Abstract: A vacuum pump control apparatus for an evacuating-type waste water collecting system is disclosed. In the system, waste water discharged from houses and facilities is collected into a water collecting tank by vacuum sewer pipes provided with a negative pressure therein, and the waste water collected in the water collecting tank is discharged by a booster pump while the air in the water collecting tank is discharged by a vacuum pump. The vacuum pump control apparatus comprises a gas-liquid ratio detection arrangement for detecting the ratio of the amount of air to the amount of waste water to be collected into the water collecting tank, and a control system for regulating the operating time of the vacuum pump based upon the gas-liquid ratio detected by the gas-liquid ratio detecting arrangement, so that a target value of the gas-liquid ratio is recovered when it falls below the target value.

Abstract: An optical bilge pump apparatus has two optical fibers with gaps cut therein and associated light transmitters-receivers. The apparatus is responsive to high and low liquid levels in the pump such that the pump is turned on when both gaps in the optical fibers are submersed in liquid, and the pump is turned off when both gaps in the optical fibers are not submersed in liquid.

Abstract: A hazardous fluid removal and storage apparatus for the fail-safe removal and storage of hazardous liquids. An electrically monitored and electro-pneumatically controlled system provides safe pumping and storage of hazardous liquids with automatic pump shut-down and warning signals for leak detection and full storage tank conditions.

Abstract: A pumping arrangement for pumping water to ships comprises a pump having an inlet and an outlet, a level sensor, connectable to the outlet, for insertion into a water tank, and a control arrangement associated with the level sensor. The control arrangement de-activates the pump in response to a signal from the level sensor so as to avoid over-filling of the tank. The pumping arrangement further comprises transmitting means to transmit a signal at the same time as the control arrangement de-activates the pump. Such a signal is transmitted to, for example, a paging unit. Several pumping arrangements may be used at once with several ships, the paging unit being adapted to inform the operator when a given pumping arrangement has filled a water tank.

Abstract: An air conditioner condensate sump pump controller comprising a self heated PTC resistor located in the sump including an ambient compensation circuit. When the condensate reaches the resistor level due to decreased resistance the sump pump is energized reduce the condensate level.

Abstract: A submersible well probe sleeve made of a cylindrical tube attached to a submersible pump. The cylindrical tube has a number of slots exposing access to a protective pouch. Inside the protective pouch a fluid level sensing probe is placed. The fluid has access to the pouch through slots in the sleeve. Opposite the pouch at a predetermined level is a number of fluid inlet holes which provide access to the center of the cylinder by fluid in the well. The submersible pump draws fluid into the center of the cylinder past the probe. The submersible well probe sleeve provides a fixed relative position between the probe and the inlets such that the level of the well can be maintained at a precise level and the probe can be installed in the well while under the protection of the sleeve.

Abstract: An oil and water separator for separating oil from water collected in a bilge of a boat prior to discharging the water overboard. A reservoir is provided for receiving the mixture of oil and water from the bilge. The oil separates from the water and floats on top of the water. When only water is positioned between a pair of spaced electrical probes carried in the reservoir, such causes a drain valve to be opened to drain the water from the reservoir. As the water is drained from the reservoir, the oil floating on top of the water will come between the probes and close the valve, stopping the draining of water from the bottom of the reservoir. A mixture of oil and water is pumped from a tank carried in the bilge at an intermittent rate so that the mixture is allowed sufficient time to separate in the reservoir.

Abstract: A drainage pump (10) in which the portion near the entrance of the impeller (12) in the suction tube (11) is open to the set lowest suction water level (L.W.L.) through an air intake pipe (15), so that air is introduced into the portion near the entrance of the impeller (12) when the suction water level (W.L.) is lower than the preset lowest suction water level. An air intake hole (14) may also be provided through the water intake suction tube (11) near the impeller.

Abstract: The present invention is a liquid level control system for selectively activating and deactivating a pump according to the liquid level indicated by capacitive sensors. Activating circuitry starts the pump motor when both upper and lower sensors indicate the presence of liquid. The pump motor is latched on by the activating circuitry and continues operation until the lower capacitive sensor indicates the absence of liquid, in which case the pump is shut off. A high frequency oscillator operates reliably over a wide range of operating conditions to maintain relatively constant charging time for the capacitive sensors.

