Oven and Method of Operating an Oven Field of the invention The invention relates to the field of ovens, typically but not exclusively domestic ovens for cooking food, and also to a humidity analyser. Background to the invention At the present time, typical domestic ovens have a number of limitations. They are wasteful of energy, which has both cost and environmental implications; they do not cook some types of food well, with poor surface texture or soggy bases; and they can take a long time to operate, including both pre-heating and cooking steps. Embodiments of the present invention seek to address these limitations, to provide professional quality food, cooked quickly and energy efficiently. Summary of the invention According to a first aspect of the invention there is provided an oven, the oven comprising an enclosure defining an oven chamber for receiving food to be cooked and having a base for supporting received food (whether directly or in a cooking container), an electrical power input, a fan, and a plurality of electrically powered heaters, the electrically powered heaters comprising one or more gas heaters (configured to heat the gasses within the oven), one or more base heaters configured to heat the base and/or a cooking container (where present) located on the base (in use), and one or more radiative heaters (configured to heat by radiation food supported on the base), and a controller,

the controller configured (e.g. programmed) to receive an instruction to start a cooking program and, responsive thereto, to cause the fan to drive gas circulation around a gas flow path, the gas flow path extending past the one or more gas heaters to cause heated gas to impinge upon received food, and to direct electricity from the electrical power input to power the plurality of electrically powered heaters. Typically, the controller is configured to vary the power consumed by the electrically powered heaters during the cooking program to independently regulate the heating of food by convection (from heated gas), conduction (from the base or the container) and radiation (from the one or more radiative heaters).

According to a second aspect of the invention there is provided a method of operating an oven, the oven comprising an enclosure defining an oven chamber for receiving food to be cooked and comprising a base (on which food rests in use, whether directly or in a cooking container), an electrical power input, a fan, and a plurality of electrically powered heaters, the electrically powered heaters comprising one or more gas heaters (which heats gas within the oven), one or more base heaters (which heat the base and/or a cooking container located on the base (where present) and thereby heat the food by conduction) and one or more radiative heaters (which direct radiation at and thereby heat food supported on the base), the method comprising receiving an instruction to start a cooking program and, responsive thereto:

causing the fan to drive gas circulation around a gas flow path, the gas flow path extending past the one or more gas heaters thereby causing heated gas to impinge upon received food, and directing electricity from the electrical power input to power one or more of the plurality of electrically powered heaters, and typically comprising varying the power consumed by the electrically powered heaters during the cooking program to independently regulate the heating of food by convection (from heated gas), conduction (from the base or the container) and radiation (from the one or more radiative heaters).

The gas heaters and fan cause heating of the food by convection (from above). The base heater causes heating of the food by conduction (from below). The radiative heaters cause heating of the food by the absorption of radiation (typically from above). These heating mechanisms can be independently controlled by controlling the electrical power consumption of the individual heaters. Typically, the program has a first program phase and a subsequent second program phase. Typically the proportion of heating of the food by radiative heating from the one or more radiative heaters is greater in the second program phase than in the first program phase. Indeed, it may be that the one or more radiative heaters are not powered in the first program phase, or consume less than 10% or less than 5% of the total electrical power consumption of the one or more heaters. It may be that the one or more gas heaters and the one or more radiative heaters are controlled such that, in the first program phase, heat is transferred to the upper surface of the food (facing towards the roof of the oven) more by convection (from the impinging gas heated by the one or more gas heaters) than by radiation. It may be that the one or more gas heaters and the one or more radiative heaters are controlled such that in the second program phase heat is transferred to the upper surface of the food more by radiation (from the one or more radiative heaters) than by convection. It may be that the one or more gas heaters and the one or more radiative heaters are controlled such that in the first program phase, the electrical power consumed by the one or more radiative heaters is less than 20% or less than 10% of the electrical power consumed by the one or more gas heaters. It may be that in the second program phase, the electrical power consumed by the radiative heaters is at least 20% or at least 40% or at least as much as the electrical power consumed by the one or more gas heaters. Thus, the proportion of heat transfer to the food by radiative heating (by the one or more radiative heating elements) relative to convective heating (by the impingement of gas heated by the one or more gas heaters) increases in the second program phase and/or as the cooking program progresses. The program thereby uses electrical power to cause heat transfer to the food predominantly by convection and conduction to begin with, to maximise the initial rate of heat transfer to the food product and then diverts electrical power from the one or more gas heaters (and optionally the one or more base heaters) to transfer heat to the food by radiation, potentially at a greater rate than by convection (and potentially even by conduction), when the food has partially heated up and convection (and conduction) become less effective means of heat transfer. Furthermore, the quality of many cooked food products is improved by radiative heating towards the end of a cooking program.

At the same time, heat can be conducted to the base of the food from the base, or from the cooking container (where present), heated by the one or more base heaters. Typically, over the duration of the cooking program, at least 25% or at least 40% or at least 50% of the heat transferred to the food is transferred by conduction from the base or cooking container.

In some embodiments, the one or more base heaters may (directly) heat the cooking container, for example the one or more base heaters may be one or more inductive heaters which inductively heats the cooking container. In this case, the cooking container should be an inductively heatable cooking container (e.g. a container comprising ferromagnetic or ferrimagnetic materials, for example stainless steel).

In some embodiments, the one or more base heaters may (directly) heat the base (which is typically thermally-conductive). The base heater may be an inductive heater which inductively heats the base. In this case, the base should be an inductively heatable cooking container (e.g. a container comprising ferromagnetic or ferrimagnetic materials, for example stainless steel). The food may nevertheless be placed on the base in a cooking container, but the cooking container would simply conduct heat. The one or more base heaters may for example be one or more resistive heaters. However, the use of an inductive heater can be especially energy efficient (and can enable direct heating of the cooking container).

As a result, electrical energy powers the one or more base heaters, leading to heating of the base and/or the cooking container (as appropriate). Heat energy is thereby conducted to food. Accordingly, the one or more base heaters regulates the transfer of heat energy to food by conduction.

