Abstract:
An arrangement ( 10 ) for controlling the liquid level in a basin ( 60 ) by selectively actuating a pump ( 64 ) has an environmentally-sealed enclosure ( 22 ) with an internal volume subject to thermal and pressure effects. A first ( 14 ) and a second ( 16 ) pressure sensing means are positioned in the enclosure and a barrier fluid ( 18 ) fills a balance of the internal volume. The first sensing means ( 14 ) is in a first chamber ( 22   a ) of the enclosure ( 22 ) and the second sensing means ( 16 ) is in a second chamber ( 22   b ). The barrier fluid ( 18 ) has a first portion ( 18   a ) in the first chamber ( 22   a ) and a second portion in the second chamber ( 22   b ). The first pressure sensing means ( 14 ) communicates an “on/off” signal to the pump ( 64 ). Diaphragms ( 26, 40 ) in the respective chambers ( 22   a   , 22   b ) allow the device to adjust to changes in atmospheric pressures. The barrier fluid ( 18 ) is electrically non-conductive, chemically non-reactive with any materials comprising the pressure sensing means and, preferably, an oil.

Description:
The present invention relates to a level control system for a pump, particularly a pump used in a sewage basin to signal when a pump should be turned on or off. Because the present invention provides a level control system which is enclosed, it provides significant advantages over the known prior art, as the critical level sensing components are not exposed to the contents of the basin. 
     BACKGROUND OF THE ART 
     In many sewage basin applications, pressure switches of conventional design are used to provide an on/off switching and an alarm signal when an alarm level is exceeded even while the pump is attempting to lower the level. Many of these designs utilize an “air bell” to isolate the switch port from the sewage environment. This “air bell,” which acts like an inverted glass under water, uses the compressibility of air in the bell to transmit level changes in the liquid at the opening of the bell. However, when small changes in liquid level, on the order of a few inches of water, need to be detected, there must be a vent to atmosphere to compensate for the atmospheric pressure changes, so that weather and elevation do not cause the set points to shift. This atmospheric venting offers an opportunity for moisture to get to the switch or its components. 
     A further problem with the air bell is that changes in the amount of gas in the air bell can also shift actuation points. One reason for air loss in the air bell is leaky fittings. Another reason is oxygen consumption due to decomposition of sewage materials in the basin, which can be especially troublesome when the sewage basin is only used for portions of the year, as with a summer cabin. A yet third reason could be the entry of methane or other decomposition gases into the air bell at the liquid-gas interface. 
     In the known prior art, these problems with air bells have been cured by lifting the level control system completely out of the sewage liquids and resetting it into place, recapturing the air in the bell. This solution has many problems, including the undesirability to have to open the sewage basin and to move the level control system. 
     It is therefore an object of the present invention to provide an environmentally sealed pressure level switch control in which at least the on/off switch is provided such that it does not use an air bell that must be vented to atmosphere. 
     SUMMARY OF THE INVENTION 
     This and other advantages of the present invention are achieved by a device for controlling the liquid level in a basin by selectively actuating a pump. The device has an environmentally-sealed enclosure, with first and second pressure sensing means positioned in the enclosure; and a barrier fluid. The enclosure has an internal volume filled with the barrier fluid. In the embodiment taught, the enclosure comprises separate first and second chambers. 
     In some embodiments, the first pressure sensing means is a differential pressure switch positioned in the first chamber and the second pressure sensing means is a differential pressure switch positioned in the second chamber. In such embodiments, a first portion of the barrier fluid is contained in the first chamber and a separate second portion of the barrier fluid is contained in the second chamber. The first differential pressure switch is in electrical communication with the pump and provides an “on/off” signal therefor. The first differential pressure switch has a low pressure side and a high pressure side, with the low pressure side thereof exposed to the first portion of the barrier fluid and the high pressure side thereof exposed to the second portion of the barrier fluid. In such an embodiment, the high pressure side of the first differential pressure switch is mounted into a portion of the first chamber that connects the first and second chambers and that isolates the respective first and second portions of barrier fluid. 
     In some embodiments, a diaphragm in a wall of the second chamber is reactive to atmospheric pressure changes external to the diaphragm, so that the pressure in the second portion of the barrier fluid at the diaphragm varies according to variations in the atmospheric pressure; and a diaphragm in a wall of the first chamber is reactive to pressure changes external to the diaphragm, so that the pressure in the first portion of the barrier fluid at the diaphragm varies according to variations in atmospheric pressure and in a liquid head exerted at the external side of the diaphragm. 
     In some embodiments, the second differential pressure switch is in electrical communication with an alarm and provides a “high level” alarm signal therefor. In these embodiments, the second differential pressure switch has a low pressure side and a high pressure side, the low pressure side thereof being exposed to the second portion of the barrier fluid and the high pressure side thereof being exposed to the pressure in the basin external to the second chamber. 