Abstract: A control circuit for a sump pump or other pump that controls the liquid level in a vessel. The liquid is maintained between upper and lower levels defined by probes in the vessel. The probes are associated with comparators and connect with one comparator input. The upper limit comparator connects with the gate of an SCR which supplies current to the lower limit comparator and also to a relay which controls the on/off condition of the pump. The output of the lower limit comparator controls a switching transistor which is arranged in a series circuit with the SCR and relay coil. Once the SCR is conductive, it is latched in the conductive state until the transistor is made nonconductive by the lower limit comparator. In a sump pump application, latching of the SCR allows the pump to remain activated after the liquid level is reduced below the upper probe.

Abstract: The present invention provides a pump pack for automatic and continuous emptying of water condensate from a dehumidifier positioned below ground level or remotely from a gravity drain. The present invention includes a small submersible electrical pump powered by a conventional 110 volt AC power source, includes an on/off float switch connected with the power source, in cooperation with a flexible hose of small diameter of variable length. Condensate collected in the water collection pan is withdrawn at a preselected water level by the pump and passes through the tubing to an elevated or remote drain means or to other appropriate areas for disposal. The present invention permits continuous operation of portable dehumidifiers which would otherwise shut off when the condensate water pan fills after approximately one day's unattended use.

Abstract: A variable-speed pump system comprising: a hydraulic machine having a flow adjusting means and including at least a pumping function; a rotary machine operatively connected to the hydraulic machine and including at least a motor function for rotating the hydraulic machine; a frequency converter for driving the rotary machine; and a control system for controlling variable-speed operation of the rotary machine according to a command signal from an external unit. The pump system also includes an electric power control system, a flow adjustment control system, and a rotational speed control system. The power control system has a negligibly small time constant, compared to the time constant of the rotational speed control system.

Abstract: The present invention is a liquid level control system for selectively activating and deactivating a pump according to the liquid level indicated by capacitive sensors. Activating circuitry starts the pump motor when both upper and lower sensors indicate the presence of liquid. The pump motor continues operation until both of the capacitive sensors indicate the absence of liquid, in which case the pump is shut off. The circuitry of the present invention operates reliably over a wide range of operating conditions.

Abstract: A water flow system for collecting and discharging waste water, storm water and the like includes a water pumping station into which water is delivered through an inflow main and from which water is pumped through a water discharge main. The pumping station has a water collection well, a pair of water pumps and a conduit arrangement by which the suction side of each pump is communicated with the well and the pressure side of each pump is communicated with the discharge main. A secondary conduit also communicates the pressure side of one pump with the suction side of the other pump. A control system is operative to actuate one of the pumps for normal pumping conditions and to actuate both pumps during periods of heavy water inflow to the station, the secondary conduit providing serial flow of water through the pumps under such conditions to substantially increase the water outflow rate from the pumping station in comparison to conventional parallel combined operation of the pumps.

Abstract: An emergency device for use in a dwelling or any similar place provided with a sump pump in the basement, in order to operate this sump pump and, if desired, to switch on some emergency light in the case of a failure of the main electric power source of this dwelling. The device includes a storage battery connected to the sump pump in such a manner as to operate it in the case of a failure of the main power source. It also includes a transformer-rectifier to permanently recharge the storage battery when the power source is in operation. It further includes external plugs connectable to the battery of an automotive vehicle parked near the dwelling with its motor running, in order to use this battery as a substitute for the indoor battery and thus allows the sump pump to continue to operate even when the storage battery indoors is down.

Abstract: A protector device connected in series with a pool pump motor. When the automated pool timer contacts close, supplying AC power to pool pump motor, the protector apparatus logic permits instantaneous motor turn on and logic reset of apparatus clock timer. After a nominal period of time, pump motor will be shut off, unless water is flowing through pump motor. In the event water is flowing through pump motor, a water flow sensing switch in the water line to pump will override clock logic, and hold pump motor on, until the automated pool timer reaches a preset trip turn off time.

Abstract: A method and apparatus for measuring low fuel level in a fuel tank having a fuel pump with an anti-slosh pot supplied with fuel from a predelivery pump in the fuel pump. Fuel measurement is effected by sensing delivery of air to the anti-slosh put by the predelivery pump when the fuel level in the tank is low, and measuring the amount of fuel in the anti-slosh pot when delivery of air is sensed. The amount of fuel measured in the anti-slosh pot accurately represents the amount of fuel remaining in the fuel tank.

