Abstract:
A watch comprises a case, an electronic movement module inside the case, a piece of closed-cell foam inside the case, and a dielectric liquid filling the remaining voids of the interior of the case. The closed-cell foam compensates for thermal expansion of the dielectric liquid over a wide range of operating temperatures. Upon heating, the liquid expands and the foam is compressed. Upon cooling, the liquid contracts and the foam is decompressed.

Description:
BACKGROUND 
     It is well known in the art that a watch case enclosing an electronic movement can be filled with a clear dielectric liquid, typically a silicone oil or a perfluorinated fluid. The dielectric liquid improves water resistance of the watch, prevents condensation of water inside the case, ensures continuous lubrication of the movement, and enhances readability of the dial by reducing undesirable refractions of light. 
     One challenge in designing a liquid-filled watch is compensating for differential thermal expansion. The thermal expansion coefficients of the known dielectric liquids are significantly larger than those of the known materials used in construction of watch cases, including steels, ceramics, and plastics. Since liquids are incompressible, their expansion inside a confined volume can exert tremendous pressure that is adequate to rupture or displace a watch&#39;s crystal. Several methods are known in the art for accommodating thermal expansion in a liquid-filled watch, thereby enabling a wide range of operating temperatures. 
     For example, U.S. Pat. No. 4,080,781 discloses a watch filled with a liquid and a device for accommodating thermal expansion. In one embodiment, the case is designed with a movable piston in the side wall. The piston extends and retracts to follow changes in the internal liquid volume. In a second embodiment, there is an elastically deformable diaphragm built into a wall of the case. The diaphragm allows the liquid to expand and contract. Both embodiments add substantial mechanical complexity to the case design, resulting in additional costs of manufacturing. 
     U.S. Pat. No. 4,459,039 discloses a watch filled with a liquid that has one internal enclosure with a deformable wall. The internal enclosure is filled with a gas, and compensates for thermal expansion of the liquid. Three embodiments are disclosed for the internal enclosure: a diaphragm sealed to the interior of the case that contains a fixed quantity of gas; a gas-filled bladder made entirely of flexible material; and a rigid tube having one closed end and a movable flexible plug that holds a fixed quantity of gas inside the tube. A disadvantage of a single interior enclosure is that a leak in the flexible material will allow all the gas to escape from the enclosure and form a large bubble in the liquid. Another disadvantage is the complexity and cost of fabricating the internal gas-filled enclosure. 
     International Patent Publication No. WO 96/35978 discloses a watch filled with a dielectric fluid in which there is a gas bubble serving to compensate for the differential coefficients of thermal expansion between the liquid and case. One disadvantage is that the gas bubble can be seen floating freely under the crystal. The bubble can be aesthetically objectionable to some users. Another disadvantage is that the procedure for injecting the liquid requires tight control in order to target a specific size of the gas bubble. 
     German Patent Publication No. DE 19647439 discloses a watch filled with a liquid where the case is designed with a movable piston in the case back. The piston extends and retracts to follow changes in the internal liquid volume. By locating the piston in the case back instead of the side wall of the case, there is room for a piston of significantly larger diameter. The larger diameter bore serves to reduce the travel distance of the piston as it responds to changes in liquid volume. The piston feature adds design complexity and manufacturing cost. 
     French Patent Publication No. FR 2752959 discloses a watch filled with a liquid where the base of the case is provided with a deformable membrane that is protected by a rigid external wall. In one embodiment, the membrane is constructed of thin stainless steel with circular corrugations to allow for expansion. Small openings in the rigid external wall allow pressure equalization between the membrane and the exterior environment. In addition to the complexity and high manufacturing cost of this design, the space between the membrane and the rigid external wall fills with water when the watch is immersed. After exiting the water, the residual water in the space gradually drains onto the wrist of the user. 
     International Patent Publication No. WO 03/083586 discloses a watch filled with a liquid where the base of the case at least partly comprises an elastic membrane, and an adjacent rigid element to limit deformation of the membrane toward the case interior. The membrane flexes in order to accommodate thermal expansion of the liquid. The membrane feature adds complexity to the case design, resulting in a higher cost of manufacturing. 
     The invention described herein seeks to provide an improved thermal expansion compensator for liquid-filled watches. 
     SUMMARY 
     In accordance with one embodiment a watch comprises: (a) a case; (b) an electronic movement module inside the case; (c) one or more pieces of closed-cell foam inside the case; and (d) a dielectric liquid filling the remaining voids of the interior of the case. The closed-cell foam compensates for thermal expansion of the dielectric liquid by compression to a smaller volume. The use of closed-cell foam allows for simple case construction and low manufacturing cost. The foam can be easily cut into various shapes that conform to interior spaces of the case. Also, closed-cell foam is more robust than a gas-filled bladder, a diaphragm, or a piston since the foam does not rely on a single sealing element for proper function. An additional advantage is that closed-cell foam can be used to quickly retrofit a watch for liquid filling even if the watch was not originally designed to contain liquid. 
     Further details and advantages will become apparent from the discussion which follows. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention is described in detail below with reference to the drawings wherein like numerals designate similar parts. In the Figures: 
         FIG. 1  is an exploded view of a watch with closed-cell foam prior to filling with dielectric liquid. 
         FIG. 2  is a sectional view of the watch of  FIG. 1 , where the watch has been filled with dielectric liquid and is equilibrated at its lowest operating temperature. 
         FIG. 3  is a sectional view of the watch of  FIG. 2 , where the watch has been heated to its highest operating temperature. 
     
