Abstract: In a capacitive sensor of the type having X electrodes which are driven and Y electrodes that are used as sense channels connected to charge measurement capacitors, signal measurements are made conventionally by driving the X electrodes to transfer successive packets of charge to the charge measurement capacitors. However, an additional noise measurement is made by emulating or mimicking the signal measurement, but without driving the X electrodes. The packets of charge transferred to the charge accumulation capacitor are then indicative of noise induced on the XY sensing nodes. These noise measurements can be used to configure post-processing of the signal measurements.

Abstract: An impedance matching apparatus 3 calculates a forward wave voltage Vfo and a reflected wave voltage Vro at an output terminal 3b, based on a forward wave voltage Vfi and a reflected wave voltage Vri at an input terminal 3a, on information on variable values of variable capacitors VC1, VC2 acquired in advance through measurement, and on a T parameter of the impedance matching apparatus 3 corresponding to the information on the variable values of variable capacitors VC1, VC2. The impedance matching apparatus 3 calculates an input reflection coefficient ?i at the input terminal 3a corresponding to the information on the variable values of the variable capacitors VC1, VC2, based on the forward wave voltage Vfo, the reflected wave voltage Vro and the T parameter.

Abstract: As regards a measurement structure which is as simple as possible and in order to influence the mixing operation or the transfer of substances to the slightest possible extent, a method for measuring the mixing ratio of a mixture of substances comprising at least two substances (10, 11) is configured in such a manner that the mixture of substances is brought into the measurement range of a capacitive sensor (1)—in particular is moved past the latter or moved through the latter—and the mixing ratio is determined from the change in the capacitance of the sensor (1), which change is caused by the mixture of substances. A corresponding sensor arrangement in which the mixture of substances is guided through a tubular area (2) is specified.

Abstract: Methods and systems for improving the sensitivity of a variety of conductivity sensing devices, in particular capacitively-coupled contactless conductivity detectors. A parallel inductor is added to the conductivity sensor. The sensor with the parallel inductor is operated at a resonant frequency of the equivalent circuit model. At the resonant frequency, parasitic capacitances that are either in series or in parallel with the conductance (and possibly a series resistance) is substantially removed from the equivalent circuit, leaving a purely resistive impedance. An appreciably higher sensor sensitivity results. Experimental verification shows that sensitivity improvements of the order of 10,000-fold are possible. Examples of detecting particulates with high precision by application of the apparatus and methods of operation are described.

Abstract: A method for detecting capacitor variation in a device comprises operating an oscillator in the device, the oscillator being an Inductive-Capacitive (LC) oscillator and including an inductor of known value and a capacitor under test, comparing an output of the oscillator to a reference output, and evaluating variation for a plurality of capacitors in the device based on the comparing.

Abstract: An occupancy detection system includes a signal generator coupled to an electrode, the signal generator configured to output a first signal at a first frequency and a second signal at a second frequency. The system further includes a voltage detection circuit connected to an output terminal of the signal generator and to an input terminal of the electrode, wherein the voltage detection circuit is configured to measure a first voltage responsive to the first signal at the first frequency and a second voltage responsive to the second signal at the second frequency. A control module in communication with the voltage detection circuit is configured to detect a state of occupancy based on the first voltage and the second voltage.

Abstract: A circuit for measuring an unknown capacitance includes a reference capacitor having a known capacitance, an oscillator timing circuit, a variable frequency oscillator and a microcontroller. The oscillator timing circuit includes switches which selectively couple the unknown capacitance and the reference capacitor to the oscillator timing circuit. The variable frequency oscillator generates time varying signals which vary in frequency proportionally to the unknown capacitance and reference capacitor selectively coupled to the oscillator timing circuit. The microcontroller receives the time varying signals from the oscillator, and compares the periods of the time varying signals to determine the value of the unknown capacitance.

Abstract: A capacitance detection circuit that compensates for the fluctuation of a reference voltage with a simple structure. A C-V circuit for a sensor element generates a detection signal by amplifying a capacitance change value of the sensor element by a predetermined gain based on a reference voltage. A voltage compensation circuit, which is connected to the C-V circuit and supplies a reference voltage to the C-V circuit, reduces the gain relative to a deviation amount when the reference voltage fluctuates by a predetermined deviation amount.

Abstract: A system includes a capacitive sensor including a first electrode and a second electrode. The system includes a measurement system configured to sense a capacitance between the first electrode and the second electrode and apply a first offset to the sensed capacitance to provide an offset compensated capacitance.