The base may be spaced away from the enclosure of the oven to minimise heating of the enclosure (which leads to a loss of waste heat to the environment). There may be a thermally insulating layer between the base and the internal oven enclosure. This contrasts with typical ovens which have a heater underneath the bottom surface of the oven chamber, leading to a substantial loss of heat to the environment. The gas which circulates is what is referred to in the art as oven atmospheric gases, i.e. a mixture of ambient air, steam and other oven gases.

The oven (e.g. the controller) may be configured to direct an amount of electrical power initially to the one or more gas heaters when the gas temperature in the oven is below a setpoint and to share at least some of the amount of electrical power to the one or more radiative heaters, instead of the one or more gas heaters, when the gas temperature in the oven is above a setpoint (and/or after a predetermined period of time). Again, this has the effect of directing available electrical power preferentially to gas heating in a first phase of a cooking program and preferentially to radiative heating in the second phase of a cooking program, when heating through convection has become slower.

It may be that the amount of electrical power consumed by the one or more base heaters is reduced in favour of electrical power consumption by the one or more radiative heaters in the second phase, although typically the ratio of mean electrical power consumption of the one or more base heaters in the second phase to the first phase is greater than the ratio of mean electrical power consumption of the one or more gas heaters in the second phase to the first phase. Heating through convection can be maintained throughout the cooking program.

The method may comprise interleaving the consumption of electrical power by the one or more radiative heaters between the consumption of electrical power by some or all of the other heaters (at least in the second phase). The oven may interleave directing electrical power to the one or more radiative heaters between directing electrical power to some or all of the one or more gas heaters (at least in the second phase). It may be that the consumption of electrical power by one or more, or all, of the radiative heaters is interleaved between the consumption of power by one or more, or all, of the gas heaters and that the proportion of time for which electrical power is consumed by the said one or more, or all of the radiative heaters, and not by the one or more, or all, of the gas heaters is higher in the second phase than in the first phase and/or increases during the cooking program. It may be that in the first program phase, the one or more radiative heaters are controlled to not heat the food but the one or more gas heaters are controlled to heat the circulating gas and during the second program phase there are gaps in the consumption of electrical power by the one or more gas heaters during which the one or more radiative heaters consume electrical power and heat the food by radiation. Similarly, there may be gaps in the consumption of electrical power by the one or more radiative heaters during which the one or more gas heaters consume electrical power.

Preferably, in the first program phase, and typically also in the second program phase, the combined electrical power consumption of the plurality of electrically powered heaters is controlled to be at at least 90% (or at least 95%) of, or ideally at a predetermined electrical power limit. The electrical power limit may be determined by a power limit of mains electricity in the territory where the oven is located, for example it may be 16A, 240V = 3.84kW in the UK and/or Europe. The electrical power limit may be selected to avoid tripping a fuse in an electric plug of the oven. Thus, the oven heats food almost, or as quickly, as is possible within the constraints of the electrical power limit. The cooking program may begin with the oven chamber at ambient temperature. The method may be carried out without a step of preheating the oven chamber to a steady state temperature (or at all). Typically, the oven chamber is not preheated when the instruction is received and/or when the cooking program begins.

Typically the one or more gas heaters and the power supply of the oven are configured to heat the one or more gas heaters to steady state temperature within 5 seconds. Typically the one or more radiative heaters and the power supply of the oven are configured to heat the one or more radiative heaters to steady state energy delivery rate within 2 seconds. Typically, the one or more base heaters and the power supply of the oven are configured to heat the base or the cooking container (as appropriate) to a steady state temperature within 20 seconds. Counterintuitively the inclusion of relatively high-powered control circuits and heaters enables power saving by avoiding the need for pre-heating and reducing the duration of the cooking program.

Avoiding a need for pre-heating reduces energy wastage (especially as users often pre-heat an over for longer than is necessary in practice, for example if they are distracted) as well as having benefits of convenience for users who only require one activity, rather than two, to heat food.

Typically, the cooking program is completed before the fabric of the enclosure, for example a metal internal enclosure and/or insulation within the oven shell, reaches a steady state temperature. For example, the cooking program may be completed before the change (increase) in thermal energy of the enclosure, as a result of the heating from the heaters during the cooking program, reaches 90%, or 80%, or 70% of a steady state amount, for example the increase in thermal energy which would occur if the one or more air heaters and base heaters were controlled to maintain target air and base setpoint temperatures indefinitely, with the one or more radiative heaters off. It may be that, at the time that the cooking program is completed, the temperature of the exterior wall of the oven is increasing at a rate of >1 °C (or >2°C) per minute. Thus, the energy consumption of oven during the cooking program is significantly reduced in comparison to known ovens with cooking protocols based on pre-heating the oven and allowing the enclosure to reach a steady state, either before or during the cooking process. Energy consumption is reduced in part due to avoiding reaching steady state temperature and in part as the enclosure will be at a relatively low temperature for a first part of the cooking program.

There may be a further (and typically final) cooking phase during which the heaters are not powered but the food continues to heat. This may for example last for less than 20% or less than 10% of the duration of the cooking program. When the cooking program is completed, it may be that the electrically powered heaters are all switched off. It may be that when the cooking program is completed, a visual or auditory indicator of completion of the cooking program is generated, for example a loudspeaker may generate a sound, or a light may be switched on or off, or change colour.

The base heater is located at or underneath the bottom of the oven chamber. Typically, the base is located at or adjacent the bottom of the oven chamber. For example, the base may be located in the lower 20% of the vertical extent of, and typically lower 10% of the vertical extent of, or on the bottom of the oven chamber. Typically, the base is located such that the distance between the base and the nozzles is at least 80%, or at least 90% of the vertical extent of the oven chamber.

