Abstract:
An optically based liquid level (liquid-vapor) detector for use with a window having a substantially planar side facing the liquid-vapor side and a substantially planar side positioned parallel to the liquid-vapor side facing the outside. The detector comprises a light source positioned on the outside of the window to direct its light through the outside planar side to the inside planar liquid-vapor side and a light detector positioned on the outside of the window to receive light transmitted from the source and returned from the planar liquid-vapor window side. The detector further includes electronic means for detecting the light intensity change affecting the transducer, from the condition of liquid on the liquid-vapor side, to the condition of vapor on the liquid-vapor side of the window, whereby an action or signal is generated in response to such change.

Description:
PRIORITY: Applicant claims priority based on disclosures contained in Provisional Patent Application Ser. No. 60/347,856 filed Nov. 7, 2001 (Nov. 7, 2002). 
    
    
     Not Federally Funded: The research leading to this invention was not Federally Funded. 
     No Microfiche or other type of program is included in this application. 
     BACKGROUND 
     1. Field of the Invention 
     Liquid level within a tank means theine of transition from liquid to vapor since there is substantially always vapor residing above any liquid body. The discussion of optical detection of liquid level through a sight glass or window by a device or detector always means the discrimination by the detector positioned on one side of the window between the characteristics of vapor and the characteristics of liquid positioned on the other side of the window. Most sight glasses or windows that are provided for visual monitoring of liquid level in a tank or vessel, such as a compressor crankcase, consist of two substantially planar parallel sides, one facing the liquid-vapor side and one facing the outside. Prior art designs directed to detecting a change from vapor to liquid have either employed floats immersed in the liquid for situating a switch or have employed a window with a prism formed in the liquid-vapor side of the window to interact with a light source and light sensor or transducer for returning to the transducer a high percentage of the light emitted by the light source when only vapor was present on the prism side of the window and for returning very little light to the transducer when only liquid was present on the prism side of the window. Such prism faced windows are not often found on vessel sight glasses. 
     Therefore this invention is directed toward an optically based liquid level (liquid-vapor) detector for use with a window having a substantially planar side facing the liquid-vapor side and a substantially planar side positioned parallel to the liquid-vapor side facing the outside. The detector comprises a light source positioned on the outside of the window to direct its light through the outside planar side to the inside planar liquid-vapor side and a light detector positioned on the outside of the window to receive light transmitted from the source and returned from the planar liquid-vapor window side. The invention further includes electronic means for detecting the light intensity change affecting the transducer, from the condition of liquid on the liquid-vapor side, to the condition of vapor on the liquid-vapor side of the window, whereby an action or signal can be generated in response to such change. 
     2. Prior Art 
     Barbier U.S. Pat. No. 5,072,595 in FIG. 1 teaches a bubble detector for a flow stream having a cavity or chamber positioned above and connected to the flow stream for trapping and accumulating bubbles, the chamber including a window having a prism shape formed on the chamber side and a planar face on the outside and a light emitter  66  directing light through the planar face and toward the prism and a light transducer  68  detecting light returned from the prism. When there is liquid surrounding the prism, little light is returned to the light transducer but when bubbles surround the prism, the prism returns much more light to the transducer. Electronic apparatus detects the light intensity and acts in response thereto. In FIG. 3 Barbier teaches a bubble collecting chamber having a planar face on the chamber side but having a prism shaped face on the outside where the light source  66  directs its light against one outside prism face and the light transducer  68  is positioned to receive light from another prism face. The planar face on the bubble or chamber side acts like a 90 degree prism because the light path from source to transducer impacts the planar face at a 45 degree angle and, in the absence of liquid or the presence of bubbles is reflected from the planar face at a 45 degree angle. 
     Barbier U.S. Pat. No. 5,276,426 teaches only a liquid-vapor discriminator having a window having a prismatic face on the liquid-vapor side and a outer planar face. A light source delivers its light from the outer planar face to the inner prism face where light is reflected in greater or lesser intensity from the prism face to a light detector adjacent the outer planar face. 
