Abstract:
A sensor for detecting levels of fluids and particulate matter is based on differences in dielectric constants of the various materials. The difference is reflected in different capacitance across electrodes immersed to different depths in such materials. A square-wave clock pulse is input into circuit composed of the capacitance and a resister in series. The output voltage is fed into a logic unit (such as exclusive-OR), which converts the exponential decay of the voltage into a pulse of uniform height with duration proportional to the time constant (proportional to the capacitance) of the exponential decay. The pulse is optionally fed through a low-pass filter to give a signal proportional to the duration.

Description:
BACKGROUND OF THE INVENTION  
         [1]    1. The present invention relates to a level sensor for detecting the level of a liquid or a powder or particulate accumulation (hereinafter referred to as a powder or particulate level).  
           [2]    2. One example of this type of level sensor is shown in FIG. 35. This sensor includes a pipe member  90  having a plurality of vertically disposed proximity switches  90   a,  and a float  91  surrounding pipe member  90  and having a magnet  91   a  on its inner peripheral side.  
           [3]    3. The magnet  91   a  moves up and down with the float in response to the rise and fall of the liquid surface. In this level sensor, the level of liquid can be determined by sensing which proximity switch  90   a  the magnet  91   a  is next to.  
           [4]    4. The following problems exist in this known level sensor:  
           [5]    5. ( 1 ) Since sensing is possible only when the float  91  comes to a certain position where one of the proximity switches  90   a  is present, the liquid level can be determined only up to certain discrete values.  
           [6]    6. (2) Since the float  91  moves up and down with respect to the pipe member  90 , dust and the like can become clogged between the float  91  and the pipe member  90 , thereby inhibiting the smooth response of the float  91 , and resulting in failure to detect the level of the liquid. In other words, many mechanical elements end up with poor reliability.  
           [7]    7. (3) This sensor cannot detect the level of the interface between an upper liquid layer and a lower liquid layer.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [8]    8. It is therefore an object of the present invention to provide a liquid level sensor which overcomes the limitations of the prior art.  
           [9]    9. It is another object of the present invention to provide a level sensor with which a liquid level or a powder or particulate level can be determined continuously with high reliability.  
           [10]    10. It is a further object of the present invention to provide a level sensor in which a level of the interface between two fluid layers can be detected.  
           [11]    11. Briefly stated, the present invention provides a sensor for detecting levels of fluids and particulate matter based on differences in dielectric constants of the various materials. The difference is reflected in different capacitance across electrodes immersed to different depths in such materials. A square-wave clock pulse is input into circuit composed of the capacitance and a resister in series. The output voltage is fed into a logic unit (such as exclusive-OR), which converts the exponential decay of the voltage into a pulse of uniform height with duration proportional to the time constant (proportional to the capacitance) of the exponential decay. The pulse is optionally fed through a low-pass filter to give a signal proportional to the duration.  
           [12]    12. According to an embodiment of the invention, there is provided a level sensor for detecting a level of one of liquid, and a particulate material comprising: a capacitance type variable capacitor having an electrode having a capacitance responsive to surrounding stray capacitance, a resistor connected with the variable capacitor to form an integration circuit, the integration circuit having a time constant responsive to the capacitance and a value of the resistor, a logic device, the integration circuit receiving a clock having a constant frequency and duty ratio, an output from the integration circuit is input to the logic device, and the integration circuit producing an output from the logic device which varies according to change in capacitance of a capacitance plus a stray capacitance of the variable capacitor in response to change in length of a portion of the electrode immersed in the material.  
           [13]    13. According to a feature of the invention, there is provided a level sensor for detecting a level of one of a liquid object and a particulate object comprising: an electrode susceptible to surrounding stray capacitance, a resistor connected with the electrode to form an integration circuit, a logic device, the integration circuit receiving clock of a constant frequency and duty ratio, an output from the integration circuit is input to the logic device, and an output from the logic device varies depending on a change in the stray capacitance produced in surroundings of the electrode in response to a change in length of a portion of the electrode immersed in the object.  
           [14]    14. According to a further feature of the invention, there is provided a level sensor for detecting a level of one of a liquid and a particulate material comprising: an electrode susceptible to surrounding stray capacitance, a capacitor, a fixed resistor, a logic device, the resistor and a combined element made up of the electrode and the capacitor constituting an integration circuit, the integration circuit receiving a clock having a constant frequency and duty ratio, an output from the integration circuit is input to the logic device, and an output from the logic device varies according to change in a capacitance of a capacitance of the fixed capacitor plus a stray capacitance of the electrode in response to a change in length of a portion of the electrode immersed in the object.  
           [15]    15. According to a still further feature of the invention, there is provided a level sensor for detecting a level of one of a liquid and a particulate material comprising: at least three electrodes mutually arranged in parallel to form at least two variable capacitors, associated fixed resistors respectively connected to the capacitors to form at least first and second integration circuits which have time constants different from each other, a logic device, the first and second integration circuits receiving a clock having a constant frequency and duty ratio, an output from the integration circuits is input to the logic device, and an output from the logic device varies depending on change in capacitance of a capacitance plus a stray capacitance of the variable capacitors in response to change in length of a portion of the variable capacitors immersed in the object.  
           [16]    16. According to another feature of the invention, there is provided a level sensor for detecting a level of an interface between an upper layer and a lower layer, the upper and lower layers having different dielectric constants comprising: a float, a capacitance type variable capacitor having two electrodes arranged mutually in parallel, the variable capacitor being supported by the float, a resistor connected to the variable capacitor to form an integration circuit, a logic device receiving an output of the integration circuit, the variable capacitor extending downward into the upper and lower layers when the float is floated on a surface of the upper layer, whereby a capacitance of the variable capacitance is varied by an amount related to a thickness of the upper layer, the integration circuit receiving a clock having a constant frequency and duty ratio, an output from the integration circuit is input to the logic device, and a duty ratio of an output of the logic device varies according to change in capacitance of a capacitance plus a stray capacitance of the variable capacitor in response to change in length of a portion of the capacitor immersed in the upper liquid layer.  
