Abstract:
A multi-tank indirect liquid level measurement device and method are disclosed. The device and method may be implemented using a sealable reservoir that holds an indicating liquid and pressurized air, a pump that supplies and/or pressurizes the air in the reservoir, a plurality of pipes each of which connect on one end to the tanks to be gaged and on their other ends to a valve for selectively and exclusively connecting to the pressurized air, and a manometer tube that displays the pressure difference between the sealable reservoir and atmospheric pressure as a height reading of the indicating liquid.

Description:
BACKGROUND 
     This disclosure relates to the measurement of liquid levels in recreational vehicle wastewater holding tanks. 
     Many recreational vehicles, such as campers, trailers, fifth wheelers, and motor homes, have one or more tanks for storing the effluent or wastewater originating in the toilet, sink, or shower. These tanks are typically called black water or gray water tanks. The effluent stored in black water and gray water tanks can easily clog or render inoperable a liquid level measurement apparatus or sensors in direct contact with the wastewater. Examples of typical direct wastewater measurement devices are ones that use conductance, capacitance, floats, or other direct means for measuring the liquid in a tank. Despite the numerous cleaning methods and chemicals that have been developed, many of the existing wastewater level measuring methods and systems can fail within several weeks, resulting in the owner of a recreational vehicle draining the wastewater tank or tanks too frequently or running the risk of a tank overflow. 
     A typical modern recreational vehicle has a plurality of wastewater holding tanks. There are normally separate tanks for black water (human waste from the toilet) and gray water (waste water from the kitchen sink). There may be a second gray water tank for effluent from a shower. 
     Indirect liquid level measurement systems exist. One example is U.S. Pat. No. 7,389,688 by John Vander Horst. These devices work, but it was desired to make a simpler purely mechanical device that requires no transducers and is capable of measuring the liquid level in multiple tanks using a single gage. 
     SUMMARY 
     In one embodiment, the present disclosure provides a multi-tank indirect liquid-level measurement device and method that is implemented using a sealable reservoir that holds an indicating liquid and pressurized air, a pump that supplies and/or pressurizes the air in the reservoir, a plurality of pipes each of which connect on one end to the tanks to be gaged and on their other ends to a valve for selectively and exclusively connecting to the pressurized air, and a manometer tube that displays the pressure difference between the sealable reservoir and atmospheric pressure as a height reading of the indicating liquid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is described in conjunction with the appended figures in which: 
         FIG. 1  shows the principle of operation and the prior art; 
         FIG. 2  shows an indirect liquid level-measuring device connected to a plurality of wastewater tanks; 
         FIG. 3  shows an embodiment of a transducerless multi-tank indirect liquid level-measuring device; 
         FIG. 4  is an exploded view of the device of  FIG. 3 ; 
         FIG. 5   a  is a top view of the connection disk of  FIG. 3  and  FIG. 4 ; 
         FIG. 5   b  is a side view of the connection disk of  FIG. 3  and  FIG. 4 ; 
         FIG. 5   c  is a bottom view of the connection disk of  FIG. 3  and  FIG. 4 ; 
         FIG. 6   a  is a side view of the selection disk of  FIG. 3  and  FIG. 4 ; 
         FIG. 6   b  is a bottom view of the selection disk of  FIG. 3  and  FIG. 4 ; 
         FIG. 7   a  is a front view of the device of  FIG. 3  mounted to a wall; 
         FIG. 7   b  is a side view of the device of  FIG. 3  mounted to a wall; 
         FIG. 7   c  is a bottom view of the device of  FIG. 3  mounted to a wall; 
         FIG. 8  shows section A-A of  FIG. 7   a ; and 
         FIG. 9  shows section B-B of  FIG. 7   a.    
     
