Abstract:
A liquid handling system having a tank with a centrifugal pump for pumping the liquid out of the tank is disclosed. The centrifugal pump is located beneath tank and has a coaxial inlet-outlet. Capacitive sensors are used to detect the level of liquid in the tank, and control circuits are connected to the capacitive sensors and control the operation of the pump.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims priority from U.S. Provisional Patent Application Ser. No. 61/414,188, filed Nov. 16, 2010, which is incorporated herein in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to a liquid handling system including a pump and a liquid level detector and more particularly to a pump for pumping liquid from a tank to a remote location. 
       BACKGROUND OF THE INVENTION 
       [0003]    Liquid handling systems for moving and storing liquids generally require a pump for moving liquid from one location to another and 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, and a centrifugal pump for pumping the condensate liquid from the tank to a remote location for disposal. The centrifugal pump may be submerged in the liquid inside the tank or may be located outside the tank, typically in a location that is lower than the liquid level in the tank. 
         [0004]    When the centrifugal pump is submerged in the liquid, the centrifugal pump is positioned at the lowest point in the tank in order to assure that the centrifugal pump can remove most of the liquid from the tank. An electric motor is typically mounted above the tank and is connected to the impeller of the centrifugal pump by means of a shaft. Likewise, the control circuitry is typically mounted adjacent the motor. The electric motor spins the impeller within a volute-shaped housing of the centrifugal pump, and through centrifugal force, the impeller expels the liquid from the volute-shaped housing through one or more pump outlets that are tangent to the impeller&#39;s direction of rotation. The centrifugal pump may be plumbed to convey the liquid from the pump outlet to an elevation higher than that of the tank. Often the plumbing circuit connected to the pump outlet includes a check valve to prevent liquid from flowing back into the tank when the pump is shut off. In order to control the operation of the motor and therefore the operation of the centrifugal pump, the control circuitry must include means for determining the level of liquid in the tank. Such means for determining the level of liquid in the tank may include mechanical means, such as floats, or may include electric means, such as capacitance plates submerged in the liquid in the tank. 
         [0005]    In condensate pumps where the centrifugal pump is positioned below the tank, the bottom of the tank may be fitted with a drain or screen-drain, the location of which is at the lowest point of the tank to receive the liquid by way of a gravity feed. The drain fitting is plumbed to the inlet of the centrifugal pump to convey the liquid to the pump&#39;s impeller. 
         [0006]    The centrifugal pump system described above, whether submerged in the liquid or connected to a drain from the tank, may be plagued with difficulties as a result of air or other gas trapped inside the volute-shaped housing of the centrifugal pump. Once the tank is filled with liquid, the centrifugal pump must start against head pressure created by liquid located above the pump in the tank and in the pump&#39;s outlet plumbing. As long as the volute-shaped housing is filled with liquid, the pump can start, expel liquid, and draw in new liquid from the tank. A problem may occur if the liquid has entrained air or gas. On the suction side of the pump (the pump inlet), trapped gas will tend to expand and separate from the liquid. This trapped gas, being less dense than the liquid, will be forced away from the outlet of the pump by the denser and higher pressure at the impeller&#39;s periphery, and the trapped gas will tend to collect at the suction or neutral pressure center of the impeller. If the fluid flow is great enough, the trapped gas will be expelled through the pump outlet along with the liquid. Consequently, the centrifugal pump can be caught in three distinct modes of operation:
       1. In a normal pumping mode, the liquid completely fills the inlet and outlet of the centrifugal pump, and the centrifugal pump continuously intakes liquid through the pump inlet and expels the liquid through the pump outlet.   2. In a second pumping mode of operation, gas expands out of the inflowing liquid, creates gas bubbles inside the volute-shaped housing, and minor cavitation results during the pumping operation. Because of the low volume of gas, some liquid flow continues, and the gas is discharged through the outlet of the volute-shaped housing. In this situation, pumping efficiency is reduced and audible noise is increased because of the cavitation.   3. In a third pumping mode, gas expands out of the inflowing liquid and creates a gas bubble at the inlet of the centrifugal pump. Liquid trapped at the discharge outlet of the pump and around the periphery of impeller creates a high pressure restriction. Between the liquid head pressure from the tank and the liquid head pressure of the outlet discharge plumbing, the centrifugal pump cannot move the gas bubble that is trapped in the pump&#39;s volute-shaved housing. Because the gas bubble cannot be cleared from the volute-shaped housing, liquid flow does not occur, and the pump simply spins gas or a gas/liquid mixture. This condition is sometimes confused with cavitation but in fact is a simple balance of liquid pressure and gas pressure within the volute-shaped housing. Often the bubble of gas will remain in the impeller&#39;s volute-shaped housing when the centrifugal pump is stopped and will continue to block liquid flow through the pump when the pump is restarted.       