           [17]    17. According to another feature of the invention, there is provided a level sensor for detecting a level of an interface between an upper liquid layer and a lower liquid layer comprising: a float, a plurality of variable capacitors having electrodes arranged mutually in parallel at equal intervals supported on the float, resistors connected to respective variable capacitors to form integration circuits having different time constants, and a logic device, the float being floated on a liquid surface with the plurality of variable capacitors extending downward immersing the electrodes in the two liquid layers, the integration circuits receiving a clock having a constant frequency and duty ratio, an output from the integration circuit is input to the logic device and a duty ratio of an output of the logic device varies depending on capacitances of a capacitance plus a stray capacitance of the variable capacitors in response to change in length of a portion of the capacitors immersed in the upper liquid layer.  
           [18]    18. According to a still further feature of the invention, there is provided a level sensor for detecting a level of an interface between an upper liquid layer and a lower liquid layer comprising: a float, an electrode supported on the float, the electrode being a variable capacitor whose capacitance varies in relation to an amount by which it is immersed in the upper and lower liquid layers, a resistor which, with the electrode constitutes an integration circuit, the electrode extending downward from the float through the upper liquid layer and partly into the lower liquid layer, a logic device receiving an output of the integration circuit, the integration circuit receiving a clock having a constant frequency and duty ratio, an output from the integration circuit is input to the logic device, a duty ratio of an output of the logic device varies depending on a capacitance of a capacitance plus a stray capacitance of the variable capacitor in response to change in length of a portion of the electrode immersed in the upper layer liquid, whereby a thickness of the upper liquid layer can be determined.  
           [19]    19. According to a final feature of the invention, there is provided a level sensor comprising: a variable capacitor, the variable capacitor having a capacitance which varies in response to a dielectric constant of a surrounding material, a resistor combined with the variable capacitor to form an integration circuit, a time constant of the integration circuit being established by values of the resistor and the capacitance, support means for supporting the variable capacitor partly immersed in the material, whereby the capacitance varies in response to an amount by which the variable capacitor is immersed, a logic device receiving an output of the integration circuit, the logic device being responsive to the time constant to produce an output signal having a characteristic related to the capacitance.  
       
    
    
       [20]    20. The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [21]    21.FIG. 1 is a section view of Embodiment 1 of a level sensor of the present invention in which a sensor body is attached to a fixed wall of a container.  
         [22]    22.FIG. 2 is a diagram of a detection circuit of Embodiment 1.  
         [23]    23.FIG. 3 is a diagram showing relationships between the input clock and various output voltages when using the detection circuit of Embodiment 1.  
         [24]    24.FIG. 4 is a graph showing the relation between an output and a water level in Embodiment 1.  
         [25]    25.FIG. 5 is a section view of Embodiment 2 of a level sensor of the present invention in which a sensor body is attached to a fixed wall of a container.  
         [26]    26.FIG. 6 is a diagram of a detection circuit of Embodiment 2.  
         [27]    27.FIG. 7 is a diagram showing the relation among the input clock and various output voltages when using the detection circuit of Embodiment 2.  
         [28]    28.FIG. 8 is a graph showing the relation between an output and a water level in Embodiment 2.  
         [29]    29.FIG. 9 is a modification of the detection circuit of Embodiment 2.  
         [30]    30.FIG. 10 is a diagram of a detection circuit of Embodiment 3.  
         [31]    31.FIG. 11 is a diagram showing the relation among the input clock and various output voltages when using the detection circuit of Embodiment 3.  
         [32]    32. FIGS.  12  to  16  are diagrams of detection circuits of other embodiments of the present invention.  
         [33]    33.FIG. 17 is a perspective view of a sensor body of a level sensor according to Embodiment 4 of the present invention.  
         [34]    34.FIG. 18 is a diagram of a detection circuit of Embodiment 4.  
         [35]    35.FIG. 19 is a section view in which the sensor body of FIG. 17 is attached to a fixed wall of a container.  
         [36]    36.FIG. 20 is a diagram showing the relation among the input clock and various output voltages when using the detection circuit of Embodiment 4.  
         [37]    37.FIG. 21 is a graph showing the relation between an output and a water level in FIG. 19.  
         [38]    38.FIG. 22 is a sectional view showing a state where liquid remains on an outer surface of the sensor body when the liquid level is lowered by letting some liquid out of the container in FIG. 19.  
         [39]    39.FIG. 23 is a diagram showing the relation among integration waveforms of the points P 2  and P 3  in FIG. 18 and an output voltage when the liquid level in FIG. 19 is lowered.  
         [40]    40.FIG. 24 is a section view of Embodiment 5 in which a sensor body is attached to a fixed wall of a container.  
         [41]    41.FIG. 25 is a graph showing the relation between an output and a water level in Embodiment 5.  
         [42]    42.FIGS. 26 and 27 are perspective views of the essential parts of sensing bodies in other embodiments.  
         [43]    43.FIG. 28 is a diagram of a detection circuit of another embodiment of the present invention.  
         [44]    44.FIG. 29 is a view of the level sensor according to Embodiment 6 of the present invention.  
         [45]    45.FIG. 30 is a diagram of a detection circuit of the level sensor of FIG. 29.  
         [46]    46.FIG. 31 is a diagram showing the relation among a clock on an input side, a clock on an output side and the like when using the detection circuit in FIG. 30.  
         [47]    47.FIG. 32 is a diagram of a detection circuit of Embodiment 7 of a level sensor of the present invention.  
         [48]    48.FIG. 33 is a diagram showing the relation among the input clock and various output voltages when using the detection circuit of Embodiment 7.  
         [49]    49.FIG. 34 is a diagram of a detection circuit of Embodiment 8 of the present invention.  
