EP3568639A2

EP3568639A2 - Oven, method of controlling oven, and sensors

Info

Publication number: EP3568639A2
Application number: EP17822731.0A
Authority: EP
Inventors: Mark Edward Williamson
Original assignee: Cambridge Sensor Innovation Ltd
Current assignee: Cambridge Sensor Innovation Ltd
Priority date: 2016-11-28
Filing date: 2017-11-28
Publication date: 2019-11-20
Also published as: WO2018096369A2; WO2018096369A3; GB201821250D0; GB2566401B; GB2567076A; GB2567076B; GB201620108D0; GB2566401A; GB201821284D0

Abstract

An oven, suitable for domestic use for cooking food, includes sensors for temperature, humidity, convective heat transfer and radiant temperature, and controls convective, radiative and conductive heat fluxes, as well as the rate of mass transfer of water to/and or from an article, such as food, throughout a cooking process. An air circulation channel has inlets and outlet nozzles, and a variable air speed regulator, the inlets, outlets and air speed being configured to provide a relatively high speed of air impingement on an article in the oven, enabling an article to rest on the base of the oven, and to minimise the loss of heat and humidity conditioned air when the oven door is opened. A radiant temperature and convective heat transfer coefficient sensor has first and second sensor spades, with high and low emissivity coatings, connected to a heat sink through conductors, a plurality of temperature sensors in the sensor spades and spaced along the conductors, and an air temperature sensor, and temperature readings are taken periodically to independently determine radiative temperature and the convective heat transfer coefficient of air.