 
         [0010]    As previously indicated, in order to control the operation of the centrifugal pump for a condensate pump, the condensate pump must be able to determine accurately the liquid level in the tank and in the pump&#39;s volute-shaped housing. In a conventional condensate pump, a float monitors and detects the water level within the pump&#39;s tank. In response to movement of the float within the tank, associated float switches and a float control circuitry 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. 
         [0011]    The prior art has also adopted capacitive sensors as liquid level detectors to determine the level of the water in the tank of the condensate pump to replace the mechanical float for controlling the operation of the pump motor, for triggering alarms, or for shutting down the HVAC system if necessary. In some conventional liquid level detectors, at least one of the capacitance plates of the capacitive sensors 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 of the capacitive sensor. 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. 
         [0012]    In order to determine 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 capacitive sensor thereby affecting the capacitance value. 
       SUMMARY OF THE INVENTION 
       [0013]    In order to solve the problems associated with the presence of gas in a centrifugal pump, the present invention provides a coaxial inlet and outlet configuration for the centrifugal pump. The centrifugal pump with its integral electric motor is mounted below the tank. The pump inlet of the centrifugal pump is located at the center of the volute-shaped housing and is connected to the small end of a funnel. The large end of the funnel is connected to the bottom of the tank so that liquid in the tank is fed through the funnel to the pump inlet by gravity. The pump outlet of the centrifugal pump is coaxially positioned within the funnel and communicates with the periphery of the volute-shaped housing. Such a coaxial inlet and outlet configuration with a funnel feed inhibits gas from being trapped at the inlet (center) of the volute-shaped impeller housing. The volute-shaped impeller housing with its funnel inlet and coaxial outlet can be molded or cast as a single piece without the need for additional machining operations. Moreover, the funnel connected to the pump inlet is configured to produce a flow of water into the volute-shaped impeller housing in a direction counter to the rotation of the impeller thereby further inhibiting gas from being trapped at the inlet to the volute-shaped impeller housing. 
         [0014]    In order to detect the level of liquid in the tank and thereby control the operation of the electric motor of the centrifugal pump, an external capacitive sensor is mounted externally on one of the walls of the tank. Particularly, the external capacitive sensor comprises a printed circuit board that extends along the height of the tank. The printed circuit board has a shield foil on the outside of the printed circuit board to shield the capacitive sensor from external electromagnetic noise and interference. The shield foil is connected to circuit ground. The shield foil may also be configured as part of a guard ring circuit to decrease the impedance of the shield foil and thereby providing greater shielding against external electromagnetic noise and interference. The printed circuit board also has one or more sensor foils (capacitance plates) positioned on the printed circuit board between the printed circuit board and the external wall of the tank. The sensor foils are connected to a control circuit that determines the liquid level based on the capacitance values measured at the sensor foils. In addition, a pump sensor terminal (capacitance plate) is positioned inside the volute-shaped impeller housing or in the inlet of the volute-shaped impeller housing for determining the presence or absence of liquid within the volute-shaped impeller housing. 
         [0015]    In one embodiment of the capacitive sensor, the sensor foil may include a single foil extending the height of the tank. In another embodiment of the capacitive sensor, the sensor foil may include a series of sensor foils extending along the height of the tank and divided from each other vertically to create separate sensor foils with recognizable transition points between the vertically separated sensor foils. Further, the sensor foil or foils may be configured in a discontinuous pattern, such as a hexagonal pattern, to create discontinuities within each sensor foil and thereby recognizable discontinuities in the sensed capacitance. 
         [0016]    As the liquid in the tank rises, the sensor foils, either as a single continuous foil or as a series of separate vertically spaced foils, provide recognizable capacitance values, which in turn are resolved by the control circuit as particular liquid levels within the tank. From the sensed liquid level, the control circuit can control the operation of the motor of the centrifugal pump to start the centrifugal pump when the tank has filled with liquid to a predetermined level, to stop the motor of the centrifugal pump when the tank has emptied to a predetermined level, to shut off the source of liquid into the tank, such as by shutting off an HVAC system, and to sound an alarm if the liquid reaches a critical high level in the tank. 
