GB2240846A

GB2240846A - Slow response liquid crystal thermometer

Info

Publication number: GB2240846A
Application number: GB9102569A
Authority: GB
Inventors: Colin David Rickson
Original assignee: Liquid Crystal Devices Ltd
Current assignee: Liquid Crystal Devices Ltd
Priority date: 1990-02-09
Filing date: 1991-02-06
Publication date: 1991-08-14
Anticipated expiration: 2011-02-06
Also published as: GB9002990D0; GB2240846B; GB9102569D0

Abstract

A slow response thermometer comprises a housing (1) having thermally insulating base, walls (3-6) and transparent window (9) defining a closed cavity (35) within which is mounted an array of liquid crystal elements with an air gap between the elements and the window. The walls and air gap increase the time required for the thermometer to reach ambient temperature. The thermometer is mounted on a stand (16) in a fridge/freezer and indicates the temperature within the fridge/freezer when it is taken out for viewing. The stand includes a ball and socket (18, 20) which allows the thermometer to pivot on the stand (16). <IMAGE>

Description

LIQUID CRYSTAL THERMOMETER This invention relates to a liquid crystal thermometer and ln particular, but not exclusively, ts a sc-called refrigerator/freezer thermometer (hereinafter referred to as a "fridge/freezer thermometer").

A known liquid crystal fridgeifreezer thermometer comprises a plurality of liquid crystal materials designed to react over a specific low temperature range, e.g. from -180C to +10 C or -240C to +1soy, to display temperatures within that low temperature range. For example the liquid crystal materials may be printed on a carrer strip, each liquid crystal material displaying occur over a different, narrow temperature range within the low temperature range.

One particular disadvantage with such a known liquid crystal fridge/freezer thermometer is that when the thermometer is taken out cf its cold operating env,ronmer,- for reading in a warmer environment, the liquid crystal mater- ials are fairly rapidly warmed and the temperature orin- ally displayed by the thermometer no longer accurately displays the pre-existing temperature cf the cold operating environment.

The present invention seeks te provide a liquid crystal thermometer having liquid crystal materials which are thermally insulated from the surrounding atmosphere.

According te the present invention a liquid crystal thermometer comprising a temperature responsive display device having a plurality of temperature responsive liquid crystal materials arranged beneath a transparent front layer and designed to react to different temperatures to provide temperature indication visible from the front through said front layer and a housing mounting the said display device, is characterised in that the housing defines a closed cavity having walls made of thermally insulating material or materials and within which the display device is mounted, in that one of these cavity walls, through which the temperature indicated by the display device can be viewed, is transparent and in that the said front layer and the saic one cavity wall are spaced apart to provide an air gap therebetween.

The air in the air gap acts as a thermal insulator or barrier. Thus if the ambient air temperature around the housing suddenly changes, the air in the air gap will prevent the liquid crystal materials from responding rapidly to this sudden temperature change. As the air in the air gap is gradually heated or cooleo to the new ambient air temperature so the liquid crystal materials will gradually respond. In certain applications a slow respcnse to changing temperatures is highly desirable.For example if the liquid crystal thermometer according to the invention is a fridge/freezer thermometer, it is desirable that there should be a time delay between the time that the thermometer is taken out of the refrigerator or freezer into warmer ambient air for inspection and the time that the temperature indicated changes. Alternativey during automatic defrosting cycles and/or when the fridge/freezer door is opened and closed, the air temperature inside the appliance may rise a few degrees fairly rapidly only to fall again when the appliance door is closed cr the defrosting cycle ceases.In such "real" situations, especially if the housing is mounted in the appliance on a thermally insulated cold appliance wall, the liquid crystal materials are insulated by the air gap against reacting rapidly to relatively small temperature increases.

Conveniently the display device is mounted directly on one of the other cavity walls, e.g. the cavity wall directly opposite the said one cavity wall. Since the cavity walls are made of thermally insulating material, e.g. a plastics material, ambient air temperatures are not quickly transmitted to the inside of the cavity by conducticn. The more efficient the cavity walls are as thermal insulators, the greater will be the time lag between the liquid crystal materials respcnding to a different ambient air temperature.

QuitaDly the said air gap distance between the said front layer and the said one cavity wall is from -tC mm, preferably from 5-7 mm.

Conveniently the housing is mounted on a stank.

Typically the housing is mounted to tne stand via a universal joint. Alternatively, for a ridge/freezer ther mometer, the housing may be adhered directly or to the inside wall of the appliance. n this case the thermometer takes account of the thermal inert-a o the thermally insulated appliance wall so that transient air temsernture changes within the appliance have virtually ne influence on the inside wall temperature. The air gaD thermal barrier cf the thermometer of the present invention also ensures that such transient air temperature changes are not transmitten to the liquid crystal materials.

An embodiment of the invention will now be described, b way of example, with reference to the accompanying drawing, in which Figure 1 is a schematic disassembled exploded view of a liquid crystal thermometer according to the invention, and Figure 2 is a longitudinal sectional view on an enlarged scale through part of the assembled housing of the liquid crystal thermometer shown in Figure 1.

Figure 1 shows the various constituent parts of a liquid crystal thermometer according to the invention. In particular the thermometer includes a shallow, generally rectangular housing part 1 having a bottom wall 2, spaced apart inwardly inclined side walls 3 and 4 and spaced apart inwardly inclined end walls 5 and 6, which walls 2-5 together define an open-topped cavity 7.The cavity is closed by a transparent top housing part 8, e.g. of "PERSPEX" (trade mark), having a top wall 9 and inwardly inclined pairs of side walls 10 and end walls 11, the inwardly inclined side and end walls being received in the open-topped cavity 7 with projections 12 on each cf the end walls 11 being received in snap fit connection within correspondingly shaped recesses 13 in the end walls 5 and 6.

