EP4672992A1

EP4672992A1 - Aerosol generating devices

Info

Publication number: EP4672992A1
Application number: EP24703818.5A
Authority: EP
Inventors: Eduardo Jose GARCIA GARCIA; Tetsuya Yoshimura; Franck Pourrat
Original assignee: JT International SA
Current assignee: JT International SA
Priority date: 2023-03-02
Filing date: 2024-02-13
Publication date: 2026-01-07
Also published as: JP2026509359A; CN120676881A; WO2024179828A1; KR20250140111A

Abstract

An aerosol generating device (10) comprises a heating chamber (18) configured to receive a consumable (100) comprising an aerosol generating substrate (102), a heating assembly operable to supply heat to the aerosol generating substrate during a usage session; and a controller (24). The controller is operable to monitor an observable indicative of a moisture content of the aerosol generating substrate during the usage session; and progressively provide to a user an indication of the moisture content of the aerosol generating substrate during the usage session. The observable may be a delay associated with an RC monitoring circuit having a predetermined time constant. A method of operating an aerosol generating device is also described.

Description

AEROSOL GENERATING DEVICES

Technical Field

The present disclosure relates generally to an aerosol generating device for heating an aerosol generating substrate to generate an aerosol for inhalation by a user of the aerosol generating device. The present disclosure is particularly applicable to a portable (hand-held) aerosol generating device. Such devices heat, rather than burn, an aerosol generating substrate, e.g., tobacco or other suitable materials, by conduction, convection, and/or radiation to generate an aerosol for inhalation by a user.

Technical Background

The popularity and use of reduced-risk or modified-risk devices (also known as aerosol generating devices or vapour generating devices) has grown rapidly in recent years as an alternative to the use of traditional tobacco products. Various devices and systems are available that heat or warm aerosol generating substances to generate an aerosol for inhalation by a user.

A commonly available reduced-risk or modified-risk device is the heated substrate aerosol generating device, or so-called heat-not-burn device. Devices of this type generate an aerosol or vapour by heating an aerosol generating substrate to a temperature typically in the range 150°C to 300°C. Heating the aerosol generating substrate to a temperature within this range, without burning or combusting the aerosol generating substrate, generates a vapour which typically cools and condenses to form an aerosol for inhalation by a user of the device.

Currently available aerosol generating devices can use one of a number of different approaches to provide heat to the aerosol generating substrate. One such approach is to employ an induction heating system. In such a device, an induction coil is provided in the device and an inductively heatable susceptor is provided to heat the aerosol generating substrate. Electrical energy is supplied to the induction coil when a user activates the device which in turn generates an alternating electromagnetic field. The susceptor couples with the electromagnetic field and generates heat which is transferred, for example by conduction, to the aerosol generating substrate and an aerosol is generated as the aerosol generating substrate is heated. Another heating approach is to use a resistive heating system. In such a device, a resistive heating element is provided to heat the aerosol generating substrate. Electrical energy is supplied to the resistive heating element when a user activates the device which in turn generates heat which is transferred, for example by conduction, to the aerosol generating substrate and an aerosol is generated as the aerosol generating substrate is heated.

In most such aerosol generating devices, the heater operates in a predetermined manner when commanded to start, for example in response to the user pushing a start button or in response to the device determining by means of an airflow sensor that the user has inhaled a puff through the device. The heater typically operates for a predetermined session duration, which may be measured via a pre-set time (for example 4.5 minutes) or may be measured via a pre-selected number of puffs. The session time or pre-set number of puffs is selected with the aim of exhausting all or substantially all of the aerosol generating substrate within the consumable.

Typically there may be some variation between different types of consumable, and even between different batches of the same type of consumable, for example due to manufacturing tolerances. This can lead to a situation where the session may be terminated even though there is still some usable aerosol generating substrate remaining within the consumable, which may be frustrating for the user. Conversely, it can alternatively lead to a situation where the aerosol generating device persists in heating the consumable even though all the aerosol generating substrate within the consumable has been exhausted. This can result in an unpleasant end to the session for the user (e.g. bad taste) and can increase the likelihood of the consumable burning within the heating chamber. It is an object of the present invention to alleviate one or more of the above problems.

Summary of the Invention

According to a first aspect of the invention there is provided an aerosol generating device comprising: a heating chamber configured to receive a consumable comprising an aerosol generating substrate; a heating assembly operable to supply heat to the aerosol generating substrate during a usage session; and a controller; wherein the controller is operable to: monitor an observable indicative of a moisture content of the aerosol generating substrate during the usage session; and progressively provide to a user an indication of the moisture content of the aerosol generating substrate during the usage session.