Description

Oven, Method of Controlling Oven, and Sensors Field of the invention The invention relates to the field of ovens, typically but not exclusively domestic ovens for cooking food. Some aspects of the invention concern heat flux and humidity sensors useful in ovens as well as other applications. Background to the invention Conventional ovens have limited controllability and reproducibility. In the case of ovens for food, they may apply heat to food incorrectly or provide results which are not reproducible. Cooking effects may vary substantially with altitude, humidity or other parameters. Except for expensive commercial ovens, they generally lack the ability to provide sophisticated time variation in cooking parameters. The present invention aims to provide an oven for heat treating an article which provides a controllable, reproducible time varying heat treatment process. Some embodiments of the invention address the problem of providing a domestic oven for cooking food which provides a controllable, reproducible time varying cooking process and which can be made cost effectively. Some aspects of the invention relate to heat flux and humidity sensing arrangements suitable for use in ovens and other applications. Summary of the invention According to a first aspect of the invention there is provided an oven, the oven comprising an oven chamber for receiving an article (e.g. food) to be heat treated (e.g. cooked), at least one (typically electric) heating element, a humidity regulator, an air flow regulator (e.g. a fan, such as a variable speed fan), a plurality of sensors, and a controller, the plurality of sensors comprising at least one temperature sensor (and typically at least one humidity analyser), the controller configured (typically programmed) to receive measurements from the plurality of sensors and to control at least the at least one heating element, the air flow regulator (and typically the humidity regulator), responsive to the measurements, to thereby regulate the heat treatment (e.g. cooking) of an article received within the oven chamber (in use). Preferably, the controller thereby regulates convective, radiative (and optionally also conductive) heat fluxes to the article during a heat treatment (e.g. cooking) cycle. Preferably, the controller thereby regulates the rate of mass transfer of water to and/or from the article during the heat treatment (e.g. cooking) process. Preferably, convective, radiative (and optionally conductive) heat fluxes, and the rate of mass transfer of water to and/or from the article, are independently regulated by the controller. As a result, the invention provides improved and reproducible control of the heat treatment cycle. In the case of an oven for cooking food, the resulting food quality is thereby better controlled. Embodiments of the invention, described below, enable an oven having a high level of control and reproducibility to be manufactured economically. The convective, radiative (and optionally also conductive) heat fluxes to the article, and typically also the rate of mass transfer of water to and/or from the article during the heat treatment (e.g. cooking) process may be varied with time during a heat treatment (e.g. cooking) cycle, in accordance with a predetermined program. Preferably, the plurality of sensors comprises at least one sensor of the hemispherically-averaged radiative temperature above the exposed upper surfaces of the article (e.g. food). Typically, the controller controls at least one said heating element (and in some embodiments also the humidity regulator and/or the air flow regulator) responsive to the at least one sensor of radiative temperature to thereby regulate the (hemispherically-averaged) radiative heat flux incident on the upper food surfaces in the oven chamber. The incident radiative temperature may be regulated to a target incident radiative temperature. A numerical value of the incident radiative temperature may be calculated. Some or all of the heating elements are typically electric. However, the at least one heating element may comprise or be another heating element, such as a gas heater. The at least one said heating element may comprise one or more radiative heating elements, typically on the roof of the oven chamber. The current to such electric heating elements may be pulsed under the control of the controller, and the controller may vary the duty cycle, peak current or peak temperature to thereby control radiative heat transfer in the oven chamber. The temperature of the radiating heating elements may also be controlled, to adjust the peak intensity wavelength band of the emitted radiation. Preferably, the plurality of sensors comprises convective heat transfer sensing means (such as one or more sensors, the outputs of which are together indicative of the convective heat transfer coefficient of air within the oven chamber) configured to measure convective heat transfer within the oven. Typically, the controller controls at least one said heating element and the humidity regulator (and optionally also the air flow regulator) responsive to the measured convective heat transfer to thereby regulate convective heat transfer to (or from) the article. Convective heat transfer to the article depends on the convective heat transfer coefficient of air within the oven chamber (which depends on the humidity and pressure of the air), the difference between the (measured) temperature of the air impinging on the article and the temperature of the surface of the article, and also the speed of impingement of air on the article (which is regulated by the air flow regulator). The controller may therefore regulate the convective heat transfer coefficient of air within the oven chamber. The controller may regulate the convective heat transfer coefficient of air within the oven chamber to a target convective heat transfer coefficient. The controller may calculate a numerical value of the convective heat transfer coefficient. Typically, the controller processes at least the humidity measured by the humidity analyser and controls at least the humidity regulator (and optionally also the air flow regulator and one or more electrical heating elements) to regulate the humidity of air in the oven chamber, for example to a target humidity. The controller may calculate a numerical value of the humidity of the air. Typically, the convective heat transfer coefficient of air within the oven and/or the rate of convective heat transfer to the article within the oven chamber is controlled independently of the humidity of air within the oven chamber. Although humidity is a factor which affects convective heat transfer, it is possible to vary other parameters, such as the speed of impingement of air on the article and thereby to independently vary these parameters. Humidity also affects radiative heat transfer (absorption of IR by asymmetric stretch and other absorption bands associated with triatomic molecules such as H₂0, C0₂). Independently controlling humidity and convective heat transfer allows mass transfer and convective heating to be independently varied, under the control of the controller, which is especially useful when heat treating articles which are moisture sensitive, notably food. The controller may control one or more heating elements (in conductive thermal communication with an article in the oven compartment in use) to regulate the amount of heat conducted to the article, for example to a target conductive heat flux or to a target temperature of a support (e.g. a shelf) which receives the article in use. For example, it may be that the oven comprises a shelf, the shelf comprising at least one said (typically electric) heating element controlled by the controller and at least one said temperature sensor. The said at least one (typically electric) heating element and at least one temperature sensor are preferably integrated within the shelf. The controller may control at least one said (e.g. electric) heating element of the shelf responsive to the temperature measured by the at least one temperature sensor of the shelf to thereby regulate the heat transmitted to food located on the shelf by conduction, in use (typically through a container in which the food is located). The controller may process temperature measurements of the at least one temperature sensor of the shelf and the time history of heat generation by the at least one heating element of the shelf to calculate heat transmitted to food located on the shelf by conduction. The shelf is typically fixed. The shelf may form the base of the oven chamber. The controller may comprise at least one processor and memory storing at least one heat treatment (e.g. cooking) program. Typically, the controller reads a stored heat treatment program which is indicative of target values of one or more operating parameters during a heating cycle. Values indicated by the program may in some embodiments be modified by user instructions (e.g. as to duration) received through a user interface and/or other parameters (such as altitude). The operating parameters determine how the controller independently controls convective, radiative (and optionally conductive) heat fluxes, and the rate of mass transfer of water to and/or from the article, and how this varies with time. The operating parameters typically comprise one or more of the said target incident radiative temperature, the said target convective heat transfer coefficient, the said humidity, and the said target conductive heat flux or temperature. The