The housing part 1 has spaced apart lugs 18 and 15 extending downward'y therefrom for connecting the housing part 1 to a stand 16 via a connector 17. The connector 17 comprises a stem 13 and a crcss-pitce axle 21 arranged in a T-shape. A ball member 18 is arranged at the lower end of the stem 19 which is received within a socket 2C in the stand 16. Opposite end portions cf the axle ^-4 are snap fit connected to openings in the lugs 14 and 15.The tall member 16 and socket 20 provide a universal joint and the connection of the axle 21 to the lugs 14 and 1 enables the housing part 1 to pivotally turn on the connector 17.

On the bottom wall 2 there is adhered a tin .flexible temperature responsive display strip 30 (see Figure 2) including a plurality of temperature responsive liquid crystal materials arranged beneath a flexible transparent front layer and backed by a dark backing layer. The display strip 30 will not be described in detail since it may be of any type well known in the art in which the liquid crystal materials respond to different temperatures within a particular temperature range. As shown the display strip 30 has printing on it visible from above indicating a temperature range of from 00 C to 100C for indicating refrigerator temperatures and one, two or three stars for indicating colder freezer temperatures. The display strip 30, being of thin construction and having thin deposits of, for example, microencapsulated liquid crystal material thereon, is of the tyoe which reacts relatively quickly te changes in the air temperature immediately surrounding it.

In the thermometer described herein, and as can be seen clearly in F-gure 2, a closed air space 35, of thick- ness C, is formed between the upper surface oe the dissta:, strip 30 and the lower surface of tne top wall 9 of the housing part 8. In use this air space acts as an air well te prevent sudden changes in ambient air temperature around the thermometer being transmitted immediately to the temperature responsive display strip 30. In practice it is found that the dimension D should be at least 2 mum, and preferably from between 5 to 7 mm, e.g. 6 mm.No great benefit is obtained if D exceeds 10 mm. The housinr parts 1 and 8 are moulded from thermally insulating plastics material and thus the relatively poor thermal conductivity of the air cavIty walls serves to thermally insulate the air cavity from the surrounding air temperature.

The provision, of the closed air space 35 is particularly useful for a fridge/freezer thermometer since it is often desirable to take the thermometer from a cold temperature-sensing environment to a warmer temperaturereading environment, e.g. when the thermometer is taken from a cold refrigerator for visual reading in a warm kitchen. The provision of the closed air space 35 between the front surface of the strip 30 and the viewing window provided by the top wall 9 ensures that the air within this air space is only heated relatively slowly. Thus the temperature responsive display strip 30 only reacts slowly to the increased air temperature outside the cavity 7 and a true reading of the refrigerator temperature can be obtained even after the thermometer has been removed from the refrigerator for a period of time.

By way of example, the following table shows the length of time it takes for the display strip 30 to change temperature from one indication to another when the top housing part 8 is and is not, attached to the housing part 1. In the example given, the thermometers were cooled to a temperature of OOC and then placed in ambient air temperature of 220 C.

Change of Temperature Time (secs) for Time (secs) for displayed from a display to display to lower temperature (oC) change - Housing change - Housto an upper tempera- part 1 with open ing part 1 with ture (OC) cavity 7 cavity 7 closed by part 8 0-2 19 27 4-6 17 6 - 10 24 41 From the above table it will be seen that the time taken for the display strip 30 to display 20C temperature rises, in each instance, is significantly greater when the cavity 7 is closed by the housing part 8 to provide a closed air space between the top wall 9 and the display strip 30. In more practical situations, e.g. during a defrosting cycle, where the rise in air temperature te which the thermometer is subjected is much less, the thermal inertia of the air gap will mean that the temperature rise of the liquid crystal materials will be much less, e.g. in the order of 0.20C per minute.

Although the liquid crystal thermometer shown in the drawings has been described as being detachably mounted on a stand 16, the lugs 14, 15 on the housing part 1 may be dispensed with and the housing part adhered directly to an appliance internal wall. In this case an adhesive backing could be provided on the back of the housing part 1.

Claims

1. A liquid crystal thermometer comprising a tempera ture responsive display device having a a plurality of temperature responsive liquid crystal materials arranged beneath a transparent front layer and designed to react to different temperatures to provide temperature indication visible from the front through said front layer and a housing mounting the said display device, characterised in that the housing defines a closed cavity having walls made of thermally insulating material or materials and within which the display device is mounted, in that one of these cavity walls, through which the temperature indicated by the display device can be viewed, is transparent and in that the said front layer and the said one cavity wall are spaced apart to provide an air gap therebetween.

2, A thermometer according to claim 1, in which the display device is mounted directly on one of the cther cavity walls.

3. A thermometer according to claim 2, in which the display device is mounted directly on the cavity wall directly opposite the said one cavity wall.

4. A thermometer according to any of claims 1 to 3, in which the said air gap distance between the said front layer and the said one cavity wall is from 2-10 mm.

5. A thermometer according to claim 4, in which the said air gap distance is from 5-7 mm.

6. A thermometer according to any of the preceding claims, in which the housing is mounted on a stand.

7. A thermometer according to claim 6, in which the housing is mounted to the stand via a universal joint.

8. A liquid crystal thermometer constructed and arranged substantially as herein described with reference to, and as illustrated in, Figures 1 and 2 of the accompanying drawing.