By locating the base adjacent or on the base of the oven chamber, and the nozzles in the roof, air can be controlled to impinge on the food at a high velocity. In contrast to food sitting on a rack or tray located centrally in an oven chamber, the food can, at least in the first phase, be heated predominantly by convection from air impinging from above and conduction through the base or cooking container (where the base heater heats the cooking container). (It will be appreciated that where the base heater heats the base and the food is placed in a cooking container heat from the base will be conducted to the food through the cooking container). The flow of air can also be made more even and consistent across the breadth and depth of the oven chamber in comparison with a conventional fan oven design in which low velocity air emerges from the back wall of the oven chamber, preferentially influencing the cooking of food placed at the back of the oven. Furthermore, the radiative elements on the roof can be configured to heat the retained food more evenly than if the food were located centrally in the oven chamber, closer to the radiative elements.

The one or more gas heaters typically heat food within the oven predominantly by convection. Heat from the one or more gas heaters is typically transferred to food within the oven chamber via the gas flow. The gas heaters heat gas on the gas flow path and the gas impinges on the food. The gas heaters may, for example, be resistive heaters.

The one or more radiative heaters typically heat food within the oven chamber predominantly by radiation. Typically, the radiative heaters have a radiating element (e.g. filament) having a temperature of at least 1500°C, or preferably at least 1800°C, or around 2000°C. These temperatures are close to flame temperatures of wood burning in air. At these temperatures, most of the radiation which is incident on the food is absorbed by water within the food. This contrasts with typical radiative heaters in use for food preparation which have a temperature of around 800°C. At that lower temperature, the radiation which is emitted is not absorbed by water in food. The one or more radiative heaters are typically incandescent lamps, e.g. tungsten lamps.

It may be that the one or more electrically powered heaters further comprises one or more microwave or RF heaters configured to heat food in the oven chamber.

Typically, in a first phase of the cooking program, the one or more radiative heaters consume less than 10%, or less than 5%, or none of the received electrical power. Typically, in a first phase of the cooking program, the one or more gas heaters and the one or more base heaters together consume more than 90%, of more than 95%, or all of the received electrical power which is directed to electrically powered heaters. Typically, in a first phase of the cooking program, the majority of the received electrical power which is directed to electrically powered heaters is directed to the one or more gas heaters. Typically, in a subsequent second phase of the cooking program, the proportion of received electrical power which is directed to the one or more gas heaters is reduced and the proportion of received electrical power which is directed to the one or more radiative heaters is increased.

Typically, the combined electrical power consumption of the one or more electrical heaters is maintained at at least 90% of the predetermined electrical power limit for the majority, or at least 75% of, or at least 90% of the duration of the cooking program. Nevertheless, heating may be stopped before the end of the cooking program to allow the food to received further heat from the residual energy content of the gas and/or base (and/or cooking container, as appropriate).

Thus, in the first phase of the cooking program, heat is supplied to food within the oven chamber predominantly by convection, and optionally also to some extent by conduction, while energy transfer by radiation is relatively low. In a subsequent phase of the cooking program a significant proportion (e.g. at least 10% or at least 20%) of the received electrical power which is directed to heaters is used by the one or more radiative heaters. This strategy can be energy efficient because initially heat can be rapidly transferred to the food by convection and conduction. Heating by thermal radiation is most useful as a top-up when circulating gas temperature and the temperature of the base and/or cooking container are at or close to the surface temperature of the food.

Nevertheless, typically the one or more gas heaters (and the power supply of the oven) are configured to be capable of raising the temperature of gas impinging on the top surface of the food to 200°C within 20s or preferably within 10s. It can be especially useful for one or more base heaters to be inductive heaters. It may be that the one or more base heaters (and the power supply of the oven) are configured to be capable of raising the base, or the cooking container as appropriate, to 200°C within 30s, within 20s or within 10s. The person skilled in the art can design gas heaters, base heaters and a power supply with these capabilities. Again, it is counterintuitive that the provision of relatively high-powered heaters can lead to overall savings in power consumption (while cooking food well).

Typically, the travel time of gas from the one or more gas heaters to the food surface is less than 0.2s. The one or more has heaters, fan and air flow path may be configured so that the travel time of gas from the one or more gas heaters to the food surface is less than 0.2s. Typically, the one or more gas heaters are controlled thermostatically using feedback from one or more gas temperature sensors. Typically, the one or more base heaters is controlled thermostatically using feedback from one or more temperature sensors which measure the temperature of the base or the cooking container (as appropriate).

It may be that in the second phase, after the first phase, of the cooking program, electrical power from the electrical power input is directed to the one or more radiative heaters in preference to, or instead of, the one or more gas heaters.

This may be determined by the implementation of a predetermined program. It may be that when the temperature of circulating gas meets a criteria (typically exceeds a predetermined temperature threshold), electrical power from the electrical power input is directed to or shared with the one or more radiative heaters in preference to, or instead of, the one or more gas heaters. This may also take place after a predetermined time in the cooking program.

It may be that when the temperature of the base, or the cooking container, as appropriate meets a criteria (typically exceeds a predetermined temperature threshold), or after a predetermined time, electrical power from the electrical power input is shared with the one or more radiative heaters in preference to, or instead of, the one or more base heaters. This may have the effect of interleaving electrical power being directed to the one or more radiative heaters between electrical power being directed to some or all of the other heaters (e.g. the one or more radiative heaters, or the one or more radiative heaters and one or more base heaters).

It may be that electrical power directed to at least one, or all, of the one or more gas heaters is reduced to less than 10%, or less than 5% of peak gas heater power consumption, or switched off, when electrical power is directed to the one or more radiative heaters in preference. Thus, when the one or more gas heaters are not required to heat the circulating gas, because the circulating is at a predetermined temperature, electrical energy is diverted instead to the one or more radiative heaters.

However, typically, when electrical power is diverted to the one or more radiative heaters, the combined electrical power consumption of the plurality of electrically powered heaters is controlled to be at least 90% (or at least 95%) of, or ideally at a predetermined electrical power limit. In practice, to avoid accidentally consuming power above the predetermined electrical power limit there may be short term drops in instantaneous combined electrical power consumption between the switching off (for example) of one or more gas heaters and the switching on (for example) of one or more radiative heaters, or vice versa.