     Harding U.S. Pat. No. 4,354,180 teaches a liquid-vapor discriminating optical device where a window is positioned adjacent the liquid-vapor chamber. The window has a conical or prismatic face on its inner or liquid-vapor side and a substantially planar face on the outside. A light source is positioned on the outside directed primarily against the planar face and a light transducer is positioned on the outside adjacent another position on the planar face where a transition from vapor to liquid on the prism side of the window changes the fraction of light emitted by the light source that is reflected to the light transducer. Reflecting means are also taught that provide a substantially constant light return from source to transducer that acts as a reference. 
     Objects and Advantages 
     A primary objective is to provide an economical optical liquid level sensing construction where the glass aperture or window has a flat internal surface in contact with the liquid and vapor instead of a more expensive conical prism-shaped surface in contact with the liquid level being sensed. 
     A second primary object is to provide an optical-electronic liquid level sensor assembly that can be installed adjacent a window or sight glass in a vessel which is primarily positioned to provide visual indication from the outside of the vessel of the presence inside the vessel of liquid or vapor at the window. 
     A further object is to proved such a sensor that employs an LED as the light emitter or source and a Darlington Transistor unit as the light transducer or detector. 
     A further object is to provide such a level sensor assembly which operates effectively when the window has a non-prismatic planar surface facing the liquid-vapor side. 
     A further object is to provide such a level sensor assembly when the window has a non-prismatic planar surface facing the outside. 
     A further object is to provide such a level sensor assembly when the window planar liquid-vapor or inside surface is substantially parallel to the outside planar surface. 
     A further object is to provide such a level sensor where the axis&#39; of the light emitter and the light transducer are parallel and substantially perpendicular to the plane of the outside window surface. 
     A further object is to provide such a level sensor where the light emitter and the light sensor are close to each other and separated by an opaque light barrier. 
     A further object is to provide such a level sensor where the axis&#39; of the light emitter and the light transducer converge at a point on the liquid-vapor inside surface of the window. 
     SUMMARY OF THE INVENTION 
     An assembly, including a walled vessel having a window, for opto-electrically differentiating between the presence of liquid and vapor within the vessel at the window, the window having a planar interior face and a planar exterior face substantially parallel with the planar interior face and further providing an electrically actuated light source outside the window positioned to direct light toward the outside surface of the window and an electrically reactive light sensor outside the window positioned to receive light from the source reflected from the window inside surface, the reflected light having a greater intensity on the presence of vapor at the window interior and lesser intensity on the presence of liquid at the window interior and electrical means for reacting differently to the greater and the lesser reflected light intensity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following descriptions and throughout the specification the term LED is employed to identify the complete LED package including the LED active element, a light focusing lens, the polymer encapsulation, if used, and the electrical leads or wires employed to bring electricity to the LED package. Where the internal elements are addressed, they are identified as such. 
     FIG. 1 illustrates typical construction of a portion of a refrigeration compressor crankcase and a flanged oil sight glass or window assembly having planar inner and outer sight glass or window faces and a module of the invention positioned on the exterior of the window for detecting high oil level  34  or low oil level  32 . 
     FIG. 2 is an isometric view of the module of FIG. 1 showing the LED light source and the light detector positioned closely adjacent each other with an intermediate light barrier. 
     FIG. 3 is a side view of a screw-in fitting having an integral planar-planar window and an opto-electrical module of the invention positioned to detect high or low oil level, where the module is secured in the sight glass fitting with a snap ring and a partial cross section illustrating an internal circuit board with LED and detector positioned against the outer window face. 
     FIG. 3A illustrates a planar sight glass with a screw-in ferrule for pressing the opto-electric module against the outer sight glass surface. 
     FIG. 3B shows a module having external threads for screwing directly into a sight glass fitting. 
     FIG. 4 (SEC-A) is a much magnified view of the sectioned portion of the module of the invention positioned against a planar-planar sight glass of FIG.  3 . 