         [50]    50.FIG. 35 is a view of a prior art level sensor.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     Embodiment 1  
       [51]    51. Referring to FIGS. 1 and 2, a level sensor in Embodiment 1, especially adapted for use in detecting the level of a liquid, includes a sensor body  1  and a detection circuit DK. The sensor body  1  has a variable capacitor C V , whose capacitance varies in a known way with the level of liquid. The detection circuit DK outputs a signal corresponding to the change in capacitance, including stray capacitance, in the variable capacitor C V  resulting from the change of a portion of the sensor body  1  immersed in the liquid.  
         [52]    52. The sensor body  1 , shown in FIG. 1, includes the variable capacitor C V  which includes parallel electrodes  1   a  and  1   b,  having a mutual capacitance which is responsive to the amount of sensor body  1  which is immersed in the liquid. A coating of synthetic resin (or rubber) covers the variable capacitor C V . The sensor body  1  is also provided with an acetabulum, or suction cup,  2  to attach it to a fixed wall  80  of a container  8 .  
         [53]    53. A potential difference between the electrodes  1   a  and  1   b  yields capacitance in the variable capacitor C V . Since the dielectric constant ε 1  in the surrounding air is smaller than the dielectric constant ε 2  in the surrounding liquid (ε 1 &lt;ε 2 ), the capacitance in the sensor body  1  varies with the liquid level.  
         [54]    54. The terms used to denote signals are as follows:  
         [55]    55. Capacitance between the electrodes  1   a  and  1   b:  C ab0    
         [56]    56. Stray capacitance in the air surroundings: C ab1    
         [57]    57. Stray capacitance in the liquid surroundings: C ab2    
         [58]    58. In the case where the liquid is water, the ratio of C ab1  to C ab2  is 1:80. The total capacitance produced by the electrodes  1   a  and  1   b  is C ab = C ab0 + C ab1 + C ab2 . C ab1  and C ab2  are respectively proportional to the length L 1  of the portions of the electrodes  1   a  and  1   b  in the air and the length L 2  of the portions of the electrodes  1   a  and  1   b  in the liquid. As a result, the total capacitance C ab  varies substantially linearly with liquid level.  
         [59]    59. The detection circuit DK, shown in FIG. 2, includes fixed resistors R 1 , R 2 , and R 3 , the foregoing variable capacitor C V , and an exclusive-OR logic unit, EX-OR logic IC1. The resistor R 1  and the variable capacitor C V  constitute an integration circuit IN with a time constant equal to R 1 C V . The resistor R 2  and the EX-OR logic IC1 constitute another integration circuit IN′ with a time constant equal to an input capacitance C in2  of the EX-OR logic IC1 multiplied by R 2 . Although the time constant established by the resistor R 1  is influenced by the input capacitance C in1  of the EX-OR logic IC1 as well as by C V ; the total capacitance is so much greater than the input capacitance (C ab &gt;&gt; C in1 ) of these elements that they can be ignored.  
         [60]    60. The operation of the detection circuit DK according to change of the liquid level in the container  8  is described below in conjunction with FIGS.  2  to  4 :  
         [61]    61. (1) A clock CLK is input to P 1 . The time constants R 1 C ab  and R 2 C in2  at positions P 2  and P 3  produce integration waveforms P 2  and P 3  as shown in FIG. 3.  
         [62]    62. (2) Since the stray capacitance in the air surroundings is smaller than that in the liquid surroundings (C ab1 &lt; C ab2 ), a rise in the liquid level increases the total capacitance, and consequently increases the time constant R 1 C ab . The waveform of the position P 2  here changes into a less inclined waveform P 2 ′ in FIG. 3. The threshold time (T th  in FIG. 3) that it takes P 2  (or P 2 ′) to follow the clock signal becomes longer. The EX-OR IC1 produces an output when, and only when, one of its two inputs exceed threshold V th . As the liquid level increases the output of the EX-OR logic IC1 changes from P 4  to P 4 ′ in FIG. 3. This lengthens the signal during each clock cycle. In other words, the change in liquid level appears as the change in the duty ratio of output of the EX-OR logic IC.  
         [63]    63. (3) The output of the EX-OR logic IC, P 4 , is passed through a low-pass filter (not shown), and thereby becomes an analog voltage, having an amplitude related to the duty ratio. As shown in FIG. 4, the output voltage increases approximately linearly with the rise of the liquid level. Although the change of the liquid level and the change of the output duty ratio of the EX-OR logic IC1 are not exactly proportional, an appropriate circuit constant insures practically sufficient linearity.  
         [64]    64.  FIG. 4 shows the relation between the output (mV) and the water level (cm) when the electrodes  1   a  and  1   b  of the variable capacitor C V  are set at a height of 600 mm. It shows that practically sufficient linearity is obtained.  
         [65]    65. (4) The detection circuit DK in FIG. 2 can be assembled with a low-pass filter in a small printed circuit substrate of about 25 mm in diameter.  
         [66]    66. (5) As apparent from the above statements (3) and (4), the level sensor according to the present invention can be made very compact and at low cost. Also, since this level sensor has no mechanical operation parts, it is very reliable.  
       Embodiment 2  
       [67]    67. Referring now to FIGS. 5 and 6 the level sensor of Embodiment 2 includes a sensor body  1  and a detection circuit DK. The detection circuit DK outputs a voltage corresponding to the capacitance variance in response to the change in the portion of the sensor body  1  immersed in the liquid.  
         [68]    68.FIG. 5 illustrates the sensor body  1  having a single electrode  1   c  designed to be susceptible to the surrounding stray capacitance. Electrode  1   c  is covered by a synthetic resin coating  10 . The sensor body  1  is affixed to a fixed wall  80  of a container  8  using an acetabulum or suction  2 . The container  8  is grounded to stabilize the operation of the detection circuit.  
         [69]    69. The difference between the dielectric constant of the liquid surrounding the sensor body  1  and that of the air surrounding the sensor body  1  causes the stray capacitance to vary depending on the level of the liquid.  