Abstract: A capacitive sensor may include a transmit electrode and a receive electrode capacitively coupled with the transmit electrode. A capacitance sensing circuit senses a capacitance between the transmit and receive electrodes by applying a signal to the transmit electrode and rectifying a current waveform induced at the receive electrode. A compensation circuit reduces the effect of a mutual and parasitic capacitances of the transmit and receive electrode pair by adding a compensation current to the rectified current.

Abstract: A capacitive sensor (10) producing an output signal (VOUT) that is insensitive to stray capacitance (CS) caused by environmental and aging conditions. The sensor includes a sensing electrode (11) that exhibits a total capacitance that is responsive to both the measured process variable and to stray capacitance (CT=CA+CS). The sensor also includes a reference electrode (19) that exhibits a stray capacitance (CS?) essentially the same as that of the sensing electrode, but that is insensitive to the process variable. Balancing circuitry (29) provides an output signal that is responsive to the measured process variable and insensitive to the stray capacitance (VOUT=CT?CS?). The reference electrode is manufactured of the same materials and dimensions as the sensing electrode and may be mounted in the sensor body proximate the sensing electrode.

Abstract: An embodiment of the present invention is directed to a method for processing a position signal. The method includes receiving a first position signal from a capacitive sensor and determining a proximity of the capacitive sensor to a connection of an array of capacitive sensors. The sensitivity of the capacitive sensor is then adjusted and a second position signal is received from the capacitive sensor. The second position signal may then be reported. The present invention facilitates more accurate readings from an array of capacitive sensors.

Abstract: An oversampling electromechanical modulator, including a micro-electromechanical sensor which has a first sensing capacitance and a second sensing capacitance and supplies an analog quantity correlated to the first sensing capacitance and to the second sensing capacitance; a converter stage, which supplies a first numeric signal and a second numeric signal that are correlated to the analog quantity; and a first feedback control circuit for controlling the micro-electromechanical sensor, which supplies an electrical actuation quantity correlated to the second numeric signal.

Abstract: Methods and systems using Pade' Approximant expansion ratios provide mappings between nonlinear sensors and a more linearized output domain. In one embodiment (a) a variable gain amplifier receives a supplied input signal, the amplifier has at least a first input terminal, an output terminal, and a gain control terminal; (b) a first summer coupled to the output terminal of the variable gain amplifier adds in a first offset signal; (c) a first multiplier coupled to an output of the first summer receives a proportional feedback factor signal and correspondingly generates a multiplied feedback; (d) a second summer coupled to an output terminal of the first multiplier adds in a corresponding second offset signal; and (e) a second multiplier coupled to an output of the second summer receives a gain factor signal and generates a multiplied gain signal; where the gain control terminal of the variable gain amplifier is operatively coupled to an output terminal of the second multiplier.

Abstract: A sensor device is disclosed that includes a sensor, in which a resistance value is changed in accordance with a change in an environment of the sensor. The sensor device also includes a beat oscillator having plural oscillators of different oscillating frequencies. The beat oscillator generates a beat signal of a frequency lower than the oscillating frequencies. The beat signal corresponds to a difference of the oscillating frequencies. The beat oscillator is electrically connected to the sensor such that the frequency of the beat signal changes in accordance with the changes in the resistance value of the sensor.

Abstract: A measuring apparatus having a probe that faces a surface of a target and is configured to supply AC current to the surface, measuring a voltage drop through a space between the probe and the surface, and obtaining a distance between the probe and the surface in accordance with the measured voltage drop. The apparatus includes a ground member facing, and apart from, the surface and configured to ground the surface by capacitive coupling, and a stage configured to hold either of the target and the probe and to move to define a measurement area on the surface. The ground member is configured so that the ground member faces all areas of the surface, with respect to each of a plurality of measurement areas on the surface defined by a position of the stage.

Abstract: A microcontroller-based temperature compensated circuit for ground-fault circuit interrupter to meet the requirements of UL 943 using a single sensor to detect both ground-fault and grounded-neutral fault conditions in both full-wave and half-wave AC power supplies as part of a ground-fault circuit breaker or a receptacle device.

