WO2024153495A1

WO2024153495A1 - A method of monitoring an aerosol generating article and an aerosol generating system

Info

Publication number: WO2024153495A1
Application number: PCT/EP2024/050381
Authority: WO
Inventors: Grzegorz Aleksander PILATOWICZ; Alec WRIGHT; Hajime Fujita
Original assignee: JT International SA
Current assignee: JT International SA
Priority date: 2023-01-16
Filing date: 2024-01-09
Publication date: 2024-07-25
Anticipated expiration: 2025-07-16
Also published as: KR20250131802A; EP4651746A1

Abstract

A method of monitoring an aerosol generating article (1) is described. The article (1) comprises a capacitor (6). The capacitor (6) comprises an electrolyte which, when heated, generates an aerosol for inhalation by a user, a pair of electrodes, and a porous separator between the electrodes. The method comprises estimating or determining the amount of electrolyte by applying to one of the pair of electrodes (16) an alternating current AC signal.

Description

A METHOD OF MONITORING AN AEROSOL GENERATING ARTICLE AND AN AEROSOL GENERATING SYSTEM Technical Field The present disclosure relates generally to a method of monitoring an aerosol generating article, and in particular to an aerosol generating article which is adapted to be received in an aerosol generating device for generating an aerosol for inhalation by a user. As part of an aerosol generating system of the present disclosure, the aerosol generating article may be received in an aerosol generating device that includes a controller adapted to implement the method. The present disclosure is particularly applicable to a portable (hand-held) aerosol generating device. Technical Background Devices which heat, rather than burn, an aerosol generating material to produce an aerosol for inhalation have become popular with consumers in recent years. A commonly available reduced-risk or modified-risk device is the heated material aerosol generating device, or so-called heat-not-burn device. Devices of this type generate an aerosol or vapour by heating an aerosol generating material to a temperature typically in the range 150°C to 300°C. This temperature range is quite low compared to an ordinary cigarette. Heating the aerosol generating material to a temperature within this range, without burning or combusting the aerosol generating material, generates a vapour which typically cools and condenses to form an aerosol for inhalation by a user of the device. Such devices may use one of a number of different approaches to provide heat to the aerosol generating material. All approaches for heating the aerosol generating material require some sort of power source such as a battery, which adds to the size and weight of the device. Embodiments of the present disclosure seek to provide a power source in the aerosol generating article which may be used to supplement or partially replace the power source in the device. This may result in a smaller and lighter device, which is P51436WO-5891 beneficial for the user, while maintaining accurate control of the heating of the aerosol generating material and optimising the characteristics of the generated aerosol. Summary of the Disclosure According to a first aspect of the present disclosure, there is provided a method of monitoring an aerosol generating article comprising a capacitor, the capacitor comprising: an electrolyte which, when heated, generates an aerosol for inhalation by a user (i.e., the electrolyte is aerosolisable); a pair of electrodes; and a porous separator between the electrodes; the method comprising estimating or determining the amount of electrolyte by applying to one of the pair of electrodes an alternating current (AC) signal. The capacitor may have any suitable construction, but in a preferred embodiment it is an electric double-layer supercapacitor. The large capacitance of an electric double- layer supercapacitor may lead to an increase in the efficiency of aerosol generation during discharging and charging. The pair of electrodes typically comprises a positive electrode and a positive electrode. The AC signal may be applied across the positive electrode and the negative electrode. The electrodes and the separator are immersed in the electrolyte. Electrical charge is stored in the electrical field between the electrodes and the capacitance is a function of the surface area of the electrodes, the distance between them, and the dielectric constant of the separator material. The capacitor has a higher power density than a conventional power source such as a battery. When the capacitor is charged by an external circuit connected to the pair of electrodes, cations in the electrolyte migrate toward the negative electrode and the anions migrate to the positive electrode, while the electrons travel through the external circuit from the negative to the positive electrode. Two layers of charge with opposite polarity (an electric double- layer) are therefore formed at the interfaces with the electrodes. When charging P51436WO-5891 finishes, positive electric charges on the positive electrode and anions in the electrolyte attract each other while negative electric charges on the negative electrode and cations in the electrolyte attract each other in order to stabilize the double layers on the electrodes. A stable voltage is generated. When the capacitor is discharged, the reverse processes happen. Each electrode may comprise at least one carbon-based electrode layer, for example, a layer of porous charcoal material or activated carbon which has a high specific surface area per volume and compatibility with the proposed electrolyte. Each electrode may further comprise a current collector, which may comprise a metal foil layer, for example, an aluminium foil layer. Each current collector may encourage electron travelling via the external circuit. A carbon-based electrode layer may be positioned adjacent one or both sides of a current collector. Each carbon-based electrode layer may be formed as a coating. Such electrodes may be manufactured relatively easily and cheaply using materials that are already known to be used in aerosol generating articles. Each current collector may encourage electron travelling via the external circuit. As will be understood by one of ordinary skill in the art, the electrolyte fulfils two functions. Firstly it permits the cation and anion migration that occurs when the capacitor is charged or discharged, and secondly, when heated, it forms an aerosol that is safe to be inhaled by the user and has good characteristics. The electrolyte should therefore be selected accordingly. The electrolyte is an aerosolisable electrolyte, i.e., capable of being converted into an aerosol by heating, which aerosol is then inhaled by the user. Heating the capacitor therefore results in the electrolyte that is contained within the capacitor being converted into an aerosol and the aerosolised electrolyte is then inhaled by the user. The electrolyte is preferably a food-grade electrolyte and may comprise one or more of sodium chloride, sodium citrate, sodium bicarbonate, potassium chloride, calcium lactate, calcium carbonate, tricalcium phosphate, magnesium citrate, magnesium carbonate, citric acid, tartaric acid, benzoic acid, glycerol and any suitable equivalents, for example. The electrolyte may optionally P51436WO-5891 include a gelling agent such as polyvinyl alcohol, gellan gum or xanthan gum, for example. In one example, the electrolyte may comprise sodium chloride and glycerol, and optionally polyvinyl alcohol as a gelling agent. Such an electrolyte has been found to permit cation and anion migration and is also safe for inhalation by the user. When all of the electrolyte has been vapourised, the capacitor may not be further discharged or charged, and the article may need to be disposed of appropriately or re- filled with electrolyte. The separator must provide dielectric separation between the pair of oppositely charged electrodes. The separator also stores electrolyte in its pores and permits the passage of cations and anions during the charging and discharging processes. The separator may comprise any suitable material. The separator may comprise a plant derived material and in particular may comprise a tobacco material, for example, a porous tobacco sheet, or it may comprise any suitable cellulose- or polypropylene-based material. When heated, the separator material may release one or more volatile compounds. The volatile compounds may include nicotine or flavour compounds such as tobacco or other flavouring. The aerosol generating article may further comprise any type of solid or semi-solid material downstream of the capacitor in an aerosol flow path. Example types of solid or semi-solid material include crumb, powder, granules, pellets, shreds, strands, particles, gel, strips, loose leaves, cut filler, porous material, foam material or sheets. The material may comprise plant derived material and in particular, may comprise tobacco material. The aerosol generated by heating the electrolyte of the capacitor will flow through the solid or semi-solid material, which may be positioned between the capacitor and a filter segment or mouthpiece through which the user inhales the aerosol, for example. The solid or semi-solid material may release one or more volatile compounds which may add flavour and nicotine to the aerosol, for example. Any heating provided by the capacitor also heats or warms the solid or semi-solid material which may promote the release of volatile compounds. P51436WO-5891 The aerosol that is inhaled by the user consists essentially of the vapourised or aerosolised electrolyte and optionally one or more volatile compounds that may be released by the separator material and/or the downstream solid or semi-solid material. The capacitor may have any suitable construction such as a spiral wound (or “jelly roll”) construction that may be substantially cylindrical or flattened so that it has more of a cuboid shape that might be more suitable for a flat-format article, a prismatic construction, a folded or serpentine construction, or a stacked construction, for example. In one embodiment a layered capacitor substrate may comprise a first electrode, a separator adjacent the first electrode, and a second electrode adjacent the separator, i.e., so that the separator is sandwiched between the first and second electrodes, and more particularly between a pair of carbon-based electrode layers. The first electrode may be a positive electrode and the second electrode may be a negative electrode or vice versa. Such a substrate may be rolled or folded into a suitable shape while maintaining an air gap or other dielectric separation between facing electrodes or different parts of the same electrode. Dielectric separation in addition to that provided by the separator may be provided by one or more layers of dielectric material, for example. The dielectric material may comprise any suitable material. The dielectric material may comprise a plant derived material and in particular may comprise a tobacco material, for example, a porous tobacco sheet, or it may comprise any suitable cellulose- or polypropylene- based material. When heated, the dielectric material may release one or more volatile compounds. The volatile compounds may include nicotine or flavour compounds such as tobacco or other flavouring. The dielectric material and the separator material may be the same or different. In another embodiment a layered capacitor substrate may comprise a first electrode, a first separator adjacent the first electrode, a second electrode adjacent the first separator, i.e., so that the first separator is sandwiched between the first and second electrodes and more particularly between a pair of carbon-based electrode layers, and a second separator adjacent the second electrode. The second electrode is sandwiched between P51436WO-5891 the first and second separators. The first electrode may be a positive electrode and the second electrode may be a negative electrode or vice versa. Such a substrate is particularly suitable for a spiral wound (or “jelly roll”) construction, which may be substantially cylindrical or may be flattened so that it has more of a cuboid shape. Dielectric separation between the turns of the spiral wound capacitor is provided by the second separator, which in the wound substrate may be sandwiched between the first and second electrodes and more particularly between a pair of carbon-based electrode layers. In yet another arrangement a layered capacitor substrate may comprise a plurality of first electrodes, a plurality of second electrodes, and a plurality of separators. The first electrodes may be positive electrodes and the second electrodes may be negative electrodes, or vice versa. The first and second electrodes are stacked alternately such that the substrate comprises a first electrode, a second electrode, a first electrode, a second electrode etc. in a stacking direction. A separator is sandwiched between each pair of electrodes and more particularly between a pair of carbon-based electrode layers to provide dielectric separation. Such a substrate may be useful for a flat-format article. The first electrodes may be electrically connected together and the second electrodes may be electrically connected together. The first electrodes may be electrically connected to a first capacitor terminal and the second electrodes may be electrically connected to a second capacitor terminal. The capacitor may be contained within a casing. More particularly, the casing may contain the capacitor substrate which includes the electrodes, separator etc., and the electrolyte. The electrolyte may be injected into the casing during manufacture or if the capacitor needs to be re-filled. The casing may electrically insulate the capacitor and may be formed of any suitable material or materials. The casing may include a paper wrapper with a metal or polymer coating, for example. The casing may include a pair of end caps of any suitable material. The casing may comprise appropriate perforations or openings, or incorporate a suitable aerosol- permeable membrane material, so that the aerosol generated when the electrolyte is P51436WO-5891 heated may be freely inhaled by the user, while also preventing leakage of the electrolyte when in a liquid or gel state. The aerosol generating article may include a filter segment, for example comprising cellulose acetate fibres, at a proximal end of the aerosol generating article. The filter segment may constitute a mouthpiece filter. One or more vapour collection regions, cooling regions, and other structures may also be included in some designs. The vapour cooling region may advantageously allow the vapour to cool and condense to form an aerosol with suitable characteristics for inhalation by a user, for example through the filter segment. In general terms, a vapour is a substance in the gas phase at a temperature lower than its critical temperature, which means that the vapour may be condensed to a liquid by increasing its pressure without reducing the temperature, whereas an aerosol is a suspension of fine solid particles or liquid droplets, in air or another gas. It should, however, be noted that the terms ‘aerosol’ and ‘vapour’ may be used interchangeably in this specification. The capacitor will preferably be pre-charged in the packaged article, i.e., it will already be charged when it is purchased by the user and before it is removably inserted into an aerosol generating device. Pre-charging the capacitor reduces the amount of energy that is required from the power source of the device for heating. This may lead to a reduction in the size and weight of the device. An aerosol generating device may be adapted to receive, in use, the aerosol generating article as described above. The aerosol generating device may comprise an external circuit (e.g., a switching circuit) that is electrically connected to the pair of electrodes or capacitor terminals when the article is received in the device. The heating of the electrolyte may be controlled by controlling the discharging of the capacitor and optionally also the charging of the capacitor – i.e., where the capacitor is cycled between being discharged and charged. In particular, the switching circuit may include a switching device which may be controlled by a controller to selectively provide a continuous or switched (i.e., a discontinuous or intermittent) short-circuit path between the pair of electrodes or capacitor terminals that allows the electrical charge stored in the capacitor to be discharged through the switching circuit. The switching device may also be controlled by the controller to charge the capacitor from the power source, P51436WO-5891 which may optionally include a suitable power converter such as a bi-directional power converter that provides a suitable direct current (DC) output voltage. The switching device may include one or more switches. For example, the switching device may include a discharging switch that can be switched on to provide a short-circuit path between the pair of electrodes or capacitor terminals to discharge the capacitor, and a charging switch that can be switched on to charge the capacitor. Each switch may be a controllable semiconductor switch such as a transistor (e.g., a metal oxide semiconductor field effect transistor (MOSFET), insulated-gate bipolar transistor (IGBT) or bipolar transistor). The one or more switches may be opened or closed or switched on and off by the controller. The switching circuit may include a first terminal that is electrically connected to the first electrode or terminal of the capacitor and a second terminal that is electrically connected to the second electrode or terminal of the capacitor when the aerosol generating article is received in the device. Prior to the article being inserted into the device, to prevent accidental or deliberate discharge of a pre-charged capacitor, it is preferred that at least one of the electrodes or terminals of the capacitor is inaccessible to the user. For example, one or both of the capacitor electrodes or terminals may be concealed within a casing of the article and are only made accessible for electrical connection with the terminals of the switching circuit after the aerosol generating article has been inserted into the device, or as it is in the process of being inserted. The electrical connection may require the casing to be ruptured at one or more locations and the device may include suitable means for rupturing, puncturing or tearing the casing. The first terminal of the switching circuit may be electrically connected directly to the first electrode at one or more locations, or may be electrically connected to a first capacitor terminal which is electrically connected in turn to the first electrode(s). Similarly, the second terminal of the switching circuit may be electrically connected directly to the second electrode at one or more locations, or may be electrically connected to a second capacitor terminal, which is electrically connected in turn to the second electrode(s). The capacitor terminals may be located anywhere on the article, e.g., near an end cap or a side of the article. The insertion orientation of the aerosol generating article into the device may be restricted to ensure correct alignment between P51436WO-5891 the respective terminals so as to provide a reliable electrical connection between the capacitor and the external switching circuit. The terminals of the switching circuit may be formed as rupturing devices that are designed to rupture, puncture or tear the casing and make an electrical connection with the electrodes or terminals of the capacitor. The rupturing devices may be fixed or stationary to the device and may be designed to rupture, puncture or tear the casing as the article is inserted into the device, e.g., into an aerosol generating space or heating chamber. The rupturing devices may also be movable. For example, in one arrangement the rupturing devices may be mounted on a panel or door of the device which is opened or removed to allow the article to be inserted and where the rupturing devices are designed to rupture, puncture or tear the casing when the panel or door is closed by the user. The panel or door may be hinged, for example. In another arrangement, the rupturing devices may be moved by a suitable actuator such as an electric motor or a piston, for example, that can force the rupturing devices through the casing and make an electrical connection. The rupturing devices may be moved through openings or slots in the part of the device that defines the aerosol generating space or heating chamber. The rupturing devices may have any suitable shape and may, for example, be formed as a needle type or crown type with one or more pointed ends, a blade type with an edge, or a punch type with a non-pointed end. The rupturing devices may be designed to work with any of the capacitor constructions mentioned above. If one of the electrodes or terminals of the capacitor is accessible, only one rupturing device may be needed. Discharging a pre-charged capacitor through an external circuit such as a switching circuit of the device will generate heat in the electrodes, which in turn heats the electrolyte in which the electrodes are immersed. Sufficient heating of the electrolyte will generate an aerosol to be inhaled by the user during a vaping session. To provide improved heating, the internal resistance of the capacitor may be increased by increasing the thickness of the separator between the oppositely charged electrodes. This may result in a capacitor having fewer turns or folds if the overall dimensions remain the same. Using the switching circuit to charge the capacitor will also generate P51436WO-5891 heat in the electrodes, which in turn heats the electrolyte to generate an aerosol to be inhaled. The device may also include an external heater for heating the capacitor to generate an aerosol for inhalation by the user. Put another way, heating of the electrolyte is not limited to the heat generated by the capacitor when it is discharged or charged, but the capacitor may be heated by an external heater in a similar way to a conventional aerosol generating material or substrate. Such heating will heat the electrolyte to generate an aerosol to be inhaled. Using an external heater may provide more controllable heating during certain phases of a vaping session and thereby optimise the experience of the user. Any suitable heater may be used, e.g., a low power thin film heater, printed heater etc. However, an induction heater may be preferred. The induction heater may comprise an induction coil and a susceptor and may be configured to heat the capacitor. For example, the induction coil may be positioned adjacent an aerosol generating space or heating chamber of the aerosol generating device that is designed to receive the aerosol generating article (or consumable). When the induction heater is used to heat the electrolyte, an alternating electromagnetic field is generated by the induction coil. The susceptor in the aerosol generating article couples with the electromagnetic field and generates heat due to eddy currents and/or magnetic hysteresis, which heat is then transferred from the susceptor to the electrolyte. To generate the alternating electromagnetic field necessary for induction heating, the device may further comprise an inverter that is electrically connected to the induction coil. The same inverter may also be used to generate the AC signal that is applied across the electrodes or terminals of the capacitor to estimate or determine the amount of electrolyte in the capacitor. In particular, the inverter may be selectively connected to one or both of the capacitor and the induction coil of the induction heater, e.g., by a suitable switching device. The susceptor of the induction heater preferably comprises the current collectors of the capacitor electrodes. In other words, the current collectors may also function as the susceptor of the induction heater and transfer heat to the electrolyte when the induction heater is being operated. Because the current collectors have at least two functions, the size and weight of the aerosol generating system may be reduced. In particular, it is not P51436WO-5891 necessary for the aerosol generating article to include one or more separate susceptors for heating the electrolyte when the induction heater is operated. The heat generated by discharging the capacitor or by cycling the capacitor between charging and discharging may be used during an initial pre-heating phase and the external heater may be used to heat the electrolyte to generate an aerosol during a subsequent heating or vaping phase, for example. The power for pre-heating may therefore be provided at least in part by the capacitor and not by the power source of the device. This may result in a smaller power source, and hence in a smaller and lighter device. Alternatively, the electrolyte may be heated during the subsequent heating or vaping phase by cycled charging and discharging of the capacitor. During the heating or vaping phase, there may be times when heating is not needed and therefore the capacitor does not need to be discharged or charged. When heating is needed, the capacitor may be discharged or charged continuously, or it may be discharged or charged intermittently using an appropriate duty cycle, for example. A pre-heating phase may generally be intended to pre-heat the electrolyte to a target temperature, and the heating or vaping phase may be generally intended to heat the electrolyte for a longer period during which an aerosol is generated and the temperature may be controlled to follow a temperature profile, for example. The method according to the first aspect of the present disclosure may further comprise notifying the amount of the electrolyte to the user. The amount of electrolyte may be notified visually, or in any other way such as by using an audible or haptic notification. The amount of electrolyte may be displayed to the user using a visual indicator or as a numerical value such as a percentage, for example, where a value of 100% indicates a maximum amount of electrolyte, i.e., that the capacitor is full of electrolyte. The article or device may comprise a visual display for displaying the amount of electrolyte to the user. The visual display may be an LED display, for example. The amount of electrolyte may be notified to the user using an external device such as a smartphone, for example, that is connected to the device by a suitable wireless communication protocol. A P51436WO-5891 message may be notified to the user if the amount of electrolyte is below a certain amount. The message may be notified visually, or in any other way. By monitoring the amount of electrolyte, and notifying this to the user, the user is better informed about how much electrolyte is remaining in the capacitor over the course of a vaping session and is consequently able to work out for how much longer a particular aerosol generating article is likely to be able to generate an aerosol. The user is also able to better understand how long an unused aerosol generating article might last until the electrolyte is consumed and/or how many aerosol generating articles the user may require over a particular period of time. The amount of electrolyte may be estimated or determined using one or more electrical parameters of the capacitor. For example, the amount of electrolyte may be estimated or determined using a function such as a linear or polynomial function that relates the electrolyte amount to the one or more electrical parameters, optionally implemented as a look-up table. The one or more electrical parameters of the capacitor may be estimated or determined using voltage and current measurements obtained in response to the applied AC signal during an electrolyte amount detection step. The one or more electrical parameters of the capacitor will be electrical parameters that are known to vary with the amount of electrolyte such as internal resistance and capacitance. The electrical parameter may also be a time constant that is related to the capacitance and/or internal resistance, for example. These parameters are directly proportional to the surface contact area between the electrolyte and the electrodes of the capacitor and will therefore vary as the amount of electrolyte decreases over the course of the vaping session. The one or more electrical parameters may be estimated or determined based on the frequency dependency of the capacitor dielectric material. The capacitance and/or internal resistance of the capacitor may be estimated or determined with reference to a frequency response plot such as a Nyquist plot (or Cole- Cole plot), for example. The frequency of the applied AC signal is swept as a parameter, resulting in a plot based on frequency. This is described in more detail below with reference to Figure 8, which shows an example of a Nyquist plot and an equivalent P51436WO-5891 electrical circuit of the capacitor. The equivalent electrical circuit includes an interface capacitance ^_^^ , a solution resistance (or ohmic internal resistance) ^_^^^ , a charge transfer resistance ^_^^ and a diffusion resistance (or Warburg resistance) ^_^ that on the Nyquist plot appears as a diagonal line with a slope of 45 degrees. The applied AC signal may have a frequency that is in a preferred frequency range. For example, the frequency range may be from about 1 mHz to about 1 kHz. A wide frequency range may be beneficial because it may allow a more precise measurement of capacitance, but it may also increase measurement duration. The preferred frequency range may be a compromise between these competing factors. The frequency may be swept across substantially the whole of the frequency range or across one or more narrower frequency ranges. For example, the frequency response plot may be constructed by focusing on one or more narrower frequency ranges such as a first frequency range between about 500 Hz and about 1 kHz – which may provide an indication of the solution resistance – a second frequency range between about 1 Hz and about 100 Hz – which may provide an indication of the charge transfer resistance and focus on the semi-circular part of the Nyquist plot shown in Figure 8 – and a third frequency range of less than about 1 Hz – which may provide an indication about diffusion resistance. The frequency of the applied AC signal may be swept one or more times over the preferred frequency range(s). Alternatively, two or more preferred frequencies may be used, e.g., one or more frequencies within each of the narrower frequency ranges mentioned above. It may be preferred that at least two frequencies within the second frequency range (i.e., about 1 Hz to about 100 Hz) are used to improve accuracy. As will be understood by one of ordinary skill in the art, measured values at the one or more preferred frequencies may be used to construct a simplified frequency response plot, e.g., a Nyquist plot where the diffusion resistance is omitted. Different frequencies may be used during the course of the vaping session – e.g., frequencies at the higher end of the range may be preferred during the heating or vaping phase because it can provide faster determination. The capacitance ^ of the capacitor may be estimated or determined from: P51436WO-5891 where ^ is the frequency of the applied AC signal (^ = ^/2^ where ^ is the angular frequency) and ^^{^^} is the imaginary part of the complex impedance. The capacitance ^ may be estimated or determined with reference to the frequency of the applied AC signal ^ once the imaginary part of the complex impedance ^^{^^} is obtained. A permittivity of the capacitor is directly proportional to the capacitance ^ and depends on the amount of the electrolyte. A look-up table that relates the capacitance ^ to the amount of electrolyte can therefore be pre-determined. In one embodiment, the value of the capacitance ^ is used directly to extract a value that corresponds to the amount of electrolyte from an array of values, for example. The imaginary part of the complex capacitance ^^{^^}(^) is directly proportional to the internal resistance of the capacitor and may be estimated or determined from: ^^{^} ^^{^^}(^) =

