GB2312954A - Monitoring temperatures in microwave chambers eg ovens - Google Patents

Abstract

A method of monitoring the temperature within a region of a microwave oven comprises placing in the microwave oven a thermochromic composition which changes colour over a desired temperature range, and monitoring the colour of the thermochromic composition, visually and/or spectroscopically. The composition is applied to an internal surface of the microwave chamber or to a substrate in the form of a container within it, by printing or as an adhesive patch. Chiral nematic or smectic liquid crystals are used.

Description

TITLE: METHOD AND DEVICE The present application relates to a method for monitoring temperature in a microwave oven. The invention extends to a composition and to a microwave container for use in the method.

BACKGROUND TO THE INVENTION: Microwave ovens heat substances by irradiating them with microwaves which cause vibration of certain molecules which absorb the microwaves, notably water molecules, and this vibration produces heat. Irradiation with microwave radiation may produce a rapid rise in temperature in any material which absorbs the microwave radiation, whilst adjacent materials that do not absorb microwaves may remain relatively cool. For example a meat pie may be heated in a paper bag within a microwave oven. The pie is heated because water in the meat and/or crust absorbs the microwaves while the paper bag is substantially transparent to microwaves and hence is heated only indirectly by conduction, convection or radiation of heat from the pie.

Heating and drying may also be promoted by Radio Frequency electromagnetic waves (RF heating). For convenience the term "microwave" will be used hereinafter to denote either conventional microwaves or radio frequency waves.

Because of their ability to heat quickly and economically, microwave ovens are well known for use in thawing frozen food, and cooking or heating food or drinks in the home kitchen or in restaurants, as well as for the commercial preparation of foodstuffs. Microwave ovens are also used for industrial applications, for example the drying or curing of adhesives and the promotion of chemical reactions.

The measurement of temperature within a microwave oven can be a problem. This is because the air inside the oven does not heat up significantly compared to the food or other material which is being heated. The temperature of the food can therefore not be reliably determined by measuring the air temperature within the oven. Furthermore, conventional thermometers may be difficult to read from within a microwave oven, and may themselves be affected by the microwaves. For example mercury, being metallic, cannot safely be used in a microwave oven without creating sparks and the danger of breakage of the thermometer. The temperature of the food is therefore conventionally measured by removing it from the oven and then sticking a thermometer in it. This is inconvenient, and may cause certain foodstuffs to deteriorate in taste or texture because of the variation in temperature experienced as a result of being removed from the oven. Furthermore, when hot food is removed from the oven its temperature will begin to drop, and the measured temperature may not correspond to the temperature of the food in the oven. When measuring the food temperature with a conventional thermometer, the initially measured temperature reading will change as the food cools down; there is no permanent recording of the initially measured temperature.

It is an object of the present invention to reduce the above problem.

SUMMARY OF THE INVENTION: According to the present invention there is provided a method of monitoring the temperature within a region of a microwave oven, which method comprises the steps of: a) placing in the microwave oven a thermochromic composition which changes colour over a desired temperature range; and b) monitoring the colour of the thermochromic composition.

A change in colour of the thermochromic material indicates that it has passed through a given temperature level. The change in colour may be monitored with or without removing the composition from the oven.

The thermochromic composition may be applied to a substrate for use in the oven. In a preferred embodiment the substrate is a microwave container so that it is possible to detect when the contents of the container have received sufficient microwave energy so that they are sufficiently thawed, heated, or cooked. The invention therefore also provides a container for use in a microwave oven, which container carries a thermochromic composition which changes colour over a desired temperature range.

The preferred temperature range over which the thermochromic material changes colour will depend upon the particular operation to which the microwave oven is being put. The thermochromic material which is used will therefore be selected so as to change colour over a temperature range appropriate for the usage of the oven. For example if frozen food is to be thawed, a container may be used which has applied to it a thermochromic material that changes colour over the temperature range 0 to 100C.

The invention may also be applied in the development and evaluation of microwave ovens, to monitor microwave energy levels in different regions of the oven by monitoring the colour change in those regions. If the relationship between the intensity of incident microwave energy and rate of change of temperature of the composition is known then the total amount of microwave energy falling on the thermochromic composition may be determined in a given area by measuring the time taken for the composition to change colour in that area.

The colour of the thermochromic composition may be monitored by direct visual observation. Alternatively or additionally the colour may be monitored by spectroscopic means which may optionally include a computer program to calculate the microwave energy required to produce a colour change. Where the change in colour is monitored spectroscopically, the colour spectrum may include infrared and ultraviolet radiation provided that a spectrophotometer or other measuring device is being used which is sensitive to infrared or ultraviolet wavelengths.