    
    
     To assist in the understanding of one embodiment of the present invention, the following list of components or features and associated numbering found in the drawings is provided herein: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Number 
                 Component or Feature 
               
               
                   
                   
               
             
             
               
                   
                 F 1   
                 First fitting 
               
               
                   
                 F 2   
                 Second fitting 
               
               
                   
                 F 3   
                 Third fitting 
               
               
                   
                 h 1   
                 First height 
               
               
                   
                 h 2   
                 Second height 
               
               
                   
                 h 3   
                 Third height 
               
               
                   
                 P 1   
                 First pipe 
               
               
                   
                 P 2   
                 Second pipe 
               
               
                   
                 P 3   
                 Third pipe 
               
               
                   
                 T 1   
                 First tank 
               
               
                   
                 T 2   
                 Second tank 
               
               
                   
                 T 3   
                 Third tank 
               
               
                   
                  99 
                 Sealable vessel 
               
               
                   
                 100 
                 Liquid level measuring system with three tanks 
               
               
                   
                 101 
                 Cylindrical tube 
               
               
                   
                 102 
                 Connection disk 
               
               
                   
                 103 
                 Selection disk 
               
               
                   
                 104 
                 Indicator liquid 
               
               
                   
                 105 
                 End cap 
               
               
                   
                 106 
                 Pressurized air 
               
               
                   
                 107 
                 Pressurization source 
               
               
                   
                 108 
                 Pressurization tube 
               
               
                   
                 109 
                 Manometer tube 
               
               
                   
                 110 
                 Pressure relief hole 
               
               
                   
                 200 
                 Multi-tank indirect liquid level measuring device 
               
               
                   
                 205 
                 End cap pump housing 
               
               
                   
                 207 
                 Pump button 
               
               
                   
                 211 
                 Assembly screw 
               
               
                   
                 212 
                 Cylindrical tube slot 
               
               
                   
                 213 
                 Selection disk rotation pin 
               
               
                   
                 214 
                 Pump button o-ring 
               
               
                   
                 215 
                 Pump spring 
               
               
                   
                 216 
                 Spring support 
               
               
                   
                 217 
                 Selection disk o-ring 
               
               
                   
                 218 
                 Connection tube o-ring 
               
               
                   
                 219 
                 Stop pin 
               
               
                   
                 220 
                 Slot 
               
               
                   
                 221 
                 Pump button o-ring groove 
               
               
                   
                 222 
                 Selection disk o-ring groove 
               
               
                   
                 223 
                 Connection tube o-ring housing 
               
               
                   
                 224 
                 Selection disk rotation pin hole 
               
               
                   
                 225 
                 Assembly spring 
               
               
                   
                 226 
                 Pump button o-ring stop 
               
               
                   
                 301 
                 Recreational vehicle wall 
               
               
                   
                 302 
                 Mounting sleeve 
               
               
                   
                 303 
                 Mounting bracket 
               
               
                   
                 304 
                 Mounting screw 
               
               
                   
                 305 
                 Slit in mounting sleeve 
               
               
                   
                 401 
                 Connection disk fitting hole 
               
               
                   
                 402 
                 Connection disk assembly screw hole 
               
               
                   
                 403 
                 Connection disk stop pin hole 
               
               
                   
                 404 
                 Anti-rotation pin hole 
               
               
                   
                 502 
                 Selection disk assembly screw hole 
               
               
                   
                   
               
             
          
         
       