where ^(^) is the complex impedance and ^^{^} is the real part of the complex impedance. The imaginary part of the complex capacitance ^^{^^}(^) may be estimated or determined with reference to the angular frequency ^ once the real part of the complex impedance ^^{^} is obtained. The internal resistance of the capacitor is directly proportional to the imaginary part of the complex capacitance ^^{^^}(^) and depends on the amount of electrolyte. A look-up table that relates the imaginary part of the complex capacitance ^^{^^}(^) or the internal resistance to the amount of electrolyte can therefore be pre-determined. In one embodiment, the value of the imaginary part of the complex capacitance ^^{^^}(^) or the internal resistance is used directly to extract a value that corresponds to the amount of electrolyte from an array of values, for example. A transformation from the imaginary part of the complex capacitance ^^{^^}(^) to the internal resistance may be needed. P51436WO-5891 In a conventional capacitor, the amount of electrolyte remains constant because the electrolyte is contained within a hermetically sealed casing. But in an article according to the present disclosure, the electrolyte will be inhaled by the user as an aerosol over the course of a vaping session and so the amount of electrolyte will gradually decrease. The one or more electrical parameters of the capacitor will therefore also vary during a vaping session as the amount of electrolyte decreases. Other factors such as the temperature of the capacitor may also affect how the one or more electrical parameters of the capacitor vary and may be taken into account when the one or more electrical parameters are used to estimate or determine the amount of electrolyte. The one or more electrical parameters of the capacitor may be estimated or determined using at least one of voltage and current measurements obtained when the AC signal is applied to the electrode or capacitor terminal. For example, the voltage and current measurements may be obtained from voltage and current sensors. The capacitor may be operated in a heating mode when the capacitor is being charged or discharged in order to deliberately heat the electrolyte and generate an aerosol for inhalation by the user. In other words, during the vaping session the capacitor will be operated in the heating mode when it is necessary to heat the electrolyte. During the heating mode, a DC current will be applied to the capacitor to charge it, or a DC current will be discharged from the capacitor through the switching circuit – e.g., by providing a short-circuit path between the capacitor electrodes or terminals. These currents may be referred to as DC charging and DC discharging currents. Applying the charging DC current will increase the charge accumulated by the capacitor. The charge will decrease when the capacitor is discharged. The discharging and/or charging of the capacitor during the heating mode, and hence the heating of the electrolyte, may be controlled by varying the power at which the capacitor is discharged and/or charged through the switching circuit. For example, the discharging and/or charging power may be varied by controlling the switching device of the switching circuit so that the capacitor is discharged or charged intermittently using an appropriate duty cycle, e.g., where the charging or discharging switch of the switching device is periodically switched on and off with a duty cycle that may be varied to control the rate at which the capacitor is P51436WO-5891 discharged or charged. More particularly, the time for which the charging or discharging switch is switched on (or “pulse width”) may be varied. It will also be readily understood that there will be times when the capacitor is operated in the heating mode when the capacitor is not actually being charged or discharged – i.e., when the charging or discharging switch is switched off according to the duty cycle. Nevertheless, the overall intention during the heating mode is to operate the capacitor so that the electrolyte is heated to generate an aerosol. The AC signal that is applied to the capacitor has a small current value so there is no significant heating of the electrolyte. As such, the AC signal is not taken into account when determining if the capacitor is operating in a heating or non-heating mode. The capacitor may also be operated in a non-heating mode when heating is not necessary during the vaping session. During the non-heating mode the capacitor is not charged or discharged. The AC signal may be applied to the electrode when the capacitor is in the heating mode and the non-heating mode. For example, the AC signal may be applied substantially continuously or when the amount of electrolyte is to be estimated or determined regardless of whether the capacitor is in the heating mode or the non-heating mode. Alternatively, the AC signal may be applied to the electrode only when the capacitor is in the non-heating mode. This may simplify the switching circuit because the AC signal does not need to be superimposed on the DC current that is applied to the capacitor when it is charged, for example. In some embodiments, for example if the frequency of the AC signal is high enough, it may be possible to apply the AC signal during a heating mode of the capacitor only when the capacitor is not being charged or discharged – i.e., only when the charging or discharging switch is switched off according to the duty cycle. The AC signal would not be applied when the charging or discharging switch is switched on to charge or discharge the capacitor during the heating mode. In this case, the switching circuit may also be simplified. P51436WO-5891 The monitoring method may comprise one or more electrolyte amount detection steps. More particularly, the method may comprise an initial step in which an initial value of the one or more electrical parameters of the capacitor is estimated or determined. The initial step may be carried out when the aerosol generating article is inserted into the device, or before a pre-heating phase, for example. The initial value may be used to define a “baseline” that is indicative of the initial amount of electrolyte in the capacitor prior to the start of a vaping session. The initial amount of electrolyte may be assumed to be a maximum amount, i.e., that the capacitor is full of electrolyte. Alternatively, the initial value may be used to estimate or determine an initial amount of electrolyte in the capacitor using a suitable linear or non-linear function or look-up table, for example, that relates the initial value to an initial amount of electrolyte. The initial amount of electrolyte may be notified to the user. Instead of an initial value that is estimated or determined in the initial step, a reference value or pre-determined value may be used to define the “baseline”. The method may further comprise one or more subsequent steps in which a subsequent value of the one or more electrical parameters of the capacitor is estimated or determined. For each subsequent step, the initial value and the respective subsequent value may be used to estimate or determine the amount of remaining electrolyte in the capacitor. For example, the amount of remaining electrolyte may be estimated by comparing the respective subsequent value with the initial (or “baseline”) value. Alternatively, for each subsequent step only the respective subsequent value is used to estimate or determine the amount of electrolyte in the capacitor using a suitable linear or non-linear function or look-up table, for example, that relates the subsequent value to a remaining amount of electrolyte. The amount of electrolyte may be estimated or determined using explicit values of the one or more electrical parameters of the capacitor or using relative values – i.e., where a change in the value of the one or more electrical parameters means that there is a relative change in the amount of the electrolyte. For example, it may be determined that an x% decrease in the value of the one or more electrical parameters of the capacitor corresponds to a y% decrease in the amount of electrolyte. P51436WO-5891 The amount of electrolyte remaining in the capacitor may be notified to the user. The value of the one or more electrical parameters and/or the amount of electrolyte that is estimated or determined in the initial step or any subsequent step may be used to control an operation of the device such as varying the heating or, if the amount of electrolyte is less than a minimum amount, further use of the device may be prevented. This ensures safe operation of the device for the user. Such operation may allow the capacitor to be heated until it is substantially fully depleted of electrolyte instead of limiting the number of puffs or the duration of a vaping session, for example. This may increase user satisfaction because the user can consume all of the available electrolyte in the aerosol generating article. The value of the one or more electrical parameters and/or the amount of electrolyte that is estimated or determined may also be used to identify a defect in the article – for example, if the initial value or the initial amount of electrolyte is too low or below a certain amount. Each subsequent step may be carried out at regular or irregular intervals during the vaping session or may be carried out in response to a puff detection, i.e., where the user inhales the generated aerosol. This helps the user to understand how much electrolyte is remaining in the capacitor after a puff has been taken. Each subsequent step may be carried out when the temperature of the capacitor is kept substantially constant. During the initial step or each subsequent step the AC signal may be applied for an appropriate length of time, e.g., that is sufficient to allow the amount of electrolyte to be estimated or determined. In some embodiments, the AC signal may be applied outside of the times when an electrolyte amount detection step is carried out. If the amount of electrolyte that is estimated or determined in any subsequent step is outside an expected range – i.e., the estimated or determined amount is not plausible because it is too low and therefore suggests that too much electrolyte has been vapourised, or because it is too high and therefore suggests that not enough electrolyte has been vapourised for the particular operating conditions of the aerosol generating device – the method may comprise notifying an amount of electrolyte to the user that is estimated or determined based on one or more previously estimated or determined P51436WO-5891 amounts of electrolyte. The expected range may be based on a previously estimated or determined amount of electrolyte, for example on the amount that was estimated or determined in the immediately preceding step. If the amount of electrolyte that is estimated or determined is not plausible, the amount of electrolyte that is notified to the user may be estimated using one or more previous amount of electrolyte that were within the expected range. This may provide the user with a more reliable notification. For example, two or more previously estimated or determined amounts of electrolyte – i.e., plausible amounts obtained during two or more previous steps – may be used to determine a rate of change of electrolyte and this rate of change may be applied to a previously estimated or determined amount to estimate the current amount of electrolyte remaining in the capacitor, which is then notified to the user. The rate of change may be applied to the amount of electrolyte estimated or determined in the preceding step, for example. The determined rate of change may be linear or non-linear, for example. According to a second aspect of the present disclosure, there is provided an aerosol generating system comprising: an aerosol generating article comprising a capacitor, the capacitor comprising: an electrolyte which, when heated, generates an aerosol for inhalation by a user; a pair of electrodes; and a porous separator between the electrodes; and an aerosol generating device in which the aerosol generating article is received, the aerosol generating device further comprising a controller adapted to implement the method described above. According to a third aspect of the present disclosure, there is provided an aerosol generating system comprising: an aerosol generating article comprising a capacitor, the capacitor comprising: an electrolyte which, when heated, generates an aerosol for inhalation by a user; a pair of electrodes; and P51436WO-5891 a porous separator between the electrodes; and an aerosol generating device in which the aerosol generating article is received, the aerosol generating device further comprising: a power source; a switching circuit electrically connected between the pair of electrodes and configured to control the discharging of the capacitor and the charging of the capacitor from the power source; and an inverter electrically connected to one of the pair of electrodes and configured to apply to the electrode an AC signal. The capacitor may be configured to be operated in a heating mode when the capacitor is being charged or discharged in order to heat the electrolyte and generate an aerosol for inhalation by the user, and a non-heating mode. The inverter may be configured to apply the AC signal to the electrode when the capacitor is in the heating mode and the non-heating mode, or only when the capacitor is in the non-heating mode. The system may further comprise a superimposing circuit electrically connected to the inverter and the switching circuit. The superimposing circuit may be configured to superimpose the AC signal on a DC current that is supplied by the power source when the capacitor is being charged. This allows the amount of electrolyte to be estimated or determined when the capacitor is in the heating mode and it is being charged or discharged. The superimposing circuit may be omitted if the AC signal is only applied when the capacitor is in the non-heating mode – i.e., when heating of the electrolyte is not necessary, and the capacitor is not charged or discharged. The system may further comprise an AC coupling capacitor electrically connected between the inverter and the superimposing circuit in series. The system may further comprise a bypass circuit electrically connected to an input and an output of the superimposing circuit in parallel. The discharge current of the capacitor may be supplied to the input of the superimposing circuit via the bypass circuit. The P51436WO-5891 bypass circuit may comprise a switch, e.g., a semiconductor switch such as a transistor, and a diode that provides