The present invention relates to a temperature supervision alarm system for refrigerated compartments, preferably freezing compartments.

Home freezers, freeze boxes, show-case freezers and the like are usually equipped with an alarm system, which shall be activated if the temperature in the freezing compartment rises in a way that can be harmful to stored goods in the freezing compartment, for example if the refrigeration system for the freezing compartment cease to function or if its capacity deteriorates in a way that the predetermined temperature no longer can be maintained in the freezing compartment or if a door or a cover of the freezing com¬ partment is left open or no longer can be closed completely depending on frost or ice deposits or for some other reason. On such an occasion the alarm shall be activated as early as possible, i.e. at an increased temperature which is as close as possible to the predetermined temperature, in order to avoid that the stored goods will be damaged. At the same time false alarms must not be activated, for example, when the freezing compartment is kept open in order to put in or take out goods, or during defrosting of a freezing compartment which is equipped with an automatic defrosting system.

The alarm systems which are used today and which fulfil the above requirements are in general electronic systems and they are comparatively expensive to produce.

The object of the present invention is therefore to provide an alarm system, which fulfils all the above requirements and which is also inexpensive to produce. This object is achieved by, among other things, a temperature sensing element, which comprises a substance with essential

qualities for thiε purpose, namely

- a solidifying/melting temperature which is close to the temperature where the alarm is activated, - a phase transformation between liquid and solid state involving a change of volume,

- a phase transformation between liquid and solid state which is essentially eutectic.

The advantages with a temperature sensing element according to the invention is partly that the time delay that you try to obtain in, for example, a freezer alarm is automatically achieved due to the requirement of a great melting heat for melting in an eutectic phase transformation and partly that the alarm temperature is determined by a physical constant, which is not changed in course of time and thus must not be adjusted depending on wear and changes in dimensions in mechanisms or the like which is the case in other alarm systems.

The characterizing features of the alarm system according to the invention are disclosed in the accompanying claims.

The invention will be described in more detail below with reference to the accompanying drawing, which schematically and by way of example illustrates one embodiment of the invention.

The embodiment of an alarm system according to the inven- tion which is shown in the drawing is designed as one unit comprising a base plate 1 and preferably a casing, not shown, which is enclosing the different components. The unit is intended to be installed inside the freezing com¬ partment, the temperature of which shall be supervised, for example, on the inside of one of the walls of the freezing compartment.

The system comprises, as a temperature sensor, a sealed container 2, which is filled with a substance with a solid¬ ifying/melting temperature which is close to the tempera¬ ture which must not for a longer period be exceeded in the freezing compartment and with a phase transformation between liquid and solid state which involves an essential volume change. Preferably, the substance should have an essentially eutectic phase transformation between solid and liquid state and a substance may well be used which shows an increased volume when it is transformed from liquid to solid state. However, it is also possible to design an alarm system according to the invention using a substance which shows a decrease in volume when it is transformed from liquid to solid state. In the shown embodiment the container 2 is designed essentially as an elastic bellows with variable length and is provided with a bottom section 2a which is stationary on the base plate 1, whereas the bellows shaped part of the container 2 is freely movable. It is obvious that the axial length of the bellows shaped part of the container 2 will vary depending on the propor¬ tions between solid phase and liquid phase of the substance in the container. If you use a substance which shows an increased volume when it is transformed from liquid state to solid state, the axial length of the bellows shaped part of the container 2 will thus increase depending on the proportions of the substance that has been transformed to a solid state. The system further comprises an electric contact 3, for example a micro switch, which is connected in an electric circuit comprising a battery 4 as a current source and a lamp as a visual alarm device 5 and an acous¬ tic alarm device 6, for example an alarm bell, a buzzer or the like. The electric contact 3 is arranged on a lever 7, which is pivotingly mounted on an axle 8 on the base plate 1. There is a frictional resistance between the pivot axle 8 and the contact lever 7 which is keeping the contact lever 7 in a set position, as long as it is not effected by any external forces. In the drawing the contact lever 7 is

shown in a normal position and from this position it is not possible for the lever to pivot any further in a counter¬ clockwise direction on the axle 8, while it is possible in a clockwise direction. The contact lever 7 is provide with a part 7a which is projecting through an opening in the casing of the unit and this part can be activated manually in a similar way as a push button for pivoting the contact lever 7 in a clockwise direction on the axle 8.

It is assumed, as mentioned above, that the drawing shows the system in its normal position, when the temperature in the freezing compartment is below a predetermined tempera¬ ture, e.g. -16 ^* C, and that all or essentially all substance in the container 2 is in a solid state. In this state the bellows shaped part of the container 2 has an axial length, that the freely movable end section 2b of the container keeps the contact 3 open as illustrated. In this position the current circuit through the contact 3 is broken and the alarm is consequently not activated.

If the temperature in the freezing compartment exceeds the solidifying/melting temperature of the substance in the container 2, e.g. above -16 ^* C, the substance in the con¬ tainer 2 starts to melt. After a specific time, which is depending on the size of the container 2 and thus the volume of the substance in the container, all of or almost all of the contents in the container 2 are melted and the volume of the substance is decreasing. As a consequence, the axial length of the elastic bellows shaped part of the container 2 is also decreasing and the free end 2b of the bellows shaped part withdraws from the contact 3 so far that the contact 3 is closed. The electric circuit is thus closed and the alarm lamp 5 is lit and the acoustic alarm device 6 is activated. To save the battery 4 the electric alarm circuit can include elements which only intermittent¬ ly activate the alarm lamp 5 and the acoustic alarm 6.

To switch off or reset the alarm until the trouble has been removed, which is especially desirable when you use an acoustic alarm, the contact lever 7 can be pivoted in a clockwise direction on the axle 8 by manually activate the projecting part 7a, until the contact 3 is reopened by operation of the free end 2b of the container 2 whereby the alarm is switched off. As a consequence of the existing friction between the contact lever 7 and the axle 8, the contact lever 7 will remain in this position until the trouble has been removed and the temperature has been reduced in the freezing compartment. Then the substance in the container 2 solidifies again and the bellows shaped part of the container increases its axial length and the contact lever 7 is pivoted in an counter-clockwise direc- tion on the axle 8 back to its normal position shown in the drawing. The alarm system is thus automatically brought back to its normal working position.

This possibility for switching off or resetting the alarm function is also advantageous when the freezing compartment is not in use and the associated refrigeration system is put out of work, as well as during the necessary refrigera¬ tion period for the freezing compartment when the freezing compartment and its refrigeration system is being started up.

The shown embodiment of an alarm system according to the invention is further provided with a manually activated mechanism, a pivoting lever 9, which can be used in order to manually and temporarily close the contact 3 to test the function of the alarm circuit and especially the condition of the battery 4.

The alarm system according to the invention, which is shown in the drawing and described above, has a battery 4 as current source. This can be advantageous if the alarm system shall be installable in an existing freezer or the

like, which earlier has been without a temperature super¬ vision alarm system. It is also advantageous from the point of view that the alarm system can work also when there is a interruption in the normal power supply to the freezer or the like. However, it is obvious that there is of course nothing that prevents that an alarm system according to the invention gets its power supply from the usual electricity supply system.

Furthermore, it is obvious that the alarm system according to the invention can be designed in many other ways than according to what has been shown on the drawing and what has been described above, especially regarding the design of the elastic container 2 and its interaction with the electric contact.