Heater apparatus and methods

Abstract

Apparatus for drying and / or styling hair comprises a heater 6a comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zone

Description

Field of the Invention The present invention relates to heater calibration apparatus and heater calibration methods. The heaters can be used for drying and / or styling hair. Such drying and / or styling of the hair may be performed by a user in respect of their own hair, for example, or by a hair stylist. The invention has particular, but not exclusive, relevance to the calibration of hair styling and hair drying devices. Background to the Invention Heated hair styling and hair drying tools use heat to increase the temperature of hair to a desired styling temperature. For example, a hair straighter having a heated plate applies heat directly via conduction to heat the hair, which may be either wet or dry, to achieve the desired temperature for styling. The hair may be heated to a temperature that is particularly suitable for styling hair (for example, to or beyond a glass transition phase temperature). At lower temperatures, the user may have to make many passes with the hair straightener over the hair to achieve a desired styling effect, whereas at higher temperatures, there is a risk of causing permanent damage to the hair. Similarly, a heated brush or hair dryer can also be used to style hair by heating the hair to a temperature suitable for styling. The hair is typically styled from wet, for example after the user has washed their hair, although the hair could also be styled from dry. Existing hair styling appliances typically use relatively thick heating plates or heating tubes that provide a certain amount of thermal mass to the hair styling appliance. These heating plates or tubes are heated by a heater that is mounted on an inner surface of the heating plate / tube. As a result of the thermal mass, the heating plates / tubes take time to heat up and, once heated, they can take quite a long time to cool down. This thermal mass makes it quite difficult to control the heating of the hair and over heating or under heating of the hair can result. There has been a general desire to move towards hair styling appliances that use heaters that have a lower thermal mass and can therefore heat up and cool down much more quickly. Such low thermal mass heaters are therefore more responsive and are easier to control. Hair styling and hair drying devices typically pass current through one or more heater electrodes to generate heat for subsequent transfer of the heat to the hair of a user (for example via conductive heating, convective heating or radiative heating). The voltage supplied to the heater is controlled in order to modulate the heat output from the heater electrodes. The heat output will depend on both the applied voltage and the resistance of the heater electrodes. However, whilst the heater electrodes are manufactured to have a particular resistance, the actual resistance of the heater electrodes will have some variation that will depend on the manufacturing tolerances. Since the heat output depends on the heater electrode resistance, the heat output at a particular applied voltage will therefore have some variation between devices. It is therefore beneficial to test and calibrate the heat output from the heaters after manufacture, to enable the heat output to be more accurate and consistent between devices. Testing the devices after manufacture also enables devices for which the heater electrode resistance falls outside of an acceptable tolerance to be identified. However, there is a need for improved methods for calibration and testing of hair styling and hair drying devices. For example, there is a need for faster and more accurate calibration methods to enable large numbers of devices to be efficiently calibrated. There is also a need for improved methods for detecting devices that have faults, for example an irregularity in the response of a heater to a supply of power. The present invention aims to address or at least partially ameliorate one or more of the above problems. Summary In a first aspect the invention provides apparatus for drying and / or styling hair, the apparatus comprising: a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein each of the heater electrodes has a resistance that varies with the temperature of the heater electrode; a memory storing separate calibration information for each of the heating zones that indicates a respective relationship between at least one of: the resistance of the corresponding heater electrode and the temperature of the corresponding heating zone, the voltage supplied to the corresponding heater electrode and the temperature of the corresponding heating zone, or a measured temperature of the corresponding heating zone and an actual temperature of the corresponding heating zone; and a controller configured to control the apparatus to: independently control the supply of power to each of the heater electrodes, based on the respective calibration information stored in the memory, to control the temperature of each of the heating zones towards a respective target temperature. The calibration information may comprise a temperature offset for each of the heating zones, for correcting the measured temperature of the heating zone. The respective relationship may be a linear relationship, wherein the calibration information comprises a gain and an offset for each of the heating zones. The apparatus for drying and / or styling hair may be a hair straightener. The apparatus for drying and / or styling hair may be configured for receiving the calibration information from a calibration apparatus. The apparatus for drying and / or styling hair may be configured for receiving the calibration information from the calibration apparatus via a wireless communication link. The apparatus for drying and / or styling hair may be configurable in a calibration mode in which the supply of power to the heater is controllable by the calibration apparatus. In a second aspect the invention provides calibration apparatus for calibrating the apparatus for drying and / or styling hair according to the first aspect, the calibration apparatus comprising: a plurality of temperature sensing zones, wherein each of the plurality of temperature sensing zones is configured for thermal contact with a respective one of the independently controllable heating zones of the hair contacting surface, and wherein each of the temperature sensing zones has a resistance that varies with the temperature of the temperature sensing zone; and communication circuitry configured for communication with the apparatus for drying and / or styling hair; wherein the calibration apparatus is configured for: sensing the temperature of each of the heating zones based on the resistance of the respective temperature sensing zone; obtaining calibration information for each of the heating zones; and transmitting the calibration information to the apparatus for drying and / or styling hair using the communication circuitry. Obtaining the calibration information may comprise generating, at the calibration apparatus, the calibration information for each of the heating zones. Obtaining the calibration information may comprise: transmitting, to a calibration controller, measurement results corresponding to the sensed temperatures for each of the heating zones; and receiving, from the calibration controller, the calibration information for each of the heating zones. The calibration apparatus may be configured for controlling a supply of power to the heater of the apparatus for drying and / or styling hair. The communication circuitry may be configured for wireless communication with the apparatus for drying and / or styling hair. The communication circuitry may be configured to use a first transmission power for wireless communication with the apparatus for drying and / or styling hair; and in a case where communication with the apparatus for drying and / or styling hair using the first transmission power is unsuccessful, the communication circuitry is configured to use a second transmission power, greater than the first transmission power, for wireless communication with the apparatus for drying and / or styling hair. The calibration apparatus may comprise a calibration status indicator for indicating a calibration status of the apparatus for drying and / or styling hair. The calibration status indicator may comprise a coloured light for indicating the calibration status. In a third aspect the invention provides a calibration system comprising: at least one calibration apparatus according to the second aspect for calibrating the apparatus for drying and / or styling hair; and a calibration controller configured for communication with the calibration apparatus; wherein the calibration controller is configured for: receiving, from the calibration apparatus, measurement results corresponding to the sensed temperatures for each of the independently controllable heating zones; and transmitting, to the apparatus for drying and / or styling hair, the calibration information for each of the heating zones. The calibration controller may be configured to transmit the calibration information to the apparatus for drying and / or styling hair via the calibration apparatus. The calibration controller may be configured to transmit the calibration information directly to the apparatus for drying and / or styling hair. The calibration controller may be configured to transmit the calibration information to the apparatus for drying and / or styling hair via wireless communication link. The calibration controller may be configured for controlling a supply of power to the heater of the apparatus for drying and / or styling hair. Brief Description of the Drawings Embodiments of the invention will now be described, by way of example only, and with reference to the drawings in which: Figure 1a shows an overview of an exemplary hair styling device; Figure 1b shows a hair styling device in use; Figure 2 shows a set of heating zones of the hair styling device; Figure 3 schematically illustrates the heating zones on the heating surface of the heater shown in Figure 2; Figure 4 schematically illustrates an alternative arrangement of heating zones; Figure 5 schematically illustrates a further alternative arrangement of heating zones that are of different sizes and shapes; Figure 6a illustrates the way in which the heating zones may be formed on a tubular substrate for use in a curling tong or the like; Figure 6b illustrates the way in which the heating zones may be arranged on a curved substrate which may be used on a heated brush or the like; Figure 7 shows a simplified block diagram illustrating the main electronic components of the hair styling device shown in Figure 1; Figure 8 illustrates a method of controlling heating zones of the hair styling device; Figure 9 shows a simplified schematic illustration of heating zones and tresses of hair on two of the heating zones; Figure 10 shows a simplified schematic illustration of heating zones and a tress of hair on the heating zones; Figure 11 a shows a table of target temperatures for a heating zone; Figure 11b shows a table of target temperatures assigned for each heating zone; Figure 12a illustrates an exploded view of a low thermal mass heater; Figure 12b illustrates a perspective see-through assembled view of the low thermal mass heater shown in Figure 12a; Figure 13 shows a simplified schematic illustration of a configuration of layers of a low thermal mass heater; Figure 14 shows a simplified schematic illustration of an alternative configuration of layers of a low thermal mass heater; Figure 15 shows a simplified schematic illustration of a further alternative configuration of layers of a low thermal mass heater; Figure 16 shows schematic circuit diagram of the hair styling device illustrating heaters, a controller, and control electronics; Figure 17 shows a flow diagram of a method of monitoring the temperature of the heating zones, and controlling the power output of each heating zone; Figure 18 illustrates a method of measuring the resistance of the heater electrodes, and determining the corresponding temperature of the respective heating zones; Figure 19 illustrates a method of determining a PID controller output; Figure 20 illustrates a method of determining a duty cycle for a heater electrode Figure 21 illustrates a method of determining to change a target temperature for a heating zone to a styling temperature from an intermediate temperature; Figure 22 shows an example in which two opposing arms of the hair styling device are provided with heating zones; Figure 23 shows a simplified schematic illustration of a heater having a number of heating zones, and a temperature sensor having a number of sensing zones; Figure 24 shows a further simplified schematic illustration of a of heater having a number of heating zones, and a temperature sensor having a number of sensing zones; Figure 25 shows a simplified schematic illustration of a pair of heaters and a corresponding pair of temperature sensors; Figure 26 shows a simplified schematic illustration of a further example of a pair of heaters and a corresponding pair of temperature sensors; Figure 27 shows a schematic block diagram of apparatus for calibrating devices; Figure 28 shows a further schematic block diagram of apparatus for calibrating devices; Figure 29 shows a schematic block diagram of apparatus for calibrating devices; Figure 30 shows a flow diagram of a method of calibrating a heater; and Figure 31 shows a table of exemplary calibration measurements. Detailed Description of Preferred Embodiments The present embodiments represent the best ways known to the applicant of putting the invention into practice. However, they are not the only ways in which this can be achieved. Device Overview Figure 1a illustrates a hand held (portable) hair styler 1. The hair styler 1 includes a first movable arm 4a and a second movable arm 4b, which are coupled at proximal ends thereof to a shoulder 2. The first arm 4a bears a first heater 6a at its distal end, and the second arm 4b bears a second heater 6b at its distal end. The first and second heaters 6a, 6b oppose one another and are brought together as the first and second arms 4a, 4b are moved from an open configuration to a closed configuration. As shown in Figure 1b, during use, a tress of hair 40 is sandwiched between the two arms 4 so that the user’s hair is in contact with, and therefore heated by, outer heating surfaces of the heaters 6a, 6b. Therefore, as the user pulls the hair styler 1 along the tress of hair 40, the tress of hair 40 is heated by conductive heating to a