controller may store current target values of one or more said operating parameters in dependence on the heat treatment program. In an oven, different programs may be associated with different food types, or different dish types (for example, different ways of cooking the same or different foodstuffs). The mass of food should be optionally entered by user - which may be used by the controller to optimise settings (most importantly the total cooking time). A recipe may include multiple stages, such as the sequential defrosting of a frozen bread dough, proofing of the same, baking of the same and finally cooling, all under optimal controlled conditions. Although the function of the controller may be implemented using a single processor (e.g. a microprocessor or microcontroller), one skilled in the art will appreciate that the function of the controller may be distributed between a plurality of processors, and may be implemented in part or in whole by electronic circuits. The controller may comprise a plurality of separate control circuits or units which receive a subset of sensor measurements and regulate a subset of the components which are controlled by the controller, for example, there may be a circuit which receives humidity measurements from the humidity analyser and controls the humidity regulator responsive thereto. There may be a circuit which receives measurements of the temperature in the fixed shelf and regulates the electrical heating element in the fixed shelf responsive thereto. Typically, the oven chamber has an oven enclosure which forms the base, roof and walls of the oven chamber (and typically also an oven door) the oven further comprises an air circulation conduit extending between one or more inlets located in the oven enclosure and one or more outlets (typically nozzles) located in the oven enclosure and spaced apart from the one or more inlets, and the air flow regulator regulates the speed of flow of air through the air circulation conduit from the one or more inlets to the one or more outlets. The air flow regulator may be a variable air impeller. The air flow regulator may be a variable speed fan. The air flow regulator may be a variable damper connecting the suction and discharge sides of a fan (which may therefore be constant speed). Thus, in use, air circulates out of the oven chamber, through the inlets, through the air circulation conduit, driven by the air flow regulator, and back into the over chamber through the outlets, thereby circulating. Typically, the one or more inlets are in a lower region of the oven enclosure (e.g. in the lower half or lowest quarter of the oven chamber). Typically, some or all of the outlets are located in an upper region of the oven enclosure (e.g. in the upper half, or upper quarter, or roof of the oven chamber). At least one said (e.g. electric) heating element may be configured to heat air in the air circulation conduit (typically between the one or more inlets and the air flow regulator so that the air flow regulator mixes heated air). Within this specification and the appended claims, upper and lower, and related terms (such as roof) refer to the orientation in which the oven chamber is configured to be oriented during operation. This is typically defined by one or more ground contacting legs, one or more oven retaining formations adapted for locating the oven in a specific orientation, one or more shelves for receiving food on an upper surface thereof, and/or indicia (such as writing) defining an orientation of the oven. The orientation is typically also defined by the inlets and outlets of the air circulation channel and the air flow regulator which are arranged to impel air downwards through the oven chamber during use. Typically, a portion of the air circulation conduit is in gaseous communication with atmospheric air (for example through the air inlet vent and/or air outlet vent discussed below). It may be that (due to the number and shape of the inlets and outlets) the combined resistance to air flow of the one or more inlets and the combined resistance to air flow of the one or more outlets are substantially the same (for example to within 25% or within 10%). Thus, the air pressure in the oven chamber will remain similar to atmospheric pressure during operation (as the pressure difference between the one or more air inlets and the air flow regulator will be similar to the pressure difference between the air flow regulator and the one or more air outlets). This has the effect that if the oven door is opened, the air in the oven chamber will escape less rapidly than is the case with conventional ovens (which typically have a higher than atmospheric pressure in the over chamber), which saves energy and improves cooking consistency (in the case of ovens for food) and also allows, if beneficial, an initial atmospheric composition / temperature to be established in the oven before the article (e.g. food) is placed in the oven. Furthermore, it may be that the combined resistance to air flow of the one or more inlets and the combined resistance to air flow of the one or more outlets amounts to greater than 85%, or greater than 90%, of the resistance to air flow through the air circulation conduit. This improves the evenness of air flow through the outlets into the oven chamber and/or increases the speed of air impinging on the article. Accordingly, the invention also extends, in a second aspect to an oven comprising an oven housing, an oven chamber within the oven housing and a door opening into the oven chamber, an air circulation conduit extending between a plurality of inlets into the oven chamber and a plurality of outlets into the oven chamber, a part of the air circulation conduit intermediate the plurality of inlets and the plurality of outlets being in gaseous communication with atmospheric air, and an air impeller (which is typically an air flow regulator) (such as a fan) within the air circulation conduit, wherein (due to the number and shape of the inlets and outlets) the combined resistance to air flow of the one or more inlets and the combined resistance to air flow of the one or more outlets are substantially the same (for example to within 25% or within 10%). Typically, the humidity regulator is configured to vary the humidity of air received into the air circulation conduit through the one or more inlets. The humidity regulator may be located in the air circulation conduit. Typically, the humidity regulator comprises a humidifier to controllably increase the humidity of the air in the air circulation conduit, for example, a water tank and a steam generator (comprising an electrical heating element) controllable to generate steam and thereby increase humidity. The humidity regulator may comprise a controllable vent (such as the air output vent described below) to allow humid air to controllably escape into the surrounding atmosphere. The air circulation conduit may comprise an air input vent (typically located between the one or more inlets and the air flow regulator) and/or an air output vent (typically located between the air flow regulator and the one or more outlets). The air input vent and/or air output vent may be regulated to control the flow of atmospheric air into the oven and the flow of humid air out of the oven to the surrounding atmosphere. The air input vent and/or air output vent may comprise dampers to vary the flow of air into or out of the air circulation conduit. The air input vent and air output vent may comprise dampers with a common actuator, for example they may comprise rotary dampers mounted on a common shaft driven by a rotary actuator under the control of the controller. Furthermore, it may be that the combined resistance to air flow of the one or more inlets and the combined resistance to air flow of the one or more outlets amounts to greater than 85%, or greater than 90%, of the resistance to air flow through the air circulation conduit. It may be that the oven comprises an air channel extending from within the oven (typically from within the air circulation conduit, typically between the air flow regulator and the outlets) and the exterior of the oven and a temperature regulator which in use regulates the temperature of air within a region of the air channel to above 100°C (to thereby prevent condensation on the walls of the air channel in said region), wherein the humidity analyser comprises a water partial pressure sensor located within the temperature regulated region of the air channel. Accordingly, the humidity of air within the oven adjacent the air channel (and therefore typically within the air circulation conduit) can be measured accurately. Thus, the invention extends in a third aspect to an oven comprising an oven housing, an oven chamber within the oven housing (and typically a door opening into the oven chamber), an air channel extending from within the oven and the exterior of the oven, through the oven housing, and a temperature regulator which in use regulates the temperature of air within a region of the air channel to above 100°C (to thereby prevent condensation on the walls of the air channel in said region), and a water partial pressure sensor located within the temperature regulated region of the air channel. It may be that the oven comprises an air circulation conduit having a plurality of inlets and a plurality of outlets and an air impeller (typically an air flow regulator) within the air circulation conduit and the air channel extends from the air circulation conduit to the exterior