This invention relates to thermometers. In particular, it relates to thermometers that can provide a clear and immediate indication to a user when a temperature of a space, such as within a cold store or a refrigerator, is no longer within a safe range.

Thermochromic liquid crystals exhibit a change of colour with a variation in temperature. This is a well-known property, and has been used in a wide variety of devices to provide an indication of temperature. Some such crystals pass through a spectrum of colours as their temperature varies across a range. Thus, a thermometer can be constructed using several patches of thermochromic material, each of which is visible only through a small temperature range. Thus, the instant temperature is indicated by which patch is visible.

Alternative formulations of crystals exhibit a single change of colour, for example changing from colourless to a single colour when viewed against a dark background, at a threshold temperature. By suitable formulation of the crystal, that single colour can be selected, for instance green, orange, or red [or a range of other colours]. Such crystals can be implemented as a cholesteric liquid crystal. In a cholesteric liquid crystal mixture, the single colour change results from the ability to control the wavelength of maximum reflection (so, controlling the colour) and the rate of change of this wavelength with temperature throughout the cholesteric phase.

One example application of a single colour-change cholesteric liquid crystal is to provide a simple binary indication of the temperature of an environment. For instance, some jurisdictions require that all domestic refrigerators provide a visual indication that the temperature within the refrigerator has not exceeded a safe threshold. Such devices for use in refrigerators intended for sale in France must have a region that displays the symbols "OK" on a green background when the interior of the refrigerator is less than 5°C, and a solid black region when the temperature is in excess of 5°C. However, this does not indicate the specific instant temperature. An aim of this invention is to provide a thermometer that can provide both a clear, binary indication of a threshold temperature being exceeded, yet can also provide an indication of the specific temperature in a space.

To this end, from a first aspect, the invention provides a thermometer that comprises a plurality of spaced-apart thermochromic patches, each of which comprising a liquid crystal that, within an operating temperature range of the thermometer, exhibits a single colour change at a respective threshold temperature within the operating range, and having exhibited that colour change, each of which liquid crystal is a colour distinct from at least one other patch that has exhibited a colour change.

While the ambient temperature is below the threshold temperature of the lowest patch, no patch is visible. Then, as the ambient temperature increases, those patches with successively higher threshold temperatures become visible, and remain so as the temperature continues to increase.

The single colour change means that, during a continuous increase in temperature, patches successively undergo a colour change that is not reversed until the temperature subsequently drops.

At least one patch may include indicia that become visible when the threshold temperature of the patch is exceeded. Alternatively, some or all of the indicia may be visible both above and below the threshold temperature, the background colour against which the indicia are visible changing at or around the threshold temperature. The indicia may be a numeric sequence. More specifically, such indicia may indicate the value of the threshold temperature. Alternatively or additionally, some or all of the indicia may include symbols, letters or shapes. There may be more than one thermochromic patch that exhibit the same colour when a plurality of distinct threshold temperatures are exceeded. In addition, the thermometer may include a permanently coloured area that serves to give the impression of a patch that is in a colour-change state, that will remain visible no matter how cold the thermometer becomes. In embodiments that do not have such a permanently coloured area, the lowest-temperature patch may be arranged to become visible below a minimum temperature, whereby the presence of any visible area indicates that a minimum temperature threshold has been passed. In the case of a thermometer placed in a refrigerator, this may indicate that there is a risk that the contents of the refrigerator are liable to freeze.

Advantageously, the colour selected for the patches may provide an indication to a user of the status of the ambient temperature of the space in which the thermometer is located. For example, in the case of a thermometer intended to indicate safe operation of a refrigerator, the status is considered to be hazardous in the event that the temperature exceeds 5°C, and detrimental to the condition of the food if the temperature falls below 1°C. In such a thermometer, indicia that appear at 1°C (and any permanently coloured area) might be coloured blue, 2°C to 5°C might be coloured green, 6°C coloured orange, and 7°C and above coloured red, the ranges being approximate. A room thermometer will, of course, have the colour change occur at a significantly higher temperature, determined by the preferences of a user, the purpose of the space to be monitored, and the intent to behind temperature monitoring (e.g., optimisation of economy, comfort, etc.) Other temperature ranges may be applied for other purposes. For example, to monitor the temperature of an infant's sleeping environment or an infant's bathwater.

The invention may provide a standalone thermometer that can be placed where required. The thermometer may include a base that can be placed on a surface or attached to a component such as a shelf of a storage cupboard, a refrigerator or a freezer. Embodiments of the invention may be resistant to immersion in water. Such embodiments may have their indica configured to indicate a safe temperature range, for instance, for immersion of a person in bathwater. Further embodiments of the invention may be intended for use as a room thermometer, in which the indicia may, for example, be configured to indicate an optimum room temperature for comfort or economy.

Alternatively, it may be incorporated into an appliance. Thus, from a second aspect, this invention provides an appliance, such as a refrigerator, that incorporates a thermometer embodying the first aspect of the invention.

An embodiment of the invention will now be described in detail, by way of example, and with reference to the accompanying drawings, in which:

Figure 1; shows an embodiment of the invention in place on the shelf of a refrigerator and Figure 2 shows a sequence of displays that can appear on a thermometer embodying the invention at successively higher temperatures.

With reference to Figure 1, an embodiment of the invention provides a thermometer to indicate the temperature of a refrigerator, such as a domestic refrigerator.

