Heater management

Abstract

An electrically operated aerosol-generating system configured to detect an adverse conditions, such as a dry heater or an unauthorized type of heater, is provided, including an electric heater including at least one heating element configured to heat an aerosol-forming substrate, a power supply, and electric circuitry connected to the electric heater and to the power supply and comprising a memory, the electric circuitry being configured to determine the adverse condition when a ratio between an initial electrical resistance of the electric heater and a change in electrical resistance from the initial resistance is greater than a maximum threshold value stored in the memory or is less than a minimum threshold value stored in the memory, and to limit power supplied to the electric heater, or to provide an indication to a user, if there is the adverse condition.

Claims

1. An electrically operated aerosol-generating system, comprising: an electric heater comprising at least one heating element configured to heat an aerosol-forming substrate; a power supply; and electric circuitry connected to the electric heater and to the power supply, and comprising a memory, the electric circuitry being configured to determine an adverse condition when a ratio between an initial electrical resistance of the electric heater and a change in electrical resistance from the initial electrical resistance is greater than a maximum threshold value stored in the memory or is less than a minimum threshold value stored in the memory, or when the ratio reaches a threshold value stored in the memory outside of an expected time period, and to limit power supplied to the electric heater, or to provide an indication, if there is the adverse condition.

2. The electrically operated aerosol-generating system according to claim 1, further comprising a device and a removable cartridge, wherein the power supply and the electric circuitry are disposed in the device and the electric heater is disposed in the removable cartridge, and wherein the removable cartridge comprises a liquid aerosol-forming substrate.

3. The electrically operated aerosol-generating system according to claim 1, wherein the aerosol-forming substrate is disposed in contact with the at least one heating element.

4. The electrically operated aerosol-generating system according to claim 1, further comprising a puff detector connected to the electric circuitry and being configured to detect when a user is puffing on the system, wherein the electric circuitry is further configured to: supply power from the power supply to the at least one heater element when a puff is detected by the puff detector, and determine if there is the adverse condition during each puff.

5. The electrically operated aerosol-generating system according to claim 1, wherein the system is an electrically heated smoking system.

6. A heater assembly, comprising: an electric heater comprising at least one heating element; and electric circuitry connected to the electric heater and comprising a memory, the electric circuitry being configured to determine that there is an adverse condition when a ratio between an initial electrical resistance of the electric heater and a change in electrical resistance from the initial electrical resistance is greater than a maximum threshold value stored in the memory or is less than a minimum threshold value stored in the memory, or when the ratio reaches a threshold value stored in the memory outside of an expected time period, and to control power supplied to the electric heater based on whether there is the adverse condition, or to provide an indication, if there is the adverse condition.

7. An electrically operated aerosol-generating device, comprising: a power supply; and electric circuitry connected to the power supply and comprising a memory, the electric circuitry being configured to connect to an electric heater and to determine an adverse condition when a ratio between an initial electrical resistance of the electric heater and a change in electrical resistance from the initial electrical resistance is greater than a maximum threshold value stored in the memory or is less than a minimum threshold value stored in the memory, or when the ratio reaches a threshold value stored in the memory outside of an expected time period, and to control power supplied to the electric heater based on whether there is the adverse condition, or to provide an indication, if there is the adverse condition.

8. Electric circuitry for an electrically operated aerosol-generating device, the electric circuitry comprising: a memory, the electric circuitry being connected to an electric heater and to a power supply, the electric circuitry being configured to determine an adverse condition when a ratio between an initial electrical resistance of the electric heater and a change in electrical resistance from the initial electrical resistance is greater than a maximum threshold value stored in the memory or is less than a minimum threshold value stored in the memory, or when the ratio reaches a threshold value stored in the memory outside of an expected time period, and to control power supplied to the electric heater based on whether there is the adverse condition, or to provide an indication, if there is the adverse condition.