         [70]    70. The terms used to denote signals are as follows:  
         [71]    71. Total stray capacitance surrounding the sensor body  1 : C s    
         [72]    72. Stray capacitance in the air surrounding the sensor body  1 : C s1    
         [73]    73. Stray capacitance in the liquid surrounding the sensor body  1 : C s2    
         [74]    74. Dielectric constant of the air: ε 1    
         [75]    75. Dielectric constant of the liquid: ε 2  (with ε 1 &lt;ε 2 )  
         [76]    76. The total capacitance C s  is the sum of C s1  and C s2 . C s1  and C s2  are respectively proportional to the lengths L 1  and L 2  of the electrode  1   c  as shown in FIG. 5. Consequently the total capacitance C s  varies in a known way in response to the liquid level.  
         [77]    77. The detection circuit DK, as shown in FIG. 6, includes fixed resistors R 1 , R 2 , and R 3 , a fixed capacitor C 1 , the above-mentioned sensor body  1  and an EX-OR logic IC1. The resistor R 1  and the combination of the fixed capacitor C 1  and the electrode  1   c  constitute an integration circuit. The resistor R 2  and the EX-OR logic IC1 constitute another integration circuit. The resistor R 1  is also influenced by the input capacitance of the EX-OR logic IC1. However, since the input capacitance is much smaller than the capacitance of the combination of the fixed capacitor C 1  and the electrode  1   c,  the input capacitance is herein neglected.  
         [78]    78. The operation of the detection circuit DK at the time of the change in the liquid level in the container  8  using water as a liquid is described with reference to FIGS.  5 - 7 . A clock CLK with a constant frequency and duty ratio is input to the point P 1  of the detection circuit DK.  
         [79]    79. (1) The voltage waveforms at the points P 2 , P 3 , and P 4  when the liquid (water) level rises to a height L 2  from the bottom of the sensor body  1  are shown in FIG. 7.  
         [80]    80. (2) When the water level further rises to a height L 2 ′ (L 2 &lt; L 2 ′) from the bottom of the sensor body  1 , the stray capacitance in the surroundings of the sensor body  1  changes to C s ′ (C s &lt; C s ′). Subsequently, the waveform at the point P 2  changes from P 2  to P 2 ′ shown in FIG. 7. Accordingly, the duty ratio of the output pulse of the EX-OR logic IC1 changes to the output P 4 ′.  
         [81]    81. (3) An additional low-pass filter (not shown) receiving the signal from the point P 4 , converts the output of the EX-OR logic IC1 to an analog voltage which increases in response to the rise of the water level, in a manner similar to Embodiment 1.  
         [82]    82. (4) FIG. 8 shows measurements of the output voltage depending on the liquid level in the detection circuit DK of FIG. 6 with the low-pass filter. At water levels near 0 cm (before the water level reaches the bottom of the sensor body  1 ), the output voltage shows a sharp change with water level below the bottom of the sensor body  1 . This is because the dielectric constant of water (about 80) is considerably larger than that of air (about 1). As soon as measurements are made with sensor body  1  at least slightly immersed in the water, the relationship between output and water level returns to the roughly linear, without such sharp change. However, the sharp change near zero can be used as a reliable method of detecting when the liquid first enters the container  8  and when the liquid level reaches the bottom of the sensor body  1 .  
         [83]    83. (5) As an alternative, a detection circuit DK of FIG. 9 may be substituted for the detection circuit DK of FIG. 6. The detector circuit DK of FIG. 9 omits the fixed capacitor C 1  of FIG. 6.  
       Embodiment 3  
       [84]    84. Referring now to FIG. 10, Embodiment 3 of the present invention, employs a detection circuit DK shown which provide improved output stabilization over a broad input frequency range.  
         [85]    85. In the detection circuit DK in FIG. 10, a voltage V CC  is applied to an electrode  1   a  of a variable capacitor C V  through a fixed resistor R. An electrode  1   b  of the variable capacitor C V  is grounded. An output electrode of a switching element SW and an input of an inverter IC U 1  are connected to junction P 5  of the variable capacitor C V  and the resistor R. The switching element SW (a Field Effect Transistor) rapidly switches the potential of the junction P 5  from its value established by the voltage Vcc through the pull-up resistor R to a level near ground (zero volts). At the end of the clock pulse, the voltage rises with a time constant established by the resistance of resistor R and the capacitances in detection circuit DK (principally, the capacitance in variable capacitor C V .  
         [86]    86.FIG. 11 shows the voltage waveform at the point P 5  and the output voltage of the inverter IC when the detection circuit DK in FIG. 10 is used.  
       Alternatives to Embodiments 1, 2, and 3  
       [87]    87. In the foregoing embodiments, the variable capacitor C V  with the electrodes  1   a  and  1   b  (Embodiment 1) and the electrode  1   c  (Embodiment 2) are coated with synthetic resin. Such a coating may not be necessary, depending on the kinds of liquid. In case of using a variable capacitor C V  without synthetic resin coating, not only do the stray capacitances C ab1  and C ab2  (C s1  and C s2 ) vary, but the capacitance C ab0  (C a ) itself also changes according to the liquid level. However, detection of the liquid level is still feasible by using the detection circuit DK as stated above. The electrodes  1   a  or  1   b  may be made of conductive wire or bar (with various cross-sectional shapes such as flat, round, or square).  
         [88]    88. A variety of detection circuits DK basically provided with an integration circuit and a logic IC1 may be selectively employed. Some examples are shown in FIGS.  12  to  16 . In the drawings, R, R 4 , R 5 , C V , U 1  and U 2  respectively denote resistors, a variable capacitor, an inverter IC and an NOR logic IC. In a detection circuit DK of FIG. 16, the time constant of C ab R 4 . R 5  is set optionally.  
         [89]    89. The above level sensors are described in an application where the level of liquid is measured. One skilled in the art will recognize that they may also be used to measure a level of any fluid-like material such as accumulated powder and granules. A liquid, having a relatively low viscosity, such as water, alcohol, petroleum, or sea water, can be measured.  
       Embodiment 4  
       [90]    90. Referring now to FIGS.  17  to  19 , a level sensor in Embodiment 4 includes a sensor body  1  and a detection circuit DK. The sensor body  1  includes variable capacitors C V1  and C V2 . The detection circuit DK outputs correspond to the change of capacitance, including the stray capacitance, of the variable capacitors C V1 , CV V2  in response to the change in the portion of the length of the capacitors C V1 , CV V2  that is immersed in the liquid.  