reverse current protection. The switch may be switched on by the controller when the capacitor is being discharged during a heating mode and when the AC signal is applied to the capacitor through the superimposing circuit to estimate or determine the amount of electrolyte. The system may further comprise a charging switch configured to turn on and off the charging of the capacitor from the power source, and a diode electrically connected between the power source and the charging switch. The diode provides reverse current protection when the discharge current flows through the bypass circuit. The switching circuit and the inverter may be electrically connected to the capacitor in parallel. The device may further comprise a visual display adapted to display the amount of electrolyte estimated or determined by the controller. Brief Description of the Drawings Figure 1 is a diagrammatic view of an example of an aerosol generating article; Figure 2 is a diagrammatic view of an example of a capacitor having a spiral wound construction; Figure 3 is a cross section view along line A-A of Figure 2; Figure 4 is a diagrammatic view of an aerosol generating device; Figure 5 is a schematic representation of a first example of an electrical circuit of the aerosol generating device; Figure 6 is a schematic representation of a second example of an electrical circuit of the aerosol generating device; Figures 7A to 7D are schematic representations of the second example of the electrical circuit of Figure 6 showing different operating modes; and Figure 8 is a schematic representation of an example of a Nyquist plot and an equivalent electrical circuit of the capacitor. P51436WO-5891 Detailed Description of Embodiments Embodiments of the present disclosure will now be described by way of example only and with reference to the accompanying drawings. Referring initially to Figure 1, there is shown diagrammatically an example of an aerosol generating article 1. The aerosol generating article 1 has a proximal end 2 and a distal end 4. The aerosol generating article 1 includes a capacitor 6 that includes an electrolyte. The capacitor 6 is surrounded by a paper wrapper 8 with a metal or polymer coating. An end cap 10a, 10b is provided at each end of the capacitor 6. The paper wrapper 8 and the end caps 10a, 10b define an outer casing for the capacitor 6 that contains the electrolyte and provides electrical insulation. The aerosol generating article 1 may be generally cylindrical. At the proximal end 2, the aerosol generating article 1 includes a mouthpiece 12 having an outlet 14 through which a user may inhale an aerosol that is generated by heating the electrolyte. Although not shown, the proximal end cap 10a may include appropriate perforations or openings, or incorporate a suitable aerosol-permeable membrane material, so that the generated aerosol may pass through the end cap to the outlet 14. Referring to Figure 2, the capacitor 6 is an electric double-layer supercapacitor and has a generally cylindrical, spiral wound (or “jelly roll”) construction. The capacitor 6 includes a positive electrode 16 and a negative electrode 18. The electrodes 16, 18 are separated by a pair of porous separators 20a, 20b. As shown more clearly in Figure 3, the positive electrode 16 includes a positive current collector 22. Each side of the positive current collector 22 is provided with a porous carbon-based electrode layer 24 such as a layer of porous charcoal material or activated carbon, for example. The negative electrode 18 includes a negative current collector 26. Each side of the negative current collector 26 is provided with a porous carbon-based electrode layer 28 such as a layer of porous charcoal material or activated carbon, for example. The positive and P51436WO-5891 negative current collectors 22, 26 are aluminium foil layers, for example. The positive current collector 22 and the negative current collector 26 may enhance charging and discharging of the capacitor 6. The separators 20a, 20b are formed from a tobacco material such as a porous tobacco sheet which releases volatile compounds when it is heated. In an alternative arrangement, which is not shown, the separators may be formed from a suitable cellulose- or polypropylene-based material and the electrolyte may flow through a tobacco material such as crumb tobacco that is downstream of the capacitor 6 in an aerosol flow path. The tobacco material may be positioned between the capacitor 6 (particularly the proximal end of the capacitor) and the mouthpiece 12. The tobacco material adds flavour and nicotine to the aerosol. The heating provided by the capacitor also heats or warms the tobacco material, which promotes the release of volatile compounds. Instead of the tobacco material, a flavour source without nicotine may be used. Additionally, or alternatively, the tobacco material may be embedded into the mouthpiece 12. The electrodes 16, 18 and the separators 20a, 20b are immersed in an electrolyte which permits cation and anion migration when the capacitor 6 is charged or discharged and generates an aerosol for inhalation by the user when it is heated. The electrolyte may comprise sodium chloride and glycerol, and optionally polyvinyl alcohol as a gelling agent. The capacitor 6 is pre-charged during the manufacturing process and is packaged and sold to the user in a pre-charged state. The aerosol generating article 1 includes a positive capacitor terminal 30 which is electrically connected to the positive electrode 16, i.e., to the positive current collector 22 at one or more locations, and a negative capacitor terminal 32 which is electrically connected to the negative electrode 18, i.e., to the negative current collector 26, at one or more locations. The capacitor terminals 30, 32 may be located inside the outer casing of the aerosol generating article 1 so that they are not accessible to the user. This helps to prevent the accidental or deliberate discharge of the capacitor 6 before the article is P51436WO-5891 removably inserted into an aerosol generating device preparatory to starting a vaping session. Figure 4 shows an aerosol generating device 34 adapted to receive the aerosol generating article 1. The aerosol generating device 34 includes a cavity 36 into which the aerosol generating article 1 may be inserted. The aerosol generating device 34 includes a pair of rupturing devices 38, 40 that are adapted to rupture the distal end cap 10b of the aerosol generating article 1 when it is inserted into the cavity 36. The angular orientation of the aerosol generating article 1 relative to the aerosol generating device 34 may be restricted when it is inserted into the cavity 36 so that the rupturing device 38 makes an electrical connection with the positive electrode 30 and the rupturing device 40 makes an electrical connection with the negative electrode 32. Other ways of ensuring a reliable electrical connection may be used. For example, the positive and negative terminals 30, 32 of the aerosol generating article 1 may have an annular construction and be located coaxial with each other so that appropriately positioned rupturing devices 38, 40 will make electrical contact with the terminals irrespective of the angular orientation of the aerosol generating article 1 relative to the aerosol generating device 34. The aerosol generating device 34 includes an electrical circuit 42 and a power source 44 such as a battery (e.g., a lithium-ion secondary battery). The aerosol generating device 34 may optionally include one or more heaters. The aerosol generating device 34 shown in Figure 4 includes an induction heater with an induction coil 46 that is arranged adjacent the cavity 36 for heating the capacitor 6 when the aerosol generating article 1 is inserted in the aerosol generating device 34. The current collectors 22, 26 of the capacitor electrodes 16, 18 may function as the susceptor of the induction heater. The aerosol generating device 34 includes a visual display 48 for displaying the amount of electrolyte to the user. The visual display 48 is used to display the amount of P51436WO-5891 electrolyte to the user as a numerical value, and more particularly as a percentage, where 100% indicates that the capacitor 6 is full of electrolyte. Other visual displays or indicators such as indicator bars and/or circles, for example, may also be used. The amount of electrolyte may be displayed or notified to the user using an external device such as a smartphone, for example, that is connected to the external device by a suitable wireless communication protocol. A first example of an electrical circuit 42A is shown in Figure 5. The electrical circuit 42A includes a switching circuit 50, a low-dropout (LDO) regulator 52, an inverter 54, and microcontroller unit (MCU) 56. A DC/DC converter 58 is electrically connected to the power source 44 and provides a charging current for the capacitor 6. It should be noted that the DC/DC converter 58 may be a buck (or step-down) converter, a boost (or step-up) converter, or a buck-boost converter and converts the DC input voltage from the power source 44 into a suitable DC output voltage. The DC/DC converter 58 may be omitted in some embodiments. The DC/DC converter 58 includes a voltage input terminal (labelled “VIN”) and a voltage output terminal (labelled “VOUT”). A ground terminal (labelled “GND”) is electrically connected to ground. The voltage input terminal is electrically connected to the power source 44. The DC/DC converter 58 includes a feedback terminal (labelled “FB”) which receives a DC output voltage feedback. The DC/DC converter 58 includes a serial data terminal (labelled “SDA”) and a serial clock terminal (labelled “SCL”) that are electrically connected to corresponding terminals of the MCU 56. The DC/DC converter 58 also includes an enable terminal (labelled “EN”) that is electrically connected to a first input/output terminal (labelled “I/O1”) of the MCU 56 and which allows the MCU 56 to enable and disable operation of the DC/DC converter 58. In this embodiment, the enable terminal of the DC/DC converter 58 works according to positive logic – that is, the DC/DC converter 58 outputs a voltage from the voltage output terminal only when a high level signal is inputted to the enable terminal. It will be understood that the enable terminal may alternatively work according to negative logic. P51436WO-5891 The LDO regulator 52 is electrically connected to the power source 44. The LDO regulator 52 includes an input terminal (labelled “IN”) that is electrically connected to the power source 44 and an output terminal (labelled “OUT”) that provides a regulated voltage supply. A ground terminal (labelled “PGND”) is electrically connected to ground. The LDO regulator 52 also includes an enable terminal (labelled “EN”) that is electrically connected to the power source 44. In this embodiment, the enable terminal of the LDO regulator 52 works according to positive logic and the input and enable terminals of the LDO regulator 52 are electrically connected to the power source 44 in parallel. This means that the LDO regulator 52 continuously outputs a regulated voltage from the output terminal unless the power source 44 is unavailable. The inverter 54 is electrically connected to the power source 44 in parallel with the DC/DC converter 58. The inverter 54 includes a positive input terminal (labelled “IN+”) that is electrically connected to the power source 44 and a negative input terminal (labelled “IN-”) that is electrically connected to ground. The inverter 54 includes a positive output terminal (labelled “OUT+”) and a negative output terminal (labelled “OUT-”). The inverter 54 includes a serial data terminal (labelled “SDA”) and a serial clock terminal (labelled “SCL”) that are electrically connected to corresponding terminals of the MCU 56. The inverter 54 also includes an enable terminal (labelled “EN”) that is electrically connected to a second input/output terminal (labelled “I/O”) of the MCU 56 and which allows the MCU 56 to enable and disable operation of the inverter 54. The enable terminal of the inverter 54 works according to either positive logic or negative logic. The switching circuit 50 includes a positive rail 60 that is electrically connected to the positive electrode 16 of the capacitor 6 and a ground rail 62 that is electrically connected to the negative electrode 18 and to ground. Although not shown in Figure 5, it will be understood that the positive and ground rails 60, 62 may be electrically connected to the capacitor 6 by the pair of rupturing devices 38, 40 that make an electrical connection with the positive and negative capacitor terminals 30, 32. The positive output terminal (labelled “OUT+”) of the inverter 54 is electrically connected to the positive rail 60, P51436WO-5891 optionally by means of an AC coupling capacitor C1. The negative output terminal (labelled “OUT-”) of the inverter 54 is electrically connected to the ground rail 62. A first semiconductor switch Q1 is electrically connected between the positive and ground rails 60, 62 and provides a short-circuit path between the positive and negative electrodes 16, 18 when it is switched on. This short-circuit path can be used to discharge the capacitor 6 and may also electrically connect the current collectors 22, 26 for eddy current flow when the external induction heater is operated to heat the capacitor 6. The discharge/short-circuit current through the first semiconductor switch Q1 is shown by the solid arrow. The positive rail 60 is electrically connected to the output voltage terminal of the DC/DC converter 58 by means of a second semiconductor switch Q2. When the DC/DC converter 58 is enabled, the second semiconductor switch Q2 can be switched on to charge the capacitor 6. The charging current through the second semiconductor switch Q2 is shown by the solid arrow. The first and second semiconductor switches Q1, Q2 may define a switching device of the switching circuit 50. The MCU 56 includes an input voltage terminal (labelled “VDD”) electrically connected to the output voltage terminal of the LDO regulator 52 and receives a regulated voltage supply. As noted above, the MCU 56 includes a serial data terminal (labelled “SDA”) and a serial clock terminal (labelled “SCL”) that are electrically connected to corresponding terminals of the DC/DC converter 58 and the inverter 54. The MCU 56 includes a ground terminal (labelled “GND”) that is electrically connected to ground. As noted above, the MCU 56 includes first and second input/output terminals that are respectively electrically connected to the enable terminals of the DC/DC converter 58 and the inverter 