Abstract: A hydraulic pump composed of a pump and an accumulator combined therewith has an outer shell divided into an accumulator cylinder of metal and a tank of resin. The accumulator cylinder and said tank have thicker side portions, respectively, which define therein an outlet oil passage and an inlet oil passage, respectively.

Abstract: A system for the automatic operation of a marine bilge pump is provided. The system comprises, in series, a fluid level detector, a fluid level discrimination circuit, and pump control circuitry. The fluid level detector uses an AC signal to determine when an unacceptable level of bilge water is present. The discrimination circuit determines when the fluid level detector is sensing a true level reading, as opposed to mere bilge water sloshing. If the water level is indeed too high, a bilge pump is activated. The system may be adapted for multiple pump operation. Furthermore, an emergency backup circuit, as well as alarms and a test circuit, may be added.

Abstract: Disclosed herein is an apparatus and method for controlling electrically powered liquid-moving pumps, particularly bilge and sump pumps. The apparatus is characterized by the fact that no portion thereof need be located in the hostile environment of the bilge or sump served by the pump controlled thereby.

Abstract: According to the vertical pump of the present invention, a reverse U-shape air suction passage is connected to a suction casing which communicates with the impeller chamber, and a U-shape branch passage is branched from a hanging portion of the air suction passage. According to this vertical pump, when the water level is raised in the air operation state, air suction into the suction casing through the air suction passage and the branch passage is stopped when the water level reaches the pumping starting water level, so that the operation of the vertical pump may be changed to the water pumping operation. Where the water level is lowered in the state of water pumping operation, air suction into the suction casing through the branch passage and the air suction passage may start at the pumping stopping water level which is lower than the pumping starting water level. Accordingly, the operation of the vertical pump may be changed to the air operation.

Abstract: A method of monitoring and controlling a pumped well having a rod string extending from a pumping unit located at the earth's surface to a subterranean pump, the rod string being sequentially reciprocated through up and down strokes and the well producing pumped fluid through a collection pipe, the method includes measuring the displacement and the load on the rod string, determining the well is pumped-off when fluid flow from the well substantially stops, recording the maximum load on the sucker rod string during pumped-off conditions, the load being measured during a selected portion of the first portion of the rod string downstroke, establishing a target pumped-off load as a selected percentage of the measured load, periodically initiating a pumping cycle, terminating each pumping cycling when the rod load equals the target rod load.

Abstract: The invention relates to an arrangement for the automatic removal of water collecting in a fuel filter having a water separator. The evacuation takes place by means of a piston acted upon by a pressure medium, preferably a fuel pressurized by the pressure of the fuel pump, the piston being guided in the container of the fuel filter containing the water. It is particularly advantageous for an already existing drive to be usable for producing the pressure medium and that therefore a simple and inexpensive solution is offered for the removal of the water.

Abstract: A control system for an in-well pump for recovery of hydrocarbons which are lighter than water. Two electrical probes and a float switch are used to control the automatic raising and lowering of the pump within the well to always maintain its input within the hydrocarbon layer. The pump is also controlled so that it only operates when its input is within the hydrocarbons to assure that it does not pump water and that it is not operated in air.

Abstract: A system for monitoring and controlling the level of a fluid in a vessel, particularly applicable to controlling the pump operation at an oil well. The system includes a fluid level sensing unit having a plurality of sensors positioned at selected levels in the well casing outside the tubing string. The sensors operate independently to detect the presence of fluid and emit electrical signals which are received by a data conversion and sending unit immediately above the fluid level sensing unit in the well casing. The data conversion and sending unit converts the signals into discreet digital data bits which can be transmitted sequentially in one conductive wire to a data management and pump control unit at a convenient location nearby, usually at the well site. Thus, there is no need for a wire extending from each sensor. The data bits received by the data management and pump control unit are converted to a visual display in a light emitting diode panel.

Abstract: The present invention provides a method and apparatus for removing floating, substantially immiscible liquids such as hydrocarbons from containers or wells containing both floating liquids and water. In one aspect of the invention, a microprocessor control receives and processes input signals from a liquid sensor, an upper water interface sensor and a skim pump. The skim pump is located in the well proximate to the water interface. The liquid sensor is located proximate to the skim pump. The upper water interface sensor is located adjacent to and below the liquid sensor. The microprocessor control is initialized by setting certain parameters such as the initial conditions for pump output values. The microprocessor controls and operates the skim pump in response to the input signals so as to operate the skim pump in a manner responsive to the input signals.