         [0017]    If the control circuit determines that the volute-shaped impeller housing has dried out based on the capacitance value measured at the pump&#39;s sensor terminal within the centrifugal pump or near the pump inlet of the centrifugal pump, the control circuit can start the centrifugal pump in a priming mode. The priming mode rapidly turns the motor of the centrifugal pump on and off in an attempt to knock any air bubbles off of the impeller blades before the pumping operation begins. 
         [0018]    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 
         [0019]      FIG. 1  is a front perspective view (front and right side) of a condensate pump having a centrifugal pump with a coaxial inlet and outlet in accordance with the present invention. 
           [0020]      FIG. 2  is a front perspective view (front and left side) of the condensate pump having a centrifugal pump with a coaxial inlet and outlet in accordance with the present invention. 
           [0021]      FIG. 3  is a front elevation view of the condensate pump having a centrifugal pump with a coaxial inlet and outlet in accordance with the present invention. 
           [0022]      FIG. 4  is a top plan view of the condensate pump having a centrifugal pump with a coaxial inlet and outlet in accordance with the present invention. 
           [0023]      FIG. 5  is a bottom plan view of the condensate pump having a centrifugal pump with a coaxial inlet and outlet in accordance with the present invention. 
           [0024]      FIG. 6  is a right side elevation view of the condensate pump having a centrifugal pump with a coaxial inlet and outlet in accordance with the present invention. 
           [0025]      FIG. 7  is a left side elevation view of the condensate pump having a centrifugal pump with a coaxial inlet and outlet in accordance with the present invention. 
           [0026]      FIG. 8  is a section view of the condensate pump having a centrifugal pump with a coaxial inlet and outlet in accordance with the present invention as seen along line  8 - 8  of  FIG. 3 . 
           [0027]      FIG. 9  is a section view of the condensate pump having a centrifugal pump with a coaxial inlet and outlet in accordance with the present invention as seen along line  9 - 9  of  FIG. 3 . 
           [0028]      FIG. 10  is a section view of the condensate pump having a centrifugal pump with a coaxial inlet and outlet in accordance with the present invention as seen along line  10 - 10  of  FIG. 3 . 
           [0029]      FIG. 11  is an exploded perspective view of the condensate pump having a centrifugal pump with a coaxial inlet and outlet in accordance with the present invention. 
           [0030]      FIG. 12  is a front elevation view of the centrifugal pump with a coaxial inlet and outlet for the condensate pump in accordance with the present invention. 
           [0031]      FIG. 13  is a top plan view of the centrifugal pump with a coaxial inlet and outlet for the condensate pump in accordance with the present invention. 
           [0032]      FIG. 14  is a right side elevation view of the centrifugal pump with a coaxial inlet and outlet for the condensate pump in accordance with the present invention. 
           [0033]      FIG. 15  is a section view of the centrifugal pump with a coaxial inlet and outlet for the condensate pump in accordance with the present invention as seen along line  15 - 15  of  FIG. 12 . 
           [0034]      FIG. 16  is an exploded perspective view of centrifugal pump with a coaxial inlet and outlet for the condensate pump in accordance with the present invention. 
           [0035]      FIG. 17  is a front elevation view of a capacitive sensor for the condensate pump in accordance with the present invention. 
           [0036]      FIG. 18  is a back elevation view of a capacitive sensor for the condensate pump in accordance with the present invention. 
           [0037]      FIG. 19  is a side elevation view of a capacitive sensor for the condensate pump in accordance with the present invention. 
           [0038]      FIG. 20  is a schematic diagram of a control circuit for the condensate pump having the capacitive sensor in accordance with the present invention. 
           [0039]      FIG. 21  is a schematic diagram of a negative amplifier (configured as an integrator) for a shielding electrode (guard ring) for the capacitive sensor in accordance with the present invention. 
           [0040]      FIG. 22  is a schematic diagram of a positive amplifier (configured as a guard ring) for the capacitance sensor in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0041]    Turning to  FIGS. 1 and 2 , a condensate pump  10  is shown comprising a tank (or reservoir)  12 , a base  26 , and a centrifugal pump  28  with an integrated electric motor  40 . The tank  12  and the base  26  are molded as a single part. The base  26  houses the centrifugal pump  28  and the integrated electric motor  40  below the tank  12 . 
