Capacitive Liquid Level Sensor

This project, a capacitive liquid level sensor from 1989,  uses an ingenious technique where the senor is located outside the liquid container.

This is a very useful technique and has been used in later designs to create a portable liquid level sensor for locating the exact height of liquid in a plumbing system.

Normal capacitive liquid level sensors work by having the sensor within the liquid and sensing the capacitance change when as they are exposed when the liquid level falls. This has the disadvantage of plate electrolysis when an ac signal is developed across the capacitive sensor. This design completely avoids that problem.

Executive Summary of the Capacitive Liquid Level Sensor

A non-intrusive fluid level detector including a single point capacitive sensor mounted on the outside surface of a receptacle such that capacitive principles can be utilized to sense the level of a liquid contained within the receptacle. The sensor assembly is disposed in a substantially fixed position on the exterior wall of the receptacle wherein the dielectric effect of the liquid changes the effective capacitance of the sensing capacitor as the liquid rises and falls within the receptacle.

This change in effective capacitance is detected by electronic circuitry included in the detector device. In one embodiment, the fluid level detector is directly mounted to a completely non-conductive receptacle. In another embodiment, the fluid level detector is mounted to a non-conductive window which is an integral part of a receptacle fabricated out of a conductive material.

Background of the Capacitive Liquid Level Sensor

Fluid level detectors which sense the level of a liquid contained within a receptacle are well known for use in conjunction with automotive engines. Historically, such detectors have been made in the form of float operated switches involving moving parts which are subject to friction and wear.

Other devices utilize an electrical probe to detect fluid levels by measuring the conductivity of the coolant. However, these devices require complicated current amplifying systems because there is often an insufficient amount of current passing through the electrodes to power an indicator lamp.

In either of these systems, the measuring sensor is located in the fluid where contaminants are likely to collect on the sensor and disturb the fluid level measurement. Furthermore, the devices in the prior art, as described, are categorized as "intrusive" in that they require an opening into the receptacle containing the fluid.

This creates an additional potential for fluid leaks as well as potential deterioration of the sensing devices.

Due to the problems discussed above, it has become desirable to employ non-intrusive means to sense fluid levels contained in receptacles. These non-intrusive systems typically involve wave propagation techniques which are implemented through a transmitter/receiver system. Typically, these systems involve the transmission of an ultrasonic signal from a transmitting transducer through a fluid to a receiving transducer.

Such ultrasonic transmission systems require a liquid transmission medium in order to carry the ultrasonic signal from the transmitting transducer to the receiving transducer. Lack of signal at the receiving transducer relates to a lack of liquid transmission medium, activating a no fluid present indication.

However, a failure in the transducer pair or in their respective electronic connections results in a lack of signal from the receiving transducer which, in turn, activates the normal failure mode thereby falsely indicating a dry or no fluid present state.

The increasing importance of monitoring fluid levels in automotive and other applications is creating a need for more reliable non-intrusive fluid level sensors. It is, therefore, important that a fluid level detection system be developed which can provide reliable data and which does not require contact with the fluid being measured.

Summary of the Capacitive Liquid Level Sensor

In accordance with the present design a non-intrusive fluid level detector is provided for mounting on the exterior wall of a receptacle. The present design provides single point sensing of predetermined low levels of fluids, as in an automobile cooling system or a windshield washer solvent reservoir, without contact with the liquid being sensed.

The design also provides an electrical signal which can energize an alarm light or other indicator. The fluid level detector of the present design incorporates capacitive sensor means established relative to a fluid receptacle in a substantially fixed relation on the outside wall of the receptacle.

The capacitive sensor can be made to be an integral part of a printed circuit containing electronic detection circuitry, thereby making the device self-contained. This sensor assembly is mounted on the exterior wall of the fluid receptacle at a position to sense the lower limit of acceptable fluid level.

The present design utilizes capacitive principles to sense the level of a liquid contained within a non-metallic receptacle. As a liquid rises and falls in the container, the dielectric effect of the liquid changes the effective capacitance of the sensing capacitor which is detected by electronic circuitry coupled to the sensor. The device remains activated whenever power is applied and provides an indication to the user only when the low liquid level is detected.

The present design is a passive device in that the device monitors the level of the liquid within the receptacle at all times and requires no interaction or other monitoring by the user.

Figure 1 : (a,b) is a schematic of the circuitry of the present invention. figure b is a schematic of an alternative capacitive sensing circuit for the capacitive liquid level sensor

View larger image here.

Description of the Capacitive Liquid Level Sensor

FIG. 1 (A) illustrates the circuit diagram of one embodiment of the fluid level detector of the present design. The supply voltage for this circuit is typically provided by a 12-volt automobile battery which is reduced to a 5-volt DC source voltage by means of a voltage regulator comprising the combination of resistor R1 and zener diode Z1. Capacitor C1 serves as a filter for this voltage supply regulator.

The detector of FIG. 1A incorporates an amplifier A1 in conjunction with a resistor/capacitor network R4 and C2 and resistors R5 and R6 to form a square wave oscillator. A reference voltage is supplied from the voltage regulator where the reference voltage value is determined by the voltage divider circuit R2/R3.

This voltage serves as a reference voltage for the square wave oscillator and voltage comparators A2, A4 located in the circuit, as will be described.