suitable temperature to facilitate styling. The heaters 6a, 6b are low thermal mass heaters and are therefore able to heat up and cool down rapidly. Whereas the heaters of a typical conventional hair styler may take, for example, around 30 seconds to reach an operating temperature of about 200°C from ambient temperature, a low thermal mass heater operating at maximum power may take less than one second to reach the same operating temperature. It will be appreciated that the exact operating temperature and heat-up time depend on the particular requirements of the device, and on the maximum power output of the device. By way of example, a low thermal mass heater may heat up at a rate of approximately 410 °Cs'1, and may cool from an operating temperature for styling to a touchable temperature in a timescale of the order of a few seconds A user interface 11 is provided to allow the user to turn the device on or off, and could also be used to enable the user to set user-definable parameters of the device 1 or to output information to the user. For example, a desired operating temperature for the heaters 6a, 6b could be input via the user interface 11. The user interface 11 may have a dial, button 14, or touch display for allowing the user to input information into the device 1 and the user interface 11 may have an indicator light, display, sound generator or haptic feedback generator for outputting information to the user. The user interface 11 may also comprise an indicator light 15 to indicate whether the device is on. A printed circuit board assembly (not shown) may be provided at any suitable location within the housing of the device 1 and carries the control circuitry for controlling the operation of the device 1 and for controlling the interaction with the user via the user interface 11. In this example, electrical power is provided to the device 1 by means of a power supply located at an end of the device, via a power supply cord 3. The power supply may be an AC mains power supply. However, in an alternative embodiment the power supply may comprise one or more DC batteries or cells (which may be rechargeable, e.g. from the mains ora DC supply via a charging lead), thereby enabling the device 1 to be a cordless product. In use, the device 1 is turned on, energising the heaters 6 to cause them to heat up. The user then opens the first and second arms 4a, 4b and, normally starting from the roots of the hair (i.e. near the scalp), a length or tress of hair 40 (which may be clumped) is introduced between the arms 4a, 4b, transversely across the heaters 6a, 6b. The user then closes the arms 4a, 4b so that the length of hair 40 is held between the first and second arms 4a, 4b and then the user pulls the hair through the closed arms (as illustrated in Figure 1b). The outer (hair contacting) surface of the heaters 6 is flat in this embodiment and so the hair styler 1 can be used to straighten the user’s hair (but alternatively the device 1 could comprise a cylindrical heater 6, or another shape of heater6 having a curved surface). The hairstyling device 1 shown in Figure 1 can also be used to curl the hair by turning the device 1 through approximately 180 degrees or more after clamping the hair between the arms 4a, 4b and before moving the device 1 along the tress of hair 40. Hair has a relatively high thermal mass, and when in contact with the heating surface of the heaters 6 the hair absorbs a significant amount of heat energy. If the temperature of the heaters 6 falls below the glass transition temperature of the hair, the hair may not retain the styled shape. However, if the hair is heated to a temperature that is too high, the hair can undergo damage. Therefore, the device 1 is configured to control the temperature of the heaters 6 so that the hair-contacting surfaces remain within a particular temperature range when in use for styling the hair. A hinge may be provided at the elbow 2. The hinge may comprise a spring for biasing the first and second arms 4a, 4b into the open configuration, such that the user is required to apply pressure to the arms 4a, 4b to close them together (overcoming the effect of the spring). For example, the hinge may include a leaf spring or a coiled spring. The spring itself can be used to couple the first and second arms 4a, 4b together, thereby avoiding the need to provide a separate mechanical hinge and simplifying the overall construction of the device. Alternatively, the first and second arms 4a, 4b may be formed in a unitary manner (e.g. from a plastics material) with a “U” shaped middle part at the elbow 2, the “U” shaped middle part being able to resiliently flex to allow opening and closing of the first and second arms 4a, 4b. Opening and closing of the arms 4a, 4b may be detected using a switch. For example, a microswitch or Hall effect sensor and passive magnet could be used detect closure and opening of the arms 4a, 4b. The present invention is not limited to the type of device illustrated in Figure 1a. For example, a device that additionally transfers heat to hair using heated air could be used. When heated air is used to heat the user’s hair, the device may comprise a heater having an electrically-powered heating coil (or any other suitable type of heating element), operable to heat air drawn in by a fan assembly. Heating Zones Independently controllable heating zones 642 of the heaters 6a, 6b will now be described with reference to Figures 2 to 6b. Figure 2 shows a set of heating zones 642, Z1 to Z7, of the hair styling device. Whilst the zones 642 are illustrated for the first heater 6a, a corresponding set of zones is also provided for the second heater 6b. The power output to each heating zone 642, and therefore the temperature of each heating zone, is independently controllable. Advantageously, this enables the temperature distribution along the longitudinal length of the heater 6 to be controllable. Moreover, the use of independently controllable heating zones 642 also enables better mitigation against reductions in temperature of the styling surface due to transfer of heat to the user’s hair 40. For example, when the user places a tress of wet or damp hair on heating zones Z3 and Z4, the power output to heating zones Z3 and Z4 can be independently increased in order to maintain the hair-contacting surface at the styling temperature. As will be described in more detail later, the use of independently controllable heating zones 642 also enables some of the zones 642 to be operated at a temperature below the styling temperature even when the device is in use (e.g. when only some of the heating zones 642 are in contact with the user’s hair), increasing the energy efficiency of the device 1. Each heating zone can be independently controlled towards a respective target temperature. Control is performed to reduce the difference between the actual temperature of the heating zone and the target temperature. The heating zones can be switched off when the device is not in use to style hair, which can be detected, for example, by sensing that the heater arms are in the open position. Figure 3 schematically illustrates the heating zones 642 on the heating surface of the heater shown in Figures 1 and 2. Whilst in the example shown in Figure 3 the temperature of the hair contacting surface is independently controllable along the longitudinal direction of the heater 6 by virtue of the arrangement of the heating zones 642, the independent temperature control is not limited to being along the longitudinal direction. For example, Figure 4 illustrates an alternative arrangement of independently controllable heating zones 644 in which the temperature of the hair contacting surface is also controllable along the transverse direction, by virtue of two heating zones (e.g. zones Z1 and Z2) being provided across the width of the heater. Whilst the heating zones 642, 644 illustrated in Figures 3 and 4 are all the same size, this need not necessarily be the case. Different sized heating zones 646 may be provided, as illustrated in Figure 5, which shows a heater 6 having seven different sized heating zones (labelled Z1 to Z7). The way in which the heater electrodes 64 would be arranged to define these different sized zones would be understood by the skilled reader and will not be described in detail here. The heating zones described above form part of a heater 6 having a flat hair-contacting surface. However, the heater 6 is not limited to having a flat hair-contacting surface and could alternatively have a tubular form (as illustrated in Figure 6a), for example for use in a hair curler device. In a further alternative the heater could have a curved form as illustrated in Figure 6b, for example for use in a heated hair brush. The heater surface may have a corrugated or ribbed shape to provide a hair crimping device. Illustrative Block Diagram Figure 7 is a simplified block diagram of control circuitry 15 that controls the operation of the hair styler device 1 shown in Figure 1. As shown, the control circuitry 15 comprises a power supply 21 that, in this embodiment, derives power from a battery power source. A mains power supply input may be provided to charge the battery via an AC to DC converter (not shown), which may be external or internal to the device 1. Alternatively, the power supply 21 may derive power from an AC mains supply input. In this example, power is provided to the heaters 6 for heating the users hair. The power supplied to the heaters 6 is controlled by a controller 28 having a microprocessor 29. The power supplied to the heaters 6 is controlled by drive circuitry 23 (which may include one or more power semiconductor switching devices (triacs)) which controls the application of an AC mains voltage, or a DC voltage derived from the AC mains or from a battery, to the heaters 6 in accordance with instructions from the microprocessor 29. The microprocessor 29 is coupled to a memory 30 (which is typically a non-volatile memory) that stores processor control code for implementing one or more control methods that control the heating of the heaters 6 in accordance with a desired operating temperature of the heaters 6 (for each of the independently controllable heating zones) and sensed temperatures of the heaters obtained from temperature measurement circuitry 25. The memory 63 may store, for example, one or more operating profiles or parameters (e.g. a target temperature for each of the heating zones). Software stored in the memory 63 may include, for example, an operating system and a heater control module suitable for implementing one or more of the methods described below. For example, the software stored in the memory 63 may include a calibration and testing module for use when the device 1 is being calibrated and tested. The temperature measurement circuitry 25 may comprise temperature sensors such as thermistors or they may use circuitry that senses the resistance of heater electrodes that are used to heat the heaters 6, which resistance depends on the temperature of the heater electrode. The temperature measurement circuitry 25 may comprise a noncontact type of temperature sensor (for example, an infrared sensor) for sensing the temperature of the user’s hair, and / or for sensing the temperature of a part of the device 1 (for example the temperature of the hair-contacting surface of the heater 6). The temperature sensor(s) could be provided inside a body portion of the device 1, or could be provided on an exterior surface of the device 1. A temperature measurement of a part of the device 1 can be measured directly or indirectly. For example, a heat pipe could be used to transfer heat via conduction from the part whose temperature is to be measured to an internal sensor. Figure 7 also shows a user interface 11 that is coupled to the microprocessor 29, for example to provide one or more user controls and / or output indications such as a visual indication or an audible alert. The output(s) may be used to indicate to the user, for example, if they have inserted too much hair between the heaters 6, if they are moving the device 1 too quickly along the hair tress 40, or simply if the device is on or off. In this example, the control circuitry 15 also comprises communications circuitry 27 to allow the device to communicate with a remote sensor, a remote server, or a remote application (e.g. on a mobile telephone). The communications circuitry 27 may use, for example, Bluetooth, Wi-Fi and / or 3GPP communication protocols to communicate with the remote device. The communications circuitry 27 could be used to receive an input parameter from a remote device, for example a desired operating temperature for the heaters 6 for styling hair. As those skilled in the art will appreciate, the device 10 does not necessarily need to have all of the blocks illustrated in Figure 7. For example, if the device 10 is a hair straightener, then there is not necessarily a need for the communications circuitry 27. Control of Heating zones A method of controlling the independently controllable heating zones of the heaters 6 will now be described with reference to Figure 8. Figure 8 shows a flow diagram of a method of controlling power output to the heating zones of the heaters 6. In step S801, the heating zones that are in use for styling the user’s hair 40 are determined. In the present example, a heating zone is determined to be in use for styling the user’s hair 40 when the user’s hair 40 is engaged with (in contact with) the hair-contacting surface of the heating zone. Figure 9 shows an example in which a tress of hair 40 is in contact with heating zone Z2, and a further tress of hair 40 is in contact with heating zone Z4. Therefore, zones Z2 and Z4 are determined to be in use for styling the user’s hair 40. Since the user’s hair 40 is not in contact with heating zones Z1, Z3, Z5 or Z6, these heating zones are determined not to be in use for styling the user’s hair 40, and can therefore be (but need not necessarily be) operated at a reduced temperature (which could be the ambient temperature, in which case the supply of power to these zones can be switched off). Figure 10 shows a further example in which a tress of hair 40 is in contact with heating zones Z2 and Z3, and therefore zones Z2 and Z3 are determined to be in use for styling the user’s hair 40. Since the user’s hair 40 is not in contact with heating zones Z1, Z4, Z5 or Z6, these heating zones are determined not to be in use for styling the user’s hair 40. Sensing the presence of hair 40 in a heating zone can be achieved by measuring the change in thermal load at the heating zone. As will be described in more detail later, the presence of the user’s hair can be determined based on a change in the power