     FIG. 5 shows a greatly magnified view of the LED and detector of FIGS. 3 and 4. 
     FIG. 6A shows a rudimmentary electronic circuit including the LED light source and the detector and an external device operated by the circuit to illustrate the principles. 
     FIG. 6B shows the LED and detector within the optical construction having external leads for connection to external electronics and relays. 
     FIG. 7A illustrate-the situation arising with higher liquid level on the inside of the window. The relative positions of the light source and light transducer of the invention positioned with parallel centerlines or axis&#39;, perpendicular to the window faces and close together are shown. The light paths generated by the LED with high oil level, whereby less light is reflected back to the detector and more is transmitted into the oil is demonstrated. 
     FIG. 7B shows a low oil level and the resultant higher intensity of light reflected from the window inner surface back to the detector. FIG. 7B also shows that with substantial light reflected back to the module the sensitive cone of response or sensitivity of the detector is ineffective to cause its response when the axis&#39; of the LED and detector are perpendicular to the window surface and spaced relatively far apart. 
     FIG. 7C shows the cone of response of the detector overlapping the illuminated zone of the LED whereby, even with the axis&#39; of the LED and detector parallel and perpendicular to the window, the detector receives sufficient light on low oil level to generate an effective response. 
     FIG. 7D illustrates the increased effectiveness of response of the detector when the axis&#39; of the detector and the LED are both non-perpendicular to the planar surfaces of the window but their axis&#39; converge at a point at the inner window surface. FIG. 7D also shows a bullet shaped detector. 
     FIG. 8A is a graphical representation of the light intensity emitted by a typical LED as a function of angular displacement from the LED centerline. 
     FIG. 8B is a graphical representation of the sensitivity or response of a typical detector as a function of the angular displacement from the detector centerline. 
     FIG. 9A shows the exterior shape of substantially externally identical LED and detector side-looking rectangular module packages. 
     FIG. 9B shows the exterior bullet shape of substantially externally identical LED and light detector packages. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1 there is shown a portion of a liquid holding tank  22 , such a compressor oil-holding crankcase, having an opening in the tank side. The level of the liquid in the crankcase-tank-vessel is considered critical. The opening is positioned so that the critical liquid level is at or about the horizontal centerline of the opening. The opening is framed by a circular boss or projection  24 . Boss  24  is provided with several threaded holes  39 . A cover plate  20  for the opening has a flange  26  with a transparent window  29  sealed to the flange central portion so that the position of window  29  allows visual observation of a range of liquid levels  34 , above and  32 , below the critical level within tank  22 . 
     In the following description the term ‘inner’, as applied to a window, means the side of the window  29  in contact with the liquid in tank  22  when the liquid level is higher or in contact with the vapor residing above the liquid in tank  22  when the liquid level is lower. The term ‘outer’ means the side of the window  29  that is normally in contact with the atmosphere and to which the module or construction of the invention is applied. 
     Window  29  is formed with an outer planar face  28  and an inner planar face  30 . Inner planar face  30  is substantially parallel to outer planar face  28 . Flange  20  bearing window  29  is secured to boss  24  of tank  22  by bolts  38  that engage and screw into threaded holes  39  of boss  24 . O-ring  36  is positioned in a circular groove provided in flange  20  to provide an effective seal against leakage past flange  20  of the liquid and vapor within tank  22 . 
     Abortive past efforts have been made to provide opto-electronic means for remotely reporting whether the liquid level in tank  22  was higher than,  34 , or lower than,  32 , the critical level employing the window  29  with its planar parallel inner  30  and outer  28  faces. The term ‘reporting’ here is intended to include the lighting of lights, sounding of alarms or the automatic instigation of some other informative or corrective action such as adding more liquid, draining off liquid, stopping a motor, starting a timer and so forth. 
     These efforts, represented by prior art described above, all were found to require replacing the planar/planar faced window with a window having either a prismatic inner face or a prismatic outer face and positioning the light detector a distance away from the light source. (Barbier U.S. Pat. No. 5,072,595). 
