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.

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.

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.

Post time: Sep-18-2019