Consumables adapted for use with aerosol generating devices typically have a certain moisture content when new. This moisture content is depleted by heating, such that a used consumable typically has a much lower moisture content, and may have little or no moisture content remaining. The amount of moisture within an unheated consumable is thus indicative of an expected session length for that consumable. Displaying an indication of the moisture content of the aerosol generating substrate within the consumable to a user progressively throughout a session may thus give the user an indication of the likely remaining session length, as well as a visual representation of the consumption status of the aerosol generating substrate.

The controller may be operable to select a time duration for each usage session by determining, using the monitored observable, when to terminate the session. Since the moisture content is indicative of an expected session length, using the monitored observable to select a time duration for each usage session may result in a more accurate session length that is bespoke to each session.

The controller may be operable to terminate the session when the monitored observable indicates that the moisture content of the aerosol generating substrate is at or below a first predetermined threshold. The first predetermined threshold may be a moisture content in the range 1-6%, for example 5%

The controller may be operable to measure an initial value for the observable on insertion of the consumable into the heating chamber, prior to initiation of heating. The initial value of the observable may thus be representative of an initial moisture content of the aerosol generating substrate. The controller may further be operable to compare the measured initial observable value to a further predetermined threshold. The controller may be operable to prevent operation of the heating assembly if the measured initial value for the observable is less than or equal to the further predetermined threshold, which may be the same as the first predetermined threshold. Alternatively, the controller may be operable to cause operation of the heating assembly only if the measured initial value for the observable equals or exceeds the further predetermined threshold, which may be an observable value representative of an expected minimum initial moisture content. An unexpectedly low initial moisture content (i.e. an initial value for the measured observable which indicates a moisture content below the expected minimum initial moisture content) may indicate that the consumable has already been heated. Thus comparison of the measured initial observable value with a further predetermined threshold may allow the aerosol generating device to prevent further heating of already heated consumables.

The controller may be operable to generate an indication that a session end is approaching. Such an indication may be provided to the user in addition to the progressive indication of moisture content that is provided throughout the session. The indication that the session end is approaching may be provided when the monitored observable indicates that the moisture content of the aerosol generating substrate is at or below a second predetermined threshold, wherein the second predetermined threshold is reached prior to the first predetermined threshold. The second threshold may be in the range 8-3%, or 7-5%, or 6-5% The second predetermined threshold may equate to an estimated remaining session duration, for example 10%, 15% or 20% (e.g. 30 seconds, 45 seconds or 1 minute) remaining session duration.

The observable may be monitored periodically throughout the usage session. The observable may be measured every 5-100ms, or every 10-50ms, for example.

The aerosol generating device may further include a monitoring circuit having a predetermined time constant, wherein the observable is a time delay associated with the monitoring circuit.

The time constant, r(tau), of a circuit is a parameter characterizing the response of the circuit to a step input. For example, in an RC circuit composed of a single resistor and capacitor, the time constant T (in seconds) = RC, where R is the resistance (in ohms) and C is the capacitance (in farads). A consumable has a non-zero capacitance, such that the introduction of a consumable into a monitoring circuit having a known time constant will change the capacitance of the circuit, and thus will change the time constant of that circuit. The consumable does not need to be in physical contact with the monitoring circuit in order to have an effect on the capacitance of the circuit.

The capacitance of a consumable is dependent, at least in part, on the moisture content of the consumable. Since the time constant of the monitoring circuit varies with capacitance, the time constant of the monitoring circuit represents an observable indicative of the moisture content of a consumable present in a heating chamber of an aerosol forming device. The change in moisture content of a consumable throughout a session can thus be observed by monitoring a change in the time constant of a monitoring circuit having a known time constant.

The monitoring circuit may comprise an RC circuit having a predetermined resistance and capacitance. An RC circuit has a simple and low cost construction, and the time constant of such a circuit is computationally straightforward to measure. It will be appreciated that the wiring of a circuit necessarily has a certain capacitance, and thus the use of an RC circuit does not necessarily imply the presence of a capacitor.

The controller may be operable to supply a signal to an input of the monitoring circuit, and to receive an altered signal from an output of the monitoring circuit. The controller may be operable to determine the time delay from the altered signal.