9. Electric circuitry for an electrically operated aerosol-generating device, the electric circuitry comprising: a memory, the electric circuitry being connected to an electric heater configured to heat an aerosol-forming substrate and to a power supply, the electric circuitry being configured to measure an initial resistance, or an initial rate of change of resistance, of the electric heater within a predetermined time period after power is supplied to the electric heater, compare the initial resistance or the initial rate of change of resistance of the electric heater with a range of acceptable values, and if the initial resistance or the initial rate of change of resistance is outside the range of acceptable values, prevent a supply of power to the electric heater, or provide an indication, until the electric heater or the aerosol-forming substrate is replaced.

10. A method of controlling a supply of power to a heater in an electrically operated aerosol-generating system, the system comprising an electric heater comprising at least one heating element configured to heat an aerosol-forming substrate, and a power supply configured to supply power to the electric heater, the method comprising: determining an adverse condition when a ratio between an initial electrical resistance of the electric heater and a change in electrical resistance from the initial electrical resistance is greater than a maximum threshold value stored in a memory or is less than a minimum threshold value stored in the memory, or when the ratio reaches a threshold value stored in the memory outside of an expected time period, and limiting power supplied to the electric heater, or providing an indication to a user, in dependence on detection of the adverse condition.

11. The method according to claim 10, further comprising measuring an initial resistance, or an initial rate of change of resistance, of the electric heater within a predetermined time period after power is supplied to the electric heater, comparing the initial resistance or the initial rate of change of resistance of the electric heater with a range of acceptable values, and if the initial resistance or the initial rate of change of resistance is outside the range of acceptable values, preventing a supply of power to the electric heater, or providing an indication, until the electric heater or the aerosol-forming substrate is replaced.

12. The method according to claim 10, further comprising detecting when the electric heater or the aerosol-forming substrate is inserted into the system.

13. A nontransitory computer-readable storage medium having a computer program stored thereon, which when executed on an internal memory of a microprocessor, in an electrically operated aerosol-generating system comprising an electric heater comprising at least one heating element configured to heat an aerosol-forming substrate, and a power supply configured to supply power to the electric heater, the microprocessor being connected to the electric heater and to the power supply, causes the microprocessor to perform the method according to claim 10.

Description

(1) The invention will be further described, by way of example only, with reference to the accompanying drawings, in which:

(2) FIGS. 1a to 1d are schematic illustrations of a system in accordance with an embodiment of the invention;

(3) FIG. 2 is an exploded view of a cartridge for use in a system as shown in FIGS. 1a to 1d;

(4) FIG. 3 is a detailed view of the filaments of the heater, showing a meniscus of liquid aerosol-forming substrate between the filaments;

(5) FIG. 4 is a schematic illustration of the change of resistance of the heater during a user puff;

(6) FIG. 5 is an electric circuit diagram showing how the heating element resistance may be measured;

(7) FIGS. 6a, 6b and 6c illustrate control processes following detection of an adverse condition;

(8) FIG. 7 is a schematic illustration of a first alternative aerosol-generating system;

(9) FIG. 8 is a schematic illustration of a second alternative aerosol-generating system; and

(10) FIG. 9 is flow chart illustrating a method for detecting an unauthorised, damaged or incompatible heater.

(11) FIGS. 1a to 1d are schematic illustrations of an aerosol-generating system, including a cartridge in accordance with an embodiment of the invention. FIG. 1a is a schematic view of an aerosol-generating device 10 and a separate cartridge 20, which together form the aerosol-generating system. In this example, the aerosol-generating system is an electrically operated smoking system.

(12) The cartridge 20 contains an aerosol-forming substrate and is configured to be received in a cavity 18 within the device. Cartridge 20 should be replaceable by a user when the aerosol-forming substrate provided in the cartridge is depleted. FIG. 1a shows the cartridge 20 just prior to insertion into the device, with the arrow 1 in FIG. 1a indicating the direction of insertion of the cartridge.

(13) The aerosol-generating device 10 is portable and has a size comparable to a conventional cigar or cigarette. The device 10 comprises a main body 11 and a mouthpiece portion 12. The main body 11 contains a battery 14, such as a lithium iron phosphate battery, electric circuitry 16 and a cavity 18. The electric circuitry 16 comprises a programmable microprocessor. The mouthpiece portion 12 is connected to the main body 11 by a hinged connection 21 and can move between an open position as shown in FIG. 1 and a closed position as shown in FIG. 1d. The mouthpiece portion 12 is placed in the open position to allow for insertion and removal of cartridges 20 and is placed in the closed position when the system is to be used to generate aerosol. The mouthpiece portion comprises a plurality of air inlets 13 and an outlet 15. In use, a user sucks or puffs on the outlet to draw air from the air inlets 13, through the mouthpiece portion to the outlet 15, and thereafter into the mouth or lungs of the user. Internal baffles 17 are provided to force the air flowing through the mouthpiece portion 12 past the cartridge.