54. A voltage sensing circuit 64 is configured to detect the voltage across the capacitor 6. The voltage sensing circuit 64 includes a voltage divider. A current sensing circuit 66 is configured to detect the current through the capacitor 6 and includes a shunt resistor 68 electrically connected in series with the ground rail 62 P51436WO-5891 of the switching circuit 50 and a current sensing amplifier 70 electrically connected with the shunt resistor 68. An additional operational amplifier 72 may be provided to improve accuracy of the current measurement because any floating capacitance is cancelled by keeping 0V at the positive terminal of the shunt resistor 68. The MCU 56 also includes: ^ A third input/output terminal (labelled “I/O”) that is electrically connected to the first semiconductor switch Q1 for switching it on and off. When the first semiconductor switch Q1 is switched on, a short-circuit path is provided between the positive and ground rails 60, 62, and hence between the first and second electrodes 16, 18 of the capacitor 6. This short-circuit path may be used to discharge the capacitor 6 or to facilitate eddy current flow between the current collectors 22, 26 which function as the susceptor when the capacitor 6 is heated by the external induction heater. When the first semiconductor switch Q1 is switched off there is no short-circuit path through the first semiconductor switch Q1. ^ A fourth input/output terminal (labelled “I/O”) that is electrically connected to the second semiconductor switch Q2 for switching it on and off. When the second semiconductor switch Q2 is switched on, the positive rail 60 is electrically connected to the output voltage terminal of the DC/DC converter 58 so that DC current is applied to the positive electrode 16 of the capacitor 6 to charge the capacitor 6. When the second semiconductor switch Q2 is switched off, the positive rail 60 is electrically disconnected from the output voltage terminal of the DC/DC converter 58 and no DC current is applied to the positive electrode 16 of the capacitor 6. ^ A fifth input/output terminal (labelled “I/O”) that is electrically connected to the voltage sensing circuit 64, optionally by means of an AC coupling capacitor C2 that may form part of the voltage sensing circuit. ^ A sixth input/output terminal (labelled “I/O”) that is electrically connected to the current sensing amplifier 70 of the current sensing circuit 66, optionally by means of an AC coupling capacitor C3 that may form part of the current sensing circuit. P51436WO-5891 The capacitor 6 is operated in a heating mode when it is necessary to heat the electrolyte. In the heating mode, the capacitor 6 is discharged, charged or cycled between being discharged and charged. This generates heat in the electrodes 16, 18, which in turn heats the electrolyte in which the electrodes are immersed to generate an aerosol which the user may inhale through the mouthpiece 12 of the aerosol generating device 34. More particularly, in the heating mode, the capacitor 6 may be discharged by periodically switching the first semiconductor switch Q1 on and off with a duty cycle. The capacitor 6 may also be charged by enabling the DC/DC converter 58 and periodically switching the second semiconductor Q2 on and off with a duty cycle so that the charging current is applied to the capacitor. When the capacitor 6 is being discharged, the DC/DC converter 58 may be disabled. In the non-heating mode, the capacitor 6 is not charged or discharged. The AC signal is only applied across the positive and negative electrodes of the capacitor 6 during the non-heating mode. The AC signal is applied by selectively enabling the inverter 54 when the capacitor 6 is being operated in the non-heating mode. In Figure 5 the applied AC signal is indicated by the dashed arrow. A second example of an electrical circuit 42B is shown in Figure 6. The second example of the electrical circuit 42B is substantially similar to the first example. The main difference is that the electrical circuit 42B includes a superimposing circuit 74 that is electrically connected to the positive output terminal of the inverter 54, optionally by means of an AC coupling capacitor C4. The negative output terminal of the inverter 54 is electrically connected to ground. The superimposing circuit 74 is configured to superimpose the AC signal on the DC current that is supplied by the DC/DC converter 58 to charge the capacitor 6. The superimposing circuit 74 includes an operational amplifier 76. The operational amplifier 76 includes: ^ A non-inverting input terminal (labelled “+”) electrically connected to the second semiconductor switch Q2 by means of a first resistor R1 and to a first P51436WO-5891 ground connection GND1 by means of a fifth capacitor C5 and a parallel second resistor R2. ^ An inverting input terminal (labelled “-“) electrically connected to the output voltage terminal of the inverter 54 by means of a series-connected third resistor R3 and the optional AC coupling capacitor C4. ^ A positive voltage terminal electrically connected to the second semiconductor switch Q2 in parallel with the non-inverting input terminal of the operational amplifier 76. ^ A negative voltage terminal electrically connected to the first ground connection GND1. ^ An output voltage terminal electrically connected to positive rail 60 of the switching circuit 50. A junction point 78 between the inverting input terminal and the third resistor R3 is electrically connected to the output voltage terminal of the operational amplifier 76 by means of a fourth resistor R4 and to a second ground connection GND2 by means of a fifth resistor R5. The electrical circuit 42B also includes a bypass circuit 80 electrically connected between an input and an output of the superimposing circuit 74 in parallel – or more particular, electrically connected to a junction point 82 between the second semiconductor switch Q2 and the positive voltage terminal of the operational amplifier 76, and the positive rail 60 that is electrically connected to the output voltage terminal of the operational amplifier 76. The junction point 82 may therefore represent an input of the superimposing circuit 74. The bypass circuit 80 is configured so that the discharge current of the capacitor 6 is also supplied to the input of the superimposing circuit 74 via the bypass circuit 80. The bypass circuit 80 includes a third semiconductor switch Q3 and a first diode D1 that prevents current from flowing through the bypass circuit 80 from the input of the superimposing circuit 74 to the output – e.g., during charging of the capacitor. The MCU 56 includes a seventh input/output terminal (labelled “I/O”) that is electrically connected to the third semiconductor switch Q3 for switching it on and off. P51436WO-5891 A second diode D2 is included between the second semiconductor switch Q2 and the voltage output terminal of the DC/DC converter 58 and prevents the discharge current from reaching the voltage output terminal of the DC/DC converter. The AC signal may be applied to the positive electrode of the capacitor 6 during the heating mode or the non-heating mode by selectively enabling the inverter 54. During the heating mode, if the capacitor 6 is being charged, the AC signal from the inverter 54 is superimposed on the DC current from the DC/DC converter 58 by the superimposing circuit 74 so that the output of the superimposing circuit – i.e., the output voltage of the operational amplifier 76 – has both AC and DC components. Figures 7A to 7D show the electrical circuit 42B when the capacitor 6 is being charged and discharged during a heating mode. In Figure 7A the capacitor 6 is being charged – i.e., the DC/DC converter 58 is enabled and a DC current is being provided to the positive electrode of the capacitor 6 through the superimposing circuit 74 (more specifically, through the operational amplifier 76) because the second semiconductor switch Q2 is switched on. An AC signal is not being applied because the inverter 54 is not enabled. The first and third semiconductor switches Q1, Q3 are switched off. In Figure 7B the capacitor 6 is being charged and an AC signal is applied to the inverting input terminal of the operational amplifier 76 because the inverter 54 is enabled. The AC signal from the inverter 54 is superimposed on the DC current from the DC/DC converter 68 by the superimposing circuit 74. The amount of electrolyte may therefore be estimated or determined by the MCU 56 even when the capacitor 6 is being charged. The first and third semiconductor switches Q1, Q3 are switched off. In Figure 7C the capacitor 6 is being discharged. The first semiconductor switch Q1 is switched on. The second semiconductor switch Q2 is switched off and/or the DC/DC P51436WO-5891 converter 58 is not enabled. An AC signal is not being applied because the inverter 54 is not enabled. The third semiconductor switch Q3 is switched off. In Figure 7D the capacitor 6 is being discharged. The first semiconductor switch Q1 is switched on. The second semiconductor switch Q2 is switched off and/or the DC/DC converter 58 is not enabled. An AC signal is being applied to the inverting input terminal of the operational amplifier 76 because the inverter 54 is enabled. The third semiconductor switch Q3 is also switched on so that current also flows through the bypass circuit 80 as shown. In particular, part of the discharge current from the capacitor 6 flows through the bypass circuit 80 to the input of the superimposing circuit 74 that is defined by the junction point 82 and then to the non-inverting input terminal of the operational amplifier 76. The amount of electrolyte may therefore be estimated or determined by the MCU 56 even when the capacitor 6 is being discharged. In both of the first and second examples of the electrical circuit 42A, 42B, an I2C communication protocol may be used for serial data communication between the MCU 56 and the inverter 54 and the DC/DC converter 58. Other suitable communication protocols such as SPI or UART may be also used. The amount of remaining electrolyte may be estimated or determined by the MCU 56 from an electrical parameter of the capacitor 6 such as internal resistance or capacitance that are known to vary with the amount of electrolyte. The electrical parameter of the capacitor 6 may be estimated or determined using voltage and current measurements obtained when the AC signal is applied across the positive and negative electrodes 16, 18 of the capacitor 6 as described above, optionally by means of the superimposing circuit 74. The MCU 56 calculates the internal resistance or capacitance of the capacitor 6 mainly according to a Nyquist plot (or Cole-Cole plot). An example of a Nyquist plot 100 is shown in Figure 8. The imaginary part of the complex impedance ^^{^^} (or -^^(^)) is the vertical axis and the real part of the complex impedance ^^{^} (or ^^(^)) is the horizontal axis. A bold line 102 is an aggregation of the complex impedances corresponding to P51436WO-5891 each frequency of the AC signal that is applied across the positive and negative electrodes 16, 18. The frequency of the AC signal may be swept by the inverter 54 across a wide frequency range (e.g., 1 mHz to 1 kHz) one or more times. Alternatively, the frequency may be swept across several narrower frequency ranges (e.g., 500 Hz to 1 kH, 1 Hz to 100 Hz and 1 mHz to 1 Hz) one or more times, or a plurality of different frequencies may be used to construct the Nyquist plot as described above. The MCU 56 calculates concrete values of the complex impedances corresponding to each frequency based on the input signal that is provided to the fifth input/output terminal that is electrically connected to the voltage sensing circuit 64 and the input signal that is provided to the sixth input/output terminal that is electrically connected to the current sensing circuit 66, and in particular to the current sensing amplifier 70. Because these input signals are AC signals, the MCU 56 may calculate the real and imaginary parts of the complex impedance ^^{^}, ^^{^^} by dividing the input signal from the voltage sensing circuit 64 by the input signal from the current sensing circuit 66. More particularly, the MCU 56 may plot a dot corresponding to the calculation results of the real and imaginary parts of the complex impedance ^^{^}, ^^{^^} for a particular frequency of the AC signal on a diagram. As a result of frequency sweeps within the preferred frequency range(s), the individual dots will form the bold line 102. In the example of the Nyquist plot shown in Figure 8, the right-hand part corresponds to a low frequency and the left- hand part corresponds to a high frequency – i.e., the bold line 102 is plotted from right to left as the frequency of the AC signal is swept from a low frequency to a high frequency and is plotted from left to right as the frequency of the AC signal is swept from a high frequency to a low frequency. The example of the Nyquist plot shown in Figure 8 may be divided into two separate regions – namely a substance transfer process region 104 and a charge transfer process region 106. The bold line 102 may be substantially linear in the substance transfer process region 104 and may be substantially semi-circular in the charge transfer process region 106 as shown. The length of the bold line 102 in the substance transfer process region 104 corresponds to a diffusion resistance ^_^ that is shown in Figure 8 as part of a real equivalent capacitor circuit (and where ^_^^ is the interface capacitance). The semi-circular part of the bold line 102 a first zero crossing point 108 and a P51436WO-5891 second zero crossing point 110. The second zero crossing point 110 is positioned on the boundary between the substance transfer process region 104 and the charge transfer process region 106. The distance between the origin and the first zero crossing point 108 corresponds to a solution resistance ^_^^^ and a distance between the first and second zero crossing points 108, 110 – i.e., the diameter of the semi-circular part of the bold line 102 – corresponds to a charge transfer resistance ^_^^. The sum of the solution resistance ^_^^^ and the charge transfer resistance ^_^^ – i.e., the distance between the origin and the second zero crossing point 110 – corresponds to the internal resistance of the capacitor 6. The real part of the complex impedance ^^{^} at the second zero crossing point 110, the complex impedance ^(^) at the second zero crossing point 110, and the angular frequency ^ at the second zero crossing point 110 are used to calculate the imaginary part of the complex capacitance ^^{^^}(^), where: ^ ^^{^^}(^) =