The observable may comprise a rise and/or fall time associated with the signal. As used herein, the term “rise time” refers to the time (in seconds) for a signal (e.g. a voltage) to change from a specified low value to a specified high value. Similarly, the term “fall time” refers to the time taken for the signal to change from a specified high value to a specified low value. Thus, rather than monitoring the time constant itself, the controller may be operable to monitor changes in rise and/or fall time. The relative change in rise time between a state where a consumable is present in the heating chamber and a state where a consumable is absent from the heating chamber may be greater than a relative change between the known time constant of the monitoring circuit and the altered time constant due to the presence of the consumable. Thus utilising the rise and/or fall time may improve the accuracy of the determination.

The signal that is input to the monitoring circuit may comprise a voltage pulse, such as a square wave. The controller may be operable to input the signal to the monitoring circuit periodically. Use of a sharp edged signal such as a pulse may make observation of the rise and/or fall time more straightforward.

The monitoring circuit, such as the RC circuit, may be electrically connected to a heating circuit of the heating assembly. The change in time constant of the monitoring circuit is typically small, particularly when the consumable is not in physical contact with the monitoring circuit. The inventors have found that electrically connecting the monitoring circuit to the heating circuit of the aerosol forming device increases the magnitude of the change in time constant, and thus increases the sensitivity of the detection. Since a heating circuit is necessarily provided in an aerosol forming device, this increased sensitivity does not come at the cost of increased complexity or additional manufacturing expense.

It is possible to measure during a heating session; however, it is preferable not to measure at the exact same time as a power pulse of the heater. Measurements of the observable may thus be timed to fall in between heating pulses, and/or the heating assembly may be turned off when the observable is monitored. This may reduce the likelihood of damage to the monitoring circuit from the voltage supplied to the heating assembly and/or may improve the monitoring accuracy by reducing interference in the monitoring signal. The observable may be measured every 5-100ms, or every 10- 50ms, for example.

The aerosol generating device may be switchable between a heating mode in which a voltage is supplied to the heating circuit and a time delay monitoring mode in which a signal is supplied to the monitoring circuit. The controller may be operable to only use the time delay associated with the monitoring circuit to determine the moisture content of the consumable when the aerosol generating device is in the time delay monitoring mode. This may prevent the monitoring circuit being damaged when a voltage supplied to the heating circuit in the heating mode. The aerosol generating device may comprise a first switch that may be closed in the time delay monitoring mode and open in the heating mode and a second switch that may be closed in the heating mode and open in the time delay monitoring mode.

The heating circuit may comprise a resistive heater, such as a resistive wire or a thin film heater.

The heating chamber may be substantially cup shaped, and may have an open first end operable to receive the consumable. For example, the heating chamber may comprise a substantially cylindrical side wall that is open at a first end, so defining the open first end, and closed at a second end, defining a base to the heating chamber. The resistive heater may be external to the heating chamber, and may be wrapped around the heating chamber.

The aerosol generating device may further comprise a user interface having an output display, wherein the aerosol generating device is operable to progressively provide the indication to the user using the display. The progressive indication may be provided to the user in any suitable manner. For example, the display may comprise one or more lighting elements, such as LEDs, and the lighting elements may be initially illuminated at the start of the session, and may progressively be switched off as the session progresses in order to provide a visual indication of the monitored moisture content. In an alternative example, the output display may comprise a display screen, and a graphic illustration may be provided to the user via the display screen. The progressive indication may thus allow the user to visually observe the depth of consumption of the consumable.

According to a second aspect of the invention we provide a method of operating an aerosol generating device that comprises: a heating chamber configured to receive a consumable comprising an aerosol generating article; a heating assembly operable to supply heat to the aerosol generating substrate during a usage session; and, a controller; wherein the method comprises: monitoring an observable indicative of a moisture content of the aerosol generating substrate during the usage session; and progressively providing to a user an indication of the moisture content of the aerosol generating substrate during the usage session.

The method may further comprise determining, using the monitored observable, when to terminate the session.

The method may further comprise providing an indication to the user that a session end is approaching.

The method may further comprise measuring an initial value for the monitored observable, prior to initiation of heating, and the controller comparing the measured initial value with a further predetermined threshold. The controller may use the comparison to determine whether or not to initiate heating.

The method may be implemented in the aerosol generating device of the first aspect of the invention and may further comprise any of the optional features of the first aspect of the invention.

Features of the above aspects of the invention may be combined together, as well as with features selected from the description, in any order unless expressly stated otherwise.

