Abstract:
A liquid level detection device includes a signal source for generating one or more unique signals on low impedance signal electrodes, a lock-in amplifier, and a reference signal directly connecting the signal source to the lock-in amplifier. The unique signals generated by the signal source are connected to one or more low impedance signal electrodes. The low impedance signal electrodes may be positioned at different levels inside the tank so that at any given level of liquid in the tank, each low impedance individual electrode may or may not be in contact with the liquid in the tank. Alternatively, the low impedance signal electrodes may be attached to the external wall of the tank at different levels. An antenna is connected to an input of the lock-in amplifier. The antenna may be placed inside the tank at the lowest level for the liquid, or the antenna may be attached to the outside wall of the tank near the bottom of the tank.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATIONS 
     This invention claims priority from U.S. Provisional Patent Application No. 61/583,339, filed Jan. 5, 2012, which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a liquid level detection device and more particularly relates to a liquid level detection device that includes low impedance signal electrodes and a lock-in amplifier for determining the level of liquid in a tank or other liquid storage vessel. 
     BACKGROUND OF THE INVENTION 
     Liquid handling systems generally require a means for determining the level of a liquid in a tank or other liquid storage vessel. One such liquid handling system is a condensate pump for use with a heating, ventilation, and air-conditioning (HVAC) system. A conventional condensate pump has a tank or reservoir for collecting condensate from the evaporator of the HVAC system. A centrifugal pump transfers the condensate from the tank to a remote location for disposal. In a simplified embodiment of the condensate pump, the operation of the centrifugal pump is controlled by a control circuit, which turns on the centrifugal pump when the liquid level in the tank has reached a certain level and turns off the centrifugal pump when the centrifugal pump has emptied the tank. Control functions for the condensate pump may also include sounding an alarm or shutting off the HVAC system when an emergency overflow level of condensate is reached in the tank of the condensate pump. Therefore, a liquid level detection device is necessary to detect the level of liquid in the tank in order to control the operation of the centrifugal pump, an alarm, or the continued operation of the HVAC system. 
     In order to determine the liquid level in the tank of a conventional condensate pump, a float is often used to monitor and detect the water level in the tank. In response to movement of the float in the tank, associated float switches and a float control circuit control the operation of the electric motor driving the impeller of the centrifugal pump, trigger alarms, or shut down the HVAC system if necessary. The condensate pump float is in contact with the water in the tank and is subject to fouling from debris and algae buildup. A molded float has seams, which may fail causing the float to sink or malfunction. The float switch that is used to control the on/off operation of the electric motor is often a specialized and costly bi-stable snap-action switch. A conventional condensate pump, which incorporates a safety HVAC shut off switch and/or an alarm switch in addition to the motor control switch, may have a separate float or linkage to operate the HVAC shutoff switch or the alarm switch further complicating the condensate pump. Further, a conventional condensate pump often requires a float mechanism retainer to prevent shipping damage, and the float mechanism retainer must be removed prior to pump use. 
     The prior art also includes capacitive sensors to sense the level of the water in the tank of the condensate pump to control the operation of the pump motor, to trigger alarms, or to shut down the HVAC system if necessary. In one conventional capacitive water level sensor, at least one of the capacitance plates of the capacitive sensor is in contact with the water in the tank in order to produce a detectable change in capacitance as the water contacts or exposes the capacitance plate. Capacitance plates that are in contact with the water in the tank are subject to fouling based on the buildup of debris or algae. The fouling of the capacitance plate adversely affects the performance of the capacitive sensor. 
