Abstract:
A water sensor includes a housing that has at least one inlet port formed therein for allowing water to pass therethrough when submerged in water. A drive piston is mounted in the housing for sliding engagement therein. A water-activated driver is coupled to the drive piston and is in communication with the inlet port(s). The water-activated driver is inert in air and reactive with water to exert pressure on the drive piston and so that the drive piston moves in the housing. A movable indicator is mounted relative to the housing and is positioned to be responsive to movement of the drive piston. The movable indicator moves from a first position to a second position in response to movement of the drive piston.

Description:
ORIGIN OF THE INVENTION 
     The invention described herein was made in the performance of official duties by employees of the Department of the Navy and may be manufactured, used, licensed by or for the Government for any governmental purpose without payment of any royalties thereon. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to water sensing devices, and more particularly to a sensor that positively detects the presence of water in a mechanical fashion. 
     BACKGROUND OF THE INVENTION 
     Water sensors are well known in the art. One conventional design approach uses water pressure to indicate the presence of water since water pressure increases rapidly with depth. However, such sensors are ineffective in shallow water (i.e., less than five feet) where the difference between the water pressure and ambient air pressure is very small. Another conventional design approach is to sense the presence of water in an electronic fashion. However, these sensors require a power source which must be checked/replaced periodically, generally have more potential failure modes, require extensive testing using fault tree analysis, and may not be robust enough to withstand harsh environmental conditions. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a water sensor. 
     Another object of the present invention is to provide a water sensor that positively detects the presence of water in a shallow water environment. 
     Still an other object of the present invention is to provide a water sensor that detects the presence of water in a simple mechanical fashion. 
     Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings. 
     In accordance with the present invention, a water sensor includes a housing that has at least one inlet port formed therein for allowing water to pass therethrough when submerged in water. A drive piston is mounted in the housing for sliding engagement therein. A water-activated driver is coupled to the drive piston and is in communication with the inlet port(s). The water-activated driver is inert in air and reactive with water to exert pressure on the drive piston and so that the drive piston moves in the housing. A movable indicator is mounted relative to the housing and is positioned to be responsive to movement of the drive piston. The movable indicator moves from a first position to a second position in response to movement of the drive piston. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view one embodiment of the water sensor of the present invention prior to its being submerged in water; 
     FIG. 2 is a cross-sectional view of the embodiment depicted in FIG. 1 after it has been submerged in water; 
     FIG. 3 is an isolated cross-sectional view of the framework used to support the compressed cotton pellets before and after their submergence in water; 
     FIG. 4 is a cross-sectional view of another embodiment of the water sensor of the present invention prior to its being submerged in water; 
     FIG. 5 is a cross-sectional view of the embodiment depicted in FIG. 4 after it has been submerged in water but before the sensor&#39;s indicator rod has been extended from the sensor housing; 
     FIG. 6 is a cross-sectional view of the embodiment depicted in FIG. 4 after it has been submerged in water and after the sensor&#39;s indicator rod has been extended from the sensor housing; and 
     FIG. 7 is a plan view taken along line  7 — 7  of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is a simple mechanical water sensor that positively indicates the presence of water even when the sensor is submerged in shallow water depths. The water sensor cannot be activated in air and is therefore well-suited to be incorporated into the safety system of an underwater explosive device to prevent premature (i.e., “in air”) activation thereof. 
     It is to be understood at the outset that the novel features of the present invention could be realized in a variety of specific embodiments. By way of example, two such embodiments will be described herein. Various features of the two embodiments are interchangeable as will be noted below. 
     Referring now to the drawings, and more particularly to FIG. 1, one embodiment of the present invention water sensor is shown in cross-section and referred to generally by numeral  10 . Water sensor  10  has a generally rigid housing  12  that includes one or more ports (e.g., two are illustrated)  14 A and  14 B formed therein. The interior portion of housing  12  defines a first cylindrical portion  12 A and a second cylindrical portion  12 B adjacent thereto. First cylindrical portion  12 A has a larger diameter than second cylindrical portion  12 B. Although portions  12 A and  12 B are shown coaxially aligned, this need not be the case. 