Brief Description of the Drawings

The present invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic cross-sectional view of an aerosol generating system comprising an aerosol generating device and a consumable positioned in a heating chamber of the aerosol generating device;

Figure 2 is a schematic illustration of a thin film heater suitable for use in the aerosol generating device of Figure 1 ;

Figure 3 shows the thin film heater of Figure 2 wrapped around a heating chamber;

Figure 4 schematically illustrates variation in a measured observable indicative of moisture content over time; Figure 5 illustrates an exemplary monitoring circuit suitable for use in the aerosol generating device of Figure 1 in a first, time delay monitoring mode, and in a second, heating mode;

Figure 6 schematically illustrates a time delay monitoring method; and

Figure 7 illustrates variations in measured rise and/or fall time before, during and after insertion of a consumable into a heating chamber;

Figure 8 illustrates variations in measured rise and/or fall time before, during and after insertion of an already-heated consumable into a heating chamber; and

Figure 9 is a close up of a region of the graph shown in Figure 7.

Detailed Description

Referring initially to Figure 1 , there is shown diagrammatically an example of an aerosol generating system 1. The aerosol generating system 1 comprises an aerosol generating device 10 and a consumable 100, also referred to herein as an aerosol generating article, for use with the device 10. The aerosol generating device 10 can have any shape that is sized to fit the components described in the various embodiments set out herein and to be comfortably held by a user unaided, in a single hand.

A first end 14 of the aerosol generating device 10, shown towards the bottom of Figure 1 , is described for convenience as a distal, bottom, base or lower end of the aerosol generating device 10. A second end 16 of the aerosol generating device 10, shown towards the top of Figure 1 , is described as a proximal, top or upper end of the aerosol generating device 10. During use, the user typically orients the aerosol generating device 10 with the first end 14 downwards and/or in a distal position with respect to the user’s mouth and the second end 16 upwards and/or in a proximal position with respect to the user’s mouth.

The aerosol generating device 10 comprises a heating chamber 18. The heating chamber 18 defines an interior volume in the form of a cavity 20 having a substantially cylindrical cross-section. The cavity 20 of the heating chamber 18 is open towards the second end 16 of the aerosol generating device 10. The heating chamber 18 has a longitudinal axis defining a longitudinal direction and is formed of a metal material, such as stainless steel. A heating assembly 15 including a heating element 22 is located in proximity to the heating chamber 18 and is operable to provide heat to the heating chamber. The heating element 22 is comprised within a heating circuit 40, which is electrically connected to a controller 24.

The aerosol generating device 10 further comprises a power source 26, for example one or more batteries which may be rechargeable. The controller 24 couples the power source 26 to the heating element 22. The controller 24 may also be connected to a user interface 23 comprising inputs such as a power button for receiving commands from a user and/or outputs such as indicator lights, a display screen or an audible or vibratory alarm for providing information to the user. The controller 24 may also be interfaced with an antenna 25 for wireless communication with a remote device such as the user’s smartphone, which can be used for input and output, as well as for relaying data between the aerosol generating device 10 and its manufacturer.

The heating chamber 18, and specifically the cavity 20, is arranged to receive a correspondingly shaped generally cylindrical or rod-shaped aerosol generating article 100. Typically, the aerosol generating article 100 comprises a pre-packaged aerosol generating substrate 102. The aerosol generating article 100 is a disposable and replaceable article (also known as a “consumable”) which may, for example, contain tobacco as the aerosol generating substrate 102. The aerosol generating article 100 has a proximal end 104 (or mouth end) and a distal end 106. The distal end 106 is inserted into the heating chamber 18 of the aerosol generating device 10 so that at least the aerosol generating substrate 102 is contained within the heating chamber 18. The aerosol generating article 100 further comprises a mouthpiece segment 108 positioned downstream of the aerosol generating substrate 102. At least part of the mouthpiece segment 108 projects from the heating chamber 18 so that the proximal end 104 of the aerosol generating article 100 is accessible to be taken into the mouth of a user. When the aerosol generating device 10 applies heat to the aerosol generating article 100, heated vapour is emitted from the aerosol generating substrate 102. As inhalation by the user draws air towards the proximal end 104 of the aerosol generating article 100, the vapour cools and condenses as it passes through the mouthpiece segment 108 to form an aerosol with characteristics suitable for inhalation. The mouthpiece segment 108 may further comprise a filter (not shown) to remove particles or drops above a certain size from the airstream. The aerosol generating substrate 102 and the mouthpiece segment 108 are arranged in coaxial alignment inside a wrapper 110 (e.g., a paper wrapper) to hold the components in position to form the rod-shaped aerosol generating article 100. The wrapper 110 typically does not cover the ends 104, 106 of the aerosol generating article 100 in order that air can flow through the aerosol generating article 100 from the distal end 106 to the proximal end 104.