     Prior art capacitance sensors are bi-directional in nature. Sensing plates or electrodes are connected to an oscillator circuit, an RC or LC timing circuit (where the C capacitive component varies in relation to the proximity between the sensed fluid and the sensor plate), or other circuit arrangement which facilitates the measurement of timing or frequency by forcing a change of charge onto or off of the sensor plate or electrode and measuring the time required to reach a certain threshold potential or oscillation frequency. In order to build capacitance sensors of a reasonably small scale the resultant capacitance values typically range between a few and a few hundred picofarads and therefore require circuitry with high sensitivity and high input impedance. These high impedance circuits work well to detect water and other fluids, but due to their sensitive nature are subject to minute leakage currents created by dirty insulators, slime and mineral buildup in the vicinity of the sensing electrodes. A high impedance sensor may also fail from buildup which functions to connect it to an adjacent sensor. Additional failure modes can occur from stray radio frequency fields, electrostatic buildup and other outside electromotive forces which can easily influence the high impedance capacitive sensor input. 
     In another prior art capacitive sensor, the capacitance plates are mounted outside of the tank and not in contact with the water in the tank. In order to sense accurately the water level, such prior art external capacitive sensors have a first capacitance plate extending the height of the tank and one or more additional capacitance plates position at anticipated transition points along the height of the tank in order to determine when the water level has reached one of the transition points. Such additional capacitance plates are deemed necessary in order to offset the effects of deposits that may form on the inside of the tank adjacent to the external capacitance plates thereby affecting the capacitance value. Further, the capacitance plates of the capacitive sensors typically represent a high impedance inputs to the control circuit. Because of the high impedance, the capacitance plates pick up extraneous background interference that can further affect the accuracy of the capacitive sensors. 
     Yet another prior art sensor includes a sonic detector for determining the level water in the tank. Such a sonic detector includes a sonic generator that emits a sound wave from above the water toward the water. The sonic detector further includes a receiver for receiving the echoed sonic wave as the sound wave bounces off of the water in the tank. By measuring the time between the transmission of the sonic wave and the reception of the echoed sonic wave, the water level in the tank may be determined. Such sonic detectors are difficult to calibrate and may be affected by extraneous sound waves such as those created by the pump motor. 
     SUMMARY OF THE INVENTION 
     In order to sense the level of liquid in the tank and thereby control the operation of the motor of the centrifugal pump, an alarm, or shut off the HVAC system if necessary, a liquid level detection device is employed that can reject false signals common to electronic liquid level detection systems, which false signals are caused by contamination of the sensing electrode and the containment tank and by external electrical noise sources. The level detection device of the present invention includes an electronic signal source for generating one or more signals (multi-phase or multi-frequency), a lock-in amplifier, and a reference signal directly connecting the signal source to the lock-in amplifier. The signals generated by the signal source are connected to one or more low impedance signal electrodes. The low impedance signal electrodes may be positioned at different levels inside the tank so that at any given level of liquid in the tank, each individual low impedance signal electrode may or may not be in contact with the liquid in the tank. Alternatively, the low impedance signal electrodes may be attached to the external wall of the tank at different levels. Because each signal electrode is connected to a low impedance signal source output, the signal electrode is low impedance and immune to the perils of the high impedance input typical of capacitance sensors. Particularly, because each level detecting signal is emitted from a low impedance output, a bridge of slime or debris between adjacent signal electrodes cannot conduct a false signal onto another level electrode (output) as can be the case with high impedance capacitive or conductance sensors where the level detection electrode is at any time used as an input. This use of low impedance outputs for the signals is significant even without the added noise rejection and gain provided by a lock-in-amplifier. 
     An antenna is connected to an input of the lock-in amplifier. The use of the lock-in-amplifier with high gain and noise immunity allows the reception of signals that are weakened by adjacent grounded electrodes, metallic vessels, and other conductive components that may be in contact with the liquid inside the tank. The antenna may be placed inside the tank at the lowest level for the liquid and displaced from the low impedance signal electrodes, or the antenna may be attached to the outside wall of the tank near the bottom of the tank and displaced from the low impedance signal electrodes. 