     The module of the invention overcomes the necessity of replacing the standard planar-planar window with a special window having either a prismatic or conical inner or prismatic or conical outer face. 
     FIG. 1 shows a module  40  of the invention simply positioned securely against planar outer surface of window  29 . Module  40  is provided with electrical leads  42  that are intended to connect with appropriate power supply circuits and devices or elements to be controlled. Module  40  is secured against the outer planar face  28  of window  29  by bracket  56 . Bracket  56  is in the form of a cup with flange parts  57  that are secured by bolts  38  thereby holding module  40  securely against exterior planar window face  28 . 
     FIG. 2 shows the active face  43  of module  40  in an isometric view. The active face  43  includes an LED or light producer  46  and a light detector  44  separated from the LED only by an opaque light barrier  58 . Typically the LED is a small polymer package within which are encapsulated the light producing element and a lens. Typical LED capsules are manufactured by Fairchild Semiconductor Company in both a “side-looker” shaped package or capsule (FIG. 9A) and a “bullet” shaped package or capsule (FIG.  9 B). The model QEE113 LED and the light detector capsule model QSE113 are the side-looker type where the LED is within a rectangular case (FIG.  9 A). The side looker model QEE113 LED generates a light in the infrared having a wavelength in the region of 980 NM (nanometer). The side looker model number QSE133 light detector (FIG. 9A) has the same dimensions as the side looker QEE113 model LED. The light detector has a major sensitivity in the same 980 NM range. Other LED capsules suited for use in the invention are the bullet shaped QED233 or QED234 (FIG. 9B) both having a diameter of about 0.200 inches and a length of about 0.300 inches called a T1-3/4 package and a light detector in a similar package having the model QSC133. 
     The above side looking capsules are 0.175 by 0.2 inches on the active face and 0.1 inches thick with the dual leads coming from the 0.1×0.175 inch end. The active faces of both these side looking capsules are positioned substantially flush with the active module face  43  as shown in FIG.  2 . An encapsulating or potting material  41  such as a polymeric material is employed to fix in position the components of the module  40  substantially flush with the module active face  43 . The potting material may be either thermoplastic or thermosetting. 
     Referring now to FIG. 3 there is shown tank  22  having boss  25  with internal threads  60 . A screw-in fitting  62  has threads  60 A that conform to the internal threads  60  of the boss  25 . The fitting  62  includes a window  29  having a planar interior face  30  and a substantially parallel planar exterior face  28 . The window  29  of FIG. 3 may have identical dimensions with the window  29  of FIG. 1 or different dimensions, but the inner and outer surfaces are both planar and are substantially parallel to each other. The fitting  62  is provided with a cavity that allows module  40  to slide within until the active face  43  of module  40  is flush with and pressed against external planar surface  28  of window  29 . A snap ring  64  is employed to hold an external ferrule  66  against a soft or compressible washer  68  for the purpose of securely positioning the active module face  43  and the accompanying active faces of the embedded LED  46  and detector  44  against the external face  28  of window  29 . 
     In FIG. 3 a circular section-A is shown that is much enlarged in FIG. 4 as SEC-A to show construction details. 
     Other fitting designs for positioning and securing module  40  against the external planar surface  28  of window  29  are shown in FIG. 3A and 3B. 
     In FIG. 3A fitting  78  is provided with integral window  29  having a planar outer face  28  and a parallel planar inner face  30 . Fitting  78  has external threads  61  for engaging and securing fitting  78  into boss  25  of tank  22  shown in FIG.  3 . Fitting  78  is provided with a recess that accommodates module  40 . In a preferred embodiment directed to constructions where window  29  has a smaller diameter than the diameter of module  40 , window  29  protrudes slightly from a recessed shoulder  79  in fitting  78 , thereby allowing the active face  43  of module  40  to seat snugly and reliably against the external planar surface  28  of window  29 . Fitting  78  is provided with internal threads  85  into which sleeve  80 , having external threads  82 , is screwed, thereby providing means for forcing module  40  and its active face  43  securely against the external planar face  28  of window  29 . 