     
     It should be understood that the drawings are not necessarily to scale. In certain instances, details that are not necessary for an understanding of the invention or that render other details difficult to perceive may have been omitted. It should be understood that the invention is not necessarily limited to the particular embodiments illustrated herein. 
     DETAILED DESCRIPTION 
     The ensuing description provides preferred exemplary embodiment(s) only, and is not intended to limit the scope, applicability or configuration of the disclosure. Rather, the ensuing description of the preferred exemplary embodiment(s) will provide those skilled in the art with an enabling description for implementing a preferred exemplary embodiment. It should be understood that various changes could be made in the function and arrangement of elements without departing from the spirit and scope as set forth in the appended claims. 
     Specific details are given in the following description to provide a thorough understanding of the embodiments. However, it will be understood by one of ordinary skill in the art that the embodiments may be practiced without these specific details. For example, circuits may be shown in block diagrams in order not to obscure the embodiments in unnecessary detail. In other instances, well-known circuits, processes, algorithms, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments. 
     In one embodiment, the present disclosure provides a device suitable for use with one or more wastewater holding tanks of a recreational vehicle. Wastewater holding tanks are typically located downstream of a toilet, sink, shower, or any other place where water is used to clean something or where water is combined with other fluids or solids. In addition to recreational vehicles, vehicle wastewater holding tanks can be used in other transportable or moving applications such as boats, trains, buses, aircraft, or portable lavatories. 
     This disclosure discusses transducers and the fact that embodiments of the present invention can be made without using a transducer (i.e. transducerless). For purposes of this disclosure, a transducer is defined as a device that converts one form of energy to another, where energy types include electrical, mechanical, electromagnetic (including light), chemical, acoustic, or thermal energy. Therefore, an example of a transducerless device would be one that does not rely on any conversion from mechanical energy to electrical, electromagnetic, chemical, acoustic, or thermal energy. 
     The present invention relies on the principle that the height of a liquid in a tank can be measured at a distance from such tank by measuring the pressure of a gas induced above the surface of an indicating liquid contained in a suitable gage reservoir, the excess gas being lead by a pipe or tube from the top of the gage reservoir to a point within and near the bottom of the distant tank, so that the pressure on the indicating liquid surface is proportional to the depth of the liquid in the distant tank. This principle is understood in the prior art and is illustrated in  FIG. 1  in which a pump  107  pressurizes the air  106  that is part of a sealable vessel  99  that serves as a gage reservoir. This pressurized air  106  has two places to go. It can push the indicator liquid  104  up a manometer tube  109  and it can push air down a pipe P 1  until excess air escapes from the distant tank T 1 . If the liquid in the tank T 1  and the indicator liquid  104  have the same density, then the height h 1  of the liquid in the manometer tube  109  as measured from the top of the indicating liquid  104  will be the same as the height h 1  of the liquid in tank T 1  above the inlet of the pipe P 1  in the tank T 1 . Note that the pressure reading does not change with the relative vertical position of the tank T 1  and sealable vessel  99 . In one embodiment, the present invention applies this principle described in  FIG. 1  to an easy-to-fabricate and easy-to-use transducerless device for measuring the level of multiple wastewater tanks in a vehicle (such as a recreational vehicle) or in a portable application (such as a portable lavatory). 
       FIG. 2  shows a system comprising an embodiment of an indirect liquid level-measuring device and a plurality of wastewater tanks is shown at  100 . Specifically shown are three tanks, labeled T 1 , T 2 , and T 3 , connected with three pipes P 1 , P 2 , and P 3 , respectively, to the measuring device, which comprises all of the other elements shown in  FIG. 2 . Note that in a typical recreational vehicle application the tanks T 1 , T 2 , and T 3 , will generally be larger, flatter, and more distant from the measuring device than the relative scale of the tanks, pipes, and device depicted in  FIG. 2 . One end of the pipes P 1 , P 2 , and P 3  are connected to either the bottom of the tanks T 1 , T 2 , and T 3 , or to the sides of these tanks as close to their bottoms as possible, in order to read as close to the full pressure head of the liquid in the tanks. The other end of the pipes P 1 , P 2 , and P 3  are connected to a connection disk, shown at  102 . The connection disk  102  is located at the base of the measuring device. The pipes P 1 , P 2 , and P 3  are typically made of tubing. This tubing may be flexible PVC, it may be some other kind of a flexible or rigid plastic, or it can be some other type of piping capable of being understood by anyone skilled in the art. The tubing may be any length or diameter. However, for performance reasons, one would like to have tubing that has a relatively thin internal diameter as this reduces the displacement required by the pump. The lengths of the pipes needed for using a gage of this type in a recreational vehicle may be a minimum of 20 feet. 
