Patent Publication Number: US-2009224929-A1

Title: Liquid level sensing systems

Description:
FIELD OF THE INVENTION 
     This invention relates to level sensing systems, and has for its object the provision of an improved form of level sensing system that can be used for sensing the level of a liquid, paste or gel in a container. The term “liquid” as used herein is accordingly to be interpreted as including pastes and gels. 
     In the composites industry, the use of meter mix machines is quite common and there is an increasing requirement for a simple level sensor for providing an indication in the levels of liquid in the catalyst tanks and solvent cleaner tanks of such machines. There is a specific need for providing a warning signal when the liquid levels in such tanks fall to a predetermined low level. 
     The liquids may be volatile and inflammable and it is a more specific object of the present invention to provide a liquid level sensing system that does not include any electrically operated components. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided a level sensing system for sensing the level of liquid in a container, the system including a magnetic member arranged for movement relative to a pneumatic switch operated by the presence or absence of a magnetic field, spring means acting on the magnetic member and urging it towards a first operative position, and a weight suspended from the magnetic member, the weight being positioned, in use, within the liquid in the container such that, when the level of liquid in the container falls to a predetermined level, the magnetic member is caused to move from its first operative position into a second operative position. 
     The weight is preferably connected to the magnetic member by a flexible pipe or tube. 
     The second operative position of the magnetic member is lower than the first operative position thereof and the arrangement is preferably such that an alarm will be operated pneumatically when the magnetic member moves into its second operative position. 
     The spring means is preferably a compression spring and the magnetic member and the compression spring are preferably contained in a housing. 
     An extension rod is preferably provided extending upwardly relative to the magnetic member, the extension rod projecting upwardly from the housing so that it can be depressed manually to move the magnetic member into its second operative position. 
     According to a second aspect of the present invention there is provided a method of sensing the level of a liquid in a container which includes the use of the sensing system defined above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a device for sensing the level of liquid in a container and for generating a signal when the level of the liquid in the container falls to a predetermined level, and 
         FIG. 2  shows the device of  FIG. 1  when a signal is generated. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The device shown in the drawings includes a housing  10  in the form of a machined aluminum block with a vertical hole in which a polished stainless steel rod  11  is mounted. A small ring magnet  12  is fixed to the top of the stainless steel rod  11  and the ring magnet  12  is acted on by one end of a lightweight compression spring  13 , the other end of which rests upon a p.t.f.e. bush  14  contained within the vertical hole in the aluminum block. 
     The rod  11  is externally threaded at its lower end and the upper end of a 4 mm. nylon pipe  15  is connected to the lower end of the rod  11 . The pipe  15  is cut to a suitable length and a solid machined weight  16  is attached to the lower end of the pipe  15  by means of a push-fit fitting screwed into the upper surface of the weight  16 . The weight  16  may be machined from nylon or from some other suitable plastics material. It is solid, as opposed to hollow, and thus not liable to leak. 
     A pushbutton  17  is positioned on top of the magnet  12  and projects from the housing  10  so that, if desired, the pushbutton  17  can be depressed manually to effect downward movement of the magnet  12  for test purposes, as described below. 
     The combined weight of the rod  11 , the pushbutton  17 , the ring magnet  12 , the pipe  15  and the weight  16  is sufficient to compress the compression spring  13  when the rod  11  is positioned vertically, as shown. 
     A micro pneumatic switch  18  is mounted in the housing  10  and is responsive to the presence or absence of a magnetic field. It is so positioned that it is operated when the ring magnet  12  moves from the position shown in  FIG. 1  into the position shown in  FIG. 2 . 
     The housing  10  can be mounted on top of a liquid container  19  by drilling a hole in the top of the container  19  through which the end of the rod  11  and the pipe  15  can be inserted. A fitting  20  is then screwed from the inside of the container  19  into a threaded bore in the base of the block  10  and the fitting is tightened to clamp the housing  10  securely to the top of the container  19 . The weight  16  is positioned within the container  19  and the lower end of the pipe  15  is connected to the weight  16 . The length of the flexible pipe  15 , which is typically of nylon, is adjusted if necessary so that the weight  16  can be suspended within the container  19  so that it can swing within the container  19  at a predetermined height clear of the base of the container. 
     With no liquid in the container  19 , the weight  16  will be in the position shown in  FIG. 2  so that, when an air supply is connected to the switch  18 , an alarm will operate as the ring magnet  12  is in the lower, alarm-switching position and the compression spring  13  is fully compressed. As the container  19  is filled with liquid, the weight  16  effectively displaces a volume of the liquid and this displacement reduces the load applied by the weight  16  to the lower end of the pipe  15 . This reduction in applied load allows the compression spring  13  to uncompress and lift the weight  16 , the pipe  15 , the rod  11  and the ring magnet  12 . Movement of the ring magnet  12  from the position shown in  FIG. 2  into the position shown in  FIG. 1  will release the pneumatic switch  18  and the alarm will be turned off. 
     With the components of the device in the positions shown in  FIG. 1 , the alarm is effectively primed waiting for the liquid level to drop and cause the switch  18  to be operated again. 
     The pushbutton  17  projects from the top of the housing  10  so that, in the quiescent or non-alarmed condition, the user can simply press down on the pushbutton  17  to effect downward movement of the magnet  12  to operate the alarm so as to be assured that the system is live. When the user releases the pushbutton  17 , it will move upwardly under the action of the compression spring  13  and the alarm will be turned off. 
     The space within the housing  10  permits the provision of an air whistle that is connected internally to the output of the switch  18  and thus provides a built-in audible alarm. The air supply to the switch  18  is connected to the associated machine&#39;s air supply so that the switch  18  becomes live whenever the machine is switched on. The switch exhaust may be connected to the internal bore of the vertical hole within which the parts  11  to  14  are mounted to purge the interior of the housing  10  of any dust particles and thus increase the reliability of the device. The output of the switch  18  has sufficient volume to operate a pressure switch or other device in addition to the built-in whistle to provide additional low level signally. 
     The rod  11  slides relative to the p.t.f.e. bush  14  so that frictional losses are minimized. The device is so designed as to minimize the likelihood of damage as a result of liquid spills or dust and all parts that might be exposed to fumes are chemically resistant and pose no potential spark or other explosion risk. 
     The shape and size of the displacement weight  16  will depend on the size and/or shape of the container  19  and on the specific gravity of the liquid within the container  19 .