of the oven. The oven of the third aspect typically comprises a humidity regulator (typically a humidifier) as set out above. Nevertheless, the humidity analyser may determine humidity in another way. For example, the humidity analyser may comprise a circuit or program code executed on a processor (e.g. the processor of the controller) which infers the humidity of air from the power consumption and rotational speed of the air flow regulator (e.g. fan), and the temperature of air passing through the air flow regulator (measured with a temperature sensor). Thus the invention extends in a fourth aspect to a method of determining the humidity of air circulating through an air circulation conduit, impelled by an electric fan, the method comprising monitoring the power consumption of the fan (and typically also the speed of the fan, or of the air in the air circulation conduit), the temperature of the air (and typically also the altitude or current atmospheric pressure) and thereby determining the humidity of the air in the air circulation conduit. The invention extends to an oven having an air circulation conduit and a controller configured (e.g. programmed) to determine the humidity of air circulating through the air circulation conduit by the said method. Typically, the speed of air flow through the nozzles is regulated by the air flow regulator. This in turn determines the speed of air impingement on the article. Typically this speed is in the range 0.2 - 10 ms^"1. The upper end of this range is much higher than is typical in a domestic oven. In in some embodiments the speed of air impingement is greater than 1 ms^" , greater than 2 ms^"1 or greater than 4 ms^" . Importantly, as the speed of air flow through the nozzles is regulated and is at least 0.2 ms^"1 and as the convective heat flux to the article is regulated, the vertical position of the article in the oven chamber is less important than would otherwise be the case. Accordingly, it may be that the oven chamber does not comprise any intermediate shelves (and in some embodiments any demountable shelf retaining formations) between the base of the oven chamber and the top of the oven chamber. Typically, the base of the oven chamber is formed by the above described shelf. As set out above, the plurality of sensors may comprise at least one sensor of (hemispherically-averaged) radiative temperature above the exposed surfaces of the article (e.g. food), and the plurality of sensors may comprise convective heat transfer sensing means. It may be that the least one sensor of (hemispherically-averaged) radiative temperature and the convective heat transfer sensing means are integrated. The plurality of sensors may comprise a heat sensor for independently determining (hemispherically-averaged) radiative temperature (above the exposed surfaces of the article, e.g. food) and the convective heat transfer coefficient of air within the chamber and comprising a plurality of temperature sensors, the outputs from which are processable (and in use are analysed by the controller) to independently derive the (hemispherically-averaged) radiative temperature within the oven and the convective heat transfer coefficient of air within the oven, or parameters related to these properties. The heat sensor may comprise a sensor body comprising insulation, a heat sink, a sensing surface having first and second surface mounted elements thereon, the first surface mounted element having a relatively lower emissivity surface and thermally coupled to the heat sink through a first thermal conduction path extending through the insulation, the second surface mounted element having a relatively higher emissivity surface and thermally coupled to the heat sink through a second thermal conduction path extending through the insulation, a temperature sensor configured to measure the temperature of the heat sink, a temperature sensor configured to measure the temperature of the first surface mounted element, a temperature sensor configured to measure the temperature of the second surface mounted element, at least two temperature sensors configured to measure the temperature at spaced apart locations along the first thermal conduction path and at least two temperature sensors configured to measure the temperature at spaced apart locations along the second thermal conduction path. The heat sensor is useful in other applications and for measuring the temperature of other gases too, accordingly, the invention extends in a fifth aspect to a heat sensor for independently determining (hemispherically-averaged) radiative temperature and the convective heat transfer coefficient of gas on a sensing surface of the heat sensor, the sensor comprising a sensor body comprising insulation, a heat sink, a sensing surface having first and second surface mounted elements thereon, the first surface mounted element having a relatively lower emissivity exterior and thermally coupled to the heat sink through a first thermal conduction path extending through the insulation, the second surface mounted element having a relatively higher emissivity exterior and thermally coupled to the heat sink through a second thermal conduction path extending through the insulation, a temperature sensor configured to measure the temperature of the heat sink, a temperature sensor configured to measure the temperature of the first surface mounted element, a temperature sensor configured to measure the temperature of the second surface mounted element, at least two temperature sensors configured to measure the temperature at spaced apart locations along the first thermal conduction path and at least two temperature sensors configured to measure the temperature at spaced apart locations along the second thermal conduction path. Typically, the sensing surface further comprises a gas temperature sensor. The gas temperature sensor typically has a cross-section area of less than 10% (or less than 1 %) of the cross-sectional area of the first and second surface mounted elements and so is much less sensitive to convective heat transfer. It is small so it has a rapid response to change, and it has a low emissivity surface (e.g. may be gold plated) so that it is unaffected by incident radiation. Typically, the first and second surface mounted elements have the same size and shape but different emissivities. It may be that the emissivity of the first and second surface mounted elements differs by at least 0.25, at least 0.5, at least 0.75 or at least 0.9. The emissivity of the first surfaced mounted element is typically less than 0.1 . The emissivity of the second surface mounted element is typically greater than 0.9. They may be formed from the same material (typically a metal) but with different coatings, for example, the first surface mounted element may have a matt black coating and the second surface mounted element may have a metal (e.g. gold) coating. The first and second surface mounted elements may be generally planar. They typically protrude into the oven chamber. They may be oriented in the same direction. They may be adjacent and parallel. They may be adjacent and parallel to the roof and base of the oven chamber. The first and second surface mounted elements thereby have a similar sensitivity to convective heat transfer but different emissivities and therefor dissimilar sensitivity to radiative heat transfer. The temperatures measured by the temperature sensors are measured repetitively to thereby determine at least one rate of change of temperature. Thus, the temperatures measured by the said temperature sensors can be processed to independently determine the (hemispherically-averaged) radiative temperature at the sensing surface and the conductive heat transfer parameter. Furthermore, these properties can be determined independently of the temperature of the heat sensor itself. The first and second surface mounted elements may be hollow (to thereby reduce their heat capacity and increase their rate of temperature response to changes in radiative temperature and/or convective heat transfer). The controller may be configured (e.g. programmed) to carry out a sensor checking cycle in which it monitors the temperature change of both the first and second surface mounted elements in response to a change of radiant temperature within the oven. Typically, the change of radiant temperature is switching on, or off, one or more radiant heating elements in the oven chamber. The change in the temperature of the first and second mounted elements as a result is monitored. If the temperature deviations exceed a predetermined amount, a fault status is generated (and typically notified to a user). The test may be carried out on start-up or during a quiescent period of the oven, for example. The controller may take into account (actual or average local) atmospheric pressure. For example, the oven may comprise a pressure sensor. However, the controller may comprise stored data concerning the altitude or average atmospheric pressure at the location of the oven and the controller may be configured to take that stored data into account during operation. The controller may be configured to receive the altitude or average atmospheric pressure data, for example through a user interface. The oven chamber is typically enclosed. The oven chamber is adapted for receiving a product (typically food) to be cooked. The cooking chamber typically comprises a door. In use, the interior of the cooking chamber is typically defined by the oven enclosure and the interior of the door. The oven