         [91]    91. As shown in FIGS. 17 and 19, the sensor body  1  includes electrodes  1   a,    1   b,  and  1   c  susceptible to the surrounding stray capacitance. Electrodes  1   a,    1   b  and  1   c  are disposed parallel to each other. Electrodes  1   a,    1   b  and  1   c  are covered with a synthetic resin coating  10  so as to form a single integrated unit containing two variable capacitors C V1 , C V2 . Here the electrodes  1   a,    1   b,  and  1   c  are conductive wires. The sensor body  1  also has an acetabulum or suction cup  2  for attaching the sensor body  1  to a fixed wall  80  of a container  8  (FIG. 19).  
         [92]    92. When a voltage is applied to the electrodes  1   a  and  1   c,  and the center electrode  1   b  is connected to ground potential, stray capacitance is produced in the variable capacitors C V1  and C V2 , which are respectively formed between the electrodes  1   a  and  1   b,  and  1   b  and  1   c.  Since there is a difference between the dielectric constant of the air surroundings and that of the liquid surroundings, stray capacitance produced around the variable capacitors C V1  and C V2  varies depending on the level of the liquid.  
         [93]    93. The terms used to denote signals are as follows:  
         [94]    94. Capacitance of the variable capacitor C V1 : C ab0    
         [95]    95. Stray capacitance of the air surrounding the variable capacitor C V1 : C ab1    
         [96]    96. Stray capacitance of the liquid surrounding the variable capacitor C V1 : C ab2    
         [97]    97. Capacitance of the variable capacitor C V2 : C bc0    
         [98]    98. Stray capacitance of the air surrounding the variable capacitor C V2 : C bc1    
         [99]    99. Stray capacitance of the liquid surrounding the variable capacitor C V2 : C bc2    
         [100]    100. Dielectric constant of the air: ε 1    
         [101]    101. Dielectric constant of the liquid: ε 2  (with ε 1 &lt;ε 2 ) The total capacitance C ab  produced by the electrodes  1   a  and  1   b  is C ab0 + C ab1 ,+ C ab2 . The total capacitance C bc  produced by the electrodes is C bc0 + C bc1 + C bc2 . The capacitances C ab1  and C bc1  are both related to portion of the length L 1  in FIG. 19. C ab2  and C ab2  are both related to the length L 2 . Therefore, the total capacitances C ab  and C bc  change in a known way depending on the liquid level.  
         [102]    102. As shown in FIG. 18, the detection circuit DK includes fixed resistors R 1 , R 2 , and R 3 , variable capacitors C V1  and C V2 , and an EX-OR logic IC1. The resistor R 1  and the variable capacitor C V1  constitute an integration circuit IN 1  (with time constant R 1 C ab ). The resistor R 2  and the variable capacitor C V2  constitute another integration circuit IN 2  (with time constant R 2 C bc ). The fixed resistor R 3 is provided only to stabilize the operation; the detection circuit can operate without it.  
         [103]    103. The operation of the detection circuit DK at the time of the change in the liquid level in the container  8  using water as a liquid is described with reference to FIGS.  18  to  20 .  
         [104]    104. (1) When a clock CLK is input to a point P 1  under the conditions R 1 &lt; R 2 , the time constants R 1 C ab  and R 2 C bc  cause integration waveforms P 2  and P 3  as shown in FIG. 20. The output of the EX-OR logic IC1 presents the output pulse P 4  in FIG. 20 in response to the integration waveforms P 2  and P 3 .  
         [105]    105. (2) When the liquid level rises from the a height L 1  and L 2  to a height L 1 ′ and L 2 ′ (with L 1 ′&lt; L 1  and L 2 ′&gt; L 2 ; see FIG. 19), the total capacitances change from C ab  and C bc  to C ab ′ and C bc ′ (with C ab ′&gt; C ab  and C bc ′&lt; C bc ). As a result, the integration waveforms at the points P 2  and P 3  change to waveforms P 2 ′ and P 3 ′ (FIG. 20), corresponding to the time constants R 1 C ab ′ and R 2 C bc ′. The output of the EX-OR logic IC1 presents a signal P 4 ′, having an increased duty ratio, as shown in FIG. 20.  
         [106]    106. (3) The output pulse of the EX-OR logic IC1 may be integrated in a subsequent low-pass filter (not shown) to produce an analog voltage corresponding to the liquid level.  
         [107]    107. (4) FIG. 21 shows measurements of the level of water using the detection circuit DK (with R 1  set to 330 KΩ and R 2  set to 680 Ω) of FIG. 18. The output voltage therein is obtained by integrating the output pulses. The output voltage (mV) varies approximately in a straight line in a range between 5 to 50 cm in the water level, showing that the detection circuit DK is suitable for use as a level sensor. Because the dielectric constant of water (approx. 80) is much larger than the dielectric constant of air (approx. 1), the output voltage sharply changes from a water height of zero cm to a water height of 5 cm, where the water surface just comes in contact with the sensor body  1 . However, levels near zero cm can be considered outside of the valid range, and cause no problem.  
         [108]    108. (5) This level sensor includes only the sensor body  1  in FIG. 17 and the detection circuit DK in FIG. 18 as stated above and therefore can be made at very low cost. Additionally, because this level sensor has no mechanical operation part, it is very reliable.  
         [109]    109. (6) This level sensor, in the sensor body  1  in FIG. 17 and the detection circuit DK in FIG. 18, can make measurements with a high accuracy even in the case of a descending level of a highly viscous liquid. In such a case, a layer of the liquid remains adhering around the sensor body  1  as shown in FIG. 22. Such adhering liquid remainder causes the total capacitances of the variable capacitors C V1  and C V2  of the sensor body  1  to be slightly higher than they would normally be for the actual liquid level. As a result, the integration waveforms at the points P 2  and P 3  appear as P 2 + and P 3 + in FIG. 23, which are slightly higher than the waveforms P 2  and P 3  without the adhering liquid.  