     Also referring to  FIG. 2 , in one embodiment the indirect liquid-level measuring device comprises a cylindrical tube  101 , the connection disk  102 , a selection disk  103 , an end cap  105 , the pressurization source  107 , a pressurization tube  108 , and a manometer tube  109 . The enclosed space in the cylindrical tube  101  between the selection disk  103  and the end cap  105  may be partially filled with the indicator liquid  104  and the multi-tank indirect liquid-level measuring device is oriented so that the remaining volume of the enclosed space between the selection disk  103  and the end cap  105  can serve as the body of a sealable vessel. A selection disk o-ring  217  is part of the system that helps maintain pressure in the measuring device. The indicator liquid  104  can be any liquid understood by someone skilled in the art of manometers, examples of which include mercury and water. If water is to be used, a colorant may be added to make the height of the water easer to see in the manometer tube  109 . The pressurization source  107  generates the pressurized air  106  that makes the device work. Note that the indicator liquid  104  is typically not necessarily part of the device, but will be added by the user during the installation process. 
     Further referring to  FIG. 2 , the pipes P 1 , P 2 , and P 3 , are attached to the connection disk  102  at three points on one end of the cylindrical disk with each of the attachment points being located the same distance from the central axis of the connection disk. The attachment points comprise axial holes through the connection disk that allow air to pass through the connection disk to one of the pipes P 1 , P 2 , and P 3 , when a pressurization tube  108  mounted in the selection disk  103  is rotated to align with the hole in the connection disk  102  opposite that pipe, The selection disk  103  is assembled into the cylindrical tube in a way that allows an operator to rotate the selection disk  103  inside the cylindrical tube. The details of one implementation will be explained later in this disclosure. This pressurization tube  108  that is mounted in the selection disk, extends through the indicator liquid  104  to the pressurized air  106 . 
     Additionally  FIG. 2  shows the end cap  105  that helps to seal the pressurized air  106  into an enclosed space. The end cap  105  can either be an integral part of the cylindrical tube  101  as illustrated in  FIG. 2 , or it can be a separate component that is attached to the cylindrical tube  105 . The pressurization source  107  shown in  FIG. 2 , can be a simple single-stroke hand-operated push button positive displacement pump cylinder  207  that, when depressed, pressurizes the air  106 . A user&#39;s thumb would typically press this push button. There is also a pressure relief hole, shown at  110 , which can be sealed by a user&#39;s fingers while pressing the push button. Examples of hand-operated positive displacement pumps include, but are not limited to piston pumps, pumps that use a bulb (such as a turkey baster), other kinds of squeeze pumps, bellows pumps, and diaphragm pumps. The pump also doesn&#39;t need to be limited to one that is operated by the operator&#39;s hands. Other examples of manual human-operated pumps can include pumps in which the user blows into a tube, presses a footpad, or uses an arm or leg action to crank a handle. 
       FIG. 2  also illustrates the operation of the system when the first tank T 1  has been selected and has its liquid level measured. The first pipe P 1  has been selected and pneumatically connected to the volume of air in the device through the pressurization tube  108 . The pump button  207  has been depressed from its relaxed state while a pressure relief hole  110  has been sealed by a user&#39;s finger, causing air to push through the first pipe P 1  and create air bubbles in the first tank T 1  as excess air escapes. The pressurized air  106  also forces water up the manometer tube  109  making the reading of the pressure head h 1  equal to the vertical height of the liquid in the first tank T 1  from the point at which the first pipe P 1  enters the first tank T 1 . If the indicator liquid  104  and the liquid in the first tank are both water, or are both liquids having the same densities, the vertical height of the indicator liquid in the manometer tube and the vertical height of the water in the first tank T 1  from the point of the inlet of the first pipe P 1  will both be the same as shown by the two h 1 &#39;s in  FIG. 2 . If liquids of different densities are used, there will be a ratio in the two heights that is directly proportional to the ratio of the densities of the two liquids. 