may be a domestic oven, for example, it may have an oven chamber with a capacity of less than 150 litres. It may have a width of less than 80 cm. It may have a depth of less than 1 m. It may have a height of less than 1 m. It may have a maximum power of less than 10kW. According to a sixth aspect of the invention there is provided a method of operating an oven, the oven comprising an oven chamber for receiving an article (e.g. food) to be heat treated (e.g. cooked), at least one (typically electric) heating element, a humidity regulator, an air flow regulator (e.g. a fan, such as a variable speed fan), a plurality of sensors, and typically a controller, the plurality of sensors comprising at least one temperature sensor (and typically at least one humidity analyser), the method comprising making measurements using the plurality of sensors, and controlling at least the at least one heating element and the air flow regulator (and typically the humidity regulator), responsive to the measurements, and thereby regulating the heat treatment (e.g. cooking) of an article received within the oven chamber (in use). Preferably, convective, radiative (and optionally also conductive) heat fluxes to the article are thereby regulated during a heat treatment (e.g. cooking) cycle. Preferably, the rate of mass transfer of water to and/or from the article during the heat treatment (e.g. cooking) process is thereby regulated. Preferably, convective, radiative (and optionally conductive) heat fluxes, and the rate of mass transfer of water to and/or from the article, are thereby independently regulated. The convective, radiative (and optionally also conductive) heat fluxes to the article, and typically also the rate of mass transfer of water to and/or from the article during the heat treatment (e.g. cooking) process may be varied with time during a heat treatment (e.g. cooking) cycle, typically in accordance with a predetermined program. Preferably, the plurality of sensors comprises at least one sensor of the hemispherically-averaged radiative temperature above the exposed upper surfaces of the article (e.g. food) and the output of at least one said heating element (and in some embodiments also the humidity regulator and/or the air flow regulator) is controlled responsive to the at least one sensor of radiative temperature to thereby regulate the (hemispherically-averaged) radiative heat flux incident on the upper food surfaces in the oven chamber. The incident radiative temperature may be regulated to a target incident radiative temperature. The temperature of the radiating heating elements may also be controlled, to adjust the peak intensity wavelength band of the emitted radiation. Preferably, the plurality of sensors comprises convective heat transfer sensing means (such as one or more sensors, the outputs of which are together indicative of the convective heat transfer coefficient of air within the oven chamber) configured to measure convective heat transfer within the oven and the controller controls at least one said heating element and the humidity regulator (and optionally also the air flow regulator) responsive to the measured convective heat transfer to thereby regulate convective heat transfer to (or from) the article. The method may comprise regulating the convective heat transfer coefficient of air within the oven chamber, for example to a target convective heat transfer coefficient. The method may comprise calculating a numerical value of the convective heat transfer coefficient. Typically, at least the humidity regulator (and optionally also the air flow regulator and one or more electrical heating elements) is regulated to regulate the humidity of air in the oven chamber, for example to a target humidity, typically taking into account the humidity measured by the humidity analyser. A numerical value of the humidity of the air may be calculated. Typically, the convective heat transfer coefficient of air within the oven and/or the rate of convective heat transfer to the article within the oven chamber is controlled independently of the humidity of air within the oven chamber. The power output of one or more heating elements (in conductive thermal communication with an article in the oven compartment in use) may be controlled to regulate the amount of heat conducted to the article, for example to a target conductive heat flux or to a target temperature of a support (e.g. a shelf) which receives the article in use. The method may be carried out by at least one processor in electronic communication with a memory storing at least one heat treatment (e.g. cooking) program. The method may comprise circulating air, using an air flow regulator, through an air circulation conduit extending between one or more inlets located in the oven enclosure and one or more outlets (typically nozzles) located in the oven enclosure and spaced apart from the one or more inlets. The method may comprise regulating the humidity of air received into the air circulation conduit through the one or more inlets. The humidity may be regulated by regulating a humidifier (e.g. a steam generator) and/or an air output vent which is selectively openable to vent humid air. The method may comprise passing air from within the air circulation conduit through a channel having a region at a temperature of at least 100°C and measuring the partial pressure of water in the air in that region. The invention extends to a computer readable storage medium storing computer program code which, when executed on the controller of an oven, causes the oven to carry out the method of the sixth aspect of the invention. Further optional features of the sixth aspect of the invention correspond to optional features set out in respect of the first five aspects of the invention and, indeed, optional features mentioned above in respect of any aspect of the invention apply to each aspect of the invention. Description of the Drawings An example embodiment of the present invention will now be illustrated with reference to the following Figures in which: Figure 1 is a schematic cross-section through an oven; Figure 2 is a schematic diagram of the controller of the oven; Figure 3 is a side elevation section through a humidity analyser; Figure 4 is a plan view of a combined convective heat and radiant heat flux sensor; and Figure 5 is an example of the variation of cooking parameters (y-axis) with time (x- axis) throughout a cooking cycle, specified by a cooking programme. Detailed Description of an Example Embodiment By way of example, Figure 1 illustrates a domestic oven (1 ) for cooking food. The oven comprises an outer oven wall (2) including a thermally insulating material, an oven chamber (4) having an oven enclosure (6) and a door (8), which together define the oven chamber, where food is cooked in use. A controller (100) is located within the oven outer wall but outside of the oven enclosure. At the base of the oven is a fixed shelf (10), which supports food (12), retained within a container (14) (such as a metal tray) during use. The fixed shelf comprises a shelf heating element (16) and a shelf temperature sensor (18). In an example embodiment, there are no other shelves. Nevertheless, for marketing reasons shelves may be retained so that users may, if so desired, operate the oven in 'conventional' mode. An air circulation conduit (20) extends around the periphery of the oven chamber between a plurality of inlet apertures (22), located near the base of the oven chamber, and a plurality of nozzles (24), located in the roof of the oven chamber, and retains a variable speed fan (32) which is controlled in use to cause air to re-circulate through the air circulation conduit and oven chamber as shown by the arrows in Figure 1 . The number and cross-section area of the inlet apertures, and the number and shape of the nozzles are selected to provide a similar level of resistance to air flow and the total resistance of the inlet apertures and nozzles makes up at least 90% of the resistance to air flow of the air circulation path through the air circulation conduit. The re-circulation air path through the air circulation conduit, extends through a controllable electrical heating element (26). A humidity regulator shown generally as (28) comprises a water tank (30) and a controllable electrical heating element (32), which is controllable by the controller to generate steam as required, and thereby controllably humidify air in the air circulation conduit. Between the nozzles and the roof of the oven chamber are located strips of radiant electrical heating elements (34) under the control of the controller. In some embodiments, the radiant electrical heating elements have maximum operating temperatures in the range of 800°C - 2000°C, which can be helpful in some types of cooking, for example to simulate the direct exposure to burning wood found in some pizza ovens. A control circuit enables the controller to vary both the maximum temperature of the radiant heating elements and the duty cycle of the electrical heating elements, and so the controller can regulate both the spectrum of radiant heat incident on the food from the radiant electrical heating elements, and also the average power delivered by the electrical heating elements. The oven chamber includes a heat sensor (300) which extends through the enclosure wall and which is configured to measure both the convective heat transfer coefficient of air adjacent the oven chamber wall and the (hemispherically-averaged) incident radiative temperature