required to maintain the heating zone at a particular temperature, or based on a change in temperature of the heating zone (which could be an increase in temperature as hair is removed from the zone, or a decrease in temperature as hair comes into contact with the zone). Alternatively, the presence of the user’s hair in a heating zone could be measured directly using a sensor. For example, an optical sensor could be used to sense presence of hair in a particular heating zone. Light pipes could be used to sense the brightness of light through openings in the casework of the device 1, arranged so that the brightness of the detected light will change depending on whether a tress of hair 40 is present on the surface of the heater 6 in a particular heating zone. In step S802, a target temperature for each of the heating zones is determined. For heating zones that are determined, in step S801, to be in use for styling the user’s hair, the target temperature is set to a styling temperature. For the heating zones that are not determined to be in use for styling hair, the target temperature is set lower than the styling temperature in order to reduce the power needed to operate the device 1. The target temperature for use in styling the hair of the user may be, for example, 200°C. For zones determined not to be in use for styling the hair 40 of the user, the lower target temperature may be, for example, 100°C. Alternatively, a target temperature may not be defined (or ‘set’) for the zones determined not to be in use for styling the hair of the user, and the zones may be allowed to cool towards the ambient temperature. If the arms 4a, 4b of the device 1 are determined to be in the open configuration (e.g. using a switch or sensor as described above) then step S801 comprises determining that none of the heating zones are in use for styling. A table indicating the target styling temperature and one or more intermediate target temperatures may be stored in the memory 30 of the device 1. Figure 11a shows an example of a table indicating that the styling temperature, Tstyie, is 200°C and an intermediate target temperature, Tint, is 120°C. Figure 11b shows a further table that could be stored in the memory 30 of the device 1, indicating the target temperature assigned to each heating zone in step S802. Therefore, step S802 may comprise updating the target temperature stored in the table for each of the heating zones. The table shown in Figure 11b corresponds to the situation illustrated in Figure 9, in which the user’s hair is in contact with heating zones Z2 and Z4. Therefore, the table indicates that the target temperature for zones Z2 and Z4 is Tstyie, whereas the target temperature for the other heating zones is Tint (which is lower than Tstyie, and therefore results in less power being required to maintain those heating zones at the target temperature). Whilst two separate tables are illustrated in Figures 11a and 11b, it will be appreciated that table 11b could simply indicate the target temperature in degrees directly, rather than by reference to an index of table 11 a. Advantageously, since the heaters 6 of the device 1 are low thermal mass heaters, each of the heating zones can be controlled to rapidly return to the styling temperature (e.g. from ambient temperature, or the intermediate temperature Tint) in response to a change in the distribution of the user’s hair amongst the heating zones. The power output to a heating zone that is not in use for styling hair can be reduced in order to increase the energy efficiency of the device 1, whilst ensuring that the heating zone can be rapidly returned to the styling temperature when hair 40 comes into contact with that heating zone. Whilst in the present example each of the heating zones is controlled towards a lower target temperature (lower than the styling temperature) if the heating zone is determined not to be in use for styling hair, the method could alternatively be applied to a subset of the heating zones. For example, referring to Figure 2, the method of reducing the power output to the heating zones when the zones are not in contact with the user’s hair could be applied to zones Z1, Z2, Z5, Z6 and Z7, whereas zones Z3 and Z4 could be operated at the styling temperature even when the presence of hair is not detected in zones Z3 and Z4. This alternative operation of zones Z3 and Z4 could be performed for heating zones that are in a region of the heater 6 that is more likely to be in regular contact with the user’s hair (e.g. zones in a central region of the heater 6), reducing the risk that hair is present in those regions but not detected. In step S803, the temperature of each heating zone is controlled towards the corresponding target temperature by controlling the power output to each heating zone. Methods of controlling the power output to each heating zone based on the difference between the target temperature and the actual temperature of the heating zone will be described in more detail later. The method then returns to step S801, in which the heating zones in contact with the user’s hair are determined. Exemplary Heater Configuration Figures 12a and 12b show an exemplary embodiment of the low thermal mass heaters 6a, 6b of the device of Figure 1. In this example, the heaters 6a, 6b comprise a stack of thin layers. Referring firstly to Figure 12a, the heaters 6a, 6b include an upper dielectric (electrically insulating) layer 62, an electrode layer 63 that has a plurality of separate heater electrodes 64, and a lower dielectric layer 66 which electrically insulates the heater electrodes 64 from other components mounted behind the heater 6a, 6b. The three layers 62,63 and 66 are bonded together (mechanically or chemically) and define a heater 6 that is very thin (the three layers have an overall thickness of between 0.3mm to 0.4mm in the case of low voltage operation (less than about 40 Volts) and 0.8mm to 1.0mm in the case of AC operation) and with very low thermal mass. The upper dielectric layer 62 provides the hair contacting surface of the heater 6, although a nonstick coating may be applied to the upper surface of the layer 62 to facilitate the passage of the user’s hair 40 over the heating surface. The bonded layers 62, 63 and 66 define a flexible heater 6 and rigidity of the heater is provided in the illustrated embodiment by mounting the heater layers 62, 63 and 66 onto a rigid support 68 which forms a base. If a flexible heater is desired, then there is no need for the rigid support 68. Thus, in this embodiment, there is no heater plate or tube that is heated by the heaters 6, and instead the heaters 6 directly heat the user’s hair 40. This provides a hair styler 1 having a very low thermal mass which can therefore heat up and cool down rapidly. In the illustrated embodiment, there are ten heater electrodes 64 that each snake across and back across the width of the heater 6, folding twice such that they each cross the width three times. The ends of each of the heater electrodes 64 are electrically connected through the lower dielectric layer 66 to electrical connections within the rigid support 68, which connect to an electrical connector 70. Drive circuitry 23 that is mounted within one of the arms 4 connects to the heater electrodes 64 via the electrical connector 70 and applies electrical power to the individual heater electrodes 64 to control the heat generated by each heater electrode 64. The electrical connector 70 extends from a surface of the rigid support 68 facing away from the surface layer 62 (shown in Figures 12a and 12b as extending directly away from the upper layer 62, but it could also be provided as extending in a perpendicular direction). Each of the heater electrodes 64 thus creates an individual heating zone on the hair contacting surface of the heater 6, which spans the width (which we shall refer to as the x-direction, as illustrated in Figure 12b) of the heater 6 and the heater electrodes 64 are arranged sequentially one after the other along the length (the y-direction as illustrated in Figure 12b) of the heater 6. The arrangement of heating zones illustrated in Figure 4, in which heating zones 644 are arranged along both the x- and y-directions, can be provided by arranging two sets of heater electrodes 64 side by side in the width (x-) direction. The heaters 6 may be separated in this way into any number of heating zones and may comprise any number of heating zones along the x- and y-directions. For example, whilst Figure 4 shows two zones along the transverse direction, a greater number of zones in the transverse direction could also be provided. The heating zones 644 of the heaters 6a, 6b can be operated (heated) independently, which can help to reduce hot / cold spots when using very low thermal mass heaters 6. As described above, the temperature of each heating zone is independently controllable. Each heating zone can be set to a target temperature. The target temperature of each heating zone may be different. A separate temperature sensor may be provided for sensing the temperature of each heating zone which is fed back to the microprocessor 29 to allow the microprocessor 29 to control the delivery of power to the heater electrode 64 of the corresponding heating zone 642. Alternatively, if the heater electrodes 64 are formed of a material having a Positive Temperature Coefficient (PTC) or a Negative Temperature Coefficient (NTC) (such that its resistance varies with its temperature), then the temperature of each heating zone can be determined by sensing the resistance of the corresponding heater electrode 64. The microprocessor 29 controls the heating in order to reduce the difference between the actual temperature of the heating zone and the target temperature for that heating zone. Figure 13 shows a simplified schematic illustration of a configuration of layers of a low thermal mass heater 6. In this example, the heater 6 comprises the layer of electrically conductive heater tracks 63, and the electrically insulating dielectric layer 62 that forms the hair-contacting surface for engaging with the hair 40 of the user. However, as illustrated in Figure 14, an additional layer 141 may be provided in between the dielectric layer 62 and the hair 40, to form the hair contacting surface. For example, a frictionreducing layer 141, or any other suitable type of coating, may be provided between the dielectric layer and the hair 40. Whilst in the examples illustrated in Figures 12a to 14 the heater electrodes 64 are provided in a layer 63 that is separate from the dielectric layer, this need not necessarily be the case. For example, Figure 15 shows an alternative in which the heater electrodes 64 are arranged within the dielectric layer 62, but are nevertheless electrically insulated from the hair 40 of the user. Exemplary Control Circuitry Control circuitry for monitoring the temperature of the heating zones, and controlling the power output to the heating zones, will now be described with reference to Figure 16. Figure 16 shows a schematic view of the way in which the heater electrodes 64 may be connected together and to the drive circuitry 23 and the power supply 21. As shown in Figure 16, each heater electrode 64 is connected at one end to a power supply and at the other end to a respective switch (in this case a metal-oxide-semiconductor fieldeffect transistor, MOSFET, switch) 95-1 to 95-3. The switches 95 are controlled by the microprocessor 29. When a heater electrode 64 is to provide heat, the corresponding switch 95 is closed thereby connecting the heater electrode 64 to ground through the resistor R. As a result, current flows from the power supply 21 to ground causing the heater electrode 64 to heat up. The microprocessor 29 can control the position of each switch 95 independently thereby allowing each heater electrode 64 to be powered independently. When the temperature of a selected heating zone is to be determined, the switch 95 of the corresponding heater electrode 64 is closed and all other switches 95 are opened. In this way, the selected heater electrode 64 is provided in series with the resistor R. Since the heater electrodes 64 are formed of a PTC or an NTC material whose resistance changes with the temperature of the heater electrode 64, the temperature of the heating zone can be determined based on the resistance of the heater electrode 64. As described in more detail below, the resistance (and therefore temperature) of the heater electrode 64 can be determined based on measurements of the attenuated power supply potential, VSuppiy, and the voltage at the output of the current sense amplifier, Vcurrent. If the determined temperature is above the desired temperature for that heating zone, then the microprocessor 29 can reduce the power applied to that heater electrode 64; or if the heating zone is at a lower temperature than that desired, then the microprocessor 29 can increase the power applied to the corresponding heater electrode 64. Any suitable ON / OFF control or PWM (pulse width modulation) control can be used to vary the power applied to the different heater electrodes 64. The microprocessor 29 can select each heater electrode 64 in turn in order to determine the temperature of each heater electrode 64 / heating zone 642. The microprocessor 29 may be a pre-programmed microcontroller unit with in-built timing references, digital outputs and analogue-to-digital converter (ADC) inputs (e.g. from the operational amplifier 97). The microprocessor 29 may also receive an input signal from a microswitch, or Hall effect sensor and passive magnet, (not shown in the figure) to detect closure / opening of the arms 4a, 4b of the device. The voltage source supply, VSS, and voltage common collector, VCC, of the microprocessor are also illustrated in Figure 16. A DC-to-DC converter arranged between the microprocessor and the heater electrodes 64, and a Vsupply monitor input for monitoring the voltage provided by the power supply, are also illustrated. Whilst all of the heater electrodes could be controlled by a single microprocessor 29 via the corresponding set of switches 95, this need not necessarily be the case. Alternatively, a plurality of microprocessors could be provided, each for controlling the operation of a respective set of heater electrodes 64. Advantageously, the use of a plurality of microprocessors 29 (and a corresponding plurality of resistors, R, and operational amplifiers 97) enables the resistance (and therefore temperature) of a plurality of heater electrodes 64 to be measured independently and simultaneously. Temperature Measurement and Thermal Load Methods of monitoring and controlling the temperature of the heating zones 64 will now be described in