The thermochromic composition may be a solid, fluid, or semi-solid. If the composition absorbs microwave energy directly it may be used in the oven without being carried on a substrate.

If the thermochromic composition does not significantly absorb microwaves it is necessary to apply it to the surface of a substrate which does absorb microwaves, or which is adapted to contain food or other materials which absorb microwaves. The substrate may be any material which is suitable for use in a microwave oven, for example a microwave oven container.

Thermochromic materials for use in the composition may change colour over a relatively narrow temperature range, or over a broad temperature range. The thermochromic materials may therefore be selected so as to indicate temperature change over virtually any preferred range. The term "composition" may include a single chemical species of thermochromic material, or a plurality of chemical species which may comprise more than one state of matter; for example solid and liquid, or solid and liquid crystal.

The thermochromic material may be inorganic or organic, and the colour change may be effected by absorption of light, for example by Basic Green 4, Solvent Yellow 56, or thermochromic polyacetylenes such as those disclosed in European Patent Application 0 036 899, or by selective reflection of a particular wavelength range of light, for example by a conventional chiral nematic liquid crystal or a liquid crystalline chiral nematic polymer. The thermochromic material may change colour irreversibly above a given temperature range, or the change may be reversible.

Where the thermochromic material is a chiral nematic liquid crystal (also known as a cholesteric liquid crystal), the liquid crystal material may be chosen or prepared so as to give selective reflection of light over virtually any temperature range which is of interest, with continuous change of colour over that temperature range. This allows the visible observation of variations in temperature, and hence in microwave energy, over a large area of the oven with the use of only a single thermochromic composition.

The thermochromic material may be formulated in a carrier for application to the substrate or to an internal surface of the microwave oven. The invention therefore also provides a carrier composition for application to a visible area of a substrate for use in a microwave oven, or to an internal surface of a microwave oven, which carrier composition contains a thermochromic material which changes colour over a desired temperature range.

The carrier composition may be fluid, solid, or semi-solid, for example an ink, paint, lacquer, paste, film or patch.

In a preferred embodiment the carrier composition is an ink suitable for ink jet printing, and the ink is printed on the substrate by means of an ink jet printer. Alternatively, the ink may be formulated for, and applied by, contact printing.

The ink may comprise the thermochromic material in solution in a solvent, or it may comprise a suspension of particles or droplets of the thermochromic material in a liquid carrier medium. Other materials may also optionally be included in the ink, for example surfactants, viscosity modifying agents, or curing agents. Where the thermochromic composition is a chiral nematic liquid crystal it is preferred that the liquid crystal be microencapsulated, coacervated, emulsified, or otherwise contained in a suitable carrier medium so that it forms a film which is dry to the touch. For example the liquid crystal may be emulsified with an aqueous solution of poly(vinyl alcohol) to give an emulsion which dries to a coherent film in which the liquid crystal is dispersed.

Alternatively, the carrier composition may comprise a thermoplastic or adhesively coated film or patch which is applied to the substrate by means of heat and/or pressure.

The film or patch may carry the thermochromic composition on its surface, or the thermochromic composition may be contained beneath the surface. The film or patch may be formed, for example by mixing an inorganic thermochromic material or a microencapsulated thermochromic liquid crystal with a monomeric resin, for example methyl methacrylate, casting a liquid film and polymerising the film by means of heat and/or catalysis. Alternatively the thermochromic material may be mixed with a suitable film-forming solution or suspension, for example a natural or synthetic latex or a solution of polystyrene in toluene. The mixture may then be cast as a film and allowed to dry by water or solvent evaporation.

The use of an adhesively coated film or patch allows the thermochromic material to be applied to a substrate or to a surface of the oven without the release of significant amounts of solvent or other liquid carrier medium.

If it is desired to know the temperature of food inside the container when the colour of the thermochromic composition changes, this may be achieved by calibration during the manufacture of the container. For example the temperature of food inside the container may be measured by means of a conventional thermometer, and this temperature may be compared with the colour of the thermochromic composition on the surface of the container. When the thermochromic composition changes colour, the corresponding temperature of food inside the container is known. The calibration may be carried out either inside or outside an oven, and the oven that is used for the calibration may be either conventional or microwave; however if a microwave oven is used for the calibration it will be necessary to use a conventional thermometer that does not significantly absorb microwave radiation.

In order to permit the measurement of temperature over a wide range of temperatures, a plurality of thermochromic compositions, each of which changes colour in a different temperature range, may be applied to the substrate in separate discrete areas and/or in a pattern.