(14) The cavity 18 has a circular cross-section and is sized to receive a housing 24 of the cartridge 20. Electrical connectors 19 are provided at the sides of the cavity 18 to provide an electrical connection between the control electronics 16 and battery 14 and corresponding electrical contacts on the cartridge 20.

(15) FIG. 1b shows the system of FIG. 1a with the cartridge inserted into the cavity 18, and the cover 26 being removed. In this position, the electrical connectors rest against the electrical contacts on the cartridge.

(16) FIG. 1c shows the system of FIG. 1b with the cover 26 fully removed and the mouthpiece portion 12 being moved to a closed position.

(17) FIG. 1d shows the system of FIG. 1c with the mouthpiece portion 12 in the closed position. The mouthpiece portion 12 is retained in the closed position by a clasp mechanism. The mouthpiece portion 12 in a closed position retains the cartridge in electrical contact with the electrical connectors 19 so that a good electrical connection is maintained in use, whatever the orientation of the system is.

(18) FIG. 2 is an exploded view of the cartridge 20. The cartridge 20 comprises a generally circular cylindrical housing 24 that has a size and shape selected to be received into the cavity 18. The housing contains capillary material 27, 28 that is soaked in a liquid aerosol-forming substrate. In this example the aerosol-forming substrate comprises 39% by weight glycerine, 39% by weight propylene glycol, 20% by weight water and flavourings, and 2% by weight nicotine. A capillary material is a material that actively conveys liquid from one end to another, and may be made from any suitable material. In this example the capillary material is formed from polyester.

(19) The housing has an open end to which a heater assembly 30 is fixed. The heater assembly 30 comprises a substrate 34 having an aperture 35 formed in it, a pair of electrical contacts 32 fixed to the substrate and separated from each other by a gap 33, and a plurality of electrically conductive heater filaments 36 spanning the aperture and fixed to the electrical contacts on opposite sides of the aperture 35.

(20) The heater assembly 30 is covered by a removable cover 26. The cover comprises a liquid impermeable plastic sheet that is glued to the heater assembly but which can be easily peeled off. A tab is provided on the side of the cover to allow a user to grasp the cover when peeling it off. It will now be apparent to one of ordinary skill in the art that although gluing is described as the method to a secure the impermeable plastic sheet to the heater assembly, other methods familiar to those in the art may also be used including heat sealing or ultrasonic welding, so long as the cover may easily be removed by a consumer.

(21) There are two separate capillary materials 27, 28 in the cartridge of FIG. 2. A disc of a first capillary material 27 is provided to contact the heater element 36, 32 in use. A larger body of a second capillary material 28 is provided on an opposite side of the first capillary material 27 to the heater assembly. Both the first capillary material and the second capillary material retain liquid aerosol-forming substrate. The first capillary material 27, which contacts the heater element, has a higher thermal decomposition temperature (at least 160 C. or higher such as approximately 250 C.) than the second capillary material 28. The first capillary material 27 effectively acts as a spacer separating the heater element 36, 32 from the second capillary material 28 so that the second capillary material is not exposed to temperatures above its thermal decomposition temperature. The thermal gradient across the first capillary material is such that the second capillary material is exposed to temperatures below its thermal decomposition temperature. The second capillary material 28 may be chosen to have superior wicking performance to the first capillary material 27, may retain more liquid per unit volume than the first capillary material and may be less expensive than the first capillary material. In this example the first capillary material is a heat resistant material, such as a fiberglass or fiberglass containing material and the second capillary material is a polymer such as suitable capillary material. Exemplary suitable capillary materials include the capillary materials discussed herein and in alternative embodiments may include high density polyethylene (HDPE), or polyethylene terephthalate (PET).