         [0042]    As shown in  FIGS. 1 and 2 , the tank  12  is generally rectangular in shape and has a tank bottom  14 , a tank top  16 , a tank front  17 , a tank back  18 , and tank sides  20 . The tank  12  has a tank inlet  22  in the tank top  16  for receiving liquid, such as condensate water from an HVAC system. The tank  12  has a tank outlet  24  ( FIG. 11 ) in the bottom  14  of the tank  12  that is generally rectangular in shape. The centrifugal pump  28  with the integrated electric motor  40  is fastened by means of screws  48  to the underside of the bottom  14  of the tank  12 . 
         [0043]    With reference to  FIGS. 12-16 , the centrifugal pump  28  comprises a volute-shaped impeller housing  30  with a pump inlet  32  ( FIG. 15 ) in the center of the volute-shaped housing  30  and a pump outlet  34  at the periphery of the volute-shaped housing  30 . An impeller  36  with impeller blades  38  is mounted for rotation within the volute-shaped impeller housing  30  ( FIG. 16 ). The integrated electric motor  40  drives the impeller  36 . 
         [0044]    The tank outlet  24  ( FIG. 11 ) and the pump inlet  32  are connected together by means of a funnel  42  ( FIG. 11 ). The funnel  42  has a large top opening  44  that is connected to the tank outlet  24  ( FIG. 11 ). The funnel  42  at its opposite end has a small opening  46  that is connected to the pump inlet  32  of the volute-shaped impeller housing  30  ( FIGS. 13 and 15 ). The funnel  42  has a substantially vertical side  56  and a facing angled side  58  ( FIGS. 14 and 15 ). The facing angled side  58  directs the flow of water into the volute-shaped impeller housing in a direction counter to the rotation of the impeller thereby inhibiting gas from being trapped at the pump inlet  32 . The volute-shaped impeller housing  30 , the pump inlet  32 , the pump outlet  34 , and the funnel  42  can all be molded as a single piece. 
         [0045]    With reference to  FIGS. 10-16 , the pump outlet  34  from the periphery of the volute-shaped impeller housing  30  is positioned coaxially within the funnel  42  and extends upwardly through the tank outlet  24  into the tank  12  ( FIG. 10 ). The pump outlet  34  is connected to an outlet connector  50  by means of a transition tube  54  and a check valve  52  ( FIG. 11 ). The outlet connector  50  is connected to tubing (not shown) for carrying the condensate water away to a disposal location. 
         [0046]    The positioning of the funnel  42  above of the pump inlet  32  in combination with the gravity fed condensate water from the tank  12  reduces the amount of air bubbles that are sucked into the volute-shaped impeller housing  30  through the pump inlet  32 . The large opening  44  of the funnel  42  allows air bubbles near the pump inlet  32  to bubble up through the funnel  42  and escape into the condensate water in the tank  12 . Consequently, the chances of the centrifugal pump  28  becoming airlock or cavitating are substantially reduced. 
         [0047]    In order to control the operation of the electric motor  40  and therefore the centrifugal pump  28 , a capacitive liquid level sensor  100  is positioned externally to the tank  12  and a pump sensor terminal  110  ( FIG. 13 ) is positioned adjacent the pump inlet  32 . Both the capacitive liquid level sensor  100  and the pump sensor terminal  110  are connected to a control circuit  112  ( FIG. 20 ) for controlling the operation of the motor  40 , the operation of water level display LEDs  114  ( FIGS. 17 and 20 ), and the operation of an HVAC shutoff relay  128  ( FIG. 20 ). 
         [0048]    As shown in  FIGS. 8 ,  11 , and  17 - 20 . the pump sensor terminal  110  ( FIG. 13 ) is positioned adjacent the pump inlet  32  and is connected to the control circuit  112  by means of a collector line  120 . The capacitive sensor  100  comprises a circuit board  102  that is positioned externally to one of the tank sides  20  and extends along the majority of the height of the tank  12 . A shield foil  104  ( FIG. 17 ) covers the side of the circuit board  102  that faces away from the tank  12 . The foil shield  104  may be continuous or patterned in order to adjust the value of the capacitance at sensor foils  106  and  108 . The shield foil  104  is connected to the circuit ground to minimize electromagnetic noise and interference from external sources. In order to provide additional shielding against electromagnetic noise interference from external sources, guard ring circuits, such as guard ring circuits  160  and  162  shown in  FIGS. 21 and 22 , may be employed to lower the impedance of the shield foil  104 . The operation of the guard ring circuits  160  and  162  will be described in greater detail below. 