Typically, the oven comprises a humidifier for humidifying gas in the gas flow path. Typically, water vapour within the gas flow path is superheated during use (e.g. after a period of time). The oven may also comprise a humidity analyser and a controller which controls the humidifier to regulate the humidity of gas in the gas flow path, e.g. taking into account the humidity of gas determined by the humidity analyser. The humidity analyser may comprise a (dedicated) moisture sensor and/or be configured to estimate humidity from a measurement of the power consumption and speed of rotation of the fan, and gas temperature (for example, but not necessarily, adjacent the fan). As described further below in connection with the third and fourth aspects of the invention, for an oven having a gas flow pathway with defined characteristics at a known atmospheric pressure, the power consumption of the fan is a function of gas temperature, fan rotational speed and humidity and so humidity can be inferred from measurements of fan power consumption, gas temperature and fan rotational speed. The fan is a variable speed fan, for example have two or more speed settings, or a continuous range of speed settings. The oven controller may be configured (e.g. programmed) to carry out a fan calibration protocol (e.g. occasionally) in which the relationship between fan speed and fan power consumption is monitored with gas at a predetermined temperature (which is typically > 200°C and may be at the top end of an operating range of gas temperatures, e.g. 250°C) and with the speed of the fan varied (for example progressively increased and then progressively decreased).

The oven may comprise a temperature sensor configured to measure the temperature at one or more locations on the oven chamber wall (typically one or more locations which are expected to be coolest during operation). The controller may control the humidifier taking into account the measured temperature of the oven chamber wall, to avoid water condensing on the oven chamber wall, typically while maximising humidification during the first phase, or at least controlling the humidifier to increase humidity at a rate which is at least 50%, or at least 75% of the rate which would cause water to condense on the oven chamber wall, during the first phase.

Measuring and taking into account the temperature of the oven chamber wall enables rapid humidification. This enables rapid heating of food by convection without excessive drying out of the food. Advantageously as well as avoiding reducing the quality of the cooked product by drying out the surface, maintaining the moisture content at the surface maintains the rate of heat transfer from the surface of the product into the interior which would otherwise be degraded. Thus, the total power consumption during the heating program can be reduced, more than offsetting the energy expended in the evaporation of water.

The fan may regulate the speed of flow of gas such that gas is incident on the food (typically located adjacent the base of the oven chamber) at a speed of greater than 1 meter per second. This speed is much greater than gas speeds in typical fan assisted ovens and assists with rapid heating. It enables the food to be located at the bottom of the oven, allowing efficient rapid conductive heating.

Although the relatively high-powered fan and energy cost of evaporation increases power consumption in the early stages of the heating cycle, they enable a short cooking cycle and reduce overall power consumption, for example by avoiding the temperature of the enclosure reaching steady state and typically by avoiding an oven pre-heating phase.

The oven may comprise a gas flow conduction circuit extending from a lower region of the oven chamber through the fan to the nozzles. The gas flow conduction circuit may be in gaseous communication with the lower region of the oven chamber through one or more inlet vents. Typically, the cross-section of the inlet vents and the nozzles are selected so that the oven chamber remains at a positive (i.e. greater than ambient) pressure during operation (e.g. by at least 10Pa). This facilitates the build up of a high humidity within the oven chamber even if there is some leakage to the environment (e.g. through a door, safety vent etc). If the oven temperature was approximately instead 10Pa less than ambient pressure relatively dry ambient air would be drawn into the oven chamber, limiting the maximum achievable humidity of the gasses inside the oven.

Typically, the oven chamber has a roof. The roof typically comprises a plurality of gas nozzles, which typically depend from the roof. The one or more gas heaters may be located, wholly or in part, within the gas nozzles, for proximity to the gas flow through the gas nozzles. The gas nozzles are typically directed at the food. At least some of the gas nozzles (e.g. those which are not in the middle of the roof) may be directed (e.g. slightly) away from the walls and roof of the oven chamber. The one or more radiative heaters may be located on the roof of the oven. The radiative heaters may be configured to direct emitted radiation towards the food. The radiative heaters may be configured to direct emitted radiation away from the walls of the oven chamber. The roof of the oven chamber may be shaped to reflect radiation from the radiative heaters to the food. The roof of the oven chamber may be shaped to reflect radiation from the radiative heaters away from the walls of the oven chamber. The interior surface of the gas nozzles may be reflective to reflect radiation from the radiative heaters to the food. The oven and oven operating method are especially useful with food products which have a relatively high surface area to volume ratio. This is because of the limited rate at which heat may be conducted to the centre of food. By avoiding a requirement for pre-heating, the cooking cycle is shortened and it is shortened by a proportionately greater amount when the food product has a high surface area to volume ratio. For example, the food may have a surface area to volume ratio of greater than 0.5 mm ² /mm ³ , or greater than 1 mm ² /mm ³ , or greater than 2 mm ² /mm ³ . Similarly, the oven and oven operating method are especially useful with food products which are relatively shallow, for example pizza, meat slices, quiches etc. It may be that the food has a ratio of depth to maximum length of less than 0.25 or less than 0.1. The food may for example be pizza. The cooking program may comprise additional phases. For example, it may comprise a food finishing phase, after the second phase, where the one or more heaters and, if required, the humidity regulator, are controlled to give a desired upper surface texture (for example using the radiative heaters at maximum power for a period of time), or to regulate the water content (e.g. dry out or dampen) the upper surface and/or base of the food product. The oven may comprise a radiation sensor configured to measure the radiation output of the one or more radiative heaters. The power output of the one or more radiative heaters may be varied responsive to measurements made by the radiation sensor. The oven controller may execute (typically automatically and typically periodically) a radiative heater calibration procedure during which the response of the radiation sensor to radiation from the one or more radiative heaters is measured and responsive to which the subsequent power output of the one or more radiative heaters is regulated. The power output may be varied by varying a duty cycle of the one or more radiative heaters. This is helpful because the spectral emission of radiative heaters, such as tungsten lamps, can vary and decline over time.