The aerosol generating substrate 102 may be provided as a solid or paste type material in shredded, pelletized, powdered, granulated, strip or sheet form, optionally a combination of these. The aerosol generating substrate may include tobacco, for example in dried or cured form, in some cases with additional ingredients for flavouring or producing a smoother or otherwise more pleasurable experience. In some examples, the aerosol generating substrate 102 such as tobacco may be treated with a vaporizing agent. The vaporizing agent may improve the generation of vapor from the aerosol substrate. The vaporizing agent may include, for example, a polyol such as glycerol, or a glycol such as propylene glycol. In some cases, the aerosol generating substrate may contain no tobacco, or even no nicotine, but instead may contain naturally or artificially derived ingredients for flavouring, volatilization, improving smoothness, and/or providing other pleasurable effects. The aerosol substrate 12 such as tobacco may comprise one or more humectants to retain moisture, such as glycol(s).

Before use, the aerosol generating substrate 102 has an initial moisture content, which may depend on its design, shape, packaging, type, flavour, etc. As used herein, the “moisture content” refers to the amount of water and any other humectants that may be present in the aerosol generating substrate 102, and may be defined by mass (e.g. a gravimetric water content), by volume (e.g. a volumetric water content) or any other measurable physical quantity of the aerosol generating substrate. It will be understood that in practice, the moisture content may vary slightly between consumables. Typically, the moisture content of a tobacco stick before use (i.e. the initial moisture content) is a value around 15%. After use, the moisture content of the tobacco stick typically decreases to a value around 5%. In the illustrated embodiments of the invention, the heating chamber 18 comprises an open first end 28 and a closed base 30 at a second end. That is, the heating chamber 18 is cup shaped. This can ensure that air drawn from the open end 28 is guided around the consumable towards the base 30, at which point the air is drawn through the aerosol generating substrate 102.

As noted above with regards to Figure 1 , the aerosol generating device 10 comprises a heating chamber 18 configured to receive a consumable 100 comprising an aerosol generating substrate 102, a heating assembly 15 configured to supply heat to the heating chamber 18, and a controller 24. The controller 24 is operable (inter alia) to monitor an observable indicative of a moisture content of the aerosol generating substrate during the usage session, and to progressively provide to a user an indication of the moisture content of the aerosol generating substrate during the usage session.

Figure 2 shows one example of a heating element 22 for use in a heating circuit 40 of the type shown in Figure 1. The heating element 22 is a resistive heating element, specifically a thin film heater. The heating element includes a heating track 32 embedded within a thin film and a pair of contacts 34, 36 permitting connection to the controller 24.

Figure 3 shows the heating chamber 18 of an aerosol generating device 10 in more detail. The heating chamber 18 has a cylindrical side wall 38 connecting the open first end 28 and the closed base 30. The heating element 22 is wrapped around an exterior surface of the side wall 38.

Figure 4 illustrates diagrammatically how a measured observable may vary during a usage session. Figure 4 illustrates an initial state 400 of an aerosol forming device in which no consumable is present in the heating chamber. During this initial time period, measurements of the observable indicate a moisture content of or around zero, since no consumable is present in the heating chamber.

During a second time period 402 a consumable is inserted into the heating chamber. Measurements of the observable indicate a sharp rise in moisture content. Once the consumable is fully inserted a first measured observable value mi may be considered indicative of an initial moisture content of the consumable. As noted above, the first measured observable value mi may equate to a moisture content within the aerosol generating substrate of around 15%.

Heating of the consumable begins at a start time 404 following insertion of the consumable, and continues for a session 406. The observable indicative of moisture content is measured during the session 406, and generally decreases throughout the session as water and/or humectant within consumable is vaporised during heating. The session ends when the measured observable reaches a second value m₂ that indicates that the water and/or humectant within consumable has been substantially exhausted. This does not necessarily mean that the moisture content is zero; rather than the moisture content has fallen below a first predetermined threshold, below which the quality of the vaping experience would be reduced. Such a threshold may be a moisture content of 6% or less, or 5% or less, or 4% or less, or lower, for example.