         [110]    110. However, since the increases in time constant are small, the dominant effect is not a change in duty ratio of the output of the EX-OR logic IC1 at the point P 4 , but instead produces a slight displacement in the timing of the output. (compare P 4  and P 4 + in FIG. 23) Therefore, the analog voltage of the output at the point  4  through the lowpass filter remains substantially unaffected by the adhering liquid. The EX-OR logic IC1 corrects the error caused by the remaining liquid adhering to the sensor body  1 .  
       Embodiment 5  
       [111]    111. Referring now to FIG. 24, Embodiment 5 of a level sensor of the present invention includes electrodes  1   a,    1   b,  and  1   c.  The electrode  1   a  is 24 cm shorter than the other electrodes  1   b  and  1   c.  The top end of electrode  1   a  is at the same level as the top ends of the other two electrodes, whereas the bottom end is 24 cm above the bottom ends of the others. This level sensor employs the same detection circuit DK as in Embodiment 4.  
         [112]    112. In this level sensor, the output increases as the level of the liquid (water) rises up to the height of 24 cm as shown in FIG. 25. Then, as the water level rises further above 24 cm, the output starts descending. This type of level sensor may be efficiently used in a system set up to maintain a certain liquid level, for example by stopping the rise of liquid surface at the time when the output starts decreasing. In this graph, the liquid level of zero cm corresponds to the position of the bottoms of the electrodes  1   b  and  1   c.    
       Alternatives to Embodiments 4 and 5  
       [113]    113. An alternative of the sensor body  1  is illustrated in FIG. 26. This sensor body includes electrodes (conductive wires)  1   a,    1   b,  and  1   c  which are arranged mutually in parallel angularly disposed at 120-degree intervals and covered with a synthetic resin coating  10  so as to constitute two variable capacitors C V1  and C V2  therein.  
         [114]    114. Another alternative is illustrated in FIG. 27. This sensor body includes electrodes (conductive wires)  1   a  and  1   b  with a conductive pipe  1   d  surrounding them. A synthetic resin filling  10  fills the space in the pipe  1   d  to retain electrodes  1   a  and  1   b  in place. Here, the electrodes  1   a  and  1   b  receive a voltage. The conductive pipe  1   d  is grounded.  
         [115]    115. The electrode  1   a  and the conductive pipe  1   d  constitute a variable capacitor and the electrode  1   b  and the conductive pipe  1   d  constitute another variable capacitor.  
         [116]    116. There may be more than three electrodes, and the electrodes may be, for example, conductive wires or bars.  
         [117]    117. As an alternative to the detection circuit DK of Embodiment 4, a detection circuit DK in FIG. 28 may perform the same function. The drawing shows inverter IC&#39;s (open collector output) as U 1  and U 2 , resistors as R 4  and R 5 , and an EX-OR logic IC2.  
         [118]    118. Liquids, the levels of which are measured in this level sensor, include, for example, water, alcohol, petroleum, sea water, edible oil. Furthermore, particulates (i.e. powder, particles or a mixture thereof) may also be measured by this level sensor. In case of detecting the particulate level, it is desirable to use conductive bars for the electrodes, rather than conductive wires in view of the pressure of the powder.  
       Embodiment 6  
       [119]    119. Referring now to FIGS. 29 and 30, Embodiment 6 of a level sensor of the present invention is designed to detect the level of the interface between an upper and a lower liquid layer (for instance, between oil and water). This level sensor includes a float  7 , a sensor body  1  fixed to the float  7 , and a detection circuit DK which outputs a voltage corresponding to the change in capacitance (capacitance+ stray capacitance) of a variable capacitor in response to the change in the immersed length of the sensor body  1  in the upper layer liquid.  
         [120]    120. As shown in FIG. 29, the float  7  includes an upper element  70  and a lower element  71  fixed together to form a buoyant chamber capable of floating on the surface of the upper liquid. The upper element  70  has an opening  70   a  for a lead RS connected to an output part of a detection circuit DK. The lower element  71  has an insertion hole  71   a  through which the upper end of the sensor body  1  is inserted.  
         [121]    121. The sensor body  1  includes electrodes  1   a,    1   b,  and  1   c,  susceptible to the surrounding stray capacitance, which are arranged mutually in parallel and covered with a synthetic resin (or rubber) coating  10  so as to form two variable capacitors C V1  and C V2 . Electrodes  1   a  and  1   b  constitute the variable capacitor C V1 , while electrodes  1   b  and  1   c  constitute the variable capacitor C V2 .  
         [122]    122. Electrodes  1   a  and  1   c  receive voltages and the electrode  1   b  is connected to a ground potential.  
         [123]    123. Since the dielectric constant of oil is different from that of water, stray capacitance produced around the variable capacitors C V1  and C V2  depend on the thickness of the upper liquid layer, the oil layer in the example. The length of the sensor body  1  immersed in the lower liquid layer is equal to the total length of the sensor body extending below float  7 , minus the thickness of the upper layer.  
         [124]    124. The terms used to denote signals are as follows:  
         [125]    125. Total capacitance produced around the whole variable capacitor C V1 : C ab    
         [126]    126. Capacitance of the variable capacitor C V1 : C ab0    
         [127]    127. Stray capacitance in the water surrounding the variable capacitor C V1 : C ab1    
         [128]    128. Stray capacitance in the oil surrounding the variable capacitor C V1 : C ab2    
         [129]    129. Total capacitance produced around the whole variable capacitor C V2 : C bc    
         [130]    130. Capacitance of the variable capacitor C V2 : C bc0    
         [131]    131. Stray capacitance in the water surrounding the variable capacitor C V2 : C bc1    
         [132]    132. Stray capacitance in the oil surrounding the variable capacitor C V2 : C bc2    
         [133]    133. Dielectric constant of the water: ε 3    
         [134]    134. Dielectric constant of the oil: ε 4  (with ε 3 &gt;ε 4 )  
         [135]    135. The total capacitance C ab  produced by the electrodes  1   a  and  1   b  equals C ab0 + C ab1 + C ab2 , while the total capacitance C bc  produced by the electrodes  1   b  and  1   c  equals C bc0 + C bc1 + C bc2 . The electrodes  1   a,    1   b,  and  1   c  may be made of conductive wires or bars.  