Abstract: A monitoring method for wastewater pump station performance and output, which method is capable of detecting an operating malfunction of the wastewater pump station such as a clog or leak within the pump, piping network or valves, or an output reduction due to pump wear. A monitoring apparatus placed at the wastewater pump staion performs continual monitoring of pump output values based on rates of rise of liquid level during pump non-run periods, time periods for the pump to reduce levels and resulting volume changes and at a controlled moment compares the values with measured reference values.

Abstract: Storm water pumps drain storm water in a pump well. In order to control the storm water pumps, a radar rain gage and ground raingages are set. Measurement data from the radar rain gage and the ground rain gages are supplied to a data processing unit. The data processing unit calibrates rainfall distribution data representing a two-dimensional rainfall distribtuion state obtained by the radar rain gate by the measurement data from the ground rain gages, and forecasts a rainfall in a predetermined time from the present from several sets of the calibrated rainfall distribution data. The data processing unit performs runoff analysis corresponding to characteristics of a drainage basin on the basis of the forecast rainfall to calculate a rainfall flow, thereby forecasting a flow of storm water flowing in the pump well.

Abstract: In a suction apparatus (1) for sucking up wet and/or dry suction material there is a single suction chamber (6) having an approximately constant inside diameter over its entire height forming in the bottom region a preseparating chamber (9) for heavier parts and optionally liquid, as well as spaced above the same in the top area a filter chamber (10) for receiving a wet or a dry filter (11) for filtering before or after the preseparation. The main container (2) forming the suction chamber (6) and to be opened level with filter (11) is immediately adjacent to an electromotive exhauster unit (3) relative to which it can be tilted for emptying the preseparation chamber (9). So that the filter (11) is not subject to liquid or heavier parts to be preseparated, there is an adjustable fill level limiting device (25) with a limit switch (26) directly responding to the suction material, which switches off when the exhauster unit (3) responds and then automatically switches in again.

Abstract: In a liquid waste collection and disposal apparatus, liquid waste gravity flows to a tank through a check valve equipped inlet line. A vertical guide float within the tank controls the liquid level therein. A two-way tank venting solenoid operated valve supplies gas, under pressure, to the tank for evacuating liquid therein. An electrical circuit connects a source of electrical energy with liquid upper and lower limit magnetic switches disposed in the path of travel of the float for energizing the actuator of the solenoid valve to evacuate the fluid therein through a check valve equipped discharge line in response to the float closing the upper limit switch. When the float reaches the limit of its downward movement, the normally closed lower magnetic switch is opened to interrupt gas pressure to the tank.

Abstract: A trough surrounding the base of a machine for trapping the spilled chemical and draining it into a pit in which a pump is located that is automatically engaged to pump the spilled chemical into a closed holding tank. A vapor detecting device will, upon the detection of toxic vapors, automatically turn on a ventilation system that will remove vapors from the area of the trough and from an optional isolation tent surrounding the machine and force the vapors through a filter having a bed of charcoal for absorbing the toxic vapors from the air. The filtered air is then vented to the atmosphere. A splash curtain is also used to direct the spilled chemicals into the trough. A substantial portion of the apparatus may be molded as a single unit or as separate sections that may be readily assembled and fixed into a unit at the installation site.

Abstract: A portable infusion pump assembly utilizing continuous drive propulsion means (8) to effect automatic and incremental displacement of a plunger (5) within a disposable syringe (3) so as to deliver accurately repeatable volumes of fluid from the syringe (3) over a predetermined period of time.Intermittent drive of the continuous drive propulsion means (8) is effected. Comparison of desired delivery rate with actual delivery rate is made by use of angular displacement signals received from a shaft encoder (10) attached to the shaft of the continuous propulsion means (8).This method of control also allows indirect measurement of fluid pressure in the syringe (3) without the need for a pressure sensor to be located in direct contact with the fluid being delivered.

Abstract: In connection with a protective circuit for an electrically driven liquid or water vacuum cleaner it is proposed to utilize the abrupt rise of the vacuum, which is encountered when a blocking element interrupts the connection between the receptacle for the liquid and the motor/blower area. The vacuum rise is detected by a diaphragm-type vacuum governor and used for triggering a lockable flip-flop which then interrupts the current supply of the drive motor. The locked condition can be released only by disconnecting the unit from the mains.