above the exposed upper surfaces of the article (e.g. food). The heat sensor is described further below with reference to Figure 4. The air circulation conduit also includes an inlet (38) for receiving ambient air and an outlet (40) for venting humid air to the surrounding atmosphere (36), and an inlet damper (42) and an outlet damper (44) which regulate air flow through the inlet and the outlet. Typically, these are controlled in common. In an embodiment, the dampers (42 and 44) are controlled using a common shaft (46) driven by a rotary actuator (48) under the control of the controller. The relative orientation of the dampers is selected to achieve a preferred pressure distribution within the air circulation conduit. The relative orientation of the dampers is fine tuned for each individual oven during manufacture to thereby adapt for manufacturing tolerances. The air circulation conduit comprises an aperture (202) communicating with a conduit (204) through which a small proportion of re-circulating air (50) may pass out of the oven, to the atmosphere (36), through a humidity sensing arrangement (200), which is described further below with reference to Figure 3. Because the humidity sensing arrangement is located in the air circulation conduit downstream of the fan, there is a slight positive air pressure which drives air out to the atmosphere, and the dimensions of the aperture and conduit are selected to have a suitable resistance to air flow such that only a small proportion of re-circulating air is lost through the humidity sensing arrangement. With reference to Figure 2, the controller (100) comprises a control module (102) comprising a microprocessor (104) and memory (106) which stores a control program (108) and various cooking programs (1 1 OA, 1 10B, 1 10C etc.). The control module has a plurality of inputs for receiving signals from the various sensors, including temperature signals (120) from each of the various temperature sensors (18, 309, 31 1 , 312, 307, 314, 316, 317, 305 and typically others), a humidity signal (122) from the humidity sensor (207) and typically other signals such as a fan power level or air speed signal or fan speed signal (124) from a tachometer in the air circulation conduit. During use, the signals are digitised using an ADC (if they are not already in digital format). They are sampled and stored as measurement data (126) in the memory. The controller may also receive user interface command signals (128) such as a door open signal, inputs (130) from manual user interface controls buttons, knobs etc., and remote user interface instructions (132) received from other devices through a wired or wireless electronic interface (e.g. via internet protocol). These may include maximum temperature or duration settings etc., or the mass of the food. The controller also stores relevant user settings (134), including preferred programs and settings and also including the altitude (136) or a parameter related to the altitude (e.g. mean pressure) of the location where the oven is installed, received through the user interface by querying the user on start-up or by other means (for example by a remote server querying a database of geographical locations associated with IP addresses for an IP address from which the oven contacts the remove server and a database of the altitude of geographical locations). The controller stores calculated parameters in the memory during use, such as calculated values of incident radiative temperature (138) in the oven chamber, the convective heat transfer coefficient (140), the humidity of air in the oven (142), air pressure in the air circulation conduits (144) and so forth. The controller also stores target values of key operating parameters during use, for example target values of incident radiative temperature (146), heat transfer coefficient (148), humidity (150) etc. These are recalculated and updated in use as a cooking program progresses. The controller has a number of outputs including a fan control output (152), fan damper actuator control output (154), humidifier control output (156), radiant heater maximum temperature (or maximum current) control output (158), radiant heater duty cycle control output (160) (e.g. a digital or analogue values communicate to a radiant heater control circuit, or a digital signal which switches a FET or other switch, to switch current to the radiant heater on and off), shelf temperature (162) or shelf electrical heater control output (164), a heater control output (166) for the electrical heater (16) in the shelf (10), a heater control output (168) for the electrical heater in the humidity analyser etc. Although in this example, the function of the controller is carried out by a module having a microprocessor executing a program, one skilled in the art will appreciate that the controller may comprise multiple distributed modules which address individual parts of the function of the controller. For example, the regulation of temperature in the shelf, or in the humidity analyser, may be implemented by a simple electronic feedback circuit in the shelf or humidity analyser respectively. The derivation of radiant temperature and convective heat transfer coefficient by the heat sensor may be carried out by a circuit or microprocessor within the heat sensor etc. With respect to Figure 3, the humidity analyser (200) measures the humidity of air within the air circulation channel (20) (and thereby introduced into the oven chamber). The air circulation channel has an aperture (202) and a channel (204) extending through the humidity analyser. The humidity analyser has a body (205) outside of the oven outer wall (2) comprising thermal insulation (206). Due to the positive pressure of air in the air circulation channel (arising from the action of the fan), air passes into channel (204) and out of an external aperture (21 1 ) to the outside air (36). Within the body there is a block of metal (209) or another thermally conductive material, and a heater element (210) and temperature sensor (208) are used to regulate the temperature of block of metal to about 105°C. At this temperature, the interior of the channel, where it passes through the block of metal, is kept clear of condensation but the maximum temperature of the temperature and humidity sensing elements is not exceeded. A combined humidity and temperature sensor (207) (for example, a capacitive sensor, such as a HIH8000 Series solid state digital humidity/temperature sensor available from Honeywell, Inc., Morris Plains, New Jersey) measures the partial pressure of water within the channel. Thus, with appropriate correction for the change in temperature due to the heating, the humidity of the air in the air circulation channel can be established and used by the controller to regulate cooking. Figure 4 is a plan view, from above, of heat sensor (300), which independently measures both the convective heat transfer coefficient of circulating air and the hemispherically averaged radiative temperature of incident radiation. The heat sensor is located on the oven enclosure with the sensing surfaces facing upwards, generally in the middle of the oven chamber and extends through the thermal insulation (2) of the oven outer wall. It has a main body (302) formed by a thin walled cylinder filled with thermal insulation (306). At one end is a sensing surface (301 ), facing into the interior of the oven chamber, and at the other end, outside the oven (36) is a heat sink (303) with outward extending cooling fins (304). The cooling fins sit in a small cylindrical enclosure, which may be connected by a tube to the suction side of the external cooling fan, a device which already exists in known ovens (to prevent the electronics and circulating fan motor from overheating). The heat sensor has a high emissivity sensor spade (313), functioning as the second surface mounted element and a low emissivity sensor spade (308), functioning as the first surface mounted element. The sensor spades are planar and hollow, and aligned parallel to the roof and base of the oven (thereby facing the radiant elements in the roof and the air nozzles). They are made of the same material with different coatings - gold in the case of the low emissivity sensor spade, and a matt black coating in the case of the high emissivity sensor spade. Each sensor spade includes a temperature sensor (314, 309) respectively and is connected through a thermally conductive (typically metal) rod (315,310) to the heat sink (303). Further temperature sensors are located in the heat sink (305), and at spaced apart locations (31 1 , 312) along the thermally conductive rod which connects the low emissivity sensor spade to the heat sink, and at spaced apart locations (316, 317) along the thermally conductive rod which connects the high emissivity sensor spade to the heat sink. The sensing surface also comprises an air temperature sensor (307) which is small to ensure rapid response to changes in temperature, and is gold coated (being a low emissivity coating to minimise convective heat transfer). Referring to the temperature measured at the temperatures sensors 309, 31 1 , 312, 307, 314, 316, 317 and 305 as Ti , T₂, T₃, T₄, T₅, T₆, T₇, T₈ respectively, it is possible to independently derive both /?, the convective heat transfer coefficient of air at the sensing surface, and Tr, the hemispherically- averaged incident radiative temperature at the sensing surface as follows: L distance between sensors (31 1 ) and (312), and between sensors (316) and (317)