     Note that the second pipe P 2  and third pipe P 3  have liquid in them to the same height as the liquid that is in the respective tanks T 2  and T 3  because these pipes have not been connected to the pressurized air  106  by the valve that is part of the multi-tank liquid-level measuring device. The volume of liquid in these two pipes h 2  and h 3 , respectively, must be purged if the liquid level were to be measured in these tanks. To minimize the amount of liquid that must be pumped from these pipes it is desirable to minimize the inside diameter of the pipes and to run the pipes directly vertically until they are a sufficient distance above the highest liquid level that could be in any of these tanks. As a reference, the system shown in  FIG. 2  has been implemented using pipe that has an inside diameter of 3/32 inch. Other inside diameters such as 1/16 inch and ⅛ inch could also be used for a typical recreational vehicle. If a 3/32 inch inside diameter pipe is used and the length of pipe is 15 inches before it is above the tank, a displacement of 0.1 cubic inches would be required to purge the liquid from the pipe. The method of attachment and sealing of the pipes P 1 , P 2 , and P 3  to the tanks T 1 , T 2 , and T 3  and to the connection disk  102  of the measuring device can be accomplished via a mechanical fitting, through ultrasonic bonding, through plastic welding, or through some other means capable of being understood by someone skilled in the art. 
     Note that the entire indirect liquid level-measuring device shown in  FIG. 2  is transducerless, in that it uses only mechanical energy (in the form of pressure) to provide a reading of liquid levels in a plurality of recreational vehicle wastewater tanks, without converting this to an other form of energy such as electrical, electromagnetic, chemical, acoustic, or thermal energy. 
     Further referring to  FIG. 2  the operation of a push button positive displacement pump can cause the indicating liquid in the manometer tube  109  to overshoot. To prevent this, a flow damper can be placed into the manometer tube  109  close to the point at which the indicator liquid  104  enters the manometer tube  109 . Placing a ⅛ inch long plug into the manometer tube  109  at the end close to the inlet for the indicator liquid  104  has been found to work well as a flow damper. 
       FIG. 3  and  FIG. 4  show another embodiment of an indirect liquid level-measuring device at  200 . The embodiment shown in  FIG. 3  and  FIG. 4  is similar to the device shown in  FIG. 2 , but the tanks and pipes have been removed to reveal more detail of the device. In the device shown in  FIG. 3  the end cap ( 105  in  FIG. 2 ) and the housing for the pressurization source ( 107  in  FIG. 2 ) have been integrated into a single end cap housing, shown at  205 . In this embodiment, the end cap housing  205  is made from a standard 90-degree polyvinylchloride (PVC) elbow joint that can be obtained at hardware stores. In one specific embodiment, the end cap housing is a 90-degree elbow joint for standard ¾ inch PVC pipe. This means the inside diameter of the elbow joint is 1.050 inches because ¾ inch PVC pipe has an inside diameter of approximately ¾ inch and a wall thickness of approximately 0.15 inches. In the embodiment shown in  FIG. 3 , one leg of the elbow joint has been reduced in length to fit the cylindrical tube, shown at  101 . There is a hole drilled into the elbow for the manometer tube, shown at  109 . In the embodiment shown, the manometer tube is made of ¼ inch outside diameter rigid acrylic tubing with a 1/16 inch wall thickness giving an inside diameter of ⅛ inch. The manometer tube  109  has lines marked on it to show the height of the indicator liquid. In the embodiment shown, the manometer tube hole is drilled so the manometer tube  109  has its one end centered in the center of the indicator liquid, shown at  104 . The manometer tube hole and the fitting of the manometer tube into this hole must be done in a way that provides a good seal for the pressurized air  106 . This can be done using any technique capable of being understood by someone skilled in the art. One technique is to use a fitting made of pliable PVC tubing that provides a hermetic seal. There are also adhesives that can be used to provide such a hermetic seal. 
     Further referring to  FIG. 3  and  FIG. 4 , there is a pressure relief hole (or vent) drilled into the elbow, which is shown at  110 . This hole serves the same function as the equivalent hole shown in  FIG. 2 . A pump button, shown at  207  is used to pressurize the air  106 . More details of the pump button  207  and other pump elements will be described later in this disclosure. The exact configuration, materials, and manufacturing process for the end cap housing  205  can be anything capable of being understood by someone skilled in the art. 
       FIG. 3  and  FIG. 4  also show the cylindrical tube at  101 . The cylindrical tube  101  in this embodiment is made from 0.80 inch inside diameter, 1.05 inch outside diameter transparent PVC pipe that has been cut to a length of approximately 2.5 inches. The cylindrical tube  101  has a cylindrical tube slot, shown at  212 , cut into it. In the embodiment shown, the cylindrical tube slot  212  is approximately 0.10 inch wide; it is located about 0.70 inches from one end (the bottom) of the cylindrical tube; and it extends approximately 90 degrees around the circumference of the cylindrical tube  101 . The end of the cylindrical tube  101  furthest from the cylindrical tube slot  212  is attached and sealed to the end cap pump housing  205  using any technique capable of being understood by someone skilled in the art. One example of an attachment and sealing technique for the cylindrical tube  101  and end cap housing  205  is the use of an adhesive. 