The radiation sensor may comprise a black metal block having a surface which is exposed to radiation from the one or more radiation sensors (at least when no food is present). The black metal block may be located at the bottom of the oven chamber. The black metal block may be thermally insulated from the enclosure (to reduce heating by conduction), for example by thermally insulating material.

The methods of the invention are typically implemented by the controller. 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 base or cooking container and regulates the base heater responsive thereto.

Within this specification and the appended claims, upper and lower, top and bottom and related terms (such as roof and base) 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 gas flow path and the fan which are arranged to impel gas downwards through the oven chamber during use.

The invention extends to a computer readable storage medium storing computer program code which, when executed by the controller of an oven, causes the oven to carry operate as the oven of the first aspect of the invention or to carry out the method of the second aspect of the invention. The invention extends in a third aspect to a humidity analyser comprising an electric fan arranged to drive gas circulation around a gas flow path, a power sensor configured to measure the power consumption of the electric fan, a temperature sensor configured to measure the temperature of gas in the gas flow path, the apparatus configured to (e.g. comprising a circuit, which may comprise a processor, configured to) estimate the humidity of gas in the gas flow path from the power consumption and speed of rotation of the fan and the gas temperature measured by the temperature sensor. The invention extends in a fourth aspect to a method of estimating the humidity of gas circulating around a gas flow pathway blown by an electric fan, the method comprising determining (e.g. measuring) the power consumption and speed of rotation of the fan and the temperature of gas in the gas flow pathway and processing the determined power consumption, speed of rotation, and temperature to thereby estimate the humidity of the gas. Typically, the atmospheric pressure at the humidity analyser is taken into account. The atmospheric pressure, or a variable related to atmospheric pressure such as altitude, may be pre-set, for example, during installation, or atmospheric pressure could be measured (e.g. averaged over a period of time). It may be that the relationship between fan power and gas volumetric flow rate is stored, for example in a look-up table (typically in a memory which is part of the circuit), and taken into account in estimating the humidity. It may be that the relationship between the pressure drop across the fan and gas volumetric flow rate is stored, for example in a look-up table (typically in a memory which is part of the circuit) and taken into accounting in estimating the humidity. A three (or even four) dimensional lookup table, storing humidity at different values of fan power consumption, fan speed of rotation and gas temperature (and atmospheric pressure) is stored and taken into account in estimating the humidity. Typically, the gas flow pathway extends through one or more orifices. The pressure rise across the fan can therefore be related to the pressure drop across the one or more orifices around the gas circulation pathway which is a predetermined function of gas density and volumetric flow rate. The apparatus may be an oven (e.g. a domestic oven), for example an oven according to the first aspect of the invention. The method may be a method of measuring the humidity of oven atmospheric gases, for example in an oven operated according to the method of the second aspect of the invention. The fan may be the fan of a said oven and the gas flow path may be the gas flow path of a said oven. Water vapour may be introduced into the gas flow path in dependence on the estimated humidity. The humidity estimate may be calculated by the controller (e.g. processor) of the oven. The humidity analyser may comprise the controller (e.g. processor) of the oven.

It may be that periodically the relationship between speed and power output of the fan is varied at a regulated temperature without humidification (e.g. in dry air) to enable calibration of the fan.

The invention extends to computer program code which when executed by a processor causes the processor to function as part of a humidity analyser according to the third aspect of the invention or according to the method of the fourth 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 diagram of an oven;

Figure 2A is a schematic diagram of air flow in a conventional oven and Figure 2B is a schematic diagram of air flow in an oven according to the invention;

Figure 3A is a cut way view of a gas heater and a Figure 3B is a schematic diagram of a nozzle depending from the roof of the oven;

Figure 4 is a schematic diagram of a control circuit of the oven;

Figure 5A is a graph of energy consumption with time during a cooking program; Figure 5B is graph of gas and food surface temperature with time during a cooking program;

Figure 5C is a graph of heat transfer to food with time during a cooking program; and

Figure 5D is a graph of the temperature of the base (solid line), gas (dashed and dotted line) and the oven enclosure (dotted line) with time during a cooking program;

Figure 6 is a flow chart of steps during the operation of a cooking program;

Figures 7A through 7C show the variation in time of electrical power consumption of the air heaters (Fig 7A), the inductive base heater (Fig 7B), and the radiative heaters (7C) and Figure 7D shows the variation in time of the total electrical power consumption of these heaters during a cooking program;

Figure 8 is a graph of the centre temperature of a pizza with time measured during a test cooking program according to the invention and a corresponding pizza cooked in a conventional domestic fan-assisted oven.

Detailed Description of an Example Embodiment

With reference to Figure 1 , an oven 1 has an enclosure 2 and a cooking chamber 4 defined by walls 6, roof 8 and oven chamber floor 10. The oven chamber floor supports a removable base 12 (functioning as the food supporting and thermally- conductive base) formed of aluminium with a thin coating of a ferromagnetic material, such as stainless steel, supported over a glass hob surface 14 by legs 16, which incorporate a base temperature sensor 18. Underneath the base is the primary driver coil 20 of an induction heater, separated from the hob surface and base by a block of thermal insulator 22 and arranged to heat the base by inductive heating.

Heat is conducted during use from the base to food 25, including any cooking containers/dish, which are placed on the base 12 during cooking. In alternative embodiments, instead of (or potentially as well as) the base being heated, the container in which the food is held is heatable by induction, for example formed of aluminium with a thick coating of stainless steel or another ferromagnetic material. Pans and dishes are commercially available in a range of size and shapes which can be heated by induction heaters. In use, the induction heater thereby heats the food container, resting on the base. This avoids metal to food container heat transfer resistance. In this case, the base may nevertheless have a high thermal conductivity so that the temperature sensor 18 located in the base can accurately measure the temperature of the food container, or a temperature sensor may be provided which measures the temperature of the container inductively.