(22) The capillary material 27, 28 is advantageously oriented in the housing 24 to convey liquid to the heater assembly 30. When the cartridge is assembled, the heater filaments 36, 37, 38 may be in contact with the capillary material 27 and so aerosol-forming substrate can be conveyed directly to the mesh heater. FIG. 3 is a detailed view of the filaments 36 of the heater assembly, showing a meniscus 40 of liquid aerosol-forming substrate between the heater filaments 36. It can be seen that aerosol-forming substrate contacts most of the surface of each filament so that most of the heat generated by the heater assembly passes directly into the aerosol-forming substrate.

(23) So, in normal operation, liquid aerosol-forming substrate contacts a large portion of the surface of the heater filaments 36. However, when most of the liquid substrate in the cartridge has been used, less liquid aerosol-forming substrate will be delivered to the heater filaments. With less liquid to vaporize, less energy is taken up by the enthalpy of vaporization and more of the energy supplied to the heating filaments is directed to raising the temperature of the heating filaments. So as the heater element dries out, the rate of increase of temperature of the heater element for a given applied power will increase. The heater element may dry out because the aerosol-forming substrate in the cartridge is almost used up or because the user is taking very long or very frequent puffs and the liquid can not be delivered to the heater filaments as fast as it is being vaporized.

(24) In use, the heater assembly operates by resistive heating. Current is passed through the filaments 36 under the control of control electronics 16, to heat the filaments to within a desired temperature range. The mesh or array of filaments has a significantly higher electrical resistance than the electrical contacts 32 and electrical connectors 19 so that the high temperatures are localised to the filaments. In this example, the system is configured to generate heat by providing electrical current to the heater assembly in response to a user puff. In another embodiment the system may be configured to generate heat continuously while the device is in an on state. Different materials for the filaments may be suitable for different systems. For example, in a continuously heated system, NiCr filaments are suitable as they have a relatively low specific heat capacity and are compatible with low current heating. In a puff actuated system, in which heat is generated in short bursts using high current pulses, stainless steel filaments, having a high specific heat capacity may be more suitable.

(25) The system includes a puff sensor configured to detect when a user is drawing air through the mouthpiece portion. The puff sensor (not illustrated) is connected to the control electronics 16 and the control electronics 16 are configured to supply current to the heater assembly 30 only when it is determined that the user is puffing on the device. Any suitable air flow sensor may be used as a puff sensor, such as a microphone or pressure sensor.

(26) In order to detect this increase in the rate of temperature change, the electric circuitry 16 is configured to measure the electrical resistance of the heater filaments. The heater filaments in this example are formed from stainless steel, and so have a positive temperature coefficient of resistance. This means that as the temperature of the heater filaments rises so does their electrical resistance.

(27) FIG. 4 is a schematic illustration of the change of resistance of the heater during a user puff. The x-axis is time after initial detection of a user puff and the resulting supply of power to the heater. The y-axis is electrical resistance of the heater assembly. It can be seen that the heater assembly has an initial resistance R1 before any heating has occurred. R1 is made up of a parasitic resistance RP resulting from the electrical contacts 32 and electrical connectors 19 and the contact between them, and the resistance of the heater filaments R0. As power is applied to the heater during a user puff, the temperature of the heater filaments rises and so the electrical resistance of the heater filaments rises. As illustrated, at time t.sub.1 the resistance of the heater assembly is R2. The change in electrical resistance of the heater assembly from the initial resistance to the resistance at time t.sub.1 is therefore R=R2R1.

(28) In this example the parasitic resistance RP is assumed to not change as the heater filaments heat up. This is because RP is attributable to non-heated components, such as the electrical contacts 32 and electrical connectors 19. The value of RP is assumed to be the same for all cartridges and a value is stored in the memory of the electric circuitry.

(29) The relationship between the resistance of the heater filaments and their temperature is given by the following equation:
R2=R0*(1+*T)+RP(1)

(30) where is the temperature coefficient of electrical resistance of the heater filaments and T is the change in temperature between an initial temperature before the application of power to the heater and the temperature at time t.sub.1.