Abstract: A collection basin for collecting leakage from an automatic dishwashing machine. A drain or pump may be provided for removing the leakage collecting in the basin. Flanges extending along the sidewalls of the basin in combination with the sidewalls facilitate installation and removal of the dishwashing machine by guiding the machine support legs.

Abstract: A moisture detection device that is responsive to moisture or foam in the vacuum port of a water extraction unit, which creates a circuit break in the electrical power to the extraction unit. The device includes two conductors mounted in the vacuum port of an extraction unit, a monitoring system, by pass switch, and delay for fool proof detection of moisture overflow from the recovery tank of water extractors.

Abstract: A bilge pump switch sensing the presence of bilge water by measuring the density of water by using an ultrasonic field penetrating the water, the field extending through the housing of the switch. Thus the internal elements of the switch are isolated from any contact with the water. The ultrasonic field detecting the presence of water activates sensing means in said switch housing to activate the pump, delay means being provided against false pump starts due to wave action as caused by a rocking boat.

Abstract: A capacitance liquid level or volume sensing and touch control system for use with a conductive liquid having some isolation from and a capacitance to ground in accordance with its volume, is characterized by a circuit for cyclically charging the liquid a predetermined amount and then discharging it for a fixed time through an impedance. The extent of discharge of the liquid at the end of the fixed time is sensed, and is used to generate a signal representative of the volume of the liquid and its level. The system also has a capacitance touch control circuit, which generates an output signal when a sensor probe is touched or closely approached by a user. The signals from the capacitance liquid level sensing circuit and the touch control circuit are applied to a discriminator circuit which, when a signal representative of a sensed minimum volume of liquid is present, enables outputs from the touch control circuit to operate peripheral equipment, such as a pump for recirculating the liquid.

Abstract: A liquid level control in a refrigeration system operates a solenoid valve to maintain the level of liquid refrigerant in a vessel within a range defined by predetermined lower and upper liquid levels, sounds an alarm if the upper level is exceeded and stops a pump if the lower level is violated. The control comprises a float assembly and a switching assembly. The float assembly comprises a float chamber connected to the vessel, a float ball therein, a vertically extending float rod connected to the float ball and movable along a vertical path, and a pair of spaced apart stop members on the float rod disposed within the switching assembly. The switching assembly comprises a switch actuator permanent magnet, having magnet poles at opposite ends, slidably mounted on the float rod between the stop members and movable thereby along the said path.

Abstract: An apparatus for measuring the quantity and flow rate of liquids in a flowing fluid stream containing liquids and gases such as found in oil well production, with an optional controller for oil well pumps to control the pumping period or speed based on pump displacement. Total number of tank fill events provide a precise measurement of total volume. Volumes are accumulated on a time base to determine flow rate and on the basis of strokes to determine pump displacement. Liquid measurement is accomplished without restriction to the flowing stream, even in the event of a valve failure. The filling of each tank with oil of low or high viscosity can be measured by level, hydrostatic pressure, or weight sensors. Measurement of total produced volumes are accumulated with the accuracy and repeatability needed when correlating producing wells with injection wells, and when diagnosing well problems.

Abstract: A water well and storage tank control system for use in isolated areas that utilizes a solar power source in conjunction with pneumatic pump apparatus for lifting well from a well bore hole for surface deposit in a storage tank. A solar power panel functions to continually charge into a storage battery which, in turn, is periodically applicable to energize a pump/compressor to generate air at increased pressure for conduction down the well bore. An airjet chamber immersed beneath the water level in the well bore receives input of the air at increased pressure and evacuates well water upward through a delivery pipe to a surface storage tank.

Abstract: A packaged hydro-pneumatic pumpset system in which a plurality of pumps JI, M2, M3 are brought into operation sequentially as water pressure in the system drops below pre-set values. The system includes an accumulator 6 for storing water to maintain pressure in the system when the pumps are not operating and a prime is maintained on the pumps by means of a footvalve in the suction pipe inlet. The system also includes a second accumulator 9, which is connected to the first and which is arranged, when the pumps are inoperative, to maintain a reservoir of stored pressurized water in the suction manifold.