Although the oven can be used to efficiently cook any kind of food it is has especial benefits described below when cooking food with a relatively high surface area / volume ratio, such as pizza, shallow pies, fillets of fish, hamburgers, trays of vegetables etc.

The roof of the oven has a plurality of nozzles 30 which depend from the roof. The inner surface of the nozzles has a thermally insulating layer 32 (e.g. mica). The nozzles are used to direct oven atmospheric gas which circulates as described below, into the cooking chamber so that it is incident on food during use. Below the roof 8 are located resistive heaters 34, which function as the gas heaters. They are arranged such that gas flowing into the cooking chamber passes close to/through the heaters to maximise heat transfer to the gas. Alongside the nozzles are located incandescent (tungsten) lamps 36, with an operating temperature of 2000°C, which function as the radiative heaters. In use they direct near infra-red radiation (shown with wavy arrows) at the food. The lamps (which have an IR reflective surface coating) are configured to direct radiation from the lamps predominantly at the food and away from the walls of the enclosure. The nozzles are also shaped to direct gas to impinge predominantly on the food rather than the walls.

At the base of the oven chamber the walls have bevelled edges 40 with apertures 42. The bevelled edges define a perimeter channel 44 which is connected to a gas conduit 46 which extends to a gas manifold 48 in the roof by way of a fan 50. Accordingly, the flow of gas from the nozzles, to impinge upon the food, and then outwards through the perimeter channel to the gas conduit and the gas manifold in the roof form a circulatory gas flow path. The fan is an electric fan and its speed of rotation is monitored to measure the speed of gas flow through the gas conduit. The electric fan and the configuration of the nozzles and other components of the circulatory gas flow path is such that the speed of gas impinging on the food can be adjusted over a wide range, typically 0.5 - 5ms ¹ . The transmit time of gas from the gas heaters to the food is less than 0.2s. The bevelled base of the oven chamber walls reduces gas circulation around the edges of the oven chamber which would otherwise lead to greater heat loss from the gas to the oven enclosure. The number and shape of the base apertures 42 and nozzles are selected so that during operation a very small (e.g. in the order of 10Pa) positive pressure (relative to the surrounding atmosphere) is established in the oven chamber.

Water 62 is introduced into the gas flow path (thereby functioning as a humidifier) as required, taking into account feedback from a humidity sensor 64 and an oven wall temperature sensor 66. Water may be injected as a mist and vapourised to steam predominantly by the gas heaters or vapourised directly using a small dedicate electrical heater (“kettle”) located outside of the main oven chamber.

Figure 2A shows gas flow 90 in a typical convention fan assisted oven, between food 25 in the middle of the oven and a typical rear wall mounted fan 50. In Figure 2A and 2B the thickness of the walls of the gas flow arrows 90 increases with gas temperature. It can be seen from Figure 2A that relatively cool gas from around the food is drawn into the fan of a conventional oven, heated and blown radiatively outwards from the fan, circulating gradually back to the food at a relatively low speed. Air flow in the oven according to the invention is shown in schematic form in Figure 2B. For clarity, only two gas heaters 34 and rows of nozzles are shown and air flow from only one perimeter channel, although in practice the perimeter channels are formed around all four walls of the base of the oven. The food 25 is located at the base of the oven chamber and very hot humid air impinges on the food, losing a good deal of heat to the food before recirculating, while relatively cold, through the oven housing. The air is at its hottest from immediately before entering the top of the oven chamber until it impinges on the food. It is guided away from the oven walls. This significantly reduces heating of the oven enclosure in comparison with the conventional oven where hot air circulates in part outside of the oven chamber, leading to greater heat loss through the walls.

Figures 3A and 3B show the details of the resistive gas flow heaters 34 within the nozzle plenum chambers 32. The resistive heating elements are formed as a plurality of parallel resistive heating plates 31 with air gaps 33 therebetween and a plurality of spaced apart grooves 35 arranged to facilitate and guide air flow towards apertures 37 through which gas enters the top of the oven chamber.

Figure 4 is a schematic circuit diagram of the oven. The oven has a power supply circuit 70 which receives and regulates electricity from a cable 72 which is connected to a domestic power supply in use and a controller 74 which regulates the functions of the oven.

The power supply circuit is electrically connected to the inductive heater 20, the gas heaters 34 and the radiative heaters 36. The fan 50 also consumes some power, although much less than the inductive heater, gas heater and radiative heaters. The controller 74 includes a processor 76 and memory 78 which stores program code including specifications of cooking programs. Typically, the controller will have stored programs indicating different variations with time of radiative, conductive and convective heating for different food types. The controller receives data from various sensors, including base temperature sensor 18, humidity sensor 64, oven wall temperature sensor 66, a gas temperature sensor 80, gas flow speed sensor (e.g. tachometer data) 84, calibration block 68 (which is used in an occasional calibration of the power output of the radiative heaters which lose output gradually with age) and various user interface elements 82, such as button, knobs etc.

The combination of sensors, heaters, gas speed and humidity regulation allows independent control of key cooking and heat transfer parameters over a wide range of values, under the control of the controller.

