Abstract:
The invention relates to the use of an indicator for testing the tightness of water conducting components in a housing which is either mixed into the water or released upon contacting water drops or generated from a generative material upon contact with water. According to the invention, the presence of the indicator in gaseous or vaporous form is detected by a sensor.

Description:
This application is a national stage application under 35 U.S.C. §371 of International Application Number PCT/EP2010/066403, filed Oct. 28, 2010, which claims priority of German Patent Application Numbers 2009 056 172.2 and 2010 005 494.1, filed Nov. 27, 2009 and Jan. 23, 2010 respectively, the entire contents of each application being herein incorporated by reference. 
     TECHNICAL FIELD 
     The invention refers to a method for testing the tightness of water conducting components in a housing. 
     BACKGROUND 
     In quality testing of mass products it is often necessary to perform tightness tests. Thus, washing machines and dish washing machines have to be tested to detect possible leaks in hoses and pipes. 
     DE 10 2007 032 250 B3 describes a device for detecting liquid leaks, comprising a large-surface, electrically non-conductive surface element on which conductor paths are provided. An electric voltage is applied to the conductor paths. By measuring the electric resistance, it is detected whether a drop of liquid is on the surface element. 
     SUMMARY OF THE APPLICATION 
     It is an object of the invention to provide a method for testing tightness, which is suited for testing the tightness of components in an enclosing housing that contains an atmosphere. 
     A first variant of the invention provides that the water conducting components are filled with a combination of water and an indicator, wherein, in the event of a leaking of drops, the indicator contained therein escapes into the atmosphere enclosed by the housing and is detected by a sensor sensitive to the indicator. 
     The indicator may be a liquid mixed with the water or it may be a gas dissolved in the water. Particularly well suited indicators are alcohols, such as ethanol, and other liquids that quickly evaporate into atmosphere. Since a drop of water hitting a surface has a large surface area, a large amount of the indicator evaporates in short time into the atmosphere contained in the housing. A corresponding sensor i.e. a gas or vapor sensor that is selectively sensitive to the indicator, detects the occurrence of the indicator in the atmosphere and signals “leakage”. Such a sensor includes a pump with which the housing atmosphere is drawn in. The housing atmosphere is supplied to a sensor that gives information about the presence of the indicator. The sensor may be a gas sensor using the measurement of infrared absorption, a mass spectrometer or the like. It is an advantage of the invention that the tightness test requires no pressure buildup in the components under test. Rather, it is possible to perform a pressure-less measurement. It is also possible for the indicator to be a gas contained in the water in dissolved form, such as CO 2  or helium, for instance. 
     A second variant of the method according to the invention provides that an indicator is introduced into a container that has at least one wall that becomes permeable upon contact with water, the container is positioned below the components to be tested, and the leakage of the indicator from the container is detected by a sensor sensitive to the indicator. Here, the effect is used that the wall of the container becomes permeable to the indicator when a drop of water falls thereon, so that the indicator escapes from inside the container into the surrounding atmosphere in the housing. The indicator is preferably gaseous or vaporous. Inert gases, in particular helium, but also CO 2 , are the indicator gases of choice. 
     A special embodiment of the second variant provides that the container is formed by microcapsules filled with an indicator which is alcohol, for instance. In case of a leak, water will contact the microcapsules that are thereby destroyed. The alcohol contained in the microcapsules is set free. A corresponding sensor detects and indicates the presence of alcohol in the air. The volume of water necessary to trigger the sensor detection is very small. It is on the order of 0.1 ml. “Micro-capsule” means an object having a core that is completely enclosed by a thin—possibly semi-permeable—wall. The size of the microcapsules generally is 1-1500 μm (micrometers), preferably 10 μm-300 μm, further preferred 30 μm-150 μm, but in particular cases it may also be larger, e.g. up to 2000, 3000, 4000, 5000 μm. 
     The microcapsules may be provided in association with other substances such as fillers, non-capsulated odorous or biological substances, possibly with inorganic or organic or even biological carriers, or together with biological materials. These may all be humidified with water, an aqueous solution or a suitable organic liquid or mixtures thereof. 
     In a preferred embodiment, microcapsules have semi-permeable walls. Through these walls, the active substance can diffuse into the water from the capsule. Here, water can penetrate from outside through the capsule wall, dissolve the ingredient in the core and diffuse out from the capsule together with the dissolved ingredient. 
     A third variant of the present method provides that a generator substance, which produces a gaseous or vaporous indicator upon contact with water, is positioned in the housing below the components under test, and that the occurrence of the indicator is detected by a sensor sensitive to the indicator. The generator substance can be stored in a water-soluble container, whereby it is protected from air humidity during storage. Only when a drop of water meets the container, whereby the wall of the container becomes permeable, will water reach the generator substance and CO 2 , for instance, is set free and can be detected within the housing. A suited generator substance is a mixture of sodium and acid which, upon contact with water, produces CO 2  as the indicator. CO 2  is well detectable using an infrared sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following is a detailed description of embodiments of the invention with reference to the drawings. 