Abstract: An oversampling electromechanical modulator, including a micro-electromechanical sensor which has a first sensing capacitance and a second sensing capacitance and supplies an analog quantity correlated to the first sensing capacitance and to the second sensing capacitance; a converter stage, which supplies a first numeric signal and a second numeric signal that are correlated to the analog quantity; and a first feedback control circuit for controlling the micro-electromechanical sensor, which supplies an electrical actuation quantity correlated to the second numeric signal.

Abstract: A method for enhancing the linearity of a differential capacitive sensor as a function of beam displacement. An ac-modulated feedback connection from a sensor output terminal to a movable beam electrode is provided through a fix-up capacitor. The output terminal signal is inverted in the feedback connection so that the fix-up capacitor reduces the capacitance from the fixed electrodes to the beam electrode. When the fix-up capacitor value is chosen approximately equal to the fixed capacitance of the differential capacitor, the feedback circuit compensates for the effect of this fixed capacitance. The linearity of the output as a function of the displacement of the beam is improved. Thus, the signal-to-noise ratio of devices such as accelerometers may be increased by allowing sensors to employ a large displacement range. The invention may be used in differential accelerometers and in other types of differential capacitive sensors.

Abstract: A method for measuring nm-scale tip-sample capacitance including (a) measuring a cantilever deflection and a change in probe-sample capacitance relative to a reference level as a function of a probe assembly height; (b) fitting out-of-contact data to a function; (c) subtracting the function from capacitance data to get a residual capacitance as a function of the probe assembly height; and (d) determining the residual capacitance at a z-position where the cantilever deflection is zero.

Abstract: A second operational amplifier (11) of a core unit (1) shorts an inverting input terminal and an output terminal. A signal line (19) is connected to a non-inverting input terminal. A capacitive sensor (18) is connected to the signal line (19). A first operational amplifier (12) earths the non-inverting input terminal. One end of a first resistance (15) and one end of a second resistance (16) are respectively connected to the inverting input terminal. The other end of the first resistance (15) is connected to an alternate current voltage generator (14). The other end of the second resistance (16) is connected to the output terminal of the first operational amplifier (11). A signal output terminal (21) of the core unit (1) is connected to an inverting amplification device (2). An alternate output terminal (22) of the core unit (1) and an inverting output terminal (42) of the inverting amplification device (2) are connected to an addition device (3).

Abstract: A circuit and system for sensing and measuring the mutual capacitance between a sensor capacitor having one grounded lead and a target and providing a direct digital output of the measured capacitance is disclosed. The circuit and system includes a relaxation oscillator coupled to a sensor capacitor and a fixed resistor. The fixed resistor and the sensor capacitor in conjunction with the relaxation oscillator provide a time varying output signal that has a period that is proportional to the mutual capacitance of the sensor capacitor and a target and resistance of the fixed resistor. The circuit and system can also include circuitry to compensate for the input capacitance of one or more amplifiers used in the relaxation oscillator. The circuit and system can also include circuitry to effectively increase the resistance of the fixed resistor by a predetermined constant.

Abstract: The present invention relates to a system for nulling drive feedthrough error in a sensor having first and second drive electrodes which impart vibratory motion to first and second proof masses in response to first and second opposite phase drive signals, and having first and second capacitances defined between the drive electrodes and their associated proof masses. A mismatch between the first and the second capacitance is measured. Drive feedthrough caused by the measured capacitance mismatch is nulled by adjusting the relative amplitudes of the first and second opposite phase drive signals, whereby the ratio of the amplitudes is proportional to the ratio of the first and second capacitances. A servo loop may adaptively effect the ratio of amplitudes.

Abstract: An impedance matching device is provided, for which the electric characteristics at an output terminal are accurately analyzed. The matching device is provided with an input detector for detecting RF voltage and current at the input terminal, and an output detector for detecting RF voltage outputted from the output terminal. The matching device also includes a controller for achieving impedance matching between a high frequency power source connected to the input terminal and a load connected to the output terminal. The impedance matching is performed by adjusting variable capacitors based on the detection data supplied from the input detector. When the impedance of the power source is matched to that of the load, the controller calculates the output impedance, RF voltage and RF current at the output terminal, based on the adjusted capacitances of the capacitors, a pre-obtained reactance-impedance data and the detection data supplied from the output detector.

