
characterized in that,
the controller (<NUM>) is configured to:
monitor a signal representing the electrical characteristic at each of a plurality of drive frequencies,
maintain data representing a trend in each signal and/or in a combination of the signals,
identify a start of a transition if a calculated value representing deviation from said trend or trends rises above a threshold,
continue (<NUM>) to monitor said change during the transition, including monitoring the aperture plate for presence of residual liquid on the aperture plate first surface,
wherein there is more than one threshold level, including a low level to provide an initial indication of potential transition, one or more intermediate level or levels which provide an indication of whether a rise is noise or due to a transition, and a top level which triggers immediate shut-down of the aerosol generator.