more detail. Heating Zone Temperature Monitoring and Control Figure 17 shows a flow diagram of a method of monitoring the temperature of the heating zones, and controlling the power output of each heating zone. In step S1701 the resistance of the heater electrode 64 corresponding to each heating zone is measured by operating the switches 95 and the operational amplifier 97 as described above. In step S1702 the measured resistances are converted into a corresponding set of temperatures of the heating zones. For example, the device may store a lookup table or formula that maps the measured resistance to the temperature of the heating zone. In step S1703, the differences between each of the temperatures determined in step S1702 and the target temperature of the respective heating zone are determined. Alternatively, rather than storing a target temperature for each heating zone, the device 1 could store a target resistance for each heating zone (which nevertheless corresponds to a target temperature). In this case, step S1702 could be omitted, and the method instead comprises determining the difference between the measured resistance and the target resistance for each heating zone. Based on the difference (or ‘error’) between the measured temperature and the target temperature for a heating zone (or based on the difference between the measured resistance and the target resistance), a proportional-integral-derivative (PID) controller output is determined for each heating zone. The PID controller output values can be used as an indication of the thermal load at each heating zone, and used as an input for determining the appropriate target temperature, for example in step S802 of Figure 8 (e.g. based on whether hair is present in a zone or not). In step S1704 the duty cycle to be applied for each heater electrode 64 is determined. For each heating zone, the PID controller output is mapped to a duty cycle for the corresponding electrode 64 (which can be applied by opening and closing the corresponding switch 95). In step S1705 a firing sequence is determined based on the duty cycle determined for each heater electrode. In other words, the timing of the opening and closing of each switch is determined in order to implement the determined duty cycles. In step S1705 the firing sequence is executed, thereby controlling the temperature of each heating zone towards the respective target temperature. The method then returns to step S1701 in which the resistance of each heater electrode 64 is measured. Resistance Measurement for Temperature Determination Exemplary measurements of the resistance of the heater electrodes 64, and the determination of the corresponding temperatures of the heating zones performed in step S1702 of Figure 17, will now be described in more detail with reference to the flow diagram shown in Figure 18. However, it will be appreciated that the resistance measurements and the determination of the temperatures of the heating zones could be performed in any other suitable manner. In step S1801 power is supplied to one of the heater electrodes 64 by operating the corresponding switch 95. In step S1802 the attenuated power supply potential, Vsuppiyas shown in Figure 16, is measured using an ADC input of the microprocessor 29. It will be appreciated that the attenuation of the power supply potential measured by the microprocessor will depend on the value of the corresponding resistors 161, 162 illustrated in Figure 16. Since the resistance of these resistors 161, 162 is known, the attenuation of the power supply potential can be compensated for to obtain the voltage applied to the heater electrodes 64. In step S1803 the current passing through the current sense resistor at time t, It, is measured. This is achieved by measuring the voltage at the output of the current sense amplifier (VCUrrent, as illustrated in Figure 16). Since the resistance of the current sense resistor R is known, the current passing through the current sense resistor (and therefore the current passing through the heater electrode 64 that is connected in series with the current sense resistor) can be calculated using the equation It = (VCUrrent / R). It will be appreciated that the measurement of VCUrrent is performed taking into account the gain of the operational amplifier 97. Steps S1802 and S1803 may be repeated a number of times, accumulating a summed voltage and summed current for the heater electrode 64. Mean values of the current through the heater electrode and the power supply potential can then be obtained by dividing the summed values by the number of times steps S1802 and S1803 are repeated. This averaging helps to mitigate against the effects of random noise on the Vsuppiy and Vcurrent signals, and also helps to mitigate against ADC rounding errors at the microprocessor 29. In step S1804 the supply of power to the heater electrode 64 is disabled, by operating the corresponding switch 95. It will be appreciated that the power supply to the heater electrode 64 could be disconnected at any suitable point following the measurements of the power supply potential and current, but before the measurements for the next heater electrode 64 (using the same current sense resistor and operational amplifier 97) are performed. In step S1805 the resistance of the heater electrode 64 is determined. The resistance of the heater electrode 64 is given by: Relectrode = Vt / It, where Vt is the voltage over the heater electrode that was determined based on the attenuated power supply potential measured in step S1802, and lt is the current passing through the current sense resistor measured in step S1803. In a case where the resistance of the heater electrode 64 is sufficiently large compared to the resistance of the current sense resistor, the resistance of the current sense resistor may be ignored when calculating the resistance of the heater electrode 64. Alternatively, the resistance of the current sense resistor may be taken into account using the formula: Relectrode = (Vt / It) - Rsense, where Rsense is the resistance of the current sense resistor. The determined resistance of the heater electrode 64, Relectrode, may be stored in the memory of the device in association with an indication of the time at which the measurement was performed (e.g. in association with a time-based index that increments following the execution of each complete firing sequence, described in more detail later). The time between each measurement of the resistance of the heater electrode 64 may be, for example, 40 ms. In step S1806 the temperature of the heater electrode 64 is determined based on the resistance of the heater electrode, Reiectrode, obtained in step S1805. The device 1 may store any suitable equation or table that maps the determined resistance value to the temperature of the heater electrode 64. For example, the relationship between Reiectrode and the temperature of the heater electrode 64 could be determined experimentally in advance, a corresponding mapping provided in a table stored in the memory of the device 1. The temperature of the heater electrode 64 could also be determined based on a stored calibration resistance for that heater electrode 64 for a particular reference temperature, and based on a known coefficient of resistance of the heater electrode material. In this case, the temperature of the heater electrode 64 may be determined using the formula: Telectrode = (Reiectrode-Rcal) / (k*Rcal) + Teal, where Teiectrode is the temperature of the heater electrode, Reiectrode is the resistance of the heater electrode 64 obtained in step S1805, Rcai is the stored calibration resistance when the heater electrode 64 is at a corresponding calibration temperature Tcai, and k is the coefficient of resistance (that corresponds to the change in the resistance of the heater electrode material with temperature). Steps S1801 to S1806 are then repeated for the next heater electrode 64 for which the temperature is to be determined 64. As described above, the measurements of the current passing through the heater electrodes 64 that are connected to the same current sense resistor are performed separately. However, when a plurality of microcontrollers are provided (or when each of a plurality of current sense resistors and operational amplifiers are connected to a respective different ADC input of the same microcontroller 29), measurements for heater electrodes connected to different current sense resistors may be performed simultaneously. Since the measurement of the current performed in step S1803 requires power to be supplied to the heater electrode 64, some heat will be generated in the corresponding heating zone whilst the measurement is being performed. This additional heat output may be taken into account when determining the duty cycle to be used for the heater electrode in step S1704 of Figure 17. Beneficially, therefore, the temperature of the corresponding heating zone can be more accurately controlled towards the target temperature. PIP Control An exemplary determination of the PID controller output in step S1703 of Figure 17 will now be described in more detail with reference to the flow diagram shown in Figure 19. However, it will be appreciated that the PID control could be performed in any other suitable manner. In step S1901 the proportional error value for a heating zone is calculated by subtracting the temperature of the heating zone determined in step S1702 of Figure 17 from the target temperature for that heating zone (e.g. determined in step S802 of Figure 8): Pt = Ttarget ~ Telectrode, where pt is the proportional error value obtained for time t (or an index t corresponding to a particular measurement cycle), Ttarget is the target temperature for the heating zone and Teiectrode is the measured temperature of the corresponding heater. Alternatively, the proportional error value could be calculated by subtracting the measured resistance of the heater electrode 64 from a target resistance using the formula: Pt = Rtarget - Relectrode, where Rtarget is the target resistance for the heater electrode 64 (e.g. based on experimental measurements of the correspondence between the temperature of the heating zone and the resistance of the heater electrode) and Reiectrode is the actual resistance of the heater electrode (e.g. determined in step S1805 of Figure 18). In step S1902 an integral value is accumulated for the heating zone, but adding the proportional value pt obtained in step S1901 the previous integral value: it - in + pt, where it is the new integral value and in is the previous integral value. In step S1903 the proportional value pt is subtracted from the previous proportional value for the heating zone to obtain a derivative value: dt = pt — pt-i, where dt is the derivative value and pt-i is the previous proportional value (based on the previous temperature or resistance measurement for the same heater electrode). In step S1904 the PID value, Ut, is determined using the formula: Ut = (pt ■ kp) + (it ■ ki) + (dt ■ ka), where kp, k and kd are predefined constants. Steps S1901 to S1904 are then repeated for the other heater electrodes 64 to obtain the corresponding PID values. Each of the PID values Ut can then be used in step S1704 of Figure 17 to determine the duty cycles to be used for the respective heater electrodes. Duty Cycle An exemplary determination of the duty cycles for the heater electrodes 64 performed in step S1704 of Figure 17 will now be described in more detail with reference to Figure 20, which shows a method of mapping the PID values Ut to corresponding duty cycles. However, the determination of the duty cycles could alternatively be performed in any other suitable manner. In step S2001 the PID value Ut is clipped to a predefined range. In this example, the PID value is clipped to be between -0.5 and 0.5: -0.5 <Ut <0.5 In step S2002 the fractional duty cycle, rt, is calculated by adding 0.5 to the clipped PID value: rt = Ut + 0.5 As described above, the fractional duty cycle may be corrected to account for power supplied to the heater electrode 64 in order to measure the resistance of the heater electrode 64 (e.g. by decreasing the value of rt based on the measurement time and the power output to the heater electrode 64). Steps 2001 and 2002 are then repeated for each heater electrode 64, to obtain a value of rt for each heater electrode 64. The power demand Pt, or thermal load, of the heating zone (which can be used to determine the presence of hair 40 in contact with the heating zone, since hair coming into contact with the heater zone will increase the power required to maintain the heater zone at the target temperature) can also be estimated at this stage using the formula, based on the assumption that the power supply voltage from the power source is constant: Pt = Vt ■ It ■ rt Determination of Firing Sequence An exemplary determination of the firing sequence for the heater electrodes 64 will now be described in more detail. However, the firing sequence could alternatively be determined in any other suitable matter. A pulse width modulation (PWM) pattern is determined for each heating zone that delivers the determined duty cycle. However, beneficially, the number of heating zones that are powered simultaneously is limited to reduce the peak instantaneous current demand on the power supply 21. This is achieved by preparing a firing pattern and interleaving the PWM pulses to reduce the number of heating zones that are powered simultaneously. The start times and finish times for the firing sequence for each heating zone are stored in the memory 30 of the device 1. Start times for each of the N heating zones may be stored as an array: S[1...N][2]. Similarly, finish times may similarly be stored as an array: F[1...N][2], An accumulator index, A, is initially set to zero. Then, for the first heating zone ‘n’ of the N heating zones a first set of value assignments is performed: S[n][0] — A S[n][1] = -1 F[n][0] = -1 F[n][1] = -1 In other words, the first start time for heating zone n is set to the value of A (which is initially zero), the second start time for heating zone n is set to -1, and the first and second finish times for heater zone n are also set to -1. (The value of -1 is an indication that the value has not been populated yet and can be ignored in the execution of the firing sequence). The duty cycle value for heating zone n, rt[n] (which can have a value between 0.0 and 1.0 in this example), (determined in step S1704 of Figure 17) is multiplied by the duration of the subsequent firing window which is set as a constant W (e.g. in milliseconds). The value of rt[n]*W is then added to the previous value of the accumulator index A. If the value of the of the accumulator index