     In one embodiment, three low impedance signal electrodes are attached to the signal source and mounted inside the tank. The first low impedance signal electrode extends to near the bottom of the tank inside the tank, representing a low water level. The second low impedance signal electrode extends to a point midway point between the bottom and top of the tank, representing an intermediate water level. The third low impedance signal electrode extends to a point near the top of the tank, representing a critical high water level. The antenna is positioned inside the tank near the bottom of the tank and displaced from the low impedance signal electrodes. 
     In a second embodiment, two low impedance signal electrodes are attached to the signal source and are mounted outside of the tank on one of the walls of the tank. The first low impedance signal electrode extends vertically from near the bottom to approximate the midway depth of the tank, representing a range of water levels from a low water level to an intermediate water level. The second low impedance signal electrode extends vertically from the midway point of the tank to near the top of the tank, representing a range of water levels from the intermediate water level to a critical high water level. The antenna is positioned outside the tank near the bottom of the tank on a wall opposite from the low impedance signal electrodes. 
     Other configurations and arrangements of the low impedance signal electrodes may be used including low impedance signal electrodes mounted outside of the tank in a diagonal pattern and combinations of externally mounted an internally mounted low impedance signal electrodes and antennas. 
     In operation, the signal source produces a set of unique signals in a timed sequence on each of the low impedance signal electrodes. Each of the unique signals identifies a specific low impedance signal electrode connected to the signal source. The antenna connected to the lock-in amplifier receives the unique signals and connects the unique signals to the input of the lock-in amplifier. For a first unique signal on a first low impedance signal electrode, the signal source also produces a reference signal that is connected to the lock-in amplifier. The reference signal matches the phase and frequency of the emitted unique signal for the specific electrode being monitored. ( FIG. 5 ). The reference signal is used to select an output from the antenna amplifier whose phase matches that of the reference, and therefore matches the phase and frequency of the emitted signal. Based on the phase of the reference signal&#39;s frequency the amplifier&#39;s true or complementary signal is routed in exact phase to an integrator. Only signals that match the phase of the reference signal exactly are added to the value accumulated by the integrator. Any signal received by the antenna whose phase does not exactly match that of the reference signal, whether it be from adjacent electrodes or an external electromagnetically generated source, are both positively and negatively integrated in matching proportions and are therefore neutralized. The integrator circuit produces an output that is proportional to the strength of the received first unique signal. The strength of the received first unique signal depends on whether the first unique signal is conducted through liquid in the tank or through air in the tank. From the strength of the received first unique signal, the lock-in amplifier can determine the water level in the tank with respect to the first low impedance signal electrode that is identified by the first unique signal. 
     In sequence, a second unique signal is produced by the signal source on the second low impedance signal electrode. Again, the reference signal produced by the signal source is connected to the lock-in amplifier to select the amplifier phase and thereby sample the received second unique signal while the second unique signal is present on the second low impedance signal electrode. Once the second unique signal terminates, the integrator output value is saved for later comparison, and the integrator is reset. Again, because the second unique signal identifies the specific second low impedance signal electrode, the lock-in amplifier can determine whether the second low impedance signal electrode is in contact with the water in the tank or not depending on the strength of the received second unique signal. Likewise, a third unique signal and a third low impedance signal electrode can be employed for determining a third level of water in the tank. Additional electrodes and unique signals may be employed to refine the measurement of the depth of the liquid in the tank. 
     Further objects, features and advantages will become apparent upon consideration of the following detailed description of the invention when taken in conjunction with the drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a prior art liquid level detection device in which capacitance plates are positioned at various levels inside a tank. 
         FIG. 2  is a schematic view of a liquid level detection device with low impedance low impedance signal electrodes mounted inside the tank and a lock-in amplifier in accordance with the present invention. 
         FIG. 3  is a schematic view of a liquid level detection device with low impedance signal electrodes mounted outside the tank and a lock-in amplifier in accordance with the present invention. 
         FIG. 4  is a schematic view of a pattern of low impedance signal electrodes for a liquid level detection device with the low impedance signal electrodes mounted outside the tank in accordance with the present invention. 