     The barrier fluid is electrically non-conductive and chemically non-reactive with any materials comprising the differential pressure switches. It is preferred to be an oil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be better understood when reference is made to the accompanying figures, wherein identical parts are identified with identical reference numerals and wherein: 
         FIG. 1  shows a perspective view of a pump switch level control system having the features of the present invention; 
         FIG. 2  shows a side section view of the invention; 
         FIG. 3  shows a side section view of the invention in a conventional use environment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIGS. 1  though  3  show the present invention device  10 , as will be explained in more detail.  FIG. 1  shows the device  10  in isolation;  FIG. 2  shows a side sectional view of the device and  FIG. 3  shows the device  10  in a conventional use environment. 
     The device  10  comprises an environmentally-sealed enclosure  12 , with a first and a second pressure sensing means  14 ,  16  positioned in the enclosure. The balance of the internal volume of the enclosure is filled with a barrier fluid  18 , although the barrier fluid will comprise first and second portions  18   a ,  18   b . As will be seen, these portions  18   a ,  18   b  are isolated from each other in operation. The barrier fluid  18  selected will be a non-conductive fluid, typically an oil, so that the barrier fluid does not adversely affect operation of the differential pressure switches. The barrier fluid should be essentially incompressible at pressures around ambient and that it be generally non-reactive with any components of the differential pressure switches  14 ,  16 . 
     In the particular embodiment of the device  10  shown in  FIG. 1 , the enclosure  12  is divided into two separate chambers  20 ,  22 . The first or lower chamber  20  contains the first pressure sensing means, which in this case is shown as a differential pressure switch  14 , which serves as an “on/off” switch for the pump. In the specific case shown, the lower chamber  20  is provided with two first pressure sensing means, in the form of two differential pressure switches  14 , so that there is a system redundancy, but the system may operate with only one first pressure sensing means, if desired. The second or upper chamber  22  contains the second differential pressure switch  16 , which serves a high-level alarm function for the pump. Although a variety of differential pressure switches are manufactured by a variety of manufacturers, a typical switch suitable for this application is a diaphragm switch from Barksdale, Inc. 
     The operation of the first pressure sensing means  14  will be understood by examining its position in the first chamber  20 . Generally, the outer wall  24  of the first chamber  20  will be sufficiently thick and rigid that it will not flex as a result of pressure changes that are due to either atmospheric pressure changes or pressure changes due to the head of water in the basin in which the chamber is positioned. However, a portion of the wall  24  is a diaphragm  26 , which is intended to be reactive to pressure changes, particularly pressure changes due to the water head in the basin. In the particular embodiment shown, the diaphragm  26  is shown as being presented on a bottom surface  28  of the first chamber  20 , which also serves as the bottom surface of the enclosure. This bottom surface  28  does not rest directly upon the bottom of the basin, so the diaphragm  26  is exposed at all times to the local pressure of the liquid in the basin at that level. That local pressure will be a function of both the head of the liquid above the diaphragm  26  and the atmospheric pressure above the liquid head. 
     In the embodiment illustrated, the first differential pressure switch  14  is a conventional switch with a low pressure side  30  and a high pressure side  32 . The low pressure side  30  will be exposed to the portion  18   a  of the barrier fluid that is contained in the first chamber  20 . The high pressure side  32  of the first differential pressure switch  14  will be exposed to the second portion  18   b  of the barrier fluid, and particularly, the head that it exerts. In addition to that head, the pressure of the second portion  18   b  of barrier fluid on the first differential pressure switch  14  will vary with atmospheric pressure variations because of a diaphragm in the second chamber  22 , as explained below. To expose the high pressure side  32  of the first differential pressure switch  14  to the second barrier fluid portion  18   b,  the switch is mounted into a top portion  34  of the first chamber. This top portion serves the purposes of isolating the first and second portions  18   a ,  18   b  of barrier fluid from each other while simultaneously isolating both portions from the sewage materials in the basin. The top portion  34  also serves to connect the first or lower chamber  20  or enclosure  12  with the second or upper chamber  22 . In setting the switch mechanism (not shown) of the first differential pressure switch  14 , appropriate levels will be determined in the basin such that the pump will be turned on when the liquid level in the basin reaches or exceeds a certain level L 1  and the pump will continue to operate until the level is reduced to a certain level L 2 , at which point the switch  14  will turn off the pump. Clearly, this switching function requires a signal communication between the switch  14  and the pump. While this signal communication is not shown explicitly in  FIG. 1 , the person of ordinary skill will know how to provide this communication, typically through a wire connecting the switch  14  to a quick-connect cord entry  36  provided at a top end of the second chamber  22 . In the particular embodiment of the invention that is anticipated, the switch  14  will be a normally-closed switch that is tripped by the higher pressure of the head of barrier fluid  18   b  until the pressure of barrier fluid  18   a  increases due to an increasing liquid level in the basin and counteracts the head of barrier fluid  18   b.    