     Further referring to the embodiment shown in  FIG. 3  and  FIG. 4 , the selection disk, shown at  103  and the connection disk, shown at  102  are attached to one another using an assembly screw, shown at  211 . The assembly screw  211  is axially centered in the selection disk  103  and connection disk  102 . An assembly spring  225  creates a constant force between the selection disk  103  and connection disk  102 , while still allowing the two disks to rotate relative to one another. In the embodiment shown in  FIG. 3  and  FIG. 4 , the connection disk  102  has three fittings, shown at F 1 , F 2 , and F 3  that allow pipes or tubes, shown as P 1 , P 2 , and P 3  in  FIG. 2 , from the tanks shown as T 1 , T 2 , and T 3  in  FIG. 1 , to attach to the liquid level measuring device  200 . In the embodiment shown in  FIG. 3  and  FIG. 4 , these fittings F 1 , F 2 , and F 3  are pressed into axial through holes in the connection disk  102 . By rotating the selection disk  103  relative to the connection disk  102 , the user can select which of the three fittings F 1 , F 2 , or F 3  in the connection disk  102  lines up with an axial through hole in the selection disk  103 . The outside diameters of the connection disk  102  and the selection disk  103  are designed to allow these to disks to fit snugly into the cylindrical tube  101 . There is an o-ring grove around the circumference of the selection disk  103  near the end of the selection disk  103  that is opposite the connection disk  102 . This groove has an o-ring, shown at  217 , placed in it to provide a seal between the pressurized air  106  and outside atmosphere when the selection disk and connection disk are placed inside the cylindrical tube  101 . The pressurization tube, shown at  108 , is placed into the hole in the selection disk. The pressurization tube  108  extends out the end of the selection disk  103  that is opposite the connection disk  102  and extends through the indicator liquid, shown at  104 , to the pressurized air  106 . 
       FIG. 3  and  FIG. 4  also show how the selection disk  103  can be rotated when the device  200  has been assembled. There is a hole in the selection disk  103  that lines up with the cylindrical tube slot  212  in the cylindrical tube  101 . In the embodiment shown in  FIG. 2 , a selection disk rotation pin, shown at  213 , is pressed into this hole in the selection disk  103 . The connection disk  102  is fixed so that it cannot move relative to the cylindrical housing  101 , which can be accomplished using any method capable of being understood by someone skilled in the art, such as the use of a pin that goes radially through the cylindrical housing  101  and into the connection disk  102 . The selection disk  103  will then rotate relative to the connection disk when a user rotates the selection disk rotation pin  213  relative to the cylindrical housing  101 . A rotary valve is created through the combination of the cylindrical selection disk  103  placed in a cylindrical tube  101  with a selection disk rotation pin  213  that is accessed through the cylindrical tube slot  212 . 
     Further referring to  FIG. 4 , the pump button  207  includes an o-ring groove, shown at  221 , into which a pump button o-ring, shown at  214 , can be placed. The pump button o-ring  214  helps ensure that there is a good seal for maintaining the pressurized air ( 106  in  FIG. 2  and  FIG. 3 ). There is a coil spring, shown at  215 , which becomes compressed when the pump button  207  is pressed. One end of the coil spring  215  is retained inside the pump button  207 . A spring support, shown at  216 , retains the other end of the coil spring  215 . When the device shown in  FIG. 4  is assembled, the spring support  216  rests against the manometer tube, shown at  109 . The pump button  207 , pump button o-ring  214 , coil spring  215 , and spring support  216  can be made of any materials capable of being understood by anyone skilled in the art. For example, the pump button  207  can be made of machined or molded plastic, the pump button o-ring  214  can be a commercially purchased rubber o-ring, the coil spring  215  can be a commercially purchased plastic or steel spring, and the spring support  216  can be molded or machined ring or cap capable of retaining the coil spring. 
       FIG. 4  also shows how the manometer tube, shown at  109 , and the cylindrical tube, shown at  101 , fit into the end cap pump housing  205 . The assembly spring, shown at  225 , provides a compressive force to ensure that the connection disk  102  and selection disk  103  are always pressed together. A connection tube o-ring, shown at  218 , is placed into a selection tube o-ring housing, shown at  223 , before the selection disk  103  and connection disk  102  are fastened together. A stop pin, shown at  219 , is also placed into a hole in the connection disk before the selection disk  103  and connection disk  102  are fastened together. One end of the stop pin  219  fits into a slot, shown at  220  in the selection disk to limit the amount of rotation between the connection disk  102  and selection disk  103  to the three through holes in the connection disk  102  that line up with the three fittings, shown at F 1 , F 2 , and F 3 , that are pressed into the connection disk  102 . A pressurization tube, shown at  108  is pressed into the selection disk  103 . A selection disk o-ring, shown at  217 , is placed into a selection disk o-ring groove, shown at  222 . 