m mass of black sensor spade (313) mg mass of gold sensor spade (308)

m mass of thermally conductive rods (310, 315)

d diameter of thermally conductive rods (310, 315)

Cp specific heat capacity of thermally conductive spades and rods

k thermal conductivity of thermally conductive spades and rods

ε, b emissivity of black sensor spade

ε emissivity of gold sensor spade

g

σ Stefan-Boltzmann constant

Qc convective heat flux to sensor spades

Qr Incident radiative heat flux to sensor spades

Qb total absorbed heat flux to black sensor spade

Qg total absorbed heat flux to gold sensor spade

Tr hemispherically-averaged incident radiative temperature

h convective heat transfer coefficient

A Surface area of gold sensor spade

Assui emissivity of black sensor spade = 1 .0

¾

ε emissivity of gold sensor spade = 0.0

g

Then

Τ₂ + Ά nd² _r

Q_g = Qc = m_gc_p— + m_r— + k— [T₂ - T₃] Q_b = m_gc_p T₆ - T₇]

Q_r = a[( _r + 273)]⁴ - σ[( ₅ + 273)]⁴

Successive sets of temperatures measurements (ΤΊ - T₈) are taken at known time intervals, and then used to calculate the differential terms in the equations above. Hence the real-time variation in both h and T_r can be derived by the controller and used to control the oven. One skilled in the art can adapt the above equations if the emissivity of either sensor spade departs significantly from 1 .0 or 0.0 respectively. In operation, the controller (100) reads data from the various sensors, and controls the various heating elements, and fan (32) to regulate the cooking of food in the oven compartment. In particular it regulates the temperature and humidity of air entering the oven chamber through the nozzles, the velocity of air passing through the nozzles and therefore the velocity of air impinging on the food, and the temperature and mean power output of the radiant heating elements, as well as controlling conductive heating through the fixed shelf (10). Accordingly, the controller can monitor and independently vary the convective, radiative and conductive heat flow to the food, as well as mass transfer of water to and/or from the food. The velocity of air impingement is at least 0.2 ms^~\ and typically several times this. Because of the level of control of heat flux to the food, and the regulation of the humidity of the air within the oven compartment, the vertical position of the food within the oven chamber is relatively unimportant. Hence the food can be located on a simple shelf forming the base of the oven chamber. During cooking the convective, radiative and conductive heat flow to the food, as well as mass transfer of water to and/or from the food are determined with reference to a program which specifies how these parameters vary during use. With reference to Figure 5, the program may specify the variation with time (x-axis) of the convective heat transfer coefficient, /?, (400), radiative heat temperature level (404), conductive heat transfer (406) through the shelf, and humidity (402) which affects mass transfer of water between the air in the oven chamber and the food. This allows sophisticated food cooking programs to be implemented automatically. A number of user input options, such as a maximum temperature or time may also be provided which modulate the program. Thus, the controller can determine a target time course of key parameters such as radiative heat temperature level, convective heat transfer coefficient, humidity, radiant heater power or temperature etc. Heating can be adapted depending on the altitude of the device. Altitude affects the density of air in the oven, and hence affects both radiative and convective heat transfer. Altitude is therefore used along with food mass to modulate the aspects of the program, such as duration. Cooking generally takes longer at altitude than sea level. During operation, the controller repetitively makes measurements from the sensors, processes the stored measurements to calculate operating parameters, compares these operating parameters with target values, and controls the sensor outputs using conventional feedback logic to achieve the target operating parameters as closely as possible. Some embodiments of the invention include an automatic self-test protocol to detect fouling of the high or low emissivity sensor spades. Before a heat treatment cycle is commenced, the fan is switched to maximum speed, the radiant heating strips in the roof of the oven chamber are switched to maximum power for a short period of time (e.g. 10 seconds) and then off again. The subsequent change in temperature of the sensor spades, indicated by temperature sensors (209, 214) is then monitored. The temperature of the high-emissivity sensor spade should increase substantially and then decay. The temperature of the low-emissivity sensor spade should be affected to only a very limited extent by the radiant heat. The measured temperature responses with time are compared with patterns of expected temperature responses with time stored in the memory. A significant departure from either of these patterns would indicate that there has been fouling of a sensor spade, for example, grime or burnt-on food increasing the emissivity of the low-emissivity sensor spade. In this case, a fault is determined and the user is typically alerted to this through a user interface so that they can clean (or replace) the heat sensor. Data from this test can also be used for calibration of the calculations of incident radiative temperature and convective heat transfer coefficient. In some embodiments, the fan is run at a fixed speed (to reduce cost) and a variable damper is provided, adjacent the fan, connecting the fan suction and discharge. Instead of regulating the speed of the fan, the position of the damper (how open it is) is controlled, to thereby regulate the air flow speed. When the damper was fully open there would still be some gentle air flow through the oven chamber to distribute humidity evenly around the system. In some embodiments, humidity is inferred rather than directly measured, for example from the power consumption or speed of the fan. The electrical power of the fan is a function of fan speed (which can be measured), temperature (which is already measured in the system), atmospheric pressure (which has previously been received and stored), and the density of the oven chamber atmosphere (which is a linear function of the ratio of air to water vapour). Thus, fan power (and fan speed, unless that is fixed) can be used to calculate the humidity. This has the advantage that there is not a humidity sensor to get contaminated (e.g. by condensation of volatile oils from food, silicone compounds vaporised from oven door seals etc.)

Claims

1 . An oven comprising an oven chamber for receiving an article to be heat treated, at least one heating element, a humidity regulator, an air flow regulator, a plurality of sensors, and a controller, the plurality of sensors comprising at least one temperature sensor and at least one humidity analyser, the controller configured to receive measurements from the plurality of sensors and to control at least the at least one heating element, the air flow regulator and the humidity regulator, responsive to the measurements, to thereby regulate the heat treatment of an article received within the oven chamber.

2. An oven according to claim 1 , wherein the controller thereby regulates convective and radiative heat fluxes to the article during a heat treatment cycle.

3. An oven according to claim 2, wherein the controller thereby further regulates conductive heat flux to the article during a heat treatment cycle.

4. An oven according to any one preceding claim, wherein the controller regulates the rate of mass transfer of water to and/or from the article during the heat treatment process.

5. An oven according to claims 3 and 4, wherein the controller regulates convective, radiative and conductive heat fluxes and the rate of mass transfer of water to and/or from the article during a heat treatment cycle, in accordance with a predetermined program.

6. An oven according to any one preceding claim, wherein the plurality of sensors comprises at least one sensor of incident radiative temperature in the oven chamber and the controller controls at least one said heating element responsive to the at least one sensor of incident radiative temperature to thereby regulate the incident radiative temperature in the oven chamber.

7. An oven according to any one preceding claim, wherein the plurality of sensors comprises convective heat transfer sensing means configured to measure convective heat transfer within the oven and the controller controls at least one said heating element and the humidity regulator responsive to the measured convective heat transfer to regulate the convective heat transfer coefficient of air within the oven chamber.

8. An oven according to any one preceding claim, wherein, the controller processes at least the humidity measured by the humidity analyser and controls at least the humidity regulator to regulate the humidity of air in the oven chamber, for example to a target humidity.

9. An oven according to claim 8, wherein the convective heat transfer coefficient of air within the oven and/or the rate of convective heat transfer to the article within the oven chamber is controlled independently of the humidity of air within the oven chamber.

10. An oven according to any one preceding claim, wherein the controller controls one or more heating elements to regulate the amount of heat conducted to the article.

1 1 . An oven according to any one preceding claim, wherein the controller comprises at least one processor and memory storing at least one heat treatment (e.g. cooking) program, the at least one stored heat treatment program being indicative of target values of one or more operating parameters during a heating cycle, the operating parameters determining how the controller independently controls convective, radiative (and optionally conductive) heat fluxes, and the rate of mass transfer of water to and/or from the article, and how this varies with time.

12. An oven according to claim 1 1 , wherein the said operating parameters comprise one or more of the said target incident radiative temperature, the said target convective heat transfer coefficient, the said humidity, and the said target conductive heat flux or temperature.

13. An oven according to any one preceding claim, wherein the oven chamber has an oven enclosure which defines the oven chamber at least in part and the oven further comprises an air circulation conduit extending between one or more inlets located in the oven wall and one or more outlets located in the oven enclosure and spaced apart from the one or more inlets, and the air flow regulator regulates the speed of flow of air through the air circulation conduit from the one or more inlets to the one or more outlets.

14. An oven according to claim 13 wherein the outlets are in the roof of the oven chamber.

15. An oven according to claim 13 or claim 14, wherein a portion of the air circulation conduit is in gaseous communication with atmospheric air and the combined resistance to air flow of the one or more inlets and the combined resistance to air flow of the one or more outlets are within 25% of each other.

16. An oven according to any one of claims 13 to 15, wherein the humidity regulator is configured to vary the humidity of air received into the air circulation conduit through the one or more inlets and comprises a humidifier to controllably increase the humidity of the air in the air circulation conduit.

17. An oven according to any one of claims 13 to 16, wherein the air circulation conduit comprises an air input vent and an air output vent, and the air input vent and the air output vent may be regulated to control the flow of atmospheric air into the oven and the flow of humid air out of the oven to the surrounding atmosphere

18. An oven according to claim 17, wherein the air input vent and the air output vent comprise dampers mounted on a common shaft driven by a rotary actuator under the control of the controller.

19. An oven according to any one preceding claim, wherein the oven comprises an air channel extending from within the air circulation conduit and the exterior of the oven and a temperature regulator which in use regulates the temperature of air within a region of the air channel to above 100°C, and a water partial pressure sensor located within the temperature regulated region of the air channel.