(31) A threshold value K is stored in the electric circuitry, where K is equal to *Tmax. If the temperature rises by more than Tmax in time t.sub.1 then there is considered to be an adverse condition, such as dry conditions at the heater.

(32) From Equation 1:
K=*Tmax=R/R0(2)

(33) So in order to detect a rapid increase in temperature indicative of dry conditions at the heater filaments the value of the ratio R/R0 can be compared with a stored value of K. If R/R0>K then there are dry conditions at the heater.

(34) This comparison can be performed by the electric circuitry but the inequality can be rearranged to suit the electronic processing operation, in particular to avoid the need to perform any division. In this example, software running on microprocessor in the electric circuitry performs the following comparison, derived from Equation 1:
If R2>(R1*(K+1)K*RP) then there are dry conditions at the heater(3)

(35) R2 and R1 are both measured values and K and RP are stored in memory. Ideally the value of R1 is measured before any heating takes place, in other words before first activation of the heater, and that measured value is used for all subsequent puffs. This avoids any error resulting from residual heat from previous puffs. R1 may be measured only once for each cartridge and a detection system used to determine when a new cartridge is inserted, or R1 may be measured each time the system is switched on.

(36) Other adverse conditions besides dry heater conditions may be detected in this way. If a cartridge having a heater formed from a material having a different temperature coefficient of resistance is used in the system, the electric circuitry can detect that and may be configured not to supply power to it. In the present example, the heater filaments are formed from stainless steel. A cartridge having a heater formed from NiCr would have a lower temperature coefficient of resistance, meaning that its resistance would rise more slowly with increasing temperature. So if a value K2, which equals *Tmin, is stored in memory, which corresponds to the lowest temperature rise in time t.sub.1 expected for a stainless steel heater element, then if R2<(R1*(K2+1)K*RP) then the circuitry determines an adverse condition corresponding to an unauthorized cartridge being present in the system. FIG. 9 illustrates a process for detecting an incompatible heater.

(37) So the system may be configured to compare R2 or R/R0, or even R/R1 with a stored high threshold and a stored low threshold in order to determine an adverse condition. R1 may also be compared with a threshold or thresholds to check that it is within an expected range. They may even be more than one high stored threshold and different actions taken depending on which high threshold is exceeded. For example, if the highest threshold is exceeded then the circuitry may prevent further supply of power until the heater and/or substrate is replaced. This may be indicative of a completely depleted substrate or a damages or incompatible heater. A lower threshold may be used to determine when the substrate is nearly depleted. If this lower threshold is exceeded, but the higher threshold is not exceeded, then the circuitry may simply provide an indication, such as an illuminated LED, showing that the substrate will soon need to be replaced.

(38) The ratio of R/R0 may be continually monitored to determine if the heater is cooling sufficiently between puffs. If the ratio does not go below a cooling threshold between puffs because a user is puffing very frequently, the electric circuitry may prevent or limit the supply of power to the heater until the ratio falls below the cooling threshold. Alternatively, a comparison may be made between a maximum value of the ratio during a puff and a minimum value for the ratio subsequent to the puff, to determine if sufficient cooling is occurring.

(39) Also, the ratio R/R0 may be continually monitored and the time at which it reaches a threshold value compared with a time threshold. If R/R0 reaches the threshold much faster or slower than expected, then it may be indicative of an adverse condition, such as an incompatible heater. The rate of change of R could also be determined and compared with a threshold. If R rises very quickly or very slowly then it may be indicative of an adverse condition. These techniques may allow for incompatible heaters to be detected very quickly.

(40) FIG. 5 is a schematic electric circuit diagram showing how the heating element resistance may be measured. In FIG. 5, the heater 501 is connected to a battery 503 which provides a voltage V2. The heater resistance to be measured at a particular time is R.sub.heater. In series with the heater 501, an additional resistor 505, with known resistance r is inserted connected to voltage V1, intermediate between ground and voltage V2. In order for microprocessor 507 to measure the resistance R.sub.heater of the heater 501, the current through the heater 501 and the voltage across the heater 501 can both be determined. Then, the following well-known formula can be used to determine the resistance:
V=IR(4)

(41) In FIG. 5, the voltage across the heater is V2-V1 and the current through the heater is I. Thus:

(42) $\begin{array}{cc}{R}_{\mathrm{heater}}=\frac{V2-V1}{I}& \left(5\right)\end{array}$

(43) The additional resistor 505, whose resistance r is known, is used to determine the current I, again using (1) above. The current through the resistor 505 is I and the voltage across the resistor 505 is V1. Thus:

(44) $\begin{array}{cc}I=\frac{V1}{r}& \left(6\right)\end{array}$

(45) So, combining (5) and (6) gives:

(46) $\begin{array}{cc}{R}_{\mathrm{heater}}=\frac{\left(V2-V1\right)}{V1}r& \left(7\right)\end{array}$

(47) Thus, the microprocessor 507 can measure V2 and V1, as the aerosol generating system is being used and, knowing the value of r, can determine the heater's resistance, R.sub.heater at different times.

(48) The electric circuitry can control the supply of power to the heater in several different ways following an adverse condition being detected. Alternatively, or in addition, the electric circuitry may simply provide an indication to the use that an adverse condition has been detected. The system may include an LED or display or may comprise a microphone, and these components may be used to issue an alert of an adverse condition to the user.

(49) FIG. 6a illustrates a first control process for a puff actuated system. In the scheme illustrated in FIG. 6a, if R/R0 exceeds the high threshold for a single puff, the electric circuitry continues to supply power to the heater. FIG. 6a shows three consecutive puffs during which the high threshold is exceeded. Only if R/R0 exceeds the high threshold for a particular number of consecutive puffs, say 3, 4, or 5 puffs, is power to the heater stopped. A single instance of the threshold being exceeded could be the result of a very long user puff, but several consecutive puffs during which the high threshold is exceeded is more likely to be the result of the cartridge becoming empty. At that point the cartridge may be disabled, for example by blowing a fuse within the cartridge, or the electric circuitry may block the supply of further power until the cartridge is replaced or refilled.

(50) FIG. 6b discloses another control process that may be used as an alternative, or in addition to the process described with reference to FIG. 6b. In the control process of FIG. 6b the electric circuitry stops the supply of power to the heater as soon as it is determined that the high threshold has been exceeded, until the end of the user puff. When a new user puff is detected power is supplied to the heater again. This may be useful to prevent the heater becoming too hot even when the user is puffing excessively. As well as stopping the power, an indication could be provided that the threshold has been reached.

(51) FIG. 6c illustrates an alternative control process in which the electric circuitry stops the supply of power to the heater as soon as it is determined that the high threshold has been exceeded. The supply of power is prevented for subsequent user puffs too. In order for power to be supplied to the heater again, the user may be required to replace the cartridge or perform a resetting operation. This control process may be used in conjunction with the processes described with reference to FIGS. 6a and 6b but on the basis of a higher threshold than is used in the processes described with reference to FIGS. 6a and 6b. The higher threshold may be indicative of a completely depleted aerosol-forming substrate or of a defective or incompatible heater.

(52) Although the invention has been described with reference to a cartridge based system, with a mesh heater, the same adverse condition detection methods can be used in other aerosol-generating systems.

(53) FIG. 7 illustrates an alternative system, which also uses a liquid substrate and a capillary material, in accordance with the invention. In FIG. 7, the system is a smoking system. The smoking system 100 of FIG. 7 comprises a housing 101 having a mouthpiece end 103 and a body end 105. In the body end, there is provided an electric power supply in the form of battery 107 and electric circuitry 109. A puff detection system 111 is also provided in cooperation with the electric circuitry 109. In the mouthpiece end, there is provided a liquid storage portion in the form of cartridge 113 containing liquid 115, a capillary wick 117 and a heater 119. Note that the heater is only shown schematically in FIG. 7. One end of capillary wick 117 extends into cartridge 113 and the other end of capillary wick 117 is surrounded by the heater 119. The heater is connected to the electric circuitry via connections 121, which may pass along the outside of cartridge 113 (not shown in FIG. 7). The housing 101 also includes an air inlet 123, an air outlet 125 at the mouthpiece end, and an aerosol-forming chamber 127.