     Mounted within first cylindrical portion  12 A is a first or drive piston  16  that is configured for sealed but sliding movement within first cylindrical portion  12 A. Such sealed fits and movement are well understood in the art of piston/cylinder design and will therefore not be discussed further herein. Mounted within second cylindrical portion  12 B is a second or driven piston  18  configured for sealed but sliding movement within second cylindrical portion  12 B. Pistons  16  and  18  are spaced apart from one another with the resulting sealed chamber formed therebetween being filled with a hydraulic fluid  20  such as a silicone hydraulic fluid. 
     A rod  22  is attached to or integral with piston  18 . Rod  22  extends from piston  18  through second cylindrical portion  12 B and through one end  12 C of housing  12 . Rod  22  is capped with a head portion  24  that prevents rod  22  from falling into housing  12 . Disposed about rod  22  is a spring  26  captured between piston  18  and end  12 C of housing  12 . Spring  26  is biased to push piston  18  towards piston  16  until head portion  24  seats against end  12 C thereby positioning rod  22  substantially in housing  12 . 
     When water sensor  10  is submerged in water, proper operation thereof requires movement of piston  16  towards piston  18 . Such movement pressurizes hydraulic fluid  20  which, in turn, drives piston  18  so that rod  22  is pushed further out of housing  12  as spring  26  compresses. 
     For water sensor  10 , movement of piston  16  is brought about by the water-activated expansion of compressed water-absorbent fibers maintained in housing  12 . That is, when water sensor  10  is submerged, water enters housing  12  via ports  14 A and  14 B and comes into contact with the compressed fibers. The water is absorbed by compressed fibers to bring about their expansion. This expansion results in an axial force being applied to piston  16  which, in turn, moves to compress hydraulic fluid  20 . 
     In the illustrated embodiment, the compressed fibers are in the form of pellets  30  of cotton fibers arranged in parallel stacks  32  within a portion of housing  12  that is in communication with ports  14 A/ 14 B and piston  16 . Each of pellets  30  could be formed from conventional cotton balls which, when pressed under a high load (e.g., 80,000 pounds per square inch), take a set form. Each of pellets  30  is inert in air. However, when pellets  30  are submerged in water, they expand. 
     When water sensor  10  is submerged in water, ports  14 A/ 14 B admit water into housing  12 . As they absorb the water, pellets  30  expand in a direction perpendicular to piston  16  as illustrated in FIG.  2 . The pellet expansion exerts a force on piston  16  causing it to move towards piston  18  thereby pressurizing hydraulic fluid  20 . Once the pressure in hydraulic fluid  20  is sufficient to overcome the spring force of spring  26 , piston  18  is driven along second cylindrical portion  12 B to force rod  22  further out of housing  12 . The extension of rod  22  from housing  12  is thus indicative of the fact that water has entered housing  12 . 
     In order to constrain the expansion force of pellets  30  perpendicular to the face of piston  16 , an open framework supports pellets  30  in housing  12  both before and after their expansion. While a variety of framework designs are possible, one is shown in FIG.  3  and is referenced generally by numeral  40 . Note that for clarity of illustration, framework  40  is not shown in FIGS. 1 and 2. Framework  40  includes an exterior cage  42  configured to allow water to pass therethrough. Cage  40  can be, but need not be, attached to or integral with piston  16 . Within cage  42  are a plurality of support disks  44  having holes  46  passing therethrough for receiving the cotton pellets, i.e., pellets  30  shown in FIGS. 1 and 2. These cotton pellets slide within holes  46  of disks  44  as they expand. 
