Abstract:
The present invention comprises a device and a method for determining if a system with multiple seals has a leak and, if so, which of the multiple seals is causing said leak. The device and method employ a change in the optical reflectivity of a transparent, plastic membrane due to pulling a vacuum on the seal to make the above mentioned determination.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without payment of any royalties thereon or therefor. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains to leak detection techniques and devices, more particularly to the use of vacuum tests and devices to detect the presence of leaks, and most particularly to using a vacuum test to detect the presence of leaks and identify the seals where leaks have occurred. 
     2. Description of the Related Art 
     Modern weapon systems, such as torpedoes and missiles, are a collection of subassemblies that are housed in shell sections. Individual shell sections are connected together with seals. After final assembly, the currently used methods to verify the integrity of the seals are by pulling a vacuum or conducting a pressure test. Seals are flexible and work by forcing the flexible sealing material into/against the two mating parts. Seals are designed for high internal pressure or high external pressure. Therefore, seals have a direction of use associated with their intended use and should be tested in that direction. For systems with high internal pressure a pressure test is used. In conducting a pressure test once the system has been pressurized an initial internal pressure reading is taken. Then the system is allowed to set for a period of time, usually about 30 minutes, and the internal pressure is again measured and, then, compared with the initial level. Pressure changes that exceed a nominal amount are an indication of a bad seal(s) in the system. The leak rate is a fraction of the pressure applied across the seal. Moderately high pressures are used to quickly indicate small leaks. 
     For systems with high external pressure (under water systems), the direction of the desired seal requires that test pressure should be greater outside the system than inside. It is impractical to construct large high pressure chambers to surround the system under test. 
     The only method remaining to verify the integrity of a seal for an underwater weapon is to pull a vacuum inside the weapon. Even if a perfect vacuum is used, only 14.7 psi can be applied across the seal. Leaks can be difficult to detect with this low of a pressure differential. A longer observation time helps to some degree in overcoming this shortfall, but is obviously more cumbersome and costly. 
     Another problem with both of the tests described above is that even if a leak is detected, this merely indicates that one or more of many seal may have leaked. A modern torpedo may have over a dozen seals that are being tested. A common practice after determining a possible leak condition, is to backfill the cavity on the weapon with a gas at a slight positive pressure to check for the presence of the gas around the individual seals. The gas would escape around the defective seal(s). The gas of choice was FREON 12. Recent regulations have attempted to discourage this practice. Other types of Freon gases are currently being used. However, extremely expensive leak detectors must be employed when using this method. 
     Therefore, it would be desired to provide a leak detection device and technique that could be used for any type of weapon system that could detect leaks and identify the faulty seal(s) associated with said leaks. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a device and a method for determining if a system with multiple seals has a leak and, if so, which of the multiple seals is causing said leak. The device and method employ a change in the optical reflectivity of a transparent, plastic membrane to make the above mentioned determination. 
     Accordingly, it is an object of this invention to provide a device and method to detect leaks in systems containing seals. 
     It is a further object of this invention to provide a device and method to detect leaks in systems containing seals that is simple and cost efficient. 
     A still further object of this invention to provide a device and method to determine the specific seal(s) that is leaking within a system containing multiple seals. 
     This invention accomplishes these objectives and other needs related to leak detection by providing an optical leak detection device for seals or joints comprising a transparent plastic membrane which is placed around the outer edges of the seal or joint being tested. Adhesive/sealant is applied around the edges in order to ensure that the membrane adheres around the seal or joint and also to form a gap between membrane and the seal or joint. Because the membrane adheres to the adhesive without contacting the seal or joint, the surface of the membrane is optically reflective and possesses a “glossy” look. A means to pull a vacuum is employed on the seal or joint on the side opposite to the membrane. If no leak exists in the seal or joint, the membrane stays in place and the optically reflective surface remains the same. However, if a leak exists, the membrane will be pulled into contact with the seal or joint, removing the gap, and the surface of the membrane becomes optically diffuse. The difference between the optically reflective and optically diffuse surfaces is obvious to the person using the device, and, thereby, one can easily detect a leak. The device and method of the present invention may be applied to seals or joints on a variety of one dimensional and two dimensional curved surfaces in addition to flat surfaces. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and, together with the description, serve to explain the principles of the invention. 
     FIG. 1 is a cut-away side view of an embodiment of the invention showing no leak. 
     FIG. 2 is a cut-away side view of the embodiment of the invention shown in FIG. 1 showing a leak. 
     FIG. 3 is a cut-away side view of an embodiment of the invention used on ship hull. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention uses two optical principles related to the optical reflectance being a function of the roughness of a surface. The first principle is that a transparent smooth surface is normally optically reflective. For example, when faded paint on a car (which is a rough surface from an optical perspective) is filled by water either through washing or rain, this creates a smooth optical surface, which is optically reflective, causing a glossy look. The second principle is that a transparent rough surface is normally optically diffuse. For example, when “scotch” tape conforms to a surface it becomes rough from an optical standpoint and is optically diffuse, thereby, almost invisible. 