     FIG. 3B displays substantially the same threaded fitting  78  as shown in FIG.  3 A. However, in FIG. 3B module  40 T is provided with external threads  84  to engage the internal threads  85  of fitting  78  thereby allowing the threaded sleeve  80  to be eliminated. Module  40 T is provided with an external boss  82  having wrench flats  83 , whereby a tool can be employed to tighten the threaded module  40 T into fitting  78  and against external planar surface  28  of window  29 . 
     In FIG. 4 (SEC-A is an enlarged detail of window  29  and module  40  of FIG. 3) window  29  is again shown having planar external surface  28  and substantially parallel planar inner surface  30 . Pressed against the external planar surface  28  of window  29  is active surface  43  of module  40 . LED  46  and light detector  44  are shown with their active faces positioned against window face  28  and separated by opaque barrier  58 . While the opaque barrier  58  is shown as a discrete sheet-like element other forms of light barriers between the light source and light detector may be employed. The LED  46  and the light detector  44  are mounted to a circuit board  48  on the reverse side of which the operative resistors and other electronic elements  50  and the ends of the leads  42  are also mounted. Potting material  41  is identified by numeral but the volume filled with the potting material  41  is not hatched for clarity in displaying the other elements. While electronic elements are shown mounted on circuit board  48  positioned within module  40 , in other embodiments of the invention a module  49  (FIGS. 5 and 6B) contains only the LED and light detector and the electronic components are positioned elsewhere. In further reference to FIG. 4, when the active elements LED  46  and detector  44  are mounted on a circuit board  48  the active face of the construction is the plane that resides or can reside against the planar outer  30  of window  29 , even when there is no molding or potting composition provided. That is, in FIG. 4 the circuit board  48  is intended to provide the structural basis for the construction. In that construction the circuit board  48  with its mounted LED  46  and light detector  44 , separated by a light barrier  58  with electronics  50  either positioned on the circuit board or elsewhere provide the basic elements of the invention. In such construction the snap ring  64  of FIG. 3 or the screw-in sleeve  80  of FIG. 3A are repositioned to hold the circuit board in position with its active elements, the LED and detector, against the outer surface  30  of window  29 . 
     FIG. 5 is a greatly enlarged crossection of the LED package or module  46  and the light detector package or module  44  showing their primary functional elements; LED module  46  has both the actual LED element  70  and the lens  72  that is employed to provide the desired conical light distribution pattern encapsulated within its package  46 . In a preferred construction the actual light emitting parts  70  and  72  are positioned near one end of the package  44 . A typical light intensity distribution for the LED package QEE113 as a function of the conical angle is shown in FIG.  8 A. 
     Continuing reference to FIG. 5, there is shown Darlington transistor light detector in a side-looking case or package  44 . The actual light detecting element  74  is embedded in the polymer detector package  44 . In a preferred construction the light detecting parts including the active element  74  and lens  76  are positioned near one end of package  46 . The sensitivity of the detector is increased by the provision of light receiving and focusing lens  76  which receives light and concentrates and focuses the light onto the actual active Darlington transistor  74 . FIG. 8B displays a graphical statement of the variation of the light sensitivity of the Darlington Light Detector Package QSE133 as a function of the angular departure from the centerline of the detector lens. FIG. 8B shows that for this detector a 50 percent cone of sensitivity arises at an angular distance of about 30 degrees from the central axis  102 C. 
     In the embodiments of the invention of FIGS. 4,  7 A and  7 C where the central axis&#39; of the LED and light detector are positioned perpendicular to the external active surface  43  of module  40 , it is an important construction feature of the invention that the LED module  44  and the light sensing module  46  be positioned so that their active elements within each module are closely adjacent, separated only by the opaque separator  58  as shown in FIG.  5  and FIGS. 7A and 7C in order to best ensure that there is adequate detector sensitivity to respond to the changes in light intensity. See also the discussion for FIG.  7 B. 