The imaginary part of the complex capacitance ^^{^^}(^) is directly proportional to the internal resistance of the capacitor 6. The amount of electrolyte may then be estimated or determined based on the imaginary part of the complex capacitance ^^{^^}(^) using a look-up table, for example. The imaginary part of the complex impedance ^^{^^} corresponds to the highest point 112 of the semi-circular part of the bold line 102 according to the vertical axis. The imaginary part of the complex impedance ^^{^^} and the frequency ^ at the highest point 112 are used to calculate the capacitance ^ of the capacitor from: 1 ^ = ∙

The amount of electrolyte may then be estimated or determined based on the capacitance ^ using a look-up table, for example. P51436WO-5891 It should be noted again that a wide frequency range that includes the first zero crossing point 108 may not be essential for estimating or determining the amount of electrolyte. Only more limited frequency ranges around the second zero crossing point 110 or the highest point 112 may be required in some cases. During a vaping session that may include a pre-heating phase and a subsequent heating or vaping phase, the MCU 56 may carry out a plurality of electrolyte amount detection steps. In an initial step which is carried out at time ^_^ before the pre-heating phase starts. an initial value ^_^ of the electrical parameter of the capacitor 6 is estimated or determined. This initial value ^_^ is therefore indicative of the initial amount of electrolyte in the capacitor 6 prior to the start of a vaping session. It is assumed that the initial value ^_^ defines a “baseline” against which subsequent values may be compared. It is also assumed that the initial amount of electrolyte is a maximum amount. The visual display 52 shows a numerical value of “100%” to indicate to the user that the capacitor 6 is full of electrolyte. In subsequent steps, which are carried out at times ^_^, ^_^ and ^ , subsequent values ^_^, ^_^ and ^ of the electrical parameter of the capacitor 6 are estimated or determined. The subsequent steps may be carried out in response to a puff detection. Puff detection may be based on a temperature change or using a standard flow sensor or microphone sensor, for example. The initial value ^_^ and each respective subsequent value ^_^, ^_^ and ^ are then used to estimate or determine the amount of remaining electrolyte. For example, the initial value ^_^ and the first subsequent value ^_^ are used to estimate or determine the amount of electrolyte at time ^_^, the initial value ^_^ and the second subsequent value ^_^ are used to estimate or determine the amount of electrolyte at time ^_^, and so on. If the electrical parameter is directly proportional to the amount of electrolyte, i.e., so that the electrical parameter decreases as the amount of electrolyte in the capacitor 6 decreases, the amount of electrolyte at a subsequent time ^_^ may be estimated or determined by: P51436WO-5891 ^ Electrolyte remaining ⁽%⁾ = _^ × 100, 3 = 1, 2, 3 …