         [136]    136. Referring now to FIG. 30, the detection circuit DK feed voltage through fixed resistors R 1 , R 2 , and R 3  (with R 1 &lt; R 2 ) to the variable capacitors C V1  and C V2 , and an EX-OR logic IC1. The resistor R 1  and the variable capacitor C V  constitute an integration circuit (with time constant T 2 = R 1 C ab ). The resistor R 2  and the variable capacitor C V2  constitute another integration circuit (with time constant T 3 = R 2 C bc ). A clock CLK having a constant frequency and duty ratio is input to a point P 1  in FIG. 30.  
         [137]    137. The detection circuit DK is formed on a print substrate PT shown in FIG. 29, and the sensor body  1  is provided in a downward vertical fashion from the print substrate PT.  
         [138]    138. When this level sensor is floated on the liquid surface, as shown in FIG. 29, a certain fixed length L of the sensor body  1  is always under the liquid. Therefore, when the thickness of the oil layer decreases, sensor body  1  protrudes further into the water layer. In other words, the original thicknesses of the oil and water layers being denoted as L 1  and L 2  and the subsequent thicknesses of them as L 1 ′ and L 2 ′ (not shown in the drawing), L 1  is smaller than L 1 ′ and L 2  is larger than L 2 ′ (L 1 &lt;L 1 ′, L 2 &gt;L 2 ′).  
         [139]    139. (1) This change in effective thicknesses of the water and oil layers causes a change in total capacitance of the variable capacitors C V1  and CV 2 . The changed total capacitances being denoted respectively as C ab ′ and C bc ′. Since ε 3  is greater than ε 4 , C ab ′ and C bc ′ are smaller than C ab  and C bc , respectively. (C ab ′ &lt; C ab  and C bc ′ &lt; C bc )  
         [140]    140. (2) Due to mechanical symmetry among the electrodes  1   a,    1   b,  and  1   c,  C ab  and C ab ′ are substantially equal to C bc  and C bc ′. (C ab ≅ C bc  and C ab ′≅C bc ′)  
         [141]    141. (3) The time constant at the point P 2  changes to T 2 ′ = R 1 C ab ′, and that at the point P 3  changes to T 3 ′ = R 2 C bc ′.  
         [142]    142. As described above, there are relations R 1 &lt; R 2 , C ab ≅C bc , and C ab′ ≅ C   bc ′, and the change in time constant at the point P 3  becomes larger than that at the point P 2  (i.e. (T 2 ′−T 2 )&lt; (T 3 ′−T 3 )).  
         [143]    143. The waveforms at points P 2 , P 3 , and P 4  before the change and those after the change P 2 ′, P 3 ′, and P 4 ′ are shown in FIG. 31. As apparent from FIG. 31, the duty ratio of the output pulse at the point P 4  increases with a decrease in the oil layer thickness.  
         [144]    144. (4) Therefore, the change in level of the interface between the oil and water layers wherein the dielectric constants are different can be detected as the change in duty ratio of the output pulse of the EX-OR logic IC1.  
         [145]    145. (5) Passing the output of the EX-OR logic IC1 through a low-pass filter (not shown) converts the output signal thereof to an analog voltage.  
         [146]    146. (6) In this level sensor employing the sensor body  1  in FIG. 29 and the detection circuit DK in FIG. 30, it is possible to detect the interface between the water and oil layers with a certain accuracy even when the thickness of the oil layer decreases and a bottom portion of part of the sensor body  1  which previously was submerged in the original oil layer is lowered down into the water layer. The reason for this is that, even though the respective total capacitances of the variable capacitors C V1  and C V2  of the sensor body  1  are slightly smaller than those within the actual oil layer, the respective integration waveforms at the points P 2  and P 3  have slightly smaller time constants than before. The result is merely a small phase shift in the signal at the point P 4 , but with negligible change in duty ratio (see FIG. 31).  
       Embodiment 7  
       [147]    147. Referring now to FIGS. 32 and 33, Embodiment 7 of a sensor level of the present invention has basically the same structure as Embodiment 6 in which the sensor body  1  is fixed to the float through the printed circuit substrate PT. There are slight differences in the structure of the sensor body  1  and the detection circuit DK as stated below.  
         [148]    148. The sensor body  1  is provided with electrodes  1   a  and  1   b  susceptible to the surrounding stray capacitance which are arranged in parallel and covered with a synthetic resin coating  10  so as to form a variable capacitor C V .  
         [149]    149. Application of a voltage to the electrode  1   a  and connection of the electrode  1   b  to a ground potential contribute to production of a capacitance in the variable capacitor C V  with the electrodes  1   a  and  1   b.  However, since the dielectric constant of oil is different from that of water, the stray capacitance produced around the variable capacitor C V  varies depending on the thicknesses of the oil and water layers.  
         [150]    150. The terms used to denote signals are as follows:  
         [151]    151. Total capacitance produced around the whole variable capacitor C V : C ab    
         [152]    152. Capacitance of the variable capacitor C V : C ab0    
         [153]    153. Stray capacitance around the variable capacitor C V  in water: C ab1    
         [154]    154. Stray capacitance around the variable capacitor C V  in oil: C ab2    
         [155]    155. Dielectric constant of the water: ε 3    
         [156]    156. Dielectric constant of the oil: ε 4  (with ε 3 &gt;ε 4 )  
         [157]    157. The total capacitance C ab  produced by the electrodes  1   a  and  1   b  equals C ab0 + C ab ,+ C ab2 . The total capacitance C ab  is proportional to the lengths L 1  and L 2  of the electrode bars  1   a  and  1   b.  Therefore, the total capacitance varies in a known manner with the oil layer thickness.  