    
    
     DETAILED DESCRIPTION 
     One embodiment of the thermal expansion compensator for a liquid-filled watch is shown in  FIGS. 1 ,  2 , and  3 .  FIG. 1  shows an exploded view of the watch prior to filling with liquid. A watch case  10  is joined to a transparent crystal  11  by a suitable gasket of conventional design (not shown). A case back  12  is joined to the bottom of the case  10  and is sealed by an o-ring  13  or similar hermetic seal. The case back  12  is joined to the case  10  by a conventional fastening method such as screw threads (not shown). A crown  14  is provided for setting time. The case  10  encloses an electronic movement module  15  that is surrounded by a movement spacer ring  16 . The ring  16  centers the movement module  15  in a position adjacent to a dial  17 . The movement module  15  drives the watch hands  18 . A piece of closed-cell foam  19  is located in an annular region around the movement module  15 . 
       FIG. 2  shows a sectional view of the watch of  FIG. 1  after it is entirely filled with a clear dielectric liquid  20  and is equilibrated at its lowest operating temperature, for example −10° C. The closed-cell foam  19  is not compressed. The watch can be filled by various conventional methods. In this embodiment, the watch was submerged in the liquid without the case back  12 . Vacuum was applied to remove residual air, then the case back was installed while still submerged in the liquid. 
       FIG. 3  shows a sectional view of the watch of  FIG. 2  after it has been heated to its highest operating temperature, for example 60° C. The closed-cell foam  19  is compressed by thermal expansion of the liquid  20 . 
     In the first embodiment, the case  10  and case back  12  are made of 316L stainless steel. The crystal  11  is made of sapphire. Over the operating range of −10 to 60° C., the internal volume of the watch changes by less than 0.5% due to thermal expansion. The liquid  20  is a perfluorinated fluid. It is a mixture of perfluoro n-alkyl morpholines and is sold as Fluorinert™ FC-770 Electronic Liquid by 3M of St. Paul, Minn. Over the operating range of −10 to 60° C., its volume changes by 10% due to thermal expansion. 
     The closed-cell foam  19  is composed of FKM fluoroelastomer. Even when cut, the foam will not allow the passage of air or fluid between cells. FKM fluoroelastomer has excellent resistance to compression set and very low permeability to liquids and gases. These properties ensure long term durability over the course of cycling between compression and decompression. The sales specification of the foam indicates a compressive strength in the range of 4 to 7 psi at 25% deflection. This is considered an extra soft foam, and limits pressure buildup inside the watch interior. In this embodiment, 3.0 cm 3  of liquid  20  was charged into the watch and the volume of the closed-cell foam  19  was 0.6 cm 3 . Over the range of −10 to 60° C., the liquid  20  expands by 0.3 cm 3 . This in turn compresses the closed-cell foam  19  by 0.3 cm 3 , equivalent to 50% deflection of the foam. 
     The second embodiment is similar to the first embodiment shown in  FIGS. 1 ,  2  and  3  except for a different selection of liquid  20  and of closed-cell foam  19 . The liquid  20  is a silicone fluid. It is polydimethylsiloxane, sold as Xiameter™ PMX-200 Silicone Fluid 1.5 cSt by Dow Corning of Midland, Mich. Over the operating range of −10 to 60° C., its volume changes by 9% due to thermal expansion. The closed-cell foam  19  is a blend of neoprene/PVC/nitrile. The sales specification of the foam indicates a compressive strength in the range of 2 to 5 psi at 25% deflection. In this embodiment, 2.7 cm 3  of liquid  20  was charged into the watch and the volume of the closed-cell foam  19  was 0.9 cm 3 . Over the range of −10 to 60° C., the liquid  20  expands by 0.25 cm 3 . This in turn compresses the closed-cell foam  19  by 0.25 cm 3 , equivalent to 30% deflection of the foam. 
     Accordingly, the reader will see that the closed-cell foam of the various embodiments can be used as a low-cost compensator for thermal expansion of the dielectric liquid. It enables production of liquid-filled watches without the need for complex and expensive case designs. It also avoids the need for a free-floating air bubble in the liquid. The closed-cell foam can be easily cut into various shapes that conform to interior spaces of the case. Also, closed-cell foam is robust for long term use as it does not rely on a single sealing element to retain proper function. An additional advantage is that closed-cell foam can be used to quickly retrofit a watch for liquid filling even if the watch was not originally designed to contain liquid. 
     While the above description contains many specificities, these should not be construed as limitations on the scope, but rather as exemplifications of various embodiments thereof. Many other variations are possible. For example, the movement module  15  may be a simple quartz model driven by a lithium battery, or it may contain additional complications such as a moon phase indicator or a date wheel. Instead of running on a battery, it may be powered by solar cells in combination with a capacitor. Or it may have a digital display instead of analog hands. The closed-cell foam  19  may be in multiple pieces, and can be located in any free space inside the watch case. It may be partially compressed even at the lowest operating temperature. The case  10  may be made of alternative materials such as ceramic or plastic. The crystal  11  can be constructed of sapphire, mineral glass, acrylic, or any other suitable transparent material. Various gasket designs can be used to seal the watch case  10 , including the mating areas around the crown  14 , crystal  11 , and case back  12 . 
     Accordingly, the scope should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.