The composition may be applied directly to the surface of the substrate, or the composition may be applied on top of one or more other coatings. For example a composition which includes a chiral nematic liquid crystal may be coated on top of a black coating on the substrate so that the black coating absorbs the wavelengths of light which are not selectively reflected by the thermochromic material, and hence enhancing the contrast between the reflected colour and the background. One or more transparent or translucent coatings may optionally be applied on top of the thermochromic composition. For example a tough lacquer may be applied to provide a scratch-resistant finish.

Where the substrate is transparent or translucent, for example a transparent microwave container, the thermochromic composition may be applied either to an outside surface or to an inside surface. Application to an inside surface has the benefit that the thermochromic composition is in closer thermal contact with the food or other contents which are to be heated. The thermochromic composition may therefore respond more quickly to changes in temperature of the contents.

The composition may be applied in the form of numbers, letters, a pattern, or other indicia. In a preferred embodiment the formulation is applied in the shape of one or more letters or numbers onto a surface which is closely matched in colour to the colour of the formulation at ambient temperatures. When the colour of the thermochromic composition changes at higher temperatures the different colour of the letters or numbers contrasts with the background colour. For example a thermochromic composition which changes from blue to yellow at 800 C may be applied in the form of the numerals "80" to a blue coating on a microwave container. When the temperature of the surface reaches 800 C this is indicated by the number 80 appearing as yellow numerals on a blue background.

If the temperature of food inside a microwave container is known from calibration, then the thermochromic composition may be applied as numerals which are consistent with the temperature of the food inside the container at the transition temperature at which the thermochromic composition changes from one colour to another.

The method may be used to measure temperatures in different regions of a microwave oven, and thereby indirectly to measure the intensity of microwaves in different regions of the oven, for use, for example in research into microwave ovens.

For these measurements thermochromic compositions may be located in a plurality of regions within the oven.

A plurality of substrates to which the composition or compositions have been applied may be used in different regions of the oven, either on the floor of the oven, or on any of the other surfaces, or suspended or supported in any other region. Alternatively a single substrate of a suitable area could be located over most or all of the floor or other area of the oven in which it is desired to measure temperature or microwave density. For example a flat sheet, to which a thermochromic formulation has been applied, could be laid on the floor of the oven and variations in microwave density could be observed as variations in colour in different regions of the sheet.

Although the invention has been described for convenience in terms of the use of a chiral nematic liquid crystal, it is not limited to this embodiment. Chiral smectic liquid crystals, for example chiral smectic C materials, may also be used, provided that they are thermochromic within the desired temperature range.

Claims (15)

1. A method of monitoring the temperature within a region of a microwave oven, which method comprises the steps of: a) placing in the microwave oven a thermochromic composition which changes colour over a desired temperature range; and b) monitoring the colour of the thermochromic composition.

2. A method as claimed in claim 1, wherein the thermochromic composition is applied to a microwave container.

3. A method as claimed in claim 1 or claim 2, wherein the colour of the thermochromic composition is monitored whilst it is inside the microwave oven.

4. A method as claimed in any one of the preceding claims, wherein thermochromic compositions are located in a plurality of regions within the oven so that variations in temperature may be measured for different regions within the oven.

5. A method as claimed in any one of the preceding claims, wherein the thermochromic composition is located over most or all of the floor of the oven.

6. A method as claimed in any one of the preceding claims, wherein the thermochromic composition is applied by ink jet printing.

7. A method as claimed in any one of Claims 1 to 5, wherein the thermochromic composition is applied by contact printing.

8. A method as claimed in any one of Claims 1 to 5, wherein the thermochromic composition is applied to a label.

9. A device for use in the method of claim 1, which device comprises a substrate for use in a microwave oven, which substrate carries a thermochromic composition that changes colour over a desired temperature range.

10 A device as claimed in claim 9 wherein the substrate is a container for use in a microwave oven.

11. A device as claimed in claim 9 or claim 10, wherein the thermochromic composition comprises a liquid crystal material.

12. A device as claimed in any one of claims 9 to 11, wherein the substrate carries the thermochromic composition in a plurality of discrete areas.

13. A device as claimed in claim 12, wherein the discrete areas are in the form of numerals which correspond to at least one temperature in the range over which the thermochromic composition changes colour.

14. A carrier composition for use in the method of claims 1 to 8, or the substrate of claims 9 to 13, which composition comprises a thermochromic material in a liquid carrier medium for application to a substrate for use in a microwave oven.

15. A carrier composition as claimed in claim 14, which is formulated for ink jet printing.