         [158]    158. The detection circuit DK has, as shown in FIG. 32, fixed resistors R 1 , R 2 , and R 3  (with R 1 &lt; R 2 ), the variable capacitor C V , and an EX-OR logic IC1. The resistor R 1  and the variable capacitor C V  constitute an integration circuit (with time constant R 1 C ab ), while the resistor R 2  and the EX-OR logic IC1 constitute another integration circuit (with time constant equal to the input capacitance C in2  of the EX-OR logic IC1 multiplied by R 2 ). In this detection circuit DK, the resistance chosen for the resistor R 1  is influenced by the input capacitance C in1  of the EX-OR logic IC. However, since C ab  is much larger than C in1  (C ab &gt;&gt;C in1 ), the influence of C in1  is here neglected.  
         [159]    159. When this level sensor is floated on the liquid surface, a certain fixed length of the sensor body  1  is always under the liquid in the same fashion as in FIG. 29. And when the thickness of the oil layer decreases, the relationship between the thicknesses of the oil and the water layer respectively change from L 1  and L 2  to L 1 ′ and L 2 ′ respectively, with L 1 &lt; L 1 ′ and L 2 &lt; L 2 ′.  
         [160]    160. (1) When a clock CLK is input to the point P 1 , according to the time constants R 1 C ab , and R 2 C in2 , the integration waveforms at the points P 2  and P 3  appear as P 2  and P 3  in FIG. 33.  
         [161]    161. (2) As the thickness of the oil layer decreases, the total capacitance increases from C ab  to C ab ′ because of the difference in dielectric constant of water ε 3  and oil ε 4  (with ε 3 &gt;ε 4 ). Therefore the time constant R 1 C ab  increases to R 1 C ab ′. Consequently, the total waveform at the point P 2  changes from P 2  to P 2 ′, having a smaller inclination as shown in FIG. 33, and the threshold time T th  from the change in clock to the accord with the threshold voltage V th  extends. As a result, the output of the EX-OR logic IC1 at the point P 4 , increases from P 4  to P 4 ′. Namely, the change in the oil layer thickness corresponds to a change in duty ratio of the output of the EX-OR logic IC1.  
         [162]    162. (3) The output of the EX OR logic IC1 is sent through a low-pass filter (not shown) where it is converted to an analog voltage proportional to the duty ratio. As the oil layer thickness decreases, the output voltage increases. Although the change in the oil layer thickness is not exactly proportional to the change in duty ratio of the output of the EX-OR logic IC, from a practical standpoint, the linearity is adequate when proper circuit constants are chosen.  
       Embodiment 8  
       [163]    163. Referring now to FIG. 34, Embodiment 8 of a sensor level of the present invention has basically the same structure as Embodiments 6 and 7 in which the sensor body  1  is fixed to the float through the print substrate PT. There are slight differences in the structure of the sensor body  1  and the detection circuit DK as stated below.  
         [164]    164. The sensor body  1  includes an electrode  1   c  which is made susceptible to the surrounding stray capacitance and covered with a synthetic resin coating  10 , as shown in FIG. 34.  
         [165]    165. Due to the difference in dielectric constant between water and oil, the stray capacitance around the sensor body  1  varies depending on the thickness of the oil layer.  
         [166]    166. The terms used to denote signals are as follows:  
         [167]    167. Stray capacitance in the surroundings of the sensor body  1 : C s    
         [168]    168. Stray capacitance in the water surrounding the sensor body  1 : C s1    
         [169]    169. Stray capacitance in the oil surrounding the sensor body  1 : C s2    
         [170]    170. Dielectric constant of the water: ε 3    
         [171]    171. Dielectric constant of the oil: ε 4  (with ε 3 &gt; ε 4 )  
         [172]    172. The total capacitance C s  produced by the electrode  1   c  is C s1 + C s2 . C s1  and C s2  are respectively proportional to the respective immersed lengths L 1  and L 2  of the electrode  1   c  (see. FIG. 29). Therefore, the total capacitance varies in a known manner with the thickness of the oil layer.  
         [173]    173. The detection circuit DK has, as shown in FIG. 34, fixed resistors R 1 , R 2 , and R 3  (with R 1 &lt; R 2 ), a fixed capacitor C, the sensor body  1 , and an EX-OR logic IC1. The resistor R 1  and the combination of the fixed capacitor C and the electrode bar  1   c  constitute an integration circuit. The resistor R 2  and the EX-OR logic IC1 constitute another integration circuit. In this detection circuit DK, the resistance value chosen for resistor R 1  is also influenced by an input capacitance of the EX-OR logic IC1.  
         [174]    174. However, since this input capacitance is much smaller than the capacitance of the combination of the fixed capacitor C and the electrode  1   c,  it is herein neglected.  
         [175]    175. This level sensor provides substantially the same function as the level sensor of Embodiment 7. That is, a clock having a constant frequency and duty ratio is input to the point P 1 . The output from the integration circuit having the resistor R 1  and the combination of the fixed capacitor C and the electrode  1   c  is input to the EX-OR logic IC. The duty ratio of the output of the EX-OR logic IC1 varies corresponding to the change of the capacitance (i.e. the capacitance of the fixed capacitor C plus the stray capacitance of the electrode  1   c ) in response to change of the immersed portion length of the electrode to in the oil layer.  
       Alternatives to Embodiments 6-8  
       [176]    176. Detection circuits employed in the present invention are not limited to the ones having the above mentioned constitution as long as they provide substantially the same function.  
         [177]    177. Liquids as objects for the level sensor include any liquids having dielectric constants different from air or a second liquid such as, for example, water, alcohol, petroleum, sea water, edible oil. Any two liquids in combination may be an object in level sensing as long as those two liquids have specific gravities that cause them to separate into two separate layers, and have different dielectric constants.  
         [178]    178. Constructed as described above (but not limited to the embodiments described above), the present invention provides level sensors with which a liquid level or a particulate level can be determined continuously with high reliability, and an interface between an upper and an lower liquid layer can be detected.  
         [179]    179. Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.