(54) In use, operation is as follows. Liquid 115 is conveyed by capillary action from the cartridge 113 from the end of the wick 117 which extends into the cartridge to the other end of the wick which is surrounded by heater 119. When a user draws on the aerosol generating system at the air outlet 125, ambient air is drawn through air inlet 123. In the arrangement shown in FIG. 7, the puff detection system 111 senses the puff and activates the heater 119. The battery 107 supplies electrical energy to the heater 119 to heat the end of the wick 117 surrounded by the heater. The liquid in that end of the wick 117 is vaporized by the heater 119 to create a supersaturated vapour. At the same time, the liquid being vaporized is replaced by further liquid moving along the wick 117 by capillary action. The supersaturated vapour created is mixed with and carried in the air flow from the air inlet 123. In the aerosol-forming chamber 127, the vapour condenses to form an inhalable aerosol, which is carried towards the outlet 125 and into the mouth of the user.

(55) In the embodiment shown in FIG. 7, the electric circuitry 109 and puff detection system 111 are programmable as in the embodiment of FIGS. 1a to 1d.

(56) The capillary wick can be made from a variety of porous or capillary materials and preferably has a known, pre-defined capillarity. Examples include ceramic- or graphite-based materials in the form of fibres or sintered powders. Wicks of different porosities can be used to accommodate different liquid physical properties such as density, viscosity, surface tension and vapour pressure. The wick must be suitable so that the required amount of liquid can be delivered to the heater when the liquid storage portion has sufficient liquid.

(57) The heater comprises at least one heating wire or filament extending around the capillary wick.

(58) As in the system described with reference to FIGS. 1 to 3, the capillary material forming the wick may dry out in the vicinity of the heater wire if the liquid in the cartridge is used up or if the user takes very long, deep puffs. In the same way as described with reference to the system of FIGS. 1 to 3, the change in resistance of the heater wire during the first portion of each puff can be used to determine if there is an adverse condition, such as a dry wick.

(59) A system of the type illustrated in FIG. 7 may have considerable variation in heater resistance, even between cartridges of the same type, because of variations in the length of heater wire wrapped around the wick. The invention is particularly advantageous as it does not require the electric circuitry to store a maximum heater resistance value as a threshold; instead it is a resistance increase relative to an initial measured resistance that is used.

(60) FIG. 8 illustrates yet another aerosol-generating system which can embody the invention. The embodiment of FIG. 8 is electrically heated tobacco device in which a tobacco based solid substrate is heated, but not combusted, to produce an aerosol for inhalation. In FIG. 8 the components of the aerosol-generating device 700 are shown in a simplified manner and are not drawn to scale. Elements that are not relevant for the understanding of this embodiment have been omitted to simplify FIG. 8.

(61) The electrically heated aerosol-generating device 200 comprises a housing 203 and an aerosol-forming substrate 210, for example a cigarette. The aerosol-forming substrate 210 is pushed inside a cavity 205 formed by the housing 203 to come into thermal proximity with the heater 201. The aerosol-forming substrate 210 releases a range of volatile compounds at different temperatures. By controlling the operation temperature of the electrically heated aerosol-generating device 200 to be below the release temperature of some of the volatile compounds, the release or formation of these smoke constituents can be avoided.

(62) Within the housing 203 there is an electrical power supply 207, for example a rechargeable lithium ion battery. Electric circuitry 209 is connected to the heater 201 and the electrical power supply 207. The electric circuitry 209 controls the power supplied to the heater 201 in order to regulate its temperature. An aerosol-forming substrate detector 213 may detect the presence and identity of an aerosol-forming substrate 210 in thermal proximity with the heater 201 and signals the presence of an aerosol-forming substrate 210 to the electric circuitry 209. The provision of a substrate detector is optional. An airflow sensor 211 is provided within the housing and connected to the electric circuitry 209, to detect the airflow rate through the device.

(63) In the described embodiment the heater 201 is an electrically resistive track or tracks deposited on a ceramic substrate. The ceramic substrate is in the form of a blade and is inserted into the aerosol-forming substrate 210 in use. The heater forms part of the device and may be used for heating many different substrates. However, the heater may be a replaceable component, and replacement heaters may have different electrical resistance.