A is less than W, then F[n][0] is set to be equal to A. Alternatively, if the value of the accumulator index A is greater than or equal to W, then the following values are set: F[n][0] = W F[n][1] = A-W A = F[n][1] S[n][1] = 0 In other words, the first finish time for heater n is set to W, and the second finish time for heater n is set to A-W. A is then set to the value of F[n][1], and the second start time for heater n is set to be equal to zero. At this stage of the method, the values of S[n][0], S[n][1], F[n][0] and F[n][1 ] have been determined for heating zone ‘n’. The value of n is then incremented by 1 (i.e. n = n+1), corresponding to the next heating zone, and the method returns to the assignments. This process is repeated (including the updating of the accumulator index A and the conditional steps depending on whether A is less than W, or greater or equal to W) until n=N (in other words until the values of S[n][0], S[n][1], F[n][0] and F[n][1] have been determined for each of the N heating zones. Advantageously, therefore, the method results in the power delivered to each heating zones being interleaved, reducing the peak power demand on the power source 21. Execution of Firing Sequence The firing sequence can then be executed as follows. The firing sequence duration is set as W milliseconds, a time index t is initially set to be equal to zero, and the heating zone index n is set to 1. Then, if S[n][0] = t, or if S[n][1] = t, then the power is supplied to the heating zone by controlling the corresponding switch 95-n. In other words, if the value of t is equal to the first start time or the second start time for heating zone n, then the corresponding heater electrode 64 is switched on. If F[n][0] = t, or F[n][1 ] = t, then the power supply to the heater electrode 64 is switched off by controlling the corresponding switch 95-n. The value of n is then incremented by one (i.e. n = n +1), and the determinations of whether S[n][0] = t, S[n][1 ] = t, F[n][0] = t, or F[n][1 ] = t are performed for the new value of n. This process is repeated until n >N. The value of t is then incremented by one (i.e. t = t+1). This increase in the value of t may be performed, for example, following a 1 ms pause. Following the value of t being incremented by one, the value of n is set to 1, and the method is repeated (but with the new value of t) until t = W. In other words, the firing sequence is performed for W milliseconds. Beneficially, the methods for controlling the temperature of the heating zones described above with reference to Figures 17 to 21 enable the temperatures to be more precisely controlled, and adjusted between different target temperatures more rapidly than conventional heaters and algorithms, on a timescale of milliseconds as opposed to seconds. The thermal load for a heating zone can also be measured on a timescale of milliseconds, compared to a conventional heater whose response time is typically of the order of several seconds. This enables the heating zones to be maintained at the appropriate target temperatures more accurately. Temperature Control Further details of controlling the temperature of the independently controllable heating zones will now be described. Device closure The target temperatures for the heating zones can be changed in response to whether the arms 4a, 4b of the device are detected in the open or closed position using a sensor. The sensor is monitored periodically by the microprocessor29, monitoring fora change of state. Two target temperature values can be pre-defined: one for when the heater arms are open, Topen, and one for when the heater arms are closed, Tciosed. Topen may be defined to be so low that no power (or no additional power - some power may be supplied in order to measure the resistance of the heater electrodes as described above) is supplied to the heater electrodes 64 when the sensor reading indicates that the styler is in the open configuration (e.g. by setting all of the duty cycles to zero). Alternatively, Topen may not be defined, and the microprocessor may be configured to determine that no power (or no additional power) is to be supplied to the heater electrodes 64. When closure of the arms 4a, 4b is detected, the target temperature for each heating zone is set to the respective value of TCi0Sed. Response to Thermal Load As described above, the target temperature for a heating zone may be set to an intermediate temperature Tint between the styling temperature Tstyie and the ambient temperature. A method of determining when to change a target temperature for a heating zone from Tint to Tstyie will now be described with reference to Figure 21. The method illustrated in Figure 21 is performed when the arms 4a, 4b of the device 1 are in the closed position (if the arms 4a, 4b of the device 1 are in the open position, then the heating zones can be allowed to cool to ambient temperature). In step S2101 the target temperature for each of the heating zones is set to the intermediate temperature Tint (cooler than the styling temperature Tstyie, but hotter than the ambient temperature), and each of the heating zones is controlled towards Tint using the PID control, duty cycles and firing sequences described above. A threshold power value Pthresh[n] is defined for each heating zone, n, of the N heating zones. The value of Pthresh[n] is initialised to a value of Pthresh-intermediate for each of the heating zones. In step S2102 a determination of whether the power required to maintain each heating zone at the target temperature Tint exceeds the threshold power value set for that heating zone. If the power required to maintain a heating zone at the target temperature Tint exceeds the threshold value Pthresh[n], then in step S2103 the target temperature for that heating zone is updated to the styling temperature Tstyie, since the relatively large amount of power required to maintain the heating zone at the target temperature indicates that hair 40 is in contact with the heating zone. The threshold power value Pthresh[n] forthat heating zone is also updated from Pthresh-intermediate to a value of Pthresh-styie. If the power required to maintain a heating zone at the target temperature Tintdoes not exceed the threshold value Pthresh[n], then the target temperature is maintained at Tint. The method then returns to step S2102, and the comparison of the power required to maintain each heating zone at target temperature with the corresponding threshold power values is performed. However, some of the heating zones may now have a target temperature of Tstyie, and a corresponding value of Pthresh[n] of Pthresh-styie, depending on the updates to the target temperatures performed in step S2103. If the target temperature for a heating zone is set to Tstyie, and the power required to maintain the heating zone at Tstyie falls below the Pthresh[n], then the target temperature for that heating zone is set to Tstyie. This because when the power required to maintain the heating zone at the target temperature falls below the threshold value, it indicates that the hair is no longer in contact with the heating zone. The threshold power value Pthresh[n] for the heating ZOne iS alSO Set tO Pthresh-intermediate. It will be appreciated, therefore, that in the method of Figure 21 the threshold temperature assigned to each heating zone can be changed from Tint to Tstyie, and from Tstyie to Tint, based on a comparison of the power required to maintain the heating zone at the target temperature with a threshold power value. In other words, the target temperature for a heating zone is determined based on the thermal load at the heating zone. Beneficially, therefore, the presence of hair 40 in a heating zone can be determined, and the heating zone can be controlled to increase in temperature to the styling temperature Tstyie, in order to style the hair. The removal of the hair 40 from the heating zone can also be detected, and the target temperature lowered to Tint in order to reduce the energy usage of the device 1. When the target temperature for a heating zone has changed from Tstyie to Tint, and the heating zone is cooling from Tstyie to Tint, there is no power demand at the heating zone (since the zone is cooling). However, since there is no power demand to maintain the zone at the target temperature, the power demand of the heating zone will be lower than the threshold value Pthresh-intermediate even when hair 40 is placed onto the heating zone, meaning that the presence of hair on the heating zone may not be detected by monitoring the power demand during the cooling period. This will result in a delay in returning the heating zone to the styling temperature Tstyie, decreasing the styling performance. However, the inventors have realised that the presence of hair 40 can nevertheless be detected during the cooling period by monitoring the rate of cooling, since the heating zone cools faster when hair is in contact with the heating zone. For example, the method of determining the temperature of the heater electrode 64 illustrated in Figure 18 can be performed to monitor the rate of cooling of the heating zone. When the rate of cooling exceeds a threshold rate of cooling (which may also be referred to as a threshold temperature gradient), a determination that hair is in contact with the heating zone can be made, and the target temperature for the heating zone is set to Tstyie. Beneficially, therefore, the temperature of the heating zone can be rapidly returned to the styling temperature Tstyie to style the hair 40. The threshold rate of cooling may be set to the rate of cooling of the unloaded heating zone (or slightly below that unloaded cooling rate, to avoid false determinations that hair 40 is present in the heating zone). Opposing Heating Zones When each of the arms 4a, 4b of the device 1 is provided with a respective heater 6a, 6b as illustrated in Figure 1a, heat can be transferred between opposing heating zones when the arms 4a, 4b are in the closed position. Figure 22 shows a simplified schematic illustration of such opposing heating zones. As shown in the figure, in this example heat transfer will occur between zones 1 and 4, between zones 2 and 5, and between zones 3 and 6. The inventors have realised that the methods for controlling the temperature of the heating zones can be further improved by taking into account this heat transfer. For example, a situation may occur in which the target temperature of Zone 1 of Figure 22 is set to Tint or ambient, but the target temperature of Zone 4 is set to Tstyie. In this situation, heat will be transferred from Zone 4 to Zone 1, and the power required to maintain Zone 4 at Tstyie will be relatively high. This may lead to a false determination that hair is present in Zone 4 (since the power to maintain the zone at the threshold temperature will be higher than Pthresh[n]), and therefore the target temperature for Zone 4 will be maintained at Tstyie. The inventors have realised that this effect can be mitigated against by determining a target temperature for a pair of opposing heating zones based on the sum of the powers (or the average of the powers) required to maintain each heating zone of the opposing pair at a target temperature. In other words, each pair of opposing heating zones can be considered to be a single logical heating zone, and control performed for the single logical heating zone rather than individually for the two separate opposing heating zones. Control of the target temperatures and threshold power values can then be performed as described above with reference to Figure 21, but by assigning target temperatures and threshold power values to the pairs of opposing heating zones rather than to each heating zone of the opposing pair separately (and by comparing on the total power supplied to the pair of opposing heating zones, or the average power supplied to the pair of opposing heating zones, to the threshold power values). Calibration Apparatus and Methods Improved methods and apparatus for calibrating the heater(s) 6 of the device 1 will now be described. As described above, the heater 6 may be provided with a number of heating zones 642, and the device 1 comprises temperature measurement circuitry 25 for monitoring the temperatures of the heating zones 642. The power supplied to the heater electrodes can then be controlled in order to control the temperature of each heating zone 642 towards a respective target temperature. However, the actual heat output at the heating zones (and the sensed temperature) at a particular supplied voltage will depend on various manufacturing tolerances. For example, the actual heat output and sensed temperature at the heating zones will depend on: • Heater track resistance tolerances • Analogue component tolerances • Analogue-to-Digital (ADC) inaccuracies • Power source voltage errors and fluctuations under load • Wiring and printed circuit board (PCB) track impedances • MOSFET drain-source resistances It is desirable, therefore, to calibrate the actual temperature of each heating zone as a function of the supplied voltage. Temperature Sensors Figure 23 shows a simplified schematic illustration of a heater 6 and a temperature sensor 230. As described above with reference to Figures 2 to 6b, the heater 6 is provided with a set of heating zones 642. The temperature sensor 230 is provided with a corresponding set of temperature sensing zones 232, and may also be referred to as a ‘temperature sensing array’ 230. As will be described in more detail later, during the calibration method the heater 6 is supplied with power, resulting in heat output at the heating zones 642. The temperature of each heating zone is measured using a respective temperature sensing zone 232. In this example each of the temperature sensing zones 232 is aligned with a respective heating zone 642 of the heater 6 during the calibration measurements. For example, temperature sensing zone S1 is generally aligned with (overlaps with) heating zone Z1 during the calibration measurements. It will be appreciated that the temperature sensing zones 232 need not necessarily be completely aligned with the heating zones 642. For example, the temperature sensor 230 may be larger than the heater 6 (temperature sensing zones 232 may be larger than the corresponding heating zones 642) and the and extend past the edges of the heater 6 as illustrated in Figure 29. Alternatively, the temperature sensing zones 232 could be smaller than the corresponding heating zones 642, and the temperature sensing zones 232 could be separated from one another by thermally insulating material (the temperature sensing zones 232 need not necessarily be adjacent). Figure 24 illustrates an alternative arrangement of temperature