     Referring now to the electrical schematic diagram of FIG. 6A, there is shown one possible circuit  87  embodied within module  40 . Three wire leads comprising lead group  42  of FIGS. 1,  2  and  3  extend from module  40 . Lead  86  of that group  42  is the lead which connects to the positive side of the power supply. Lead  90  is the wire of that group  42  which connects to the negative side of the power supply. Lead  88  of group  42  is the lead whose voltage with respect to lead  90  varies as a function of the conductivity and therefore of the amount of light impinging on light detector  44 . It is across these two leads of group  42 , lead  88  and lead  90 , that relay  92  is connected. LED  46  is connected across the 12 volt DC supply through voltage dropping or limiting resistor  96 . Resistor  96  has a typical value of 5500 ohms for use with the QEE113 LED. Light sensitive Darlington transistor  44  has a high electrical resistance in the absense of light, and a sharply reduced electrical resistance when illuminated. Resistor  94 , typically having a resistance of 169,000 ohms, acts as load resistor. Under low light conditions there is minimum current flow through Darlington transistor  44  and therefore low voltage across resistor  94  and higher voltage across the Darlington  44 . As the Darlington light sensor  44  receives more light, it allows greater current flow and a higher voltage drop across resistor  94  and a lower voltage across Darlington  44  arises. 
     A high impedance relay  92  is connected across Darlington light sensor  44 . The relay acts in response to the voltage across the Darlington  44 . The voltage across Darlington  44  is higher when the resistance of the Darlingon  44  is higher. This condition occurs when less light reaches the Darlington  44  and its resistance is higher. Device  93 , typically an alarm, an electric valve or a timer, acts in response to the voltage across Darlington light sensor  44  to activate and alarm or perform other activities a designer deems appropriate at conditions of higher liquid level  34  or lower liquid level  32  (FIG.  1 ). It should be emphasized that other arrangements of electronic components can be employed to coact with the light source  46  and detector  44  to secure the desired results of alerting, warning and acting on high or low oil or liquid levels. 
     Referring now to FIGS. 7A,  7 B,  7 C and  7 D there is shown in FIG. 7A an approximate graphical representation of the effect of the presence of higher liquid level  34 . In FIG. 7A, light  96 , emitted by LED  46  in a conical range of intensities along central axis  96 C (see also FIG.  8 A), is transmitted in greater part  98  into the liquid at the higher liquid level  34  (see FIG. 1) and reflected  100  in lesser part. The cone of response (see also FIG. 8B) of light detector  44  (Darlington  44 ) is not shown to allow clear representation of the light pattern of the LED. 
     FIG. 8A displays the approximate light intensity (Vertical axis, 0 to 1.0) provided by the LED as a function of the angular displacement from a central axis or centerline (Horizontal axis, 60-0-60). 
     Referring again to FIG. 7A there is shown LED light emitting diode  70  and lens  72  positioned within LED package  46 . In FIG. 7A the directions and variations in light intensity is very approximately illustrated by the directions and lengths of lines  96 . Heavier line  96 C represents the intensity and direction of the central axis of light delivery by the LED at zero angular displacement, corresponding to  96 C centerline or central axis of FIG.  8 A. 
     Referring now to FIG. 7B, two different effects are displayed. First: the monitored liquid level  32  is lower than the zone of light emitted by LED  46 . Therefore only a small fraction  98  of the light emitted by LED  70  is transmitted across the vapor glass interface (inner planar surface  30  of window  29 ) into the vapor space. A correspondingly large fraction of the light emitted from the LED is reflected at the window inner planar surface  30  in intense light rays  100 . Second: the light detector  44  cannot respond to the higher reflected light intensity because its zone of response  102 , having a central axis  102 C positioned perpendicular to the active face of the device, is separated from LED  45  by a distance  106  and in particular does not include the point of intersection of the central axis of light delivery of the LED  46  with a cone of 50 percent or greater sensitivity to ensure sufficient light pickup from the LED. 