         FIG. 5  is a schematic view of a liquid level detection device with low impedance signal electrodes and an alternative lock-in amplifier in accordance with the present invention. 
         FIG. 6  is a schematic view of a selectable phase integrator for substitution into the liquid level detector shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning to  FIG. 1 , a prior art capacitive liquid level detection device  1  is shown for detecting the level of liquid  4  in a tank  2 . The prior art liquid level detection device  1  has one or more capacitance plates  8   a .  8   b , and  8   c  of different lengths that are connected to a microprocessor/capacitance detector  3 . As the liquid  4  rises to contact the capacitance plates  8   a ,  8   b , and  8   c , the capacitance value for the individual capacitance plate changes. That change in capacitance value is detected by the microprocessor  3  thereby indicating the level of the liquid  4  in the tank  2 . Over time, dirt, debris, algae, and other slime  5  can build up inside the tank  2  and on the capacitance plates  8   a ,  8   b , and  8   c . The build up of dirt, debris, algae, and other slime  5  causes the value of the capacitance to vary in an unpredictable fashion thereby compromising the accuracy of the liquid level detection device  1 . 
       FIG. 2  discloses a first embodiment of a liquid level detection device  10  for detecting the level of liquid  14  in a tank  12  in accordance with the present invention. The liquid level detection device  10  comprises a signal source  16 , a first low impedance signal electrode  18   a  (low water level), a second low impedance signal electrode  18   b  (intermediate water level), a third low impedance signal electrode  18   c  (critical high water level), an antenna  20 , a sensing circuit comprising a lock-in amplifier  24 , and a detector  42 . 
     In accordance with the present invention, the term “low impedance” means that, by way of example, the signal electrodes are connected to a microcontroller output, a logic gate output, or other low-impedance solid-state output with an impedance in the range of 0 to 500 ohms and therefore significantly less apt to be perturbed than prior art level sensing electrodes that are capacitance sensors or signal inputs with typical input impedances in the range of 10,000 to Ser. No. 10/000,000 ohms. Further, in accordance with the present invention, the operation of the antenna  20  can be electro-static, electro-magnetic, or conductive. 
     The low impedance signal electrodes  18   a ,  18   b , and  18   c  are of different lengths and extend vertically into the tank  12  to different depths so that as the liquid  14  in the tank  12  rises, the liquid  14  sequentially contacts the signal electrodes  18   a ,  18   b , and  18   c . The signal source  16  generates a unique signal on each of the low impedance signal electrodes  18   a ,  18   b , and  18   c  in sequence. The unique signals may be multi-phase or multi-frequency. By assigning different phases and/or frequencies to each signal generated by the signal source  16 , the signal source  16  can create a large number of unique signals for use in connection with the liquid level detection device  10 . The signal source  16  generates a first unique signal on the first low impedance signal electrode  18   a  for a first preselected time period. After the end of the first preselected time period for the first unique signal, the signal source  16  generates a second unique signal on the second low impedance signal electrode  18   b  for a second preselected time period. After the end of the second preselected time period for the second unique signal, the signal source  16  generates a third unique signal on the third low impedance signal electrode  18   b  for a third preselected time period. The preselected time periods may be the same value, or they may be different values. With the transmission of each of the unique signals, the signal source  16  transmits a reference signal  26  to control the operation of the lock-in amplifier  24  as will be described in greater detail. 