sensing zones 232 in which there are two temperature sensing zones 232 provided for each heating zone 642. In this example, temperature sensing zone S1a overlaps half of heating zone Z1, and temperature sensing zone S1 b overlaps with the other half of heating zone Z1. This configuration can be used to provide a redundant temperature sensing zone 232 for each heating zone 642, to mitigate against failure of a temperature sensing zone 232 (or, for example, to verify an abnormal temperature measurement from one of the temperature sensing zones 232). It will be appreciated that any other suitable arrangement and size of temperature sensing zones 232 could alternatively be used. Figure 25 illustrates an example in which the device 1 is provided with a pair of heaters 6a, 6b. For example, the device 1 may be of the type illustrated in Figure 1a, in which a heater 6 is provided in each of the two arms 4 of the device 1. A corresponding pair of temperature sensors 230a, 230b are provided for temperature calibration measurements. Whilst in the example illustrated in Figure 25 the two temperature sensors 230a, 230b are provided separately, this need not necessarily be the case. Figure 26 shows an example in which the two temperature sensors 230a, 230b are provided together as part of a single temperature sensing device 262 for sensing the temperatures of the heating zones 642 of both heaters 6a, 6b. In the example of Figure 26 the two temperature sensors 230a, 230b are separated by a layer of thermally insulating material 260, to prevent heat transfer between the temperature sensors 230a, 230b. The layer of thermally insulating material 260 may comprise, for example, a high temperature glass-filled polymer. The heaters 6a, 6b are moved into thermal contact with the temperature sensing device 262 before performing the calibration measurements, for thermal conduction from the heating zones 642 to the temperature sensing zones 232. For example, when the device 1 is of the type illustrated in Figure 1a, the arms 4 of the device 1 can be closed to bring the heaters 6 into contact with the temperature sensors 230a, 230b (by sandwiching the temperature sensing device 262 between the heaters 6). Alternatively, for example when separate temperature sensors 230a, 230b are provided as illustrated in Figure 25, each temperature sensor 230 could be clipped or otherwise fastened onto the respective heater 6, to engage the temperature sensor 230 with the heater and to align the temperature sensing zones 232 with the corresponding heating zones 642. Whilst the temperature sensors 230 illustrated in Figures 23 to 26 are generally rectangular, and the temperature sensing zones 232 are also rectangular, this need not necessarily be the case. The temperature sensors 230 and the temperature sensing zones 232 could have any other suitable shape. For example, when the heater 6b is a curved heater as illustrated in Figure 6b, the temperature sensor 230 could also be curved to ensure good thermal contact with the heater 6b. The temperature sensing zones 232 need not necessarily have a uniform size or shape, for example when the corresponding heating zones 646 do not have a uniform size or shape as illustrated in Figure 5. The temperature sensors 232 may be formed from any suitable material and may have any suitable form. For example, each temperature sensing zone 232 may comprise a silicone heatsink coupled to a thermocouple. As described above, a heater 6 can be provided with power in order to determine the temperature of a heating zone 642 of the heater 6 by measuring the resistance of the corresponding heater electrode. The heater electrodes 64 can be formed of a PTC or an NTC material whose resistance changes with the temperature of the heater electrode 64, and the temperature can be determined based on the measured resistance of the heater electrode 64. Therefore, alternatively, the temperature sensor 230 illustrated in Figures 23 to 26 could simply be an additional heater 6 but operated in a temperature measurement mode (or in the example illustrated in Figure 26, a pair of heaters 6 separated by the layer of thermally insulating material 260 to form the temperature measurement device 262). The temperature sensor 230 could be provided with an arrangement of layers as described above with reference to Figures 13 to 15, and temperature measurement circuity as described above with reference to Figures 7 and 16. In the present example, the temperature of each of the temperature sensing zones 232 is determined by measuring a resistance of each temperature sensing zone 232 (which varies with temperature) as described above. However, the temperature sensor 230 need not necessarily be arranged for thermal conduction from the heater 6 to the temperature sensor 230. For example, a temperature sensor 230 could be arranged at a particular distance from the heater 6 for measuring the radiative heat output of the heater 6 (for example, using an array of infrared sensors). Moreover, even when the temperature sensor 230 is arranged for thermal conduction from the heater 6 to the temperature sensor 230, the temperature sensor 230 and the heater 6 need not necessarily be in direct physical contact. For example, a removable thermally conductive pad could be placed between each heating zone 642 and the respective temperature sensing zone 232 to reduce the risk of damage to the temperature sensor 230 or to the heater 6, or the temperature sensor 230 could be provided with a protective coating. The temperature sensor 230 can be calibrated by heating the temperature sensor 230 (e.g. in an oven) to two or more reference temperatures. The resistances (or voltages) for each temperature sensing zone 232 can then be measured at each reference temperature, enabling the measured temperature to be calibrated as a function of the measured resistance (e.g. using a linear fit to the calibration data, or any other suitable type of calibration fit). Advantageously, this calibration method takes into account both the sensing zone resistances and variations in the measurement electronics as a whole, including manufacturing tolerances in any amplifiers, ADCs, or the like. The method also advantageously does not require the resistances of the temperature sensing zones 232 to have previously been measured. However, the resistances of the temperature sensing zones 232 could nevertheless be measured in order to detect defects in the manufacturing of the temperature sensor 230. Calibration System A system 2700 for calibrating the devices 1 will now be described with reference to Figures 27 to 29. Figure 27 shows a schematic block diagram of the system 2700 for calibrating the devices 1. The calibration system 2700 comprises a number of calibration stations 274 (or ‘bays’). Each calibration station 274 is provided with a temperature sensor 230 and is configured for receiving a device 1 for calibration. Calibration circuitry 280 of the calibration stations 274 will be described in more detail later. The calibration station 274 may comprise a device holder for receiving the device 1, configured such that each of the heating zones 642 of the heater(s) 6 of the device 1 is brought into alignment with the respective temperature sensing zones 232 of the temperature sensor 230 when the device 1 is positioned in the device holder, for subsequent temperature calibration measurements. The calibration station 274 may comprise a flat surface (e.g. formed of plastic) with the temperature sensor 230 rigidly mounted perpendicular to the surface. A power source 270 is provided for supplying the devices 1 with power, for example to power the heaters 6 of the devices. The power source 270 could be, for example, a supply of mains power. The calibration system 2700 may be provided with any suitable number of calibration stations 274 (e.g. 10, 50, or 100 calibration stations). Each calibration station 274 is provided with communication circuity 292 for communicating with a calibration controller 272. The calibration station 274 may communicate with the calibration controller 272 to receive calibration measurement instructions, and to transmit measurement results to the calibration controller 272. The calibration controller 272 performs overall control of the calibration method. The calibration controller 272 is configured to determine calibration information for each device 1, based on the corresponding calibration measurement results, to be stored on the device 1, and to transmit the calibration information to the device 1 via the calibration station 274. Alternatively, the calibration information could be transmitted directly from the calibration controller 272 to the device 1, for example via a wireless communication link (e.g. Wifi or Bluetooth). In a further alternative, the calibration information could be determined at each of the calibration stations 274 rather than at the calibration controller 272. The calibration information, and methods of determining the calibration information, will be described in more detail later. The calibration controller 272 may also communicate with the power source 270 to control the supply of power from the power source 270 to each of the devices 1. Alternatively, the supply of power to the devices 1 may be controlled by a controller 284 provided at the calibration station 274 (which may be under the overall control of the calibration controller 272). In the example shown in Figure 27 the calibration controller 272 has a separate communication connection to each of the calibration bays 274. However, this need not necessarily be the case, and Figure 28 shows an alternative arrangement in which the communication connections are ‘daisy chained’ along the calibration stations to the calibration controller 272. Any suitable communication circuity 292 could be used to connect the calibration stations 274 to the calibration controller, for example an ethernet connection or a wireless communication link. The supply of power from the power source 270 could also be daisy chained along the calibration stations. Figure 29 shows a schematic block diagram of the calibration station 274 including the calibration circuitry 280. A device 1 to be calibrated is situated in the calibration station 274, and is arranged so that the temperature of each of the heating zones 642 of the heater 6 of the device 1 can be measured using the respective temperature sensing zone 232 of the temperature sensor 230 (e.g., when the device is of the type illustrated in Figure 1a, by clamping the temperature sensor 230 between the heater plates. During the calibration measurements, the device 1 is supplied power from the power source 270, for example between 25 V and 40 V of DC power. Alternatively, if the device 1 is a corded device, the device 1 could simply be plugged into a mains power source. The calibration circuitry 280 comprises a calibration station controller 284 that operates in accordance with software stored in the memory 285. The calibration station controller 284 controls the communication circuitry 292 to communicate with the overall calibration controller 272 (e.g. via any suitable wired or wireless connection). The calibration circuitry 280 also comprises a sensor interface 286 for communicating with the temperature sensor 230 in order to obtain the temperature measurements. The sensor interface 286 may also be configured to communicate with a sensor for measuring the power drawn by the device 1 from the power source 270 (or to communicate with the power source 270 directly to obtain the power draw information). The power draw information could be used to detect that the device 1 has a fault, for example if the device 1 is drawing an abnormally large amount of power, or is not drawing power at all. The connection from the power source 270 to the device 1 for supply of power to the device may be provided using a magnetic power jack. However, any other suitable connection could alternatively be used. A power cable of the magnetic power jack may be sacrificial, for example designed to be replaced every 1000 calibrations, with an industrial grade power connector provided for connection to the device 1. If the power is provided to the device 1 using a calibrated power source (e.g. calibrated laboratory power supply), then advantageously the voltage sense scaling of the device 1 can be accurately calibrated. The calibration circuitry also comprises a device interface 282 for communicating with the device 1 (for example, to upload the final calibration information to the device 1). The device interface 282 may comprise any suitable wired or wireless connection (e.g. Wifi or a short-range wireless communication link such as Bluetooth). The provision of a wireless device interface 282 is particularly advantageous since it removes the need to connect a physical wire from the calibration circuitry 280 to the device 1 for communication, improving the efficiency of the calibration method. The calibration circuitry 280 also includes a status indicator 290 for indicating a status of the calibration station 274 ora status of the device 1. The status indicator 290 may comprise a coloured light for indicating the status. For example, the status indicator light may illuminate green to indicate that the device 1 has been successfully calibrated, or could illuminate red to indicate that the calibration has been unsuccessful (or that the device 1 or calibration station 274 has a fault). The light could also illuminate blue when power is first supplied to the device 1, and illuminate orange whilst the calibration measurements are in progress. When the device interface 282 comprises a wireless communication link, the communication link may initially be configured to use a relatively low transmission power to minimize radio interference with the other calibration stations 274 and devices 1. If communication with the device 1 is unsuccessful, the transmission power could then be increased to attempt to achieve communication. Radio frequency (RF) shielding could be provided around each calibration station 274 to reduce the risk of interference. Calibration Method Figure 30 shows a flow diagram of a method of calibrating the device 1 using the calibration station 274. The calibration method enables efficient and accurate calibration of large numbers of devices 1. Beneficially, the assembled device 1 can be calibrated, rather than calibrating the individual components of the device 1 separately (for example, by separately measuring the resistance of the heater tracks, the resistance of the current sense resistor, and so on). This is more efficient than performing separate calibration procedures on the un-assembled components, and advantageously takes into account all of the