     Second: if the light detector  44  had been closely adjacent LED  46 , as shown in FIG. 7A, the cone of response  102  of light detector  44  would have encompassed the more intense light rays  100  reflected from the inner window surface  30  light detector  44  would have had enough light  100  to activate the electronics and provide an alarm or a corrective action. Therefore, to meet the requirements of the invention, the zone of response or sensitivity  102  of the light detector  44  of at least 25% and preferably 50% must include the point of intersection of the central axis with inner surface  30  of window  29  as shown in FIG.  7 C. In another preferred embodiment of the invention, the central axis of the light detector  102 C will intersect and substantially coincide with the intersection of the central axis of the light emitter thereby ensuring adequate zone of sensitivity coverage. 
     The closely adjacent structure of the assembly is shown in FIG.  7 C. There LED  46  is closely adjacent the detector  44  and the detector zone of sensitivity  102  encompases the intersection point  30  of the central axis of the LED on the inner surface of window  29 . The detector  44  is therefore able to react effectively to the presence of both lesser light intensity on a condition of higher liquid level on the inside surface  30  of window  29  and higher light intensity when the liquid level is lower. 
     Referring again to FIG. 7B, LED-lens combination  70 ,  72  provides maximum light intensity along centerline  96 C and reduced light intensities along other light paths  96 . Low intensity light rays  98  are transmitted through the planar inner window side  30  into the vapor above liquid level  32 . Relatively higher intensity light rays are reflected at inner planar window surface  30  back over a range of light paths  100 , some of which reach Darlington detector  74  though its collecting lens  76 . The illumination level of the Darlington detector  74  (package  44 ) causes it to have a lower electrical resistance thereby causing a lower electrical potential to arise across the Darlington detector  44  (FIG.  6 A). 
     FIG. 8B illustrates the response of the Darlington detector QSE133 to light. The graph of FIG. 8B shows that for the QSE133 the detector response varies as a function of the conical angle of the field from its central axis (horizontal axis having a range of 45, 0, 45). The “cone of sensitivity” of the detector  44  is defined, for the purposes of this specification, as that conical angle at which the detector response is 0.5. The maximum response “1.0” arises at the centerline “0” with lesser responses as the light source deviates from the central axis “0” of the detector  74 . For the QSE113 the “cone of suitable sensitivity” is 30 degrees. However, depending on the sensitivity of the electronics and other external components a lower zone of sensitivity such as 25% may be suitable. 
     In FIG. 7D LED, element  70  and accompanying lens  72  have been positioned at an angle other than 90 degrees to the active surface of the construction and to the outer face  28  and the parallel inner face  30  of window  29 . The LED angle has been selected to bring the point of intersection of the central axis  96 C of the LED closer to the position of the light detector  76 . In FIG. 7D a bullet shaped light detector  76  has been positioned at such an angle that its axis of sensitivity  102 C intersects the inner window surface  30  at the a position  96 D that is coincident with the point of intersection of the central axis  102 C of the LED, thereby providing a high degree of responsiveness by the detector  76  to the light reflected from the inner surface  30  of window  29 . 
     It should be noted that potting material  41  of module  40  is identified by the numeral  41  in FIG. 4 but no hatching is shown to avoid confusion. In preferred embodiments of the invention there is no potting material between the active face  43  of the module and either the active parts of light emitter  46  or light detector  44 . 
     From the foregoing description, it can be seen that the present invention comprises an unobvious and untaught construction for electro-optically reacting to changes in liquid level through a non-prismatic planar-planar window. It will be appreciated by those skilled in the art that changes could be made to the embodiments described in the foregoing description without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiment or embodiments disclosed and claimed, but is intended to cover all modifications and equivalents of claimed elements which are within the scope and spirit of the invention as Set forth in the claims and described in the specification.