     Further referring to  FIG. 4 , the completed subassembly comprising the connection disk  102 , the selection disk  103 , and various attachments can be placed into the cylindrical tube  101  once the operations described in the previous paragraph have been completed. The completed selection disk+assembly disk subassembly is pushed in far enough so that the selection disk rotation pin hole, shown at  224 , is in the cylindrical tube slot, shown at  212 , allowing the selection disk rotation pin, shown at  213 , to be placed in the selection disk rotation pin hole  224 . 
     Referring to  FIG. 3  and  FIG. 4 , the easiest way to fill the indirect liquid level measuring device with an indicator liquid, shown at  104  in  FIG. 3 , is to remove the pump button  207  and pump button o-ring  214  in  FIG. 3 , as well as the coil spring  215  in  FIG. 4 , and spring support  216  in  FIG. 4  from the end cap pump housing  205 . The user can then pour the indicator liquid  104  in  FIG. 3  directly into the vessel. 
       FIG. 5   a  is a top view of the connection disk  102 .  FIG. 5   b  is a side view of the connection disk  102 .  FIG. 5   c  is a bottom view of the connection disk  102 . These views illustrate the fitting holes, shown at  401 , which connect to the three fittings (F 1 , F 2 , and F 3  in  FIG. 4 ).  FIG. 5   a ,  FIG. 5   b , and  FIG. 5   c  also illustrate the assembly screw hole, shown at  402 , that houses the assembly screw ( 211  in  FIG. 4 ) and assembly spring ( 225  in  FIG. 4 ). Additionally,  FIG. 5   a ,  FIG. 5   b , and  FIG. 5   c  illustrate the stop pin hole, shown at  403 , which houses the opposite end of the stop pin ( 219  in  FIG. 4 ) 
       FIG. 6   a  shows a side view of the selection disk at  103 .  FIG. 6   b  shows a bottom view of the selection disk at  103 .  FIG. 6   a  and  FIG. 6   b  depict the o-ring groove at  222 .  FIG. 6   a  and  FIG. 6   b  depict the connection tube o-ring housing at  223 .  FIG. 6   a  and  FIG. 6   b  depict the selection disk rotation pin hole at  224 .  FIG. 6   a  and  FIG. 6   b  depict the assembly screw hole at  502 . 
       FIG. 7   a ,  FIG. 7   b , and  FIG. 7   c  provide three orthogonal views of a liquid level-measuring device mounted on the wall of a recreational vehicle. In these three views, the wall of the recreational vehicle is shown at  301 . The device is mounted using a mounting sleeve, shown at  302 . The mounting sleeve  301  surrounds the device near the bottom of the device. This sleeve has a slot, shown at  305 , which allows the sleeve  302  to fit snugly around the device regardless of tolerances of the device and the sleeve  302 . The sleeve is surrounded by a mounting bracket, shown at  303 , which is attached to the wall, shown at  301 , using two mounting screws, shown at  304 . 
       FIG. 7   b  also shows how an operator&#39;s index finger might be used to seal the pressure relief hole  110 . Typically, the thumb of the operator&#39;s other hand (not shown) would simultaneously the press the pump button  207  to make a liquid level reading using the embodiment of the device shown. 
       FIG. 8  is section A-A of  FIG. 7   a  that shows the wall of the recreational vehicle at  301 . The mounting sleeve is shown at  302 . There is a groove, shown at  306 , in the mounting sleeve  302 . This groove  306  facilitates the placement of an anti-rotation pin, shown at  307 , that goes through the wall of the cylindrical tube, shown at  101 , to prevent the connection disk, shown at  102 , from rotating relative to the cylindrical tube  101 . The mounting bracket, shown at  303 , goes over the mounting sleeve  302 . One end of the stop pin, shown at  219 , is pressed into the connection disk  102 . The selection disk rotation pin, shown at  213 , is pressed into the selection disk  203  and is accessible to the user through a slot in the cylindrical tube  101 . 
     Further referring to  FIG. 8 , the assembly screw, shown at  211 , compresses the assembly spring, shown at  225 , to provide a pressure that keeps the connection disk  102  pressed against the selection disk  103 . This ensures that the connection tube o-ring, shown at  218 , provides a proper seal. This seal is needed to ensure that the pressurized air delivered by the fitting, shown at F 2 , is transmitted to the pressurization tube, shown at  108  without leakage. 
       FIG. 9  is section B-B of  FIG. 7   a . This shows the end cap pump housing  205  and pressure relief hole  110 . It also shows how the spring cap  216  rests against the manometer tube  109 . It further shows the pump spring  215  being compressed between the spring cap  216  and pump button  207 . The pump button o-ring  214  rides in the pump button o-ring groove  221 . There is also a small ring machined out of the end cap pump housing  205  that helps serve as a pump button o-ring stop shown at  226 . Also shown at  101  is a partial section of the cylindrical tube. For reference purposes, one embodiment of the design shown in  FIG. 9  has an end cap pump housing inside diameter of approximately 1.05 inches and a pump button stroke of about 0.5 inches giving a displacement in excess of 0.4 cubic inches in a single stroke. The following is a list of minimum pump displacements (Minimum Displacement) needed assuming various choices of pipe inside diameters (Pipe IDs for pipes shown P 1 , P 2 , and P 3  in  FIG. 2 ), manometer tube ( 209  in  FIG. 2 ) inside diameters (Tube IDs), and tank heights (h 1 , h 2 , and h 3  in  FIG. 2 , heights in list below) that are suitable for various typical recreational vehicles: 
     