contributing errors using a single calibration. Moreover, the calibration method does not require the device 1 or component to be placed within a controlled environmental chamber, and the calibration can be performed particularly fast (e.g. a few seconds). Furthermore, the method also enables the detection of hardware faults and anomalies for example, by detecting that the calibration parameters are outside of an acceptable range. In step S301, pre-calibration configuration is performed. The calibration station 274 receives the device 1, bringing the heater 6 into thermal contact with the temperature sensor 230 for thermal conduction from each of the heating zones 642 to the respective temperature sensing zone 232. The device 1 is connected to the power source 270 for supply of power to the heater 6, and establishes communication via the device interface 282. The device interface 282 can be used to communicate with the device 1 to register the device 1 when it is received by the calibration station 274. The device 1 may be configured to enter a calibration mode when the device 1 is first powered on, but could alternatively be controlled to enter the calibration mode by the controller 284 via the device interface 282. In step S302, calibration measurements are performed. The calibration of the device 1 can be performed in an automatic manner, with calibration status indicated by the status indicator 290. The device 1 outputs power to the heater(s) 6, to heat the heating zones 642 to a target temperature as measured using the temperature sensing circuitry of the device 1. The device sequentially increases the target temperature under the control of the calibration controller 272. Two or more target temperatures are used for the calibration, for example a first target temperature of 100 °C and a second target temperature of 240 °C, but it will be appreciated that the actual calibration temperatures used will depend on the temperatures that the device 1 is expected to operate at during normal use. The actual temperatures of the heating zones 642 are measured using the temperature sensor 230, and the measurements are transmitted to the calibration controller 272. Based on the differences between the target temperatures and the temperatures measured using the temperature sensor 230, the calibration controller generates calibration information to be stored at the device, to calibrate the device 1. The generation of the calibration information will be described in more detail later. The calibration measurements could also be performed using a plurality of different supply voltages, for example if the design of the device 1 means this is a potential source of errors. If the calibration controller 272 detects an unexpected or anomalous relationship between the target temperatures and the temperatures measured by the temperature sensor 230, then the device 1 may be rejected as faulty or requiring further investigation. Self-measurements of voltage and current within the device 1 can also be performed in order to identify possible hardware faults, and can also be transmitted to the calibration controller. Large temperature errors or anomalous power readings at this stage can be used as a trigger to abort of the calibration, and the device 1 is rejected with diagnostic information stored. The calibration controller 272 monitors and controls calibration process and records the calibration results automatically (each device 1 could have a unique, pre-programmed software-readable serial number that can be stored in association with the calibration results). In step S303, the calibration information generated by the calibration controller 272 is uploaded to the device 1. Temperature, power or voltage corrections are determined and stored in the device 1, so that the device 1 can accurately control the heating zones 642 towards target temperatures using the calibration information. The calibration information is provided for each heating zone 642. The calibration information includes the equivalent of a gain and offset per heating zone (if a linear calibration is used). A calibration report for the device 1 may also be stored at the calibration controller 272. The device 1 is then controlled by the calibration controller 272 to exit the calibration mode and to shut down, and will enter a standard, calibrated operation mode when the device 1 is next powered on. The device 1 is then removed from the calibration station 274. Temperature Calibration Example Figure 31 shows a table of exemplary calibration measurements, obtained as part of the method of Figure 30, that can be used to generate the calibration information to be stored and used by the device 1. As shown in the table, in this example target temperatures of 100°C and 240°C are used. In this example, when the device controlled the temperature of the heater to 100 °C, the actual temperature of the heating zones 642 was measured by the temperature sensor 230 to be 103 °C. Therefore, the uncalibrated device 1 has a temperature error of 3°C at a target temperature of 100°C. Similarly, when the device controlled the temperature of the heater to 240 °C, the actual temperature of the heating zones 642 was measured by the temperature sensor 230 to be 232 °C. Therefore, the uncalibrated device 1 has a temperature error of 8°C at a target temperature of 100°C. Therefore, calibration information (e.g. gain and offset) is to be generated to correct the uncalibrated temperature of 100°C sensed by the device to 103°C, and to correct the uncalibrated temperature of 240°C sensed by the device to 232°C. If the device 1 measures a zone temperature of TSENSEraw°C, then the device 1 can correct its internal temperature measurement using the following equation: tsensecal = (TSENSEraw-TCALQraw)X(TCAL1actual-TCAU)actual) (TCAL1raw-TCAL0raw) + TCALQactual, Where TSENSEcal is the calibrated temperature of a heating zone 642 sensed by the device 1, TSENSEraw the uncalibrated temperature of the heating zone 642 sensed by the device 1, TCALOraw is a first uncalibrated calibration temperature sensed by the device 1, TCALOactual is the first calibration temperature as measured by the temperature sensor 230, TCALIraw is a second uncalibrated calibration temperature sensed by the device 1, and TCALIactual is the second calibration temperature as measured by the temperature sensor 230. In this example, the calibration information is generated so that if the device 1 senses the uncorrected temperature to be 180.013, the calibration information can be used to determine the actual temperature of: (180 - 100) x (232 - 103) TSENSECAL = ...........................y—------ + 103 = 176.71 l / w — 1UU J Therefore, given a target temperature of 180 C, the device 1 will set the heater 6 temperature higher than it would otherwise have done to achieve an actual temperature of 180 °C. By way of example, during a calibration procedure the heater 6 may be controlled from a target temperature of 125C to a target temperature of 19513, in 10C increments. For each target temperature, the actual temperature obtained for each heating zone is measured using the temperature sensor 230. These measurements enable a relationship between the target temperature and the actual temperature to be determined. This relationship may be, but need not necessarily be, a linear relationship. For example, a slope parameter ‘m’ and an offset parameter ‘C’ may be determined for a linear relationship between the target temperature and the actual temperature. For a given temperature of a heating zone sensed by the device 1, the device 1 may determine a corrected temperature for the heating zone based on the calibration information comprising the slope parameter and the offset parameter. The corrected temperature, TCorrected, for a heating zone is given by: -rCorrected _ -rMeasured / ^-. -rMeasuredx q where TMeasured is the temperature of a heating zone as measured by the device 1. The value of TCorrected for each heating zone can then be controlled towards the respective target temperature. Alternatively, rather than storing a gain and an offset for each temperature zone, the calibration information stored on the device may comprise an indication (e.g., a table) of a temperature offset per heating zone at a number of calibration temperatures. Linear interpolation could then be used to determine the value of the temperature offset, C, for any measured temperature, using the table. Modifications and alternatives Detailed embodiments and some possible alternatives have been described above. As those skilled in the art will appreciate, a number of modifications and further alternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein. It will therefore be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto. Whilst in the examples described above the temperature sensor 230 is provided with a plurality of temperature sensing zones 232, this need not necessarily be the case. For example, if the heater 6 of the device 1 comprises only a single heating zone, then the temperature sensor 230 could also be provided with only a single temperature sensing zone 232. The temperature sensor 230 need not necessarily be arranged for thermal conduction between the heater 6 and the temperature sensor 230. For example, the temperature sensor 230 may comprise a thermal camera for measuring the temperature of the heating zones of the heater 6, by measuring infrared radiation emitted from the heater 6. Whilst in some of the above examples apparatus and methods for calibrating the heaters and associated electronics of the device 1 as a whole have been described (advantageously taking into account manufacturing tolerances in the amplifiers, ADCs, or the like), additional calibration of the electronic components of the device 1 may also be performed. For example, the electronic components of the device 1 may be calibrated prior to final assembly of the device 1. This additional calibration step can provide gain and offset corrections that enable accurate measurements of the absolute values of the voltages, currents, resistances and powers (e.g. PSU current and power output, resistance of the current-sense resistor, voltage sensing, and the gain and offset of the operational amplifiers). The invention has been described above by way of implementation in a hair styling device for straightening hair (‘hair straighteners’) which employ flat hair styling heaters 6. However, it could alternatively be implemented in any form of hair styling device, such as (but not limited to) crimpers, curlers or heated brushes. The heaters 6 may define a heating surface that is flat, curved, ridged or in the shape of a barrel. The hair styling device may have two arms like the device illustrated in Figure 1 or it may be a single armed device. The heaters described above may also be used in hair dryers or in combination devices that use conductive heating and air to dry and style the user’s hair (such as those described in the applicant’s earlier PCT application WO 2021 / 019239. In embodiments where air is used, the heaters 6 may be perforated so that air passes through the heater and is warmed by the heater as the air passes through. Whilst a target styling temperature of 200°C has been described above, the target styling temperature need not necessarily be 200°C. The target temperature for styling hair can range from, for example, 30°C to 230°C. This advantageously allows for a variety of styling options, including “wet to style” where the hair styler is applied to wet hair. In this case, the water content of the hair can be measured, for example by using a detector which compares the amount of radiant energy in two absorption bands in the spectrum of light emitted by an infra-red source and reflected by the hair. Based on this measurement, the target temperature can be adjusted accordingly to stop damage to the hair or ‘sizzle’ occurring. The target temperature could also be adjusted, for example, based on a hair type of the user. In the above embodiments, Metal Oxide Semiconductor Field Effect Transistor (MOSFET) switches were used to control powering and sensing of the heater electrodes. As those skilled in the art will appreciate, other switches could be used instead. For example, Field Effect Transistors (FETs) could be used, such as Gallium Nitride FETs or bipolar junction transistors (BJTs). Whilst in the examples described above the heater electrodes 64 were used for temperature sensing by the device 10, in alternative embodiments, the device 10 may be provide with separate temperature sensors may be provided for sensing the temperature of each heating zone defined by the heater electrodes 64. For example, referring to Figure 12a, a separate layer of temperature sensors may be provided under dielectric layer 66 or on top of the layer 62. In the above embodiments, a DC power source was used to provide electrical power for heating the heater electrodes 64. This DC power source will typically be a battery, although DC supplies that derive their power from a mains power AC signal may be used. In embodiments where separate temperature sensors are provided, then AC mains power may be used to heat the heater electrodes. Thicker dielectric layers may be provided in this case between the heater electrodes 64 and the hair contacting surface of the hair styler 1. The device 10 may be partially or entirely formed of a unitary structure, e.g. by 3D printing. In the above-described examples the device 10 may comprise a single heater 6, or may alternatively comprise two or more heaters 6 (e.g. one provided for each heater arm 4a, 4b). Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “containing”, means “including but not limited to”, and is not intended to (and does not) exclude other components, integers or steps. The expressions “to dry hair”, “drying hair” or “decrease a moisture level of hair” and the like, as used in the present disclosure, can refer both to the removal of “unbound” water that exists on the outside of hair when wet, or the removal of “bound” water, which 5 exists inside individual hairs, and which can be interacted with when heat styling hair. The “bound” water need not necessarily be removed when drying hair, although removal of some bound water may occur during a drying or styling process. It should also be noted that the term “wet” as used in the present disclosure should be interpreted broadly, to encompass not only hair wetted by water, but also hair wetted 10 by liquids other than water. For example, hair may be wetted by a solvent-based colourant, which the invention may be used to dry and / or style. Various other modifications will be apparent to those skilled in the art and will not be described in further detail here. 15