20. An oven according to any one preceding claim, which is a domestic oven, and wherein the speed of air flow through the nozzles is regulated by the air flow regulator such that the speed of air impingement on the article is at least 0.2 ms^"1.

21 . An oven according to any one preceding claim, wherein the plurality of sensors comprises a heat sensor for independently determining incident radiative temperature within the oven chamber and the convective heat transfer coefficient of air within the chamber and comprising a plurality of temperature sensors, the outputs from which are processable to independently derive the incident radiative temperature within the oven and the convective heat transfer coefficient of air within the oven, or parameters related to these properties.

22. An oven according to claim 21 , wherein the heat sensor comprises a sensor body comprising insulation, a heat sink, a sensing surface having first and second surface mounted elements thereon, the first surface mounted element having a relatively lower emissivity surface and thermally coupled to the heat sink through a first thermal conduction path extending through the insulation, the second surface mounted element having a relatively higher emissivity surface and thermally coupled to the heat sink through a second thermal conduction path extending through the insulation, a temperature sensor configured to measure the temperature of the heat sink, a temperature sensor configured to measure the temperature of the first surface mounted element, a temperature sensor configured to measure the temperature of the second surface mounted element, at least two temperature sensors configured to measure the temperature at spaced apart locations along the first thermal conduction path and at least two temperature sensors configured to measure the temperature at spaced apart locations along the second thermal conduction path.

23. A heat sensor for independently determining (hemispherically-averaged) radiative temperature and the convective heat transfer coefficient of gas on a sensing surface of the heat sensor, the sensor comprising a sensor body comprising insulation, a heat sink, a sensing surface having first and second surface mounted elements thereon, the first surface mounted element having a relatively lower emissivity exterior and thermally coupled to the heat sink through a first thermal conduction path extending through the insulation, the second surface mounted element having a relatively higher emissivity exterior and thermally coupled to the heat sink through a second thermal conduction path extending through the insulation, a temperature sensor configured to measure the temperature of the heat sink, a temperature sensor configured to measure the temperature of the first surface mounted element, a temperature sensor configured to measure the temperature of the second surface mounted element, at least two temperature sensors configured to measure the temperature at spaced apart locations along the first thermal conduction path and at least two temperature sensors configured to measure the temperature at spaced apart locations along the second thermal conduction path.

24. Apparatus according to claim 22 or claim 23, wherein the sensing surface further comprises a gas temperature sensor.

25. Apparatus according to any one of claims 22 to 24, wherein the first and second surface mounted elements have the same size and shape but different emissivities.

26. Apparatus according to claim 25, wherein the emissivity of the first surface mounted element is less than 0.1 and the emissivity of the second surface mounted element is greater than 0.9.

27. Apparatus according to any one of claims 22 to 26, wherein the first and second surface mounted elements are generally planar, adjacent and parallel.

28. Apparatus according to any one of claims 22 to 27, comprising a controller configured to carry out a sensor checking cycle comprising monitoring the temperature change of both the first and second surface mounted elements in response to a change of radiant temperature within the oven.

29. An oven comprising an oven housing, an oven chamber within the oven housing and a door opening into the oven chamber, an air circulation conduit extending between a plurality of inlets from the oven chamber and a plurality of outlets to the oven chamber, a part of the air circulation conduit intermediate the plurality of inlets and the plurality of outlets being in gaseous communication with atmospheric air, and an air impeller within the air circulation conduit, wherein the combined resistance to air flow of the one or more inlets and the combined resistance to air flow of the one or more outlets are substantially the same.

30. An oven according to claim 29, wherein the combined resistance to air flow of the one or more inlets and the combined resistance to air flow of the one or more outlets amounts to greater than 85%, or greater than 90%, of the resistance to air flow through the air circulation conduit.

31 . An oven comprising an oven housing, an oven chamber within the oven housing and a door opening into the oven chamber, an air channel extending from within the oven and the exterior of the oven, through the oven housing, and a temperature regulator which in use regulates the temperature of air within a region of the air channel to above 100°C, and a water partial pressure sensor located within the temperature regulated region of the air channel. It may be that the oven comprises an air circulation conduit having a plurality of inlets and a plurality of outlets and an air impeller within the air circulation conduit and the air channel extends from the air circulation conduit to the exterior of the oven.

32. An oven according to any one preceding claim adapted for cooking food.

33. A method of operating an oven, the oven comprising an oven chamber for receiving an article to be heat treated, at least one heating element, a humidity regulator, an air flow regulator and a plurality of sensors, the plurality of sensors comprising at least one temperature sensor and at least one humidity analyser, the method comprising making measurements using the plurality of sensors and controlling at least the at least one heating element and the air flow regulator, responsive to the measurements, and thereby regulating the heat treatment of an article received within the oven chamber.

34. A method according to claim 33, wherein convective, radiative and conductive heat fluxes to the article are thereby regulated during a heat treatment cycle.

35. A method according to claim 33 or claim 34, wherein the rate of mass transfer of water to and/or from the article during the heat treatment process is thereby regulated.

36. A method according to any one of claims 33 to 35, wherein the convective, radiative and conductive heat fluxes to the article, and the rate of mass transfer of water to and/or from the article are varied with time during a heat treatment cycle.

37. A method according to any one of claims 33 to 36, wherein the plurality of sensors comprises at least one sensor of the hemispherically-averaged radiative temperature above the exposed upper surfaces of the article and the output of at least one said heating element is controlled responsive to the at least one sensor of radiative temperature to thereby regulate the radiative heat flux incident on the upper food surfaces in the oven chamber.

38. A method according to any one of claims 33 to 37, wherein the plurality of sensors comprises convective heat transfer sensing means configured to measure convective heat transfer within the oven and the controller controls at least one said heating element and the humidity regulator responsive to the measured convective heat transfer to thereby regulate convective heat transfer to the article.

39. A method according to any one of claims 33 to 38, wherein convective heat transfer coefficient of air within the oven and/or the rate of convective heat transfer to the article within the oven chamber is controlled independently of the humidity of air within the oven chamber.

40. A method of cooking food according to any one of claims 33 to 39.

41 . A computer readable storage medium storing computer program code which, when executed on the controller of an oven, causes the oven to carry out the method of any one of claims 33 to 40.

42. A method of determining the humidity of air circulating through an air circulation conduit, impelled by an electric fan, the method comprising monitoring the power consumption of the fan (and typically also the speed of the fan, or of the air in the air circulation conduit), the temperature of the air (and typically also the altitude or current atmospheric pressure) and thereby determining the humidity of the air in the air circulation conduit.