Abstract: The present invention relates to a method for nulling drive feedthrough error in a sensor having first and second drive electrodes which impart vibratory motion to first and second proof masses in response to first and second opposite phase drive signals, and having first and second capacitances defined between the drive electrodes and their associated proof masses. A mismatch between the first and the second capacitance is measured. Drive feedthrough caused by the measured capacitance mismatch is nulled by adjusting the relative amplitudes of the first and second opposite phase drive signals, whereby the ratio of the amplitudes is proportional to the ratio of the first and second capacitances. A servo loop may adaptively effect the ratio of amplitudes.

Abstract: The present invention relates to a system for nulling drive feedthrough error in a sensor having first and second drive electrodes which impart vibratory motion to first and second proof masses in response to first and second opposite phase drive signals, and having first and second capacitances defined between the drive electrodes and their associated proof masses. A mismatch between the first and the second capacitance is measured. Drive feedthrough caused by the measured capacitance mismatch is nulled by adjusting the relative amplitudes of the first and second opposite phase drive signals, whereby the ratio of the amplitudes is proportional to the ratio of the first and second capacitances. A servo loop may adaptively effect the ratio of amplitudes.

Abstract: The present invention relates to a system for nulling drive feedthrough error in a sensor having first and second drive electrodes which impart vibratory motion to first and second proof masses in response to first and second opposite phase drive signals, and having first and second capacitances defined between the drive electrodes and their associated proof masses. A mismatch between the first and the second capacitance is measured. Drive feedthrough caused by the measured capacitance mismatch is nulled by adjusting the relative amplitudes of the first and second opposite phase drive signals, whereby the ratio of the amplitudes is proportional to the ratio of the first and second capacitances. A servo loop may adaptively effect the ratio of amplitudes.

Abstract: Method for detecting movements through a micro-electric-mechanical sensor, having a fixed body and a moving mass, forming at least one first and one second detection capacitor, connected to a common node and to a first, respectively a second detection node and having a common detection capacitance at rest and a capacitive unbalance in case of a movement. The method includes the steps of: feeding the common node with a constant detection voltage of predetermined duration; generating a feedback voltage to maintain the first and the second detection node at a constant common mode voltage; generating a compensation electric quantity, inversely proportional to the common detection capacitance at least in one predetermined range; supplying the compensating electric quantity to the common node; and detecting an output quantity related to the capacitive unbalance.

Abstract: The present invention relates to a system for nulling drive feedthrough error in a sensor having first and second drive electrodes which impart vibratory motion to first and second proof masses in response to first and second opposite phase drive signals, and having first and second capacitances defined between the drive electrodes and their associated proof masses. A mismatch between the first and the second capacitance is measured. Drive feedthrough caused by the measured capacitance mismatch is nulled by adjusting the relative amplitudes of the first and second opposite phase drive signals, whereby the ratio of the amplitudes is proportional to the ratio of the first and second capacitances. A servo loop may adaptively effect the ratio of amplitudes.

Abstract: A method is provided for sensing and measuring an electrical current. The current to be measured is applied to a deflectable member in a sensing module in the presence of a magnetic field. Deflection of the member is detected as an indication of the current. A nulling current may be applied to one or more additional deflectable members mechanically linked to the deflectable member. Feedback or readout signals indicative of deflection of the members are monitored. The nulling current is modulated to drive the feedback signals to a desired level. The nulling current value is converted to a value representative of the current to be measured.

Abstract: An underground storage tank monitoring system and method uses special micropower impulse radar probes, with one such probe being inserted into each storage tank. The micropower impulse radar probes use flexible waveguides which extend downward to the tank bottom such that micropower radar impulses travel to and from the liquid surface via a wave guide, with the lapse between emission and reception of the impulse indicating a distance from the probe, and thus a liquid level. Each micropower impulse radar probe is connected to a dedicated spread spectrum transmitter which collects level and status information from the probe, encodes and transmits it to a matching spread spectrum receiver nearby. The receiver converts the coded signals into electrical signals and forwards them to a processor, a modem and a back-up battery are located. The processor stores the liquid level information for later transmission, along with liquid dispensed and liquid replaced information, to a central monitoring site via the modem.

Abstract: A static capacitance-to-voltage converter is capable of converting a static capacitance into a voltage without suffering from a stray capacitance formed between a signal and a shielding line or a stray capacitance formed between an exposed portion of the signal line and its surroundings. The static capacitance-to-voltage converter is formed of an operational amplifier placed in an imaginary short-circuit state between an inverting input and a non-inverting input thereof; a signal line having one end connected to the inverting input and the other end capable of being connected to a static capacitance; a shielding line surrounding the signal line and connected to the non-inverting input; an alternating current signal generator for applying the non-inverting input with an alternating current signal; and zero adjusters for adjusting the output of the static capacitance-to-voltage converter to minimum when no static capacitance is connected to the signal line.