Figure 2 : A-e are signal diagrams of signals at certain test points in the schematic diagram of fig. 1 for the capacitive liquid level sensor

The output of the square-wave oscillator described above, is shown as the square wave of FIG. 2 (A). The oscillator produces an alternating electrical output which causes capacitors C3 and C4 to charge through diodes D1 and D2, respectively, where capacitor C4 is a sensor capacitor and capacitor C3 is a reference capacitor.

As stated, capacitor C4 is the sensor capacitor wherein the two conductive surfaces of the capacitor are plate P1 and the residual ground of the receptacle 10 through its mounting arrangement. The dielectric of the capacitor is the fluid in the receptacle 10 such that the capacitance value of C4 varies relative to the fluid level in the receptacle.

The capacitance value of capacitor C3 is adjustable to match the value of capacitor C4 when a predetermined liquid level is sensed, i.e., at the critical level.

FIG. 1B shows an alternative embodiment of the present design wherein plates P2 and P3 are added to the configuration of sensing capacitor C4. The plates P2 and P3 are connected to circuit ground. A lower level capacitance is provided by plates P1 and P2, and an upper level capacitance is provided by plates P1 and P3.

When the voltage from the oscillator circuit at test point 1 (TP1) makes a transition from positive to negative, capacitors C3 and C4 begin to discharge through resistors R9 and R10, respectively. It is the difference in voltage decay across resistors R9 and R10 due to the capacitance values of capacitors C3 and C4 which enables the circuit to determine if there is sufficient fluid in the receptacle 10.

Referring again to FIG. 1A, voltage comparator A2 compares the voltage at test point 2 (TP2) with the reference voltage set by resistors R2 and R3. FIG. 2 (B and C) illustrate that when the voltage at TP2 decreases below the reference voltage V1, due to the discharge of capacitor C3, the output of the comparator A2 reference voltage at TP3 drops to zero.

The time between the point where capacitor C3 begins to discharge and the output of the comparator A2 drops to zero is used as a reference time T1. Similarly, the comparator A3 compares the square wave output of the comparator A2 with the voltage decay signal across resistor R10 due to sensor capacitor C4 at test point TP4.

As shown in FIGS. 2C-2E, if the discharge time T3 of capacitor C4 is longer than the reference time T1 established by the comparator A2, the output of the comparator A3 remains high. Conversely, if discharge time T3 is shorter than reference time T1, due to a faster discharge rate in C4, the output of the comparator A3 pulses low for a period of time in which the discharge voltage at TP4 is less than the output voltage of the comparator A2 (TP3). In application, this pulse is actually a current pulse rather than a voltage pulse.

The output of the comparator A3 is an open collector type output which allows capacitor C5 to charge through resistor R14. If the output of A3 pulses low, capacitor C5 discharges to ground and the voltage at test point 5 (TP5) goes to zero.

Finally, the reference voltage set by resistors R2 and R3 is compared to the voltage at TP5 through the comparator A4. If the voltage at TP5 is high, representing capacitance C4 to be of a relatively large value and corresponding to a sufficient fluid in the reservoir, the output of A4 will pull to ground and transistor TR1 will not conduct.

However, if the voltage at TP5 is low, which means that capacitance C4 is not of a sufficient value, the output of A4 will bias transistor TR1 such that current will flow through the collector and energize lamp L1.

In operation, when the fluid level in the receptacle is full, capacitor C4 takes a certain length of time to discharge; when the fluid is at a lower level, capacitor C4 takes a shorter period to discharge. Thus, as the fluid level in the receptacle decreases, the capacitance of capacitor C4 also decreases, which, in turn, decreases the discharge time of capacitor C4.

This causes a current pulse to be output by comparator A3 during the period in which the discharge voltage from capacitor C4 is less than the voltage output from comparator A2. The current pulse causes capacitor C5 to discharge, thereby allowing the output of comparator A4 to rise and allowing current to flow through transistor TR1 to lamp L1.

When current flows through lamp L1, the indicator lamp is lit.

Figure 3 : A and b are exploded perspective views of a preferred embodiment of the liquid level sensor of the present invention; and for the capacitive liquid level sensor

An exploded perspective view of the present design is shown in FIG. 3 (A and B) where sensor plate P1 is shown as an integral part of the assembled unit. In an alternative embodiment, capacitor plate P1 can be physically separated from the electronic circuitry of the design such that the sensor plate P1 is attached to the fluid receptacle and is electronically coupled to the corresponding electronic circuitry located at a position which is isolated from the receptacle.

Figure 4 : A and b are two-dimensional views of alternative mounting configurations of the present invention for the capacitive liquid level sensor

In accordance with the present design, as shown in FIG. 4 (A and B), the sensing capacitor can be mounted to the receptacle 10 in various ways depending on whether the receptacle 10 is made of a conductive or a non-conductive material. In one embodiment of the design, shown in FIG. 4A, the fluid receptacle 10 is completely made of a non-conductive material and the sensing capacitor plate P1 is mounted directly to the surface of the receptacle 10.

In another embodiment of the design, shown in FIG. 4B, the receptacle 10 is made of a conductive material except for a relatively small window 12 of non-conductive material making up a portion of a wall of the receptacle 10 upon which the capacitive plate P1 is mounted.

The fluid level detector of the present design permits the monitoring of a level of a fluid contained in a receptacle wherein there is no engagement between the sensing device and the liquid. The disclosed design is not to be limited by what has been particularly shown and described except as indicated by the present claims.

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