       
         
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Pipe ID 
                 Tube ID 
                 Height 
                 Minimum Displacement 
               
               
                   
                   
               
             
             
               
                   
                  1/16″ 
                  3/32″ 
                  5 inches 
                 0.05 cubic inches 
               
               
                   
                  1/16″ 
                  3/32″ 
                 10 inches 
                 0.10 cubic inches 
               
               
                   
                  1/16″ 
                  3/32″ 
                 15 inches 
                 0.15 cubic inches 
               
               
                   
                  1/16″ 
                  3/32″ 
                 20 inches 
                 0.20 cubic inches 
               
               
                   
                  1/16″ 
                  3/32″ 
                 25 inches 
                 0.25 cubic inches 
               
               
                   
                  3/32″ 
                 ⅛″ 
                  5 inches 
                 0.10 cubic inches 
               
               
                   
                  3/32″ 
                 ⅛″ 
                 10 inches 
                 0.20 cubic inches 
               
               
                   
                  3/32″ 
                 ⅛″ 
                 15 inches 
                 0.30 cubic inches 
               
               
                   
                  3/32″ 
                 ⅛″ 
                 20 inches 
                 0.40 cubic inches 
               
               
                   
                  3/32″ 
                 ⅛″ 
                 25 inches 
                 0.50 cubic inches 
               
               
                   
                   
               
             
          
         
       
     
     A number of variations and modifications of the disclosed embodiments can also be used. The principles described here can also be used for in applications other than recreational vehicles such as bioreactors, etc. While the principles of the disclosure have been described above in connection with specific apparatuses and methods, it is to be clearly understood that this description is made only by way of example and not as limitation on the scope of the disclosure.