Abstract: A test unit for measuring crosstalk in twisted pair cable. The test unit has an output signal balance (OSB) circuit that compensates for parasitic capacitance at its output terminals. The OSB circuit has a voltage controlled capacitance connected in circuit with each output terminal to control the effective capacitance between the output terminals and ground. The bias voltage for the variable capacitances is calibrated by a method in which the voltage for one of the variable capacitors is held constant while the voltage for the other capacitor is varied in voltage levels. A test signal frequency sweep is applied to the test unit output terminals. First and second voltage values are obtained and a final bias voltage value is calculated from using these two values.

Abstract: Method and apparatus are described for speeding parameter measurements of piezoelectric-resonators. A large driving voltage is applied to the resonator until the resonator current reaches its desired steady-state value. Then the driving voltage is reduced to a level which will maintain that desired current level. This procedure allows the resonator to reach its steady state conditions faster than if a constant drive voltage were used.

Abstract: A method is provided for sensing and measuring an electrical current. The current to be measured is applied to a deflectable member in a sensing module in the presence of a magnetic field. Deflection of the member is detected as an indication of the current. A nulling current may be applied to one or more additional deflectable members mechanically linked to the deflectable member. Feedback or readout signals indicative of deflection of the members are monitored. The nulling current is modulated to drive the feedback signals to a desired level. The nulling current value is converted to a value representative of the current to be measured.

Abstract: A circuit arrangement for measuring the diameters of metal bodies is provided which permits reliable and stable measurement of changes in diameter in a simple way. Measuring signals of relatively high accuracy are formed by compensating for environmental parameters such as ambient temperature and humidity. A dynamic reference signal which depends on the environmental parameters is formed for this purpose.

Abstract: A method of reducing noise in a mass flow measuring system includes equalizing the ratio of stiffness to mass of a sleeve located in a flow path to the ratio of stiffness to mass of a rod located within the sleeve. The method further includes recognizing a frequency signal from two capacitive sensors mounted to sense axial deflection of the sleeve relative to the rod. The frequency signal is twice the frequency signial from two capacitive sensors mounted to sense lateral deflection of the sleeve relative to the rod. An output of the capacitive sensors for sensing lateral deflection is connected to a conductance circuit and an output of two additional capacitive sensors mounted to sense lateral deflection is connected to the another conductance circuit. The method includes adjusting an output of one of the conductance circuits such that the sum of the outputs is zero.

Abstract: Accuracy of capacitance measurements taken with flying probes--probes that are movable relative to each other and surfaces of an object containing circuits being tested (e.g. a printed circuit board)--is improved by dynamically applying corrections for stray capacitance as individual probes are selected for measurements. Measuring circuitry to which the probes are linked includes multiplexor circuitry. The latter stray capacitance encompassing the stray capacitance between the one movable probe and the other movable probes as well as the cabling between the latter probes and the multiplexor circuitry. For these measurements, the reference point is contacted either by a fixed conductor or another one of the movable probes (other than the probe aligned with the test point) that is currently usable for that purpose.

Abstract: The invention consists of a system to compensate for the parasitic signal induced by a parasitic impedance in the signal path. An auxiliary signal, outside of the frequency band of the desired signal, is introduced to evaluate the effects of the parasitic impedance. A compensating signal is then generated by simulating the effects of the parasitic impedance on the desired signal. Those simulated effects are then superimposed over the normal signal path with a phase adjusted to eliminate the effects of the parasitic impedance on the total resulting signal.

Abstract: The apparatus includes a first transmitter for transmitting a first test signal from a first end of the cable and a second transmitter for transmitting a second test signal from a second end of the cable. The first test signal is transmitted on a first conductor pair at a first frequency and the second test signal is simultaneously transmitted on a second conductor pair at a second frequency. The apparatus also includes a first receiver for receiving a first coupled signal at the first end of the cable and a second receiver for simultaneously receiving a second coupled signal at the second end of the cable. The first coupled signal exhibits the first frequency and is received on a third conductor pair, while the second coupled signal exhibits the second frequency and is received on a fourth conductor pair. The transmit frequencies are offset such that interference and noise are not detected by the receivers as coupled signals.