     With continuing reference to  FIG. 2 , the lock-in amplifier  24  comprises an input  30 , an input filter  41 , an amplifier  25 , a sampling switch  38 , an integrator  40 , and an output  44 . The output  44  of the lock-in amplifier  24  is connected to the detector  42 . The input  30  is connected to the antenna  20 . The antenna  20  is positioned in the tank  12 , is displaced from the low impedance signal electrodes  18   a ,  18   b , and  18   c , and extends to the bottom of the tank  12  in order to contact the liquid  14  in the tank  12  during the entire time in which the liquid  14  rises and falls. The antenna  20  receives the unique signals generated by the signal source  16  on the low impedance signal electrodes  18   a ,  18   b , and  18   c . The received unique signal on line  30  is connected through an input filter  41  that is roughly tuned to the frequency of the unique signal. The amplifier  25  amplifies the received unique signal. The output of the amplifier  25  is connected to the integrator  40  by sampling switch  38 . The operation of the sampling switch  38  is control by the reference signal  26  from the signal source  16 . 
     In operation, the signal source  16  generates a first unique signal on the first low impedance signal electrode  18   a  for a first preselected time period. At the same time, the signal source  16  produces a reference signal  26  that closes the sampling switch  38  of the lock-in amplifier  24  for the first preselected time period. If the liquid  14  in the tank  12  is in contact with the low impedance signal electrode  18   a , the antenna  20  will receive the first unique signal as the first unique signal passes through the liquid  14 . The received first unique signal at the antenna  20  is connected to the lock-in amplifier  24  via input  30 . The input filter  41  is tuned to the frequency of the first unique signal and passes the first unique signal through to the amplifier  25 . Because the sampling switch  38  is closed, the amplified first unique signal is connected to the integrator  40 , which produces an output signal on output line  44  that is proportional to the strength of the first unique signal received by the antenna  20  through the liquid  14 . The detector  42  receives the output signal on line  44  and determines whether the strength of the output signal on line  44  is consistent with transmission of the first unique signal through the liquid  14  or consistent with the transmission of the first unique signal through air. Based on that determination, the detector  42  can determine whether the liquid  14  is in contact with the first signal electrode  18   a.    
     The liquid level detection device  10  then repeats the process by transmitting a second unique signal on the second low impedance signal electrode  18   b  (intermediate water level) for a second preselected time period and by transmitting a third unique signal on the third low impedance signal electrode  18   c  (critical high water level) for a third preselected time period. In each case, the sampling switch  38 , under the control of the reference signal  26 , gates the received unique signal into the integrator  40  in order to generate an output signal on output line  44 . Based on the value of the output signal on line  44 , the detector  42  can determine whether the liquid  14  is in contact with either the second low impedance signal electrode  18   b  and/or the third low impedance signal electrode  18   c . Based on the values of the output signal on line  44  for each of the unique signals generated by the signal source  16 , the detector  42  can determine the level of the liquid  14  in the tank  12 . 
     Because the lock-in amplifier  24  accepts only the generated unique signals while the sampling switch  38  is closed, the lock-in amplifier  24  rejects all of the signals received when the sampling switch  38  is open. Therefore, noise is rejected during the time that the sampling switch  38  is open. In addition, degradation of the unique signals, as a result of slime build up on the low impedance signal electrodes  18   a ,  18   b , and  18   c , does not adversely affect the performance of the lock-in amplifier  24 . 
       FIG. 3  shows a second embodiment of a liquid level detection device  110  for determining the level of liquid  14  in the tank  12 . The liquid level detection device  110  differs from the liquid level detection device  10  based on the arrangement of a first low impedance signal electrode  118   a  and a second low impedance signal electrode  118   b  and on the location of an antenna  120 . The low impedance signal electrodes  118   a  and  118   b  are plates, and as shown in  FIG. 3 , the low impedance signal electrodes  118   a  and  118   b  are attached to the outside of a wall  119  of the tank  12 . The antenna  120  is attached to the outside of an opposite wall  121  of the tank  12 . The first low impedance signal electrode  118   a  extends across the width of the wall  119  and extends vertically from near the bottom of the tank  12  to a mid-point gap  123 . The second low impedance signal electrode  118   b  extends across the width of the wall  119  and extends vertically from near the top of the tank  12  to the mid-point gap  123 . The low impedance signal electrodes  118   a  and  118   b  are separated at the mid-point gap  123 . 