     Attention is now directed to the second pressure sensing means, which in the embodiment illustrated is a differential pressure switch  16 , located in the second chamber  22 . Note that this second chamber really comprises an upper portion  22   a  and a lower portion  22   b , which are in liquid communication so that barrier fluid  18   b  moves freely between them. The second chamber  22 , and particularly upper portion  22   a,  will have an outer wall  38  that is sufficiently thick and rigid that it will not flex as a result of pressure changes that are due to atmospheric pressure changes. However, a portion of the wall  38  is a diaphragm  40 , which is intended to be reactive to pressure changes, particularly atmospheric pressure changes. In the particular embodiment shown, the diaphragm  40  is shown as being presented on a side surface  42  of the second chamber  22 , particularly at a point well above the highest liquid level anticipated to be encountered. The reaction of the diaphragm  40  is directly transmitted to the second portion  18   b  of barrier fluid in the second chamber  22 . For that reason, the pressure acting on the high pressure side of first switch  14  will vary with changes in the atmospheric pressure. It should also be understood that the diaphragm  26  in the first chamber  20  will also be reactive to atmospheric pressure changes, since the total pressure bearing upon the diaphragm  26  will be the sum of the atmospheric pressure and the head pressure due to liquid in the basin. 
     In the embodiment disclosed, the second differential pressure switch  16  will be a conventional switch with a low pressure side  44  and a high pressure side  46 , and will typically be identical to the first differential switch  14  used in the first chamber  20 . The low pressure side  44  will be exposed to the portion  18   b  of the barrier fluid that is contained in the second chamber  22 . The high pressure side  46  of the second differential pressure switch  16  will be exposed to an alarm air bell  48  constructed to expose the high pressure side to the pressure internal to the basin. The air bell  48  will generally be of conventional construction and should be effective, since the normal operational levels of the basin liquid level will be far below the bottom  50  of the air bell, allowing the air bell to be continuously recharged. In setting the switch mechanism (not shown) of the second differential pressure switch  16 , appropriate levels will be determined in the basin such an alarm is triggered if the liquid level in the basin reaches or exceeds a certain level L A . Clearly, this switching function requires a signal communication between the switch  16  and the pump. While this signal communication is not shown explicitly, the person of ordinary skill will know how to provide this communication, typically through a wire connecting the switch  16  to the quick-connect cord entry  36  provided at a top end of the second chamber  22 . A solid-state relay  52  may be provided in some cases where it is necessary to condition the output signal of one or more of the switches  14 ,  16 , and use of such a relay would be within the knowledge of one of ordinary skill. 
     The lower portion  22   b  of the second chamber is provided to provide an appropriate head of the barrier fluid  18   b , while also allowing the diaphragm  26  in the first chamber to be positioned sufficiently low in the basin to assure proper operation by keeping it below the liquid level in the basin. Lower portion  22   b  needs to maintain fluid communication for barrier fluid  18   b  throughout the second chamber  22 , so that variations in atmospheric pressure detected at diaphragm  40  are transmitted through barrier fluid  18   b  to the high pressure side  32  of first pressure switch  14 . In the embodiment shown, this lower portion  22   b  is essentially a cylindrical conduit with a rigid wall. 
     The ability of the device  10  of the present invention to react appropriately to liquid level and atmospheric pressure changes will be determined by a few factors, and the exact design will be understood to one of ordinary skill once the overall concept of the device is understood. First, the area of the two diaphragms  26 ,  40  and the flexibility thereof (inwardly or outwardly) will define a reactive volume, which must be considered relative to the volumes of the incompressible barrier fluid in the two chambers. It is important to keep the reactive volume of diaphragms as high as possible when compared to the volumes of the incompressible barrier fluid to assure good operation. For this reason, the actual volumes of the two portions of the barrier fluid should be maintained as low as possible. This may be achieved in several different ways. One way is to minimize the amount of volume inside the device which is subject to thermal or pressure effects. 
     Because both the first and second chambers have a diaphragm associated therewith, the barrier fluid may be sealed in place at a place of manufacture and the completed device, in this sealed condition, may be used at various altitude and pressure conditions without any adjustment being required. 
     In  FIG. 3 , the device  10  is shown in the environment of a basin  60  having a removable basin cap  62  and containing a pump  54 , typically a grinder pump. The switch set levels L 1 , L 2  and L A  are also shown.