     The second embodiment of the present invention water sensor is illustrated prior to its submergence in water in FIG.  4  and after its submergence in water in FIGS. 5 and 6, and is referenced by numeral  100 . Similar to water sensor  10 , water sensor  100  has a generally rigid housing  112  with a single port  114  formed therein. Port  114  could also be realized by a plurality of small ports contained within a single area such as that defined by port  114 . In this embodiment, the interior portion of housing  112  can define a constant diameter cylinder. Mounted within housing  112  is a drive piston  116  configured for sealed but sliding movement within housing  112 . A rod  122  is attached to or integral with piston  116 . Rod  122  extends from piston  116  through one end  112 C of housing  112 . Rod  122  is capped with a head portion  124  that prevents rod  122  from falling into housing  112 . To fix the position of piston  116 /rod  122 , a spring could be used as in water sensor  10 . However, other positioners can be used. One such alternative positioner is illustrated in FIG. 4 where the pre-submergence position is maintained by a shear pin coupling one of piston  116  and rod  122  to housing  112 . While a variety of shear pin placements are possible, one is shown by way of example in the figures. More specifically, a shear pin  126  passes through housing  112  and into piston  116 . 
     For water sensor  100 , movement of piston  116  (to drive rod  122  further from housing  112 ) is brought about by gas expansion acting on piston  116 . In order to prevent such gas expansion from occurring in air while assuring the occurrence of such gas expansion in water, a material that is inert in air but reacts with water to produce gas is used in water sensor  100 . The material used is placed in housing  112  such that is in communication with both port  114  and piston  116 . In the illustrated embodiment, the material is in the form of tablets  130 . The material could also be in the form of pellets, powder, etc. The composition of tablets  130  can be any one of a variety of material compositions that is inert in air but reactive with water to produce gas. Some inexpensive and readily available compositions include pure sodium, calcium carbide and common antacid tablets that are made primarily from citric acid and sodium bicarbonate. 
     A thin plate or diaphragm  132  is positioned loosely in housing  112  between port  114  and tablets  130 . Plate  132  is sized to be larger than port  114 . To facilitate the placement of tablets  130  and plate  132 , end  112 D of housing  112  could be removable. Plate  132  will be used to seal off port  114  as will now be explained. 
     In operation, water sensor  100  is submerged in water such that water can flow into port  114 . The loose placement of plate  132  allows water to flow into housing  112  through port  114 . As tablets  130  begin to react with water to produce a gas  134 , the gas pressure is initially sufficient to press the loosely disposed plate  132  up against end  112 D to seal off port  114  as illustrated in FIG.  5 . Sealing of port  114  constrains gas expansion within housing  112 . Accordingly, as the reaction between the water and tablets  130  continues, gas expansion exerts pressure on piston  116  until it is sufficient to break shear pin  126  into pieces  126 A and  126 B as illustrated in FIG.  6 . When this occurs, piston  116  moves to drive rod  122  further out of housing  112 . 
     In FIG. 7, a plan view is shown of one embodiment of plate  132  fitted over port  114 . Notches  136  can be formed about the periphery of plate  132  and sized so that small amounts of water/gas can pass into/out of port  114  even when plate  132  is over port  114 . 
     The advantages of the present invention are numerous. Each embodiment of the water sensor will positively sense water regardless of the depth thereof. Each cannot be inadvertently activated in air thereby making the design of the present invention a good candidate for use in an underwater explosive device&#39;s safety system. Each is of simple mechanical construction and requires no energy of activation other than that made readily available when the sensor is submerged in water. 
     Although the invention has been described relative to specific embodiments thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. For example, the housing could be constructed as a unitary body or in sections that are assembled. The hydraulic link used in the expanding fiber embodiment could also be in the expanding gas embodiment. Positioning devices other than a spring (i.e., spring  26  in FIG. 1) and a shear pin (i.e., shear pin  126  in FIG. 4) could be used to set the pre-submergence position of the indicator rod. An indicating means other than a rod could be used. For example, an indicator could be movably mounted in the housing such that movement of the drive piston changed the position of the indicator to indicate that submergence of the sensor had occurred. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.