For example, if the subsequent value ^_^ is three quarters of the initial value ^_^, the amount of remaining electrolyte may be calculated to be 75% of the initial amount at the start of the vaping session and numerical value “75%” may be displayed to the user on the visual display 48. Similarly, if the subsequent values ^_^ and ^ are respectively one half and one third of the initial value of ^_^ the amount of remining electrolyte may be calculated to be 50% and 33% of the initial amount at the start of the vaping session and numerical values “50%” and “33%” may be displayed to the user on the visual display 48. If the subsequent value ^ is not plausible – i.e., because it is either too low and therefore suggests that too much electrolyte has been vapourised since time ^_^ , or because it is too high and therefore suggests that not enough electrolyte has been vapourised since time ^_^ – or it falls outside an expected range, a different amount of electrolyte may be displayed to the user on the visual display 48. The expected range for value ^ may be based on a previously estimated or determined amount of electrolyte, for example on the value ^_^ that was estimated or determined at time ^_^. For example, it may be expected that the value ^ should be between about 60% and about 70% of the previous value ^_^ and if the value ^ that is estimated or determined at time ^ is outside this range (e.g., it is only 20% of value ^_^) it may not be plausible. If the value of ^ is not plausible, the amount of electrolyte that is notified to the user using the visual display 48 may instead be estimated using one or more previous amounts of electrolyte that were within the expected range. This may provide the user with a more reliable notification. For example, if the previous values ^_^ and ^_^ were plausible, they may be used to determine a rate of change of electrolyte. This rate of change may then be applied to the value ^_^ that was estimated or determined in the previous step at time ^_^ to derive an alternative value for time ^ . The determined rate of change may be linear or non-linear, for example. The alternative value is then P51436WO-5891 notified using the visual display 48 instead of the value ^ that was estimated or determined using the electrical parameter of the capacitor 6. 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. Thus, the breadth and scope of the claims should not be limited to the above-described exemplary embodiments. Any combination of the above-described features in all possible variations thereof is encompassed by the present disclosure unless otherwise indicated herein or otherwise clearly contradicted by context. Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like, are to be construed in an inclusive as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”. P51436WO-5891