(64) A system of the type described in FIG. 8 may be a continuously heated system in which the temperature of the heater is maintained at a target temperature while the system in on, or it may be a puff actuated system in the temperature of the heater is raised by supplying more power during periods when a puff is detected.

(65) In the case of a puff actuated system, the operation is very similar to that described with reference to the preceding embodiments. If the substrate is dry in the vicinity of the heater, the heater resistance will rise more quickly for a given applied power than if the substrate still contains aerosol-formers that can be vaporized at relatively low temperature.

(66) In the case of a continuously heated system, there will be a temperature drop of the heater initially when a used puffs on the system due to the cooling effect of airflow past the heater. The heater resistance can be measured when a puff is first detected and recorded as R1 and the subsequent resistance R2 as the system bring the heater back up to the target temperature can be measured at time t.sub.1 after puff detection, in a similar manner as described. R and R0 can then be calculated as previously described and the ratio of R/R0 can then be compared to a stored threshold, as previously described to determine if the substrate is dry in the vicinity of the heater. The substrate may be dry because it has been depleted through use or because it is old or has been improperly stored, or because it is counterfeit and has a different moisture content to a genuine aerosol-forming substrate.

(67) The system of FIG. 8 includes a warning LED 215 in the electric circuitry 209 which is illuminated when an adverse condition is detected.

(68) FIG. 9 is flow chart illustrating a method for detecting an unauthorised, damaged or incompatible heater. In a first step 300, the insertion of a cartridge, including the heater, into the device is detected. Then the electrical resistance of the heater R.sub.1 is measured in step 300. This occurs a predetermined time period after power is supplied to the heater, such as 100 ms. In step 320 the measured resistance R.sub.1 is compared with a range of expected or acceptable resistances. The range of acceptable resistances takes account of manufacturing tolerances and variations between genuine heaters and substrates. If R.sub.1 is outside of the expected range then the process proceeds to step 330, in which an indication, such as an audible alarm, is provided and power is prevented from being supplied to the heater as it is considered to be incompatible with the device. The process then returns to step 300, waiting for detection of the insertion of a new cartridge.

(69) As an alternative, or in addition, to measuring an initial resistance R.sub.1 in step 300, an initial rate of change of resistance may be measured within a pretermined time period, say 100 ms, after power is supplied to the heater. This may be done by taking a plurality of resistance measurements at different times during the predetermined time period and then calculating an initial rate of change of resistance from the plurality of resistance measurements and the times at which those measurements were taken. In the same way that a particular design of heater can be expected to have an initial resistance within a range of acceptable values, a particular design of heater can be expected to have an initial rate of change of resistance for a given applied power within an acceptable range of rate of change of resistance values. The calculated initial rate of change of resistance can be compared to an acceptable range of rate of change of resistance values and if the calculated rate of change of resistance is outside of the acceptable range, then the process proceeds to step 330.

(70) If in step 320 it is determined that R.sub.1 is in the range of expected resistance, then the process proceeds to step 340. In step 340, power is applied to the heater for a time period t.sub.1, after which the ratio R/R0 is calculated. Advantageously, t.sub.1 is chosen to be a short time period, before significant generation of aerosol. In step 350 the value of the ratio R/R0 is compared with a range of expected or acceptable values. The range of expected values again takes account of variations in the manufacture of the heater and substrate assembly. If the value of R/R0 is outside of the expected range, the heater is considered incompatible and the process goes to step 330, as previously described, and then returns to step 300. If the value of R/R0 is inside the expected range, then the process proceeds to step 360, in which power is supplied to the heater to allow for the generation of aerosol on demand by the user.

(71) Although the invention has been described with reference to three different types of electrical smoking systems, it should be clear that it is applicable to other aerosol-generating systems.

(72) It should also be clear that the invention may be implemented as a computer program product for execution on programmable controllers within existing aerosol-generating systems. The computer program product may be provided as a downloadable piece of software or on a computer readable medium such as a compact disc.

(73) The exemplary embodiments described above illustrate but are not limiting. In view of the above discussed exemplary embodiments, other embodiments consistent with the above exemplary embodiments will now be apparent to one of ordinary skill in the art.