     A signal source  116  is essentially the same as the signal source  16  except that the signal source  116  generates only two unique signals, a first unique signal for the first low impedance signal electrode  118   a  and a second unique signal for the second low impedance signal electrode  118   b . As previously indicated, the unique signals may be multi-phase or multi-frequency. The signal source  116  generates a first unique signal on the first low impedance signal electrode  118   a  for a first preselected time period. After the end of the first preselected time period for the first unique signal, the signal source  116  generates a second unique signal on the second low impedance signal electrode  118   b  for a second preselected time period. With the transmission of each of the unique signals, the signal source  116  transmits a reference signal  26  to control the operation of the sampling switch  38  of the lock-in amplifier  24 . 
     With continuing reference to  FIG. 3 , the lock-in amplifier  24  as previously described is connected to the antenna  120 . The antenna  120  receives the unique signals generated by the signal source  116  on the low impedance signal electrodes  118   a  and  118   b . The received unique signal on line  30  is connected through an input filter  41  that is roughly tuned to the frequency of the unique signal. The amplifier  25  amplifies the received signal. The output of the amplifier  25  is connected to the integrator  40  by the sampling switch  38 . The operation of the sampling switch  38  is control by the reference signal  26  from the signal source  116  as previously described. 
     In operation, the signal source  116  first generates a first unique signal on the first low impedance signal electrode  118   a  for a first preselected time period. At the same time, the signal source  116  produces the reference signal  26  that closes the sampling switch  38  of the lock-in amplifier  24  for the first preselected time period. If the liquid  14  in the tank  12  is between the first low impedance signal electrode  118   a  and the antenna  120 , the antenna  120  will receive the first unique signal as the first unique signal passes through the liquid  14 . The received first unique signal at the antenna  120  is connected to the lock-in amplifier  24  via input  30 . The input filter  41  is tuned to the frequency of the first unique signal and passes the first unique signal through to the amplifier  25 . Because the sampling switch  38  is closed during the first preselected time period, the amplified first unique signal is connected to the integrator  40 , which produces an output signal on output line  44  that is proportional to the strength of the first unique signal received by the antenna  120  through the liquid  14 . The detector  42  receives the output signal on line  44  and determines whether the strength of the output signal on line  44  is consistent with transmission of the first unique signal through the liquid  14  or consistent with the transmission of the first unique signal through air. Based on that determination, the detector  42  can determine whether the liquid  14  is between the first low impedance signal electrode  118   a  and the antenna  120 . Moreover, based on the strength of the output signal on line  44 , the detector  42  can also determine how high the liquid  14  has risen along the vertical dimension of the first low impedance signal electrode  118   a.    
     The liquid level detection device  110  then repeats the process by transmitting a second unique signal on the second low impedance signal electrode  118   b  for a second preselected time period. The sampling switch  38  under the control of the reference signal  26  from the signal source  116  gates the received second unique signal into the integrator  40  during the second preselected time period in order to generate an output signal on output line  44 . Based on the value of the output signal on line  44 , the detector  42  can determine whether the liquid  14  is between the second low impedance signal electrode  118   b  and the antenna  120 . Moreover, based on the strength of the output signal on line  44 , the detector  42  can also determine how high the liquid  14  has risen along the vertical dimension of the second low impedance signal electrode  118   b.    