Once the first predetermined threshold is reached heating ceases at time 408, and hence the second value of the measured observable m₂, which is indicative of a residuary moisture content, remains constant during a post-session period 410.

To assist in monitoring the observable the aerosol generating device 10 illustrated in Figure 1 further comprises a monitoring circuit 50. The monitoring circuit may be comprised within the controller 24, or in signal communication with the controller 24. The monitoring circuit may be operable to monitor any observable that is indicative of a moisture content of the aerosol generating substrate. One example of such an observable is a time delay associated with a monitoring circuit having a known time constant.

Referring now to Figure 5, an exemplary monitoring circuit 50 is shown in more detail, together with the heating circuit 40. The monitoring circuit 50 includes a resistor R and an optional capacitor C, and thus constitutes an RC circuit. The monitoring circuit comprises an electrical input 52 and an electrical output 54 connected to a microcontroller pC. It will be appreciated that the microcontroller pC may be comprised within the controller 24, or may be separate from the controller 24 and under the command of the controller 24. The monitoring circuit 50 is shown in Figure 5 connected to the heating circuit 40, and specifically to a contact 34 of the heating circuit 40. This is not strictly necessary however, and the monitoring circuit 50 may be connected to another part of the heating assembly 15 if required, or to another part of the aerosol forming device, such as the heating chamber 18.

The resistor R is connected between the input 52 and the output 54 and is connected in parallel with the capacitor C, which is connected to ground. The resister R has a value in the range 500kQ - 10MQ. The capacitor C is optional, but if present, assists in cleaning the signal.

The monitoring circuit 50 further comprises a first switch 56 connected between the monitoring circuit 50 and the heating circuit 40, and specifically between the monitoring circuit 50 and a contact 34 of the heating track 32. When the first switch 56 is in a closed position the monitoring circuit is electrically connected to the heating circuit 40. In this state, the aerosol generating device 10 can be considered to be in a time delay monitoring mode. When the first switch 56 is in an open position the monitoring circuit 50 is not electrically connected to the heating circuit 40, such that no current may flow between the monitoring circuit 50 and the heating circuit 40.

The heating circuit 40 includes a second switch 42 located between the heating element 22 and an input voltage (e.g. supplied by the power supply 26), and a third switch 44 located between the heating element 22 and ground. When the second and third switches 42, 44 are closed the heating element 22 is electrically connected to the input voltage such that a voltage may be supplied to the heating element to cause the heating element 22 to generate heat. In this state, the aerosol generating device can be considered to be in a heating mode. When the second and third switches 42, 44 are open the heating element 22 is disconnected from the input voltage, and cannot generate heat.

The aerosol generating device may be in the heating mode and the time delay monitoring mode simultaneously, such that all three switches 56, 42, 44 are closed at the same time. In the present example however, the aerosol generating device is operable to switch between the time delay monitoring mode and the heating mode, such that time delay monitoring does not occur during heating. Referring now to Figure 6, the operation of the aerosol generating device 10 in a time delay monitoring mode will now be described. In the time delay monitoring mode, the microcontroller pC is operable to send a signal 60 to the input 52 of the monitoring circuit 50 (e.g. from a pin labelled “send”). The signal 60 is a voltage pulse, such as a square wave defined by a low value 62 and a high value 64, and may be periodically repeating.

The signal passes through the monitoring circuit 50 and also the heating circuit 40, since in the time delay monitoring mode the switch 56 is closed. The signal is received from the output 54 by the microcontroller pC (e.g. at a pin labelled “return”); however, the received signal is altered due to the capacitance in the system. This alteration is characterised by the rise time d, which constitutes the time for the signal to change from the low value 62 to the high value 64, and/or the fall time, which constitutes the time for the signal to return from the high value 64 to the low value 62. In the original signal these changes are effectively instantaneous, but in the received signal there is a delay as the signal rises from low to high, and as the signal falls from high to low. The rise time is proportional to the time constant of the circuit. Similarly, the fall time is also proportional to the time constant of the circuit.

Figure 6 includes a graph 70 which plots rise and fall time 72 against time 74, thus allowing variations in rise and fall time over time to be perceived. It can be seen that the rise time di at a first time 76 is smaller than the rise time d₂ at a second time 78. This indicates that the capacitance in the system has changed between the first time and the second time. In particular, the capacitance in the system has increased between the first time 76 and the second time 78.