Claims

1. Apparatus for drying and / or styling hair, the apparatus comprising:a heater comprising a hair contacting surface for heating hair that contacts the hair contacting surface by conduction, the heater comprising a plurality of independently controllable heater electrodes that define a plurality of independently controllable heating zones of the hair contacting surface, wherein each of the heater electrodes has a resistance that varies with the temperature of the heater electrode;a memory storing separate calibration information for each of the heating zones that indicates a respective relationship between at least one of:the resistance of the corresponding heater electrode and the temperature of the corresponding heating zone,the voltage supplied to the corresponding heater electrode and the temperature of the corresponding heating zone, ora measured temperature of the corresponding heating zone and an actual temperature of the corresponding heating zone; anda controller configured to control the apparatus to:independently control the supply of power to each of the heater electrodes, based on the respective calibration information stored in the memory, to control the temperature of each of the heating zones towards a respective target temperature.

2. The apparatus according to claim 1, wherein the calibration information comprises a temperature offset for each of the heating zones, for correcting the measured temperature of the heating zone.

3. The apparatus according to claim 1 or 2, wherein the respective relationship is a linear relationship, and wherein the calibration information comprises a gain and an offset for each of the heating zones.

4. The apparatus according to any preceding claim, wherein the apparatus for drying and / or styling hair is a hair straightener.

5. The apparatus according to any preceding claim, wherein the apparatus for drying and / or styling hair is configured for receiving the calibration information from a calibration apparatus.

6. The apparatus according to claim 5, wherein the apparatus for drying and / or styling hair is configured for receiving the calibration information from the calibration apparatus via a wireless communication link.

7. The apparatus according to claim 5 or 6, wherein the apparatus for drying and / or styling hair is configurable in a calibration mode in which the supply of power to the heater is controllable by the calibration apparatus.

8. Calibration apparatus for calibrating the apparatus for drying and / or styling hair according to any one of claims 1 to 7, the calibration apparatus comprising:a plurality of temperature sensing zones, wherein each of the plurality of temperature sensing zones is configured for thermal contact with a respective one of the independently controllable heating zones of the hair contacting surface, and wherein each of the temperature sensing zones has a resistance that varies with the temperature of the temperature sensing zone; andcommunication circuitry configured for communication with the apparatus for drying and / or styling hair;wherein the calibration apparatus is configured for:sensing the temperature of each of the heating zones based on the resistance of the respective temperature sensing zone;obtaining calibration information for each of the heating zones; andtransmitting the calibration information to the apparatus for drying and / or styling hair using the communication circuitry.

9. The calibration apparatus according to claim 8, wherein obtaining the calibration information comprises generating, at the calibration apparatus, the calibration information for each of the heating zones.

10. The calibration apparatus according to claim 8, wherein obtaining the calibration information comprises:transmitting, to a calibration controller, measurement results corresponding to the sensed temperatures for each of the heating zones; andreceiving, from the calibration controller, the calibration information for each of the heating zones.

11. The calibration apparatus according to any one of claims 8 to 10, wherein the calibration apparatus is configured for controlling a supply of power to the heater of the apparatus for drying and / or styling hair.

12. The calibration apparatus according to any one of claims 8 to 11, wherein the communication circuitry is configured for wireless communication with the apparatus for drying and / or styling hair.

13. The calibration apparatus according to claim 12,wherein the communication circuitry is configured to use a first transmission power for wireless communication with the apparatus for drying and / or styling hair; andin a case where communication with the apparatus for drying and / or styling hair using the first transmission power is unsuccessful, the communication circuitry is configured to use a second transmission power, greater than the first transmission power, for wireless communication with the apparatus for drying and / or styling hair.

14. The calibration apparatus according to any one of claims 8 to 13, wherein the calibration apparatus comprises a calibration status indicator for indicating a calibration status of the apparatus for drying and / or styling hair.

15. The calibration apparatus according to claim 14, wherein the calibration status indicator comprises a coloured light for indicating the calibration status.

16. A calibration system comprising:at least one calibration apparatus according to any one of claims 8 to 15 for calibrating the apparatus for drying and / or styling hair; anda calibration controller configured for communication with the calibration apparatus;wherein the calibration controller is configured for:receiving, from the calibration apparatus, measurement results corresponding to the sensed temperatures for each of the independently controllable heating zones; andtransmitting, to the apparatus for drying and / or styling hair, the calibration information for each of the heating zones.

17. The calibration system according to claim 16, wherein the calibration controller is configured to transmit the calibration information to the apparatus for drying and / or styling hair via the calibration apparatus.

18. The calibration system according to claim 16, wherein the calibration controller is configured to transmit the calibration information directly to the apparatus for drying and / or styling hair.

19. The calibration system according to claim 18, wherein the calibration controller is configured to transmit the calibration information to the apparatus for drying and / or styling hair via wireless communication link.

20. The calibration system according to any one of claims 16 to 19, wherein the calibration controller is configured for controlling a supply of power to the heater of the apparatus for drying and / or styling hair.

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