The convection heat transfer rate to the food, Q _c , will be given by:

Qc hcA (Tgas Tfood surface) (W)

he = f([Vgas]° ⁷ [H vol] ^{0 33} )

Where:

h _c is the convective heat transfer coefficient (W/m ² K)

A is the surface area of the food exposed to the gas jets (m ² )

V _gas is the gas impingement velocity at the food surface (m/s)

H _V0| is the humidity of the oven atmosphere (vol% H ₂ 0)

The radiation heat transfer rate to the food, Q _r , will be given by:

Q _r = es(T ⁴ source T ⁴ f ₀ od surface) (W)

Where:

e is the surface emissivity of the food (-)

A is the surface area of the food in view of the radiative lamps (m ² )

o is the Stefan Boltzmann constant (W/m ² K ⁴ )

T _source is the hemispherically-averaged radiation temperature (K) The conduction heat transfer rate to the food, Q _con d, will be given by:

Qcond h _cond A(T ot platc'T food surface) ON)

Where:

h _COnd is the conduction heat transfer coefficient (W/m ² K)

A is surface area of the food in contact with the base (m ² )

The condensation/evaporation transfer rate to/from the food, Q _eva p will be given by: Qevap = Af(H _VO| ) (W)

A is surface area of the food exposed to the atmosphere (m ² )

H _V0| is the humidity of the oven atmosphere (vol% H ₂ 0)

Accordingly, the controller can regulate the convection heat transfer rate by measuring and controlling the gas speed, gas humidity and impingement gas temperature; the controller can regulate the radiation heat transfer rate by measuring and controlling the incandescent lamp power output (e.g. by regulating its duty cycle) and the oven internal wall temperature; the controller can regulate the conduction heat transfer rate by measuring/controlling the hot plate temperature; and the controller can regulate the gain or loss of heat by the food by condensation or evaporation by measuring and controlling humidity. Thus the controller can regulate radiative, convective and conductive heating of the food, and gas impingement speed and humidity independently across wide ranges.

With reference to Figures 5 and 6, a food cooking program starts 100 with the oven and its components at ambient temperature. Although the invention can be applied with some pre-heating stage this is not necessary. The program may start in response to a direct user input (e.g. pushing a button, touching a panel) or automatically, e.g. under the instruction of a timer.

The controller then switches on the fan to start gas circulation and controls the gas heaters and induction heater to heat the circulating gas and base or cooking container (as appropriate) to predetermined setpoint temperatures, in an example 180°C and 250°C respectively. The gas and base temperature sensors are used to thermostatically control the gas heaters and induction heater to maintain the desired set point.

As can be seen from Figure 5A, the power consumption 102 of the gas heaters and power consumption 104 of the induction heater is controlled so that the total power consumption of the heaters 106 is close to, or at, a predetermined power limit 108, typically the rated power of the power supply with a margin for other components requiring electric power (e.g. the controller, sensors, fan, humidifier, any lights, user interface elements etc.). The controller regulates the power consumption of the gas heaters and induction heater by varying the duty cycle of the electricity supply to these heaters. The power consumption is typically smoothed by suitable filtering in the control circuit. As can be seen in Figure 5D, during the initial heating phase 1 10, the temperature of the circulating air 1 12 and of the base 1 14 (or cooking container, where the cooking container is directly heated by the base heater) typically reach set point within 20 seconds.

During this initial heating stage 1 10, the water content of the circulating gas is increased by the humidifier. The humidity is increased as quickly as possible while taking into account the temperature measured by the oven wall temperature sensor to avoid condensation on the oven wall. By increasing the humidity as quickly as possible, Q _c and Q _evap can be kept high. Furthermore, this maintains water content at the food surface which maintains the conduction heat transfer coefficient within the food itself, keeping up the rate of heat transfer to the interior of the food. Still further, this avoids the surface of the food drying out and enables relatively aggressive heating by the radiative heaters described below.

As the food begins to heat, the rate of heat transfer into the food by convection and conduction progressively drops. This is because the temperature difference 1 16 between the gas temperature 1 12 and the top surface of the food 1 18 drops progressively, reducing Q _c . The same applies for Q _cond which decreases as the temperature difference between the base (or cooking container as appropriate) and the bottom surface of the food reduces.

After a period of time, the incandescent lamps are switched on 120 to begin radiative heating of the top surface of the food. As the filaments of the incandescent (tungsten) lamps operate at around 2000°C, radiative heating intensity is essentially unaffected by the increasing temperature of the top surface of the food. The electrical power consumed by the incandescent lamps 124 becomes a substantial component of the total 125 electrical power consumption of the oven.

The incandescent lamps are controlled by regulating their duty cycle. It is important that they are used at full power (giving a temperature of around 2000°C) as the wavelength of light emitted at that temperature is predominantly below 2.2 pm, where the light is strongly absorbed by water in the food. It remains important that the humidity of the gas, and so the top surface of the food, has been maintained as this maintains food quality and enables efficient transfer of received heat into the interior of the food, continuing heating.

The controller regulates the power of the incandescent lamps, gas heaters and inductive heater to continue to keep the combined electrical power 106 close to or at the predetermined limit 108 which may for example be the rated power limit of the power supply with a predetermined margin. Accordingly, as the gas temperature and base temperature reach their setpoints and the rate of transfer of heat into the food by convection and conduction reduces, the electrical power required to keep the base/cooking container and gas at setpoint temperatures decreases and they can be switched off for periods of time during which electrical power is instead consumed by the incandescent lamps. Thus, as can be seen in Figure 5C, the radiation heat transfer rate Q _r into the food exceeds the combined convective Qc and conductive Q _cond heat transfer rates. The use of radiative heat from a high temperature source also enables food to be cooked with a finish resembling that which arises from flame cooking.

Accordingly, there has been a first phase of cooking 200 where heat transfer to the food has predominantly been by conduction and convection and a later second phase of cooking 210 where heat transfer to the food has been supplemented by radiation. Although in the example of Figure 5, the power consumption by the radiative heaters in the second phase exceeds that of the gas heaters, that is not necessarily required for the rate of radiative heating to exceed the rate of conductive and convective heating due to the reduce effectiveness of conductive and convective heating. In this example there is a third phase of the cooking program 220 in which the heating is stopped 130. In the example of Figures 5A to 5D, power to the gas heaters and inductive heater is switched off shortly before the radiative heaters are switched off although this is optional. Shortly thereafter the oven makes a sound 140 or otherwise indicates to a user that the program is complete.