Figure 7 illustrates how the variations in rise and/or fall time shown in Figure 6 can be used to determine the moisture content of a consumable 100 within a heating chamber 18 of an aerosol generating device 10. Figure 7 includes a graph plotting measured rise and fall time 72 (in this example, the rise and fall time is represented by the transmission time of the signal from the input to the output) against time 74 between an initial time to and an end time t₅. During a first time period 82 between time to and time ti , a reference transmission time, such as an average rise and fall time, is a first reference time ai. During this time period no consumable is present in the heating chamber. The rise and fall time experienced by a signal during this time period is thus indicative of the time constant T of the monitoring circuit absent any consumable, and can be considered to be a predetermined time constant.

At time ti the measured rise and fall time increases, and averages a third reference time a₃ for a second time period 84 between time ti and time t₂. The increase in rise and fall time indicates that a capacitance of the system has increased, and thus that the time constant T of the monitoring circuit has changed, and in particular, increased. During this time period a consumable is being inserted into the heating chamber 18 (i.e. is in the process of being inserted and is not yet fully inserted but is instead partially inserted and/or in close proximity to the heating chamber). The increased capacitance during this time period is due to the presence of the consumable 100 in proximity to and/or partially within the cavity of the heating chamber, and potentially may also be influenced by contact between the consumable and the fingers of a user.

At time t₂ the measured rise and fall time decreases, and averages a second reference time a₂ for a third time period 86 between time t₂ and time t₃. The decreased rise and fall time indicates that a capacitance of the system has decreased, and thus that the time constant T of the monitoring circuit has changed, and in particular, decreased. However, it is noted that the value a₂> ai, meaning that the capacitance in the system, and so the time constant of the system, is greater than the predetermined time constant T of the monitoring circuit absent any consumable. During this time period a consumable 100 is present within the heating chamber 18 of the aerosol generating device. The consumable is heated during this time, and thus the third time period 86 may also be considered a vaping session.

A close up of the measurements made during the third time period 86 is shown in Figure 9. It can be seen that although the measured rise and fall time in the third period 86 averages the second reference time a₂, in fact the measured rise and fall time generally decreases 94 throughout the third time period 86. This is because, as discussed above in relation to Figure 4, the rise and fall time is indicative of the moisture content of the consumable within the heating chamber, and the moisture content decreases over the course of the session as water and/or humectant within the aerosol forming substrate is depleted.

At time t₃ the rise and fall time increases, and averages a further rise/fall time for a fourth time period 88 between time t₃ and time t4. The fourth time period 88 represents the reverse of the second time period 84, in that during the fourth time period a consumable is being removed from the heating chamber. The average rise and fall time during the fourth time period is thus similar to and approximately the same as the average rise and fall time a₃ during the second time period.

Similarly, the average rise and fall time in a fifth time period 90 between times t₄ and t₅ is similar to and approximately the same as the first average rise and fall time ai during the first time period, indicating that the consumable is no longer present in the heating chamber.

The change in time constant of the monitoring circuit, measured, for example, using the change in time delay, such as the change in rise and/or fall time as discussed above, can thus be used to monitor the moisture content of a consumable present in the heating chamber.

In one example, the controller would be operable to begin monitoring the observable only after stick insertion and heating has started. A state machine or scheduler or time interrupt or watchdog timer would disconnect the heater and connect the measurement lines every 10-50 ms in order to periodically acquire measurements of the observable. Data acquired may be stored in a memory (internal to the controller or outside), and signal processing of the stored data may be used to provide the indication of the moisture level depletion.

The aerosol generating device 10 is configured to use the monitored observable to display an indication to a user of the device of the moisture content of the consumable progressively throughout a session. The indication may be displayed to the user on a user interface 23 of the aerosol generating device, and/or may be displayed on a connected device, such as a user’s smartphone, for example via an app paired with the aerosol generating device. The indication may provide the user with a visual representation of the depth of consumption of the consumable progressively throughout the session, such that at any given time the user may have an instantaneous representation of how much aerosol generating substrate has been consumed.

For comparison, Figure 8 illustrates the variations in rise and/or fall time when a consumable that has already been heated is inserted into the heating chamber of an aerosol forming device. During a first time period 96 and during a third time period 97 no consumable is present in the heating chamber. However, during a second time period 98 a consumable 100’ is inserted into the heating chamber. The consumable 100’ has already been heated, such that the measured observable value indicative of its moisture content is approximately at or below the observable value m₂ indicative of a residuary moisture content. The controller of the aerosol generating device may thus be able to determine, using the measured observable, that a consumable inserted into the heating chamber has already been used. In such a situation the controller may prevent further heating of an already heated stick. For instance, the controller may prevent operation of the heating circuit in such a circumstance.