Abstract: Method and apparatus for generating data and analyzing the same to determine fluid depth in a well. A gun assembly includes a pressurized chamber for initiating an acoustic pulse and a transducer for measuring pressure variations. The gun assembly is mounted on a well casing at the top thereof. An acoustic pulse is initiated and reflections from tubing collars, well equipment, perforation zones and the fluid level surface are detected by the transducer and converted to an analog signal which is digitized and stored in a memory. The stored data is processed to detect the fluid pulse reflection in view of a set of preselected pulse criteria. Acoustic pulse velocity is calculated by correlating the tubing collar depths, which are stored in the memory, with peaks in the acoustic log which indicate a corresponding tubing collar.

Abstract: A water pump control system, such as for a boat, including a battery operated main motor-and-pump unit and a backup motor-and-pump unit. A pump turn-on transistor operates a relay to turn on the main motor-and-pump unit when the water level rises to a high level. A touch switch may be used to turn on the pump turn-on transistor independent of the water level. A pump turn-off transistor, which is turned on by the relay, keeps the main motor-and-pump unit on until the water level drops to a low water level. A backup motor-and-pump unit comes on if the main motor-and-pump unit has not come on after the pump turn-on transistor has been turned on.

Abstract: The invention includes a fuel tank containing an emergency supply of fuel for an internal combustion engine in the general proximity of the engine, with the arrangement being such that proper communication from this alternate source of fuel directly to the air-fuel mixer of the engine is effected. The preferred example of the air-fuel mixer is a carburetor. In this way, by utilizing a reliable arrangement for the delivery of fuel directly to the carburetor, a temporary problem with the primary fuel supply can be circumvented.

Abstract: A probe is mounted on a water submersible pump housed within a transformer vault. The probe will extend into any water which accumulates in the bottom of the transformer vault enabling a conductive path to be established through an appropriate electric circuit to the pump motor to permit operation of the pump so that the water which is on the bottom of the vault can be safely pumped from the vault. As the water level falls as it is pumped, oily fluids, which are immiscible in the water and will normally rise to a level above the water level in the vault, will come in contact with the probe to form an electrical insulator to inactivate the circuit to disconnect a power source from the pump motor.

Abstract: A system for controlling production in an oil well comprises a first level sensor mounted upon the outer surface of the tubing near the lower end of the tubing and a second level sensor mounted upon the outer surface of the tubing above the first level sensor. The first level sensor is tuned to a first electrical signal frequency. The second level sensor is tuned to a second electrical signal frequency. A control device at spaced intervals transmits electrical signals of the first and second frequencies down the oil well and senses the liquid level by detecting impedance.

Abstract: An electronic circuit that controls the operation of a motor whose output shaft drives an impeller for pumping a liquid includes a common grounded electrode positioned for immersion in the liquid, and a stopping electrode positioned above the common electrode. A first, latching transistor is connected to the stopping electrode which is connected to ground through the liquid when the level thereof reaches or exceeds the level of the stopping electrode. A second, power transistor operates the motor when the second transistor conducts in response to its grounding, the second transistor being connected to the starting electrode so as to be grounded when the level of liquid reaches the starting electrode.

Abstract: A two pump skimmer system for recovery of lighter-than-water hydrocarbons from water wells. A water pump is used to depress the water table and a contaminant pump is used to remove the hydrocarbons. The pumps are located in individual chambers which are interconnected, with the water chamber below the hydrocarbon chamber and with limited one-way flow into the water chamber. Both pumps are independently controlled by sensors in the upper chamber to assure that each pumps only the proper liquid.

Abstract: A monitoring assembly (20) which has a plurality of sensors (24) arranged in spaced relationship along a body (23) which may be suspended from a coded multicore cable (25) in a reservoir to monitor liquid levels therein. Each sensor (24) is connected to a respective coded core (43) of the multicore cable (25) such that any sensor (24) may be selectively coupled to remote control apparatus.

Abstract: A system for controlling production in an oil well comprises a first level sensor mounted upon the outer surface of the tubing near the lower end of the tubing and a second level sensor mounted upon the outer surface of the tubing above the first level sensor. The first level sensor is tuned to a first electrical signal frequency. The second level sensor is tuned to a second electrical signal frequency. A control device at spaced intervals transmits electrical signals of the first and second frequencies down the oil well and senses the liquid level by detecting impedance.