     The invention, as embodied herein, uses these optical principles to provide a device and method for leak detection that is efficient and cost-effective. In general, the invention is an optical leak detection device for seals or joints. The device comprises a transparent, flexible membrane that is large enough to be placed over the seal or joint to be tested. An adhesive is applied to the edges of the seal or joint in order to hold the membrane in place. The membrane is initially held tightly across the seal or joint by the adhesive, thereby forming a gap between the membrane and the seal or joint. In this configuration, the membrane is smooth, and, therefore is optically reflective. Thus, the membrane will have a glossy look. A means for pulling a vacuum is employed on the side of the seal or joint opposite to that of the membrane. Therefore, if a leak exists in the seal or joint, the membrane will be pulled against the seal or joint, creating a rough, optically diffuse surface. 
     Referring to FIG. 1, which shows an embodiment of the invention where no leak is present in the seal  100  being tested, the invention comprises a transparent, flexible membrane  102  that is large enough to completely cover the seal  100 . Adhesive  104  is applied to the outer edges of the seal  100  in order to hold the membrane  102  in place and form a gap  106  between the membrane  102  and the seal  100 . Because the membrane  102  is stretched across the seal  100 , but not in contact with it, the membrane  102  has an optically reflective glossy look. Means for pulling a vacuum  108  is located on the side of the seal  100  opposite to the membrane  102 . As seen in this figure, no leak is present and, therefore, the means for pulling a vacuum  108  has no affect upon the membrane  102 , and, thus, the membrane  102  retains its optically reflective, glossy look. 
     Referring to FIG. 2, the device of FIG. 1 is portrayed with a leak  210  within the seal  100 . The membrane  102  is pulled substantially against the seal  100  by the means for pulling a vacuum  108 . This causes the membrane  102  to become optically diffuse, thereby becoming dull. Therefore, to an operator using the device, a leaking seal is easily detected by observing the membrane  102 . 
     Referring to FIG. 3, an embodiment of the device may also be employed to determine if a leak exists in a seal within an object such as a ship hull  312 . Often ship hulls are modified to place an object within the hull  312  such as a water velocity sensor  313 . Therefore, the hull must be sealed around the object. In order to detect a leak in the hull  312  using the present invention, a first membrane  314  having a port  316  is placed around one side of the sensor  313 . The first membrane  314  is secured using adhesive  104  as described above. A second membrane  318  is placed over the opposite side of the sensor  313 , having a gap  315  formed between the second membrane and the hull  312  and sensor  313 , so that the second membrane  318  is optically reflective with a glossy look. This second membrane  318  is also secured with adhesive  104 . A means for pulling a vacuum  108  is connected to the port  316 . When a vacuum is pulled using the means for pulling a vacuum  108 , and if a leak exists in the seal between  313  and  312 , the second membrane will come into direct contact with the hull  312  and sensor  313  and become optically diffuse and dull looking. 
     One other preferred embodiment of the present invention draw elements from all three of the FIGS. discussed above. If a system operates with a pressure that is higher internally than the ambient pressure, the membrane  102  is placed over the seal  100  and is held by adhesive  104  as shown in FIG.  1 . In this embodiment, the membrane  102  is placed directly against the seal  100  so that the membrane  102  is optically diffuse with a dull look. A port  316  is attached to the membrane  102  as shown in FIG.  3 . The means for pulling a vacuum  108  is attached to the port  316  as shown in FIG. 3, rather than placed on the side of the seal  100  opposite to the membrane  102 . When a vacuum is pulled, if a leak exists, the membrane  102  will rise, forming a gap  106  so that the membrane  102  becomes optically reflective with a glossy look. 
     Regarding the elements of the device described above, the membrane  102 ,  314 ,  318  should comprise a transparent flexible material that is optically reflective when suspended over an object and optically diffuse when in contact with an object. Many such materials are known and may be selected by one skilled in the art. One preferred material is plastic wrap or cellophane because it is flexible, simple to obtain and inexpensive. The adhesive  104  should comprise any substance that can be applied to the edges of a seal  100  and will hold the membrane  102  in place. The adhesive  104  must also provide the gap  106  between the membrane  102  and the seal  100 . Therefore, the adhesive  104  should not be a liquid type material, but more of a gel. Again, many such substances are known and may be selected by one skilled in the art. One preferred adhesive  104  is grease because it normally is used in association with seals and joints, so is readily available. The means for pulling a vacuum  108  should provide at least about 10 pounds per square inches of vacuum. Most commercial vacuum pumps are capable of providing this vacuum, and, therefore, one skilled in the art may select any such vacuum pump. 
     Finally, the present invention also comprises a method of detecting leaks in a seal or joint using the different embodiments of the device as described above. The general steps of the method include the following. First, one would apply adhesive to outer edges of the seal or joint. Next, the membrane is placed over the seal or joint so it is held in place by the adhesive, and a gap is formed between the membrane and the seal or joint. In this configuration, the membrane comprises a reflective surface. Next, a vacuum is pulled on a side of the seal or joint opposite to the membrane. If a leak exists in the seal or joint, the membrane will directly contact the seal or joint, removing the gap, so that the membrane comprises an optically diffuse surface. 
     What is described are specific examples of many possible variations on the same invention and are not intended in a limiting sense. The claimed invention can be practiced using other variations not specifically described above.