Claims

Claims 1. A method of monitoring an aerosol generating article (1) comprising a capacitor (6), the capacitor (6) comprising: an electrolyte which, when heated, generates an aerosol for inhalation by a user; a pair of electrodes (16, 18); and a porous separator (20b) between the electrodes (16, 18); the method comprising estimating or determining the amount of electrolyte by applying to one of the pair of electrodes (16) an alternating current AC signal.

2. A method according to claim 1, wherein the amount of electrolyte is estimated or determined using one or more electrical parameters of the capacitor (6).

3. A method according to claim 2, wherein the one or more electrical parameters of the capacitor are estimated or determined using at least one of voltage and current measurements obtained in response to the applied AC signal.

4. A method according to claim 2 or claim 3, wherein if the amount of electrolyte that is estimated or determined using the one or more electrical parameters is outside an expected range, notifying an amount of electrolyte to the user that is estimated or determined based on one or more previously estimated or determined amounts of electrolyte.

5. A method according to any preceding claim, further comprising at least one of discharging and charging the capacitor (6) to heat the electrolyte and thereby generate an aerosol for inhalation by a user.

6. A method according to any preceding claim, further comprising operating the capacitor (6) in a heating mode and a non-heating mode, wherein the AC signal is applied to the electrode (16) when the capacitor (6) is in the heating mode and the non- heating mode. P51436WO-5891

7. A method according to any of claims 1 to 5, further comprising operating the capacitor (6) in a heating mode and a non-heating mode, wherein the AC signal is applied to the electrode only when the capacitor (6) is in the non-heating mode.

8. An aerosol generating system comprising: an aerosol generating article (1) comprising a capacitor (6), the capacitor (6) comprising: an electrolyte which, when heated, generates an aerosol for inhalation by a user; a pair of electrodes (16, 18); and a porous separator (20b) between the electrodes (16, 18); and an aerosol generating device (34) in which the aerosol generating article (1) is received, the aerosol generating device (34) further comprising a controller (56) adapted to implement the method according to any of claims 1 to 7.

9. An aerosol generating system comprising: an aerosol generating article (1) comprising a capacitor (6), the capacitor (6) comprising: an electrolyte which, when heated, generates an aerosol for inhalation by a user; a pair of electrodes (16, 18); and a porous separator (20b) between the electrodes (16, 18); and an aerosol generating device (34) in which the aerosol generating article (1) is received, the aerosol generating device (34) further comprising: a power source (44, 58); a switching circuit (50) electrically connected to the pair of electrodes (16, 18) and configured to control the discharging of the capacitor (6) and the charging of the capacitor (6) from the power source (44, 58); and an inverter (54) electrically connected to one of the pair of electrodes (16) and configured to apply to the electrode (16) an AC signal. P51436WO-5891

10. An aerosol generating system according to claim 9, wherein the capacitor (6) is configured to be operated in a heating mode and a non-heating mode, wherein the inverter (54) is configured to apply the AC signal to the electrode (16) when the capacitor (6) is in the heating mode and the non-heating mode.

11. An aerosol generating system according to claim 9 or claim 10, further comprising a superimposing circuit (74) electrically connected to the inverter (54) and the switching circuit (50) and configured to superimpose the AC signal on a direct current DC current that is supplied by the power source (44, 58).

12. An aerosol generating system according to claim 11, further comprising an AC coupling capacitor (C4) electrically connected between the inverter (54) and the superimposing circuit (74) in series.

13. An aerosol generating system according to claim 11 or claim 12, further comprising a bypass circuit (80) electrically connected to an input (82) and an output of the superimposing circuit (80) in parallel, the discharge current of the capacitor (6) being supplied to the input (82) of the superimposing circuit (74) via the bypass circuit (80).

14. An aerosol generating system according to any of claims 9 to 13, further comprising a charging switch (Q2) configured to turn on and off the charging of the capacitor (6) from the power source (44, 58), and a diode (D2) electrically connected between the power source (44, 58) and the charging switch (Q2).

15. An aerosol generating system according to claim 9, wherein the switching circuit (50) and the inverter (54) are electrically connected to the capacitor (6) in parallel, wherein the capacitor (6) is configured to be operated in a heating mode and a non-heating mode, wherein the inverter (54) is configured to apply the AC signal to the electrode (16) only when the capacitor (6) is in the non-heating mode. P51436WO-5891