Abstract: The fast sampling method for taking a position measurement with an electronic measurement system incorporating a capacitive position transducer of this invention allows such capacitive position transducers to be used in contaminant filled environments, such as on a shop floor or in the outdoors. By sampling the output signals within 25 ns-35 ns from when the input signals were applied to the capacitive position transducer, the adverse effects of resistive contamination can be avoided.

Abstract: Any shape lead formed on various types of drum may be measured without employing an exclusively-used profiling plane by a lead shape measuring apparatus. The lead shape measuring apparatus includes a shape measuring unit for measuring the shape of a lead formed on a drum used in a VTR while a theoretical value suitable for this lead shape is selected based on the type of drum.

Abstract: In a capacitive sensor system, first and second driving signals opposite in phase are applied to a sensor capacitor and a reference capacitor, respectively, which are both connected to a switched capacitor circuit. An output signal of the switched capacitor is sampled by two sample-and-hold circuits which are operated in different phases of the driving signals. A differential amplifier generates a sensor signal in response to the difference in potential between the outputs of the sample-and-hold circuits.

Abstract: The gas fraction and the slip velocity are proportional to the difference in dielectric readings taken in a vertically upward flowing stream of oil/water/gas and in a vertically downwardly flowing stream of the same mixture. Actual water cuts are obtained by appropriately adjusting the dielectric water cut readings.

Abstract: An instrument to measure crosstalk between conductor pairs is configured to transmit a signal at one pair and receive the signal at another pair. The spureous or parasitic crosstalk at the connectors is supressed using a vectorial substraction from the received signal.

Abstract: Apparatus is provided for continuously compensating for the dielectric absorption effect in a measuring capacitor in a circuit which employs means for charging the capacitor to develop an output signal thereacross and wherein the absorption effect causes an error in the output signal following a charging interval. The dielectric absorption effect is represented by at least one impedance branch connected in parallel with the capacitor and this branch includes a resistor and a capacitor connected together in series and having a RC time constant. Compensating means is provided having a correction means associated with the impedance branch and wherein the correction means continuously receives the output signal and provides therefrom a compensating signal. The correction means has a time constant that corresponds with that of the associated impedance branch. The compensating signal is combined with the output signal to provide a compensated output signal.

Abstract: Apparatus to measure the crosstalk between pairs of conductors in a cable. The measurements are affected by the crosstalk inherent to the connectors that are part of the apparatus. The improvement consists of adding a compensating signal which equals that from the connector crosstalk in magnitude but has opposite sign.

Abstract: A closed loop circuit for controlling a differential capacitive sensor has been provided wherein the differential capacitive sensor includes a rigid top and bottom plate, and a movable middle plate. The closed loop circuit has a clock period which is divided into three separate sub-periods: the force period, the reset period and the sense period. During the force period, a top or a bottom electrostatic force is applied such that the middle plate is attracted towards the top plate or the bottom plate, respectively. During the reset period, substantially equal voltages are applied to all three plates. Finally, during the sense period, the voltage on the middle plate is sensed to determine whether the middle plate is closer to the top plate or to the bottom plate. Further, the result during the sense period will be utilized to determine whether to apply a top force or a bottom force during the next subsequent force period.

Abstract: A capacitive position transducer with reduced sensitivity to particulate and fluid contaminants. The electrodes of the position transducer are provided with a relatively thick coating of dielectric material so that the series impedance of the dielectric coatings is at least equal to the maximum capacitive impedance between the coatings whether or not there are any contaminants positioned therebetween. In one embodiment, the coatings are spaced-apart from each other to create an air gap while in another embodiment the coatings are resiliently biased toward each other so that the coatings slide along each other.

Abstract: An improved capacitance level probe is shown which can be positioned within the return vapor passage of a coaxial vapor recovery refueling system for detecting changes in liquid level within an underground fuel storage tank. The level probe has an outer conductor, a spaced inner conductor, a closed upper end and a lower end with an end opening which allows liquid to enter and leave the interior of the probe as liquid level changes within the underground tank being monitored. A tubular impact vent is located at the upper end of the outer conductor and collects tank vapors from the return vapor passage in order to conduct such vapor pressure to the open interior of the outer conductor above the level of liquid within the outer conductor. This action prevents sudden pressure drops within the outer conductor as the level of fuel changes within the underground storage tank, thereby eliminating inaccuracies in the sensed capacitance of the unit.