Figure 5D shows a key benefit of the invention. Lines 1 12, 1 14 and 122 illustrate the temperature of the gas, the base and the oven enclosure, during the cooking process and shortly thereafter. It can be seen that the cooking process can be completed before the oven enclosure reaches a steady state temperature. Accordingly, the amount of heat lost to the environment during and after the cooking process is significantly reduced, thereby reducing overall power consumption, with both environmental and economic benefits.

Figures 7A through 7D illustrate in more depth the relative power consumption of the gas heaters 102, base heater 104 and radiative heaters 124 and the sum 125 of the power consumption of these heaters through an example heating program. It can be seen that electrical power consumption by the radiative heaters starts after a period of time, once convective heating becomes less effective and is interleaved between electrical power consumption by the gas heaters, thereby keeping total electrical power consumption close to or at a predetermined power limit. In some embodiments, electrical power consumption by the one or more radiative heaters is interleaved between electrical power consumption by the inductive base heater.

Figure 8 illustrates the measured core temperature of a 335g mass-produced cheese pizza purchased from Dr. Oetker Limited (Leyland, UK) when cooked according to the invention 300 and using a conventional fan-assisted oven 310 with preheating. The pizza was cooked with an oven as set out herein in 7.5 minutes using 0.41 kWhr and lost 25g of mass. No pre-heating step was required. Using the conventional oven to achieve the same mass loss required 0.488kWhr and took 26.5 minutes including a 10 minute oven pre-heating step. Furthermore, the pizza cooked with an oven as set out herein was cooked to a high standard, with a top finish resembling that obtained with a specialist pizza oven, whereas the pizza cooked in a conventional oven had a wet base and un-browned toppings.

Accordingly, although the oven has employed a control strategy requiring relatively high-power cooking, it has reduced overall power consumption. The use of a high gas impingement velocity on the food and humidity regulation has enabled rapid and efficient heat transfer. It has also been practical to use relatively high temperature incandescent lamps, which provide a cooking finish resembling that of flame cooking, giving a higher quality finished product. The cooking process has taken one third of the time required with a conventional fan assisted oven and has required only one user activity (putting the food in the oven and using the user interface to start the cooking program).

Referring back to Figure 1 , the oven may further comprise a black metal block 68, which is flush with the base of the oven and insulated from the enclosure, and which is in contact with a temperature sensor. Occasionally, when no food is in the oven, a calibration cycle is carried out. Some or all of the incandescent lamps are switched on, but not the gas heaters or fan, and the rate at which the black metal block absorbs radiation and heats up is measured and used to calibrate the radiative power delivered by the incandescent lamps. This is important because tungsten filament lamps are known to age over many hours or operation and hence without recalibration the oven’s cooking characteristic would change for a given recipe.

Although in this example, the function of the controller is carried out by a processor 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 base/cooking container, or the gas temperature, or the regulation of humidity may be implemented by a simple electronic feedback circuit external to the controller.

The switching of electrical power from one or some types of heater (e.g. the gas heaters and the inductive base heater) to another type of heater (e.g. the radiative heaters) and vice versa may be achieved by control of switches (FETs, relays etc.) under software control of the processor or wholly or in part by electronic circuits, which may be integrated with a rectifier for example in order to switch current to the different heaters.

The invention also extends to a method of inferring rather than directly measuring, the humidity of the gases circulating within an oven and to a humidity analyser (which is an integral part of the oven). To this effect, the oven controller 74 measures the power consumption and speed of the fan, and the temperature of circulating gas using a temperature sensor 80 (which may be located within the gas conduit 46, instead of humidity sensor 60 shown in Figure 1).

The electrical power of the fan is a function of fan speed (which can readily be measured), the measured gas temperature, atmospheric pressure (which will have previously been received and stored, or obtained by calibration), 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. The resulting humidity measurement is used instead of the output of the humidity sensor 60 described above with reference to Figure 1. In more depth, the following two power characteristic curves of the fan are determined and stored in firmware in a look-up table:

P Q

versus (Curve 1)

pN ³ D ⁵ ND ³

AP Q

versus (Curve 2)

pN ² D ² ND ³

Where:

P = fan power (W)

p = gas density (kg/m ³ )

N = rotational speed of fan (s ¹ )

D = fan impeller diameter (m)

Q = gas volumetric flow rate (m ³ /s)

DR = pressure rise across fan (Pa)

Where:

M = molecular mass of gas (kg/kmol)

p = gas pressure, i.e. local atmospheric pressure (Pa)

R = universal gas constant (J/kmol ^* K)

T = gas temperature (K)

M = 29(1 - H) + 18 H

H = volume fraction of water vapour in gases, i.e. the humidity

The fan pressure rise is equal and opposite to the pressure drop across the various orifices (e.g. nozzles 30, apertures 42) in the gas flow path, which are fixed. For a given orifice:

Q = c/pAP

Where c is a known constant.

Accordingly, the oven controller can process sensor measured values of T (gas temperature), N (rotational speed of the fan) and P (fan power) and a stored value of local atmospheric pressure (or altitude) which may for example be set during initial configuration of the oven, a continuous measurement of H can be indirectly measured.

This has the advantage that there is not a humidity sensor per se to get contaminated (e.g. by condensation of volatile oils from food, silicone compounds vaporised from oven door seals etc.). Furthermore, humidity can be measured reliably over the long term (e.g. 5 or 10 years) without replacing components. This is because fan speed can be measured accurately and reliably (e.g. by detecting a mark attached to the rotatable shaft of the fan passing a sensor), and the person skilled in the art is familiar with temperature sensors which work reliably for years. In order to adapt to variations in the performance of the fan over time (e.g. due to bearing wear) the oven controller may occasionally implement a fan calibration protocol in which gas in the oven is heated to a temperature setpoint (typically towards or at the upper of an operating temperature range, e.g. 250°C) without the addition of water vapour. The fan is switched on and the fan speed varied, e.g. progressively increased and then progressively decreased. This enables calibration as to the fan response in dry air.