The thermometer comprises a base 10 that is formed from an elongate extrusion of a resilient plastic material. The base 10 has a flat upper surface 12, a flat display surface 14 adjoining the upper surface 12, and a clip region 16.

When in use, the upper surface 12 overlies an edge portion of a shelf 20 of a refrigerator, and any fitting 22 applied to the edge of the shelf 20. The display surface 14 extends downwardly across a front edge of the shelf 20, preferably angled upwardly from vertical. The clip region 16 extends from the display surface 16, below an edge region of the shelf 20, an edge of the clip region making contact with a lower surface of the shelf 20. The base is preferably shaped and dimensioned whereby, in this arrangement, the clip region 16 is resiliently deflected by the thickness of the shelf 20 from a neutral condition to effect a grip upon the shelf 20 and thereby resist removal of the thermometer from the shelf 20.

An elongate rectangular temperature display strip 30 is carried on the display surface 14, such that it is clearly visible to a user looking into a refrigerator when the thermometer is in place on a shelf. The temperature display strip 30 is covered by a protective cover 32 of transparent plastic material.

The temperature display strip 30 has a background that is, in this embodiment, dark in colour. On the background, towards the left, when the thermometer is arranged in normal use, is a coloured patch 34, which is permanently coloured, and not temperature-sensitive. Spaced uniformly along the temperature display strip 30, to the right of the coloured patch 34, when the thermometer is arranged in normal use, is a sequence of single-colour liquid-crystal thermochromic patches 36i, 362 ... 36„. Each thermochromic patch 36i, 362 ... 36 _n changes in appearance from transparent to coloured at a threshold temperature ti, t ₂ ... , where +i>ti for i = 1 ... n-1. When transparent, a thermochromic patch 36i, 362 ... 36„ is only slightly visible, the background being clearly visible through it. Above its threshold temperature, each thermochromic patch 36i, 36 ₂ ... 36„ becomes visible against the background, the colour of each thermochromic patch 36i, 362 ... 36 _n being chosen to contrast with the background.

A respective indicium 38i, 382 ... 38„ is positioned on the temperature display strip 30 such that each is associated with (in this case, to overlie] a respective one of the thermochromic patches 36i, 362 ... 36„. In this embodiment, the indicia 38i, 382 ... 38„ read as numbers displaying the number z ^' +l for = 1 ... n. The coloured patch 34 shows the number 1. In this example, the indicia 38i, 382 ... 38„ are chosen to be in visual contrast with both the background and the thermochromic patches 36i, 362 ... 36 _n , such that each is visible irrespective of the state of the associated thermochromic patch 36i, 362 ... 36„. Alternatively, the indicia 38i, 38 ₂ ... 38„ could be coloured to closely match the background, whereby they become visible only when the associated thermochromic patch 36i, 362 ... 36„ becomes coloured above its threshold temperature.

When the temperature T of the environment in which the thermometer is placed is less than ti, all of the thermochromic patches 36i, 362 ... 36„ are transparent, so only the coloured patch 34 is clearly visible. This is represented by the top row of Figure 2, which in this embodiment is coloured blue, to give a visual indication that the ambient temperature is below an ideal range.

If the temperature T exceeds ti, but is below £¾ then the first thermochromic patch 36i (which is associated with an idicium 38i that shows the figure "2"), becomes visible, as shown in the second row of Figure 2. In this embodiment, the first four thermochnomic patches 36i ... 36 (showing indicia "2" to "5") are coloured green, to indicate that the ambient temperature is within a preferred range.

While the ambient temperature T is in the range ts< T < tg, the fifth, sixth or seventh thermochromic patches 36s, 36β, 367become visible, showing indicia "6" to "8", as shown in the third row of Figure 2. In this embodiment, these patches are coloured yellow, amber or orange, to indicate that the ambient temperature is heading above the optimum range, indicating that action should be taken.

As the ambient temperature T exceeds t _¾ t tw, and tn, the eighth to eleventh thermochromic patches 36e, 36 _¾ 36io, 36n become visible, as shown in the lower three rows of Figure 2. In this embodiment, these patches are coloured red, to indicate that the ambient temperature is above the optimum range.

Thus, as temperature increases, the thermometer of this embodiment displays an increasingly-long row of coloured patches, thereby mimicking the appearance of a conventional mercury or spirit thermometer, when the length of visible fluid indicates the temperature. In addition, the indicated colours give an immediate indication of the whether or not the ambient temperature is too cold, optimal, getting too warm, or is certainly too warm. This indication can be reinforced by the patches being formed as indicia such as shapes, numerals or other alphanumeric symbols, or may rely upon the colour of the indicia alone to indicate a status of the temperature detected by the thermometer.

In embodiments of the invention, patches, or permanently-coloured area, will be of the same colour over a plurality of temperature ranges suited to the particular purpose of the thermometer. For a thermometer for a refrigerator, the temperature ranges and typical colours might be:

< 1°C Blue

2°C to 5°C Green

6°C Yellow/Amber/Orange

>7°C Red

For a thermometer for an infant's sleeping environment, the temperature ranges might be:

< 15°C Blue

16°C to 20°C Green

21 to 23°C Yellow/Amber/Orange

>24°C. Red

For a thermometer for an infant's bathwater, the temperature ranges might be:

< 35°C Blue

36°C to 38°C Green

39°C Yellow/Amber/Orange

>40°C Red