         FIG. 1  illustrates a housing of a washing machine during the tightness test according to the first variant of the invention, 
         FIG. 2  is a side elevational view of a container for carrying out the second variant of the method of the invention, 
         FIG. 3  is a top plan view of the container of  FIG. 2 , illustrating two alternative types of welding of the film layers, and 
         FIG. 4  illustrates a material made from a substrate coated with the generator substance for performing the third variant of the method of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  illustrates a housing  10  containing components  11  to be tested for tightness. These components include, for instance, a suds container  110  connected to a detergent compartment  112  through a water jet tube  111 , the compartment being in turn connected to a water supply  113 . Further, the components include a suds container vent tube  114  that connects the suds container  110  with the detergent compartment  112 , and a water drain tube  115  which has a water drain  116  at its upper end and is connected to a suds pump  117  at the bottom end. Provided at the suds container  110  is a sensor  118  for filling level control and a heater element seal  119  which also is to be tested for tightness. All parts  110  to  119  form components  11  that are subjected to the tightness test. These components are contained in the interior  12  of the housing  10  which contains an atmosphere that corresponds to ambient air. The housing  10  encloses the interior  12  on all sides, while an absolute gas sealing of the interior is not required. It is important that the interior contains a gas volume that is delimited over the environment of the housing  10  so that an indicator substance in the interior  12  can be detected. 
     A sensor  13  serves to detect the indicator, the sensor being connected with the interior  12  through a line  14 . The sensor  13  includes a suction pump (not illustrated) that draws gas from the housing  10  and allows it to escape into the ambient atmosphere. The sensor may be of the type HLD5000 manufactured by INFICON GmbH. This sensor is an infrared cuvette. 
     The test for tightness is to show whether small quantities of water in the form of drops escape from the components  11 . A measurement of the air humidity in the housing  10  would not be sensitive enough or would take too much time. According to the invention, an indicator, such as alcohol or the like, is added to the water. Drops  15  that fall through a water leak  16  of a component  11 , fall on the floor  17 , where they burst. A drop of 0.1 ml of a 10% ethanol/water mixture in a volume of the size of a washing machine can be detected within three minutes by the HLD5000 device. It would be advantageous to have a convection of the atmosphere in the interior  12  by means of a blower or by a drive of the washing machine (not illustrated). 
       FIGS. 2 and 3  illustrate a device for performing the second variant of the method. This device comprises a container  20  formed by a carrier film  21  and a wall  22 , formed by another film arranged on the carrier film, the wall becoming permeable upon contact with water. Cavities  23  exist between the carrier film  21  and the wall  22 , which are filled with an indicator. The wall  22  is connected with the carrier film  21  either at isolated spot-welding points  24  or by line-shaped welding seams. In the case of spot-welding points, a uniform space is formed, while separated film pockets are formed by line-shaped welding seams. 
     Polyvinyl alcohol films (PVOH, also PVAL) are a suitable material for the wall  22 . Such films are water soluble. They are commercially available under the trade name SOLUBLON or SOKUFOL. PVOH films are gas-tight to many gases and are mechanically comparatively robust. They are not hygroscopic, but establish a balance with the ambient humidity, i.e. they absorb humidity and also release it again. They are well weldable using commercially available film-welding machines. 
     Various gases could be used as the indicator inside the container  20 , in particular inert gases, but also CO 2 . 
     The flexible container  20  designed as a film bag forms a water-soluble gas storage spread on the bottom of the housing. When a drop falls on the wall  22 , the same becomes permeable to the indicator so that the indicator escapes into the atmosphere in the interior of the housing. The sensor detects the present of the indicator in the interior. 
       FIG. 4  shows a device  30  for carrying out the third method variant. This device comprises a substrate  31  of a carrier film. The substrate  31  carries a coating  32  including a generator substance that releases a gaseous indicator  33  upon contact with water. The two-dimensional device  30  is placed on the bottom of the housing. When a drop falls on the device  30 , the generator substance  32  produces the indicator  33  that is detected by the sensor connected to the housing. 
     The indicator gas is generated through a chemical reaction. For example, the generator substance is a sodium-tartaric acid powder 2 NaHCO 3 +C 4 H 6 O 6 . When water is added, the following reaction occurs:
 
2NaHCO 3 +C 4 H 6 O 6 →C 4 H 4 O 6 Na 2 +2H 2 O+2CO 2  
 
     The indicator gas CO 2  is produced thereby, which gas can be detected by suitable sensors. 
     It would also be possible to use potassium hydrogen carbonate, sodium carbonate, potassium carbonate or calcium carbonate instead of sodium hydrogen carbonate. Instead of tartaric acid, other acids or acidulous substances could also be used.