     Turning to  FIG. 4 , an alternate configuration for the low impedance signal electrodes used with the liquid level detection device  110  comprises low impedance signal electrodes  218   a  and  218   b . The low impedance signal electrodes  218   a  and  218   b  are each triangular shaped plates with a gap  223  extending diagonally from a point near the top of the tank to a point near the bottom of the tank. The low impedance signal electrodes  218   a  and  218   b  are attached to the outside of the wall  119  ( FIG. 3 ) of the tank  12 . Because the signal transmitted through the liquid  14  from each of the low impedance signal electrodes  218   a  and  218   b  is proportional to the surface area of the triangle shaped low impedance signal electrode that faces the liquid in the tank, each of the triangular shaped low impedance signal electrodes  218   a  and  218   b  produces a proportional signal strength at the antenna  120  in  FIG. 3 . Therefore, as the liquid in the tank rises, the signal strength from the low impedance signal electrode  218   a  gradually increases as more of the surface area of the low impedance signal electrode  218   a  has liquid between the low impedance signal electrode  218   a  and the antenna  120 . Similarly, the signal strength from the low impedance signal electrode  218   b  gradually increases as more of the surface area of the low impedance signal electrode  218   b  has liquid between the low impedance signal electrode  218   b  and the antenna  120 . By comparing the signal strength at the output of the lock-in amplifier  24  for each of the low impedance signal electrodes, the depth of the liquid  14  in the tank  12  can be determined. 
     Turning to  FIG. 5 , an alternative lock-in amplifier  224  is shown for the liquid level detection device  10  with the low impedance signal electrodes  18   a ,  18   b , and  18   c  and the antenna  20  shown in  FIG. 2 . Particularly, the signal source  16  and the detector  42  are implemented by a microprocessor  28 . The lock-in amplifier  224  comprises an input amplifier  232 , a+1 amplifier  234 , a−1 amplifier  236 , a sampling switch  238 , and an integrator  240 . In operation, the signal source  16  generates a first unique signal on the first low impedance signal electrode  18   a  (low water level). The first unique signal is transmitted on the low impedance signal electrode  18   a  for a first preselected time period. The first unique signal is then received on the antenna  20 . The strength of the received first unique signal received by the antenna  20  depends on whether the first low impedance signal electrode  18   a  is in contact with the liquid  14  in the tank  12 . At the same time that the signal source  16  is transmitting the first unique signal on the first low impedance signal electrode  18   a , the signal source  16  also transmits a reference signal  26  to the sampling switch  238  in the lock-in amplifier  10 . The first unique signal at the antenna  20  is connected through the lock-in amplifier input  232  to the input amplifier  232 . The output of the input amplifier  232  is connected to the inputs of the +1 amplifier  234  and the −1 amplifier  236 . Depending on the polarity of the first unique signal, the reference signal  26  controls the sampling switch  238  to select either the output of the +1 amplifier  234  or the output of the −1 amplifier  236 . For example, if the polarity of the first unique signal has a positive polarity, the sampling switch  238  selects the +1 amplifier  234 . On the other hand, if the polarity of the first unique signal has a negative polarity, the sampling switch  238  selects the −1 amplifier  236 . The selected output from the +1 amplifier  234  or the −1 amplifier  236  is connected to the integrator  240 , which produces an output signal  244 . Signals that do not match the phase and frequency of the reference signal  26  positively and negatively integrate to an average of zero and are therefore rejected. The output signal  244  is proportional to the strength of the first unique signal. The output signal  244  is connected to detector  42  of the microprocessor  28 . Based on the strength of the output signal  244 , the detector  42  then determines whether the first low impedance signal electrode  18   a  is in contact with the liquid  14  or not and uses that determination to generate a control signal  246  to control the operation of the pump motor. 
     The liquid level detector  10  shown in  FIG. 5  may also be implemented by using a selectable phase integrator  340 , such as that shown in  FIG. 6 . The selectable phase integrator  340  is substituted for the integrator  240  in  FIG. 5 . Particularly, an input  330  of the selectable phase integrator  340  is connected to the antenna  20  shown in  FIG. 5 , and an output  344  of the selectable phase integrated  340  is connected to the detector circuit  42  of the microprocessor  28  shown in  FIG. 5 . The reference signal  26  is connected to a switch  338  that controls passage of the signal from the antenna  20  into the integrator  340 . 
     While this invention has been described with reference to preferred embodiments thereof, it is to be understood that variations and modifications can be affected within the spirit and scope of the invention as described herein and as described in the appended claims.