Although exemplary embodiments have been described in the preceding paragraphs, it should be understood that various modifications may be made to those embodiments without departing from the scope of the appended claims. For example, although the invention has been primarily described in connection with a resistive heating assembly, the invention may find utility in connection with another type of heating assembly, such as an inductive heating assembly. Thus, the breadth and scope of the claims should not be limited to the above-described exemplary embodiments.

Claims

1. An aerosol generating device (10) comprising: a heating chamber (18) configured to receive a consumable (100) comprising an aerosol generating substrate (102); a heating assembly operable to supply heat to the aerosol generating substrate during a usage session; and a controller (24); wherein the controller is operable to: monitor an observable indicative of a moisture content of the aerosol generating substrate during the usage session; and progressively provide to a user an indication of the moisture content of the aerosol generating substrate during the usage session.

2. The aerosol generating device of claim 1 , wherein the controller is operable to select a time duration for each usage session by: determining, using the monitored observable, when to terminate the session.

3. The aerosol generating device of claim 1 or claim 2, wherein the controller is operable to terminate the session when the monitored observable indicates that the moisture content of the aerosol generating substrate is at or below a first predetermined threshold.

4. The aerosol generating device of any preceding claim, wherein the controller is operable to generate an indication that a session end is approaching when the monitored observable indicates that the moisture content of the aerosol generating substrate is at or below a second predetermined threshold, wherein the second predetermined threshold is reached prior to the first predetermined threshold.

5. The aerosol generating device of any preceding claim, wherein the observable is monitored periodically throughout the usage session.

6. The aerosol generating device of any preceding claim, further including a monitoring circuit (50) having a predetermined time constant, wherein the observable is a time delay associated with the monitoring circuit.

7. The aerosol generating device of claim 6, wherein the controller (24) is operable to supply a signal to an input of the monitoring circuit (50), and to receive an altered signal from an output of the monitoring circuit, the controller being operable to determine the time delay from the altered signal.

8. The aerosol generating device of claim 7, wherein the monitoring circuit (50) comprises an RC circuit having a predetermined resistance and capacitance, and the observable is a rise and/or fall time associated with the signal.

9. The aerosol generating device of claim 8, wherein the RC circuit is electrically connected to a heating circuit (40) of the heating assembly.

10. The aerosol generating device of any preceding claim, wherein the heating assembly is turned off when the observable is monitored.

11 . The aerosol generating device of any one of claims 6 to 9, wherein the aerosol generating device (10) is switchable between a heating mode in which a voltage is supplied to the heating assembly and a time delay monitoring mode in which a signal is supplied to the monitoring circuit (50) .

12. The aerosol generating device of claim 11 , further comprising a first switch (56) that is closed in the time delay monitoring mode and open in the heating mode and a second switch (42) that is closed in the heating mode and open in the time delay monitoring mode.

13. The aerosol generating device of any preceding claim, wherein the heating circuit comprises a resistive heater (22), and preferably a thin film heater.

14. The aerosol generating device of any preceding claim, further comprising a user interface (23) having an output display, wherein the aerosol generating device is operable to progressively provide the indication to the user using the display.

15. The aerosol generating device of any preceding claim, wherein the controller is further operable to: measure an initial value for the observable, the initial value being indicative of an initial moisture content; compare the measured initial value with a further predetermined threshold, and determine whether to permit activation of the heating assembly based on the comparison.

16. The aerosol generating device of claim 15, wherein the controller is operable to prevent operation of the heating assembly if the measured initial value for the observable is at or below the further predetermined threshold.

17. The aerosol generating device of claim 15 or 16, wherein the further predetermined threshold is the same as the first predetermined threshold or the second predetermined threshold.

18. A method of operating an aerosol generating device that comprises: a heating chamber (18) configured to receive a consumable comprising an aerosol generating article; a heating assembly operable to supply heat to the aerosol generating substrate during a usage session; a controller (24); wherein the method comprises: monitoring an observable indicative of a moisture content of the aerosol generating substrate during the usage session; and progressively providing to a user an indication of the moisture content of the aerosol generating substrate during the usage session.