         [0049]    With reference to  FIGS. 18 and 20 , one or more sensor foils, such as first (lower) sensor foil  106  and second (upper) sensor foil  108  cover the other side of the circuit board  102  that faces the tank side  20 . The sensor foils  106  and  108  each represent a capacitance plate of a capacitor for which the liquid inside the tank  12  forms part of the capacitor&#39;s dielectric. Consequently, as the liquid inside the tank  12  rises and falls, the capacitance value of the sensor foils increases and decreases thereby providing a representation of the level of the liquid inside the tank  12 . In the embodiment of the capacitive sensor  100  shown in  FIGS. 17-20 , the sensor foils  106  and  108  are configured in a hexagonal pattern with the individual hexagonal foils in the first foil  106  interconnected by foil connector lines  116  and the individual hexagonal foils in the second foil  108  interconnected by foil connector lines  118  ( FIG. 20 ). 
         [0050]    As can be seen with reference to  FIGS. 8 ,  17 , and  18 , the first foil  106  extends from a position near the bottom  14  of the tank  12  to a gap  124  positioned about halfway up the height of the tank  12 . After the gap  124 , the second foil  108  extends from the gap  124  to near the top  16  of the tank  12 . As the liquid in the tank  12  rises above the bottom  14  of the tank  12 , the dielectric value for the first sensor foil  106  changes, and as a result, the capacitance value for the first sensor foil  106  changes accordingly (while the capacitance value for the second sensor  108  remains substantially constant). Once the liquid in the tank  12  bridges the gap  124  and then engages the second foil  108 , the discontinuity between the first foil  106  and the second foil  108  is recognized by the control circuit  112  so that the halfway reference point of the tank is established and used as a calibration point for the control circuit  112 . As the liquid in the tank  12  continues to rise along the height of the second foil  108 , the capacitance value for the second foil  108  continues to change accordingly (while the capacitance value for the first sensor  106  remains substantially constant). 
         [0051]    In addition to establishing the calibration point by means of the gap  124  between the sensor foils  106  and  108 , the calibration point positioned between the top and the bottom of the tank can also be established by mechanical means such as having a tank wall thickness below the calibration point greater than the tank wall thickness above the calibration point or vice versa. Because the wall of the tank represents part of the dielectric that also includes the liquid in the tank, an abrupt change in the tank wall thickness serves to establish an abrupt change in capacitance value, the calibration point, when the water in the tank reaches the transition point between the thick wall of the tank and the thinner wall of the tank. 
         [0052]    Turning to  FIG. 20 , the control circuit  112  controls the operation of the pump motor  40 , the liquid level display LEDs  114 , an alarm  126 , and the HVAC shutoff relay  128 . The functions of the control circuit  112  are implemented by a microprocessor  130 . The inputs to the control circuit  112  include the first sensor foil line  132 , the second sensor foil line  134 , and the collector line  120 . Each of the lines  132 ,  134 , and  120  connects a capacitance value for the first sensor foil  106 , the second sensor foil  108 , and the pump sensor terminal  110  to the control circuit  112 . In order to determine the capacitance value for the first sensor foil  106 , the second sensor foil  108 , and the pump sensor terminal  110 , the microprocessor  130  has a drive pin  136  that drives a first foil input pin  138  through an RC timing circuit that includes the capacitance value of the first sensor foil  106 . The microprocessor drive pin  136  also drives a second foil input pin  140  through an RC timing circuit that includes the capacitance value of the second sensor foil  108 . Likewise, the microprocessor drive pin  136  drives a pump sensor input pin  142  through an RC timing circuit that includes the capacitance value of the pump sensor terminal  110 . 
         [0053]    Particularly, when the microprocessor  130  initiates a sense cycle, the microprocessor  130  starts a counter for each of the input pins  138 ,  140 , and  142 , and then the microprocessor  130  begins driving each of the input pins  138 ,  140 , and  142  positively through their respective RC timing circuits. Once each of the input pins  138 ,  140 , and  142  reaches a predetermined threshold value its respective counter is suspended. Once all of the input pins  138 ,  140 , and  142  have reached their respective predetermined threshold values, the microprocessor drive pin  136  reverses polarity and begins discharging the capacitance in the RC timing circuits. At the same time, each of the counters resumes counting. When each of the input pins  138 ,  140 , and  142  reaches a zero value, each of the counters is stopped. The count on each of the counters is thereby proportional to the capacitance value for the first sensor foil  106 , the second sensor foil  108 , and the pump sensor terminal  110 , which is in turn indicative of the level of the liquid in the tank  12 . The charge/discharge sequence is employed to minimize any residual dc build up on the capacitance plates or in the circuit components. 
         [0054]    While the microprocessor  130  can determine the capacitance value for the first sensor foil  106  (input pin  138 ), the second sensor foil  108  (input pin  140 ), and the pump sensor terminal  110  (input pin  142 ) in parallel fashion as described above, the microprocessor  130  can also determine the capacitance value for the first sensor foil  106  (input pin  138 ), the second sensor foil  108  (input pin  140 ), and the pump sensor terminal  110  (input pin  142 ) in serial fashion. In the serial sensing case, the input pins that are not being sensed are driven to ground and act as an additional shields and ground references for the capacitance plate attached to the input pin that is being sensed. Further, in the serially sensing case, sensing the capacitance values for the first sensor foil  106  (input pin  138 ), the second sensor foil  108  (input pin  140 ), and the pump sensor terminal  110  (input pin  142 ) requires only a single counter implemented by software in microprocessor  130 . 
         [0055]    Based on the level of the liquid in the tank  12 , the microprocessor activates the liquid level display LEDs using a multiplex scheme to give a visual indication of the liquid level in the tank  12 . Further, when the liquid in the tank reaches a certain height, the microprocessor  130  starts the motor  40  in order to empty the tank  12 . The microprocessor  130  also controls the speed of the motor  40  by varying the pulse width of a speed control signal line  146  to the MOSFET speed control switch  144 . Once the level of the liquid in the tank  12  drops below a predetermined level, the microprocessor  130  shuts off the motor  40  until the next pumping cycle is required to empty the tank  12 . If the level of liquid in the tank  12  rises above a certain predetermined emergency level, the microprocessor  130  can control the operation of an HVAC shutoff relay  128  to stop the HVAC system and thereby cut off further flow of condensate water into the tank  12 . At the same time, the microprocessor  30  can trigger the alarm  126 . 
         [0056]    In the circumstance where the condensate pump  10  has not received any condensate water for an extended period of time and where all of the condensate water in the tank  12  and in the volute-shaped impeller housing  30  has evaporated, the air within the dried out volute-shaped impeller housing  30  may be trapped by the initial reintroduction of condensate water into the pump inlet  32 . The microprocessor  130  determines that the volute impeller housing  30  has dried out by reference to the capacitance value of the pump sensor terminal  110 . Once the microprocessor  130  has determined that the volute impeller housing  30  is dry and that air bubbles may be present inside the volute-shaped impeller housing  30 , the microprocessor  130  initiates a priming mode startup for the motor  40 . In the priming mode, the motor  40  is rapidly turn on and off by the microprocessor  130  in an attempt to dislodge air bubbles that may be attached to the impeller blades  38  of the impeller  36  ( FIG. 16 ). 
         [0057]    Turning to  FIGS. 21 and 22 , guard ring circuits  160  and  162  serve to shield the sensor foils, such as sensor foil  106 , from electromagnetic noise and interference. As previously described, the sensor foils, such as sensor foil  106 , are driven positively and negatively by the drive pin  136  of microprocessor  130 . With respect to guard ring circuit  160  shown in  FIG. 21 , an inverting amplifier  150  drives the shield foil  104  with an opposite polarity to that of the charging voltage of drive pin  136 . The capacitance between shield foil  104  and sensor foil  106  then becomes the capacitor of an integrator. By adjusting the capacitance between the shield foil  104  and the sensor foil  106 , interfering signals superimposed on the sensor foil  106  may be canceled. The guard ring circuit  162  shown in  FIG. 22 , has an operational amplifier  152  connected as a voltage follower. In this voltage follower configuration, leakage currents that might flow to or from the sensor foil  106  are nullified by the surrounding shield foil  104 , which is driven to the same electrical potential as the sensor foil  106 . 
         [0058]    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.