Device for determining the level of a medium in a container

#The invention relates to a device for determining the level of a medium in a container by means of a capacitive sensor, wherein a voltage source is provided, which subjects the sensor to a measuring frequency, wherein a measuring/evaluating circuit is provided, which ascertains the measured capacitance and compares it with a predetermined capacitance and furnishes the information about the level of the medium in the container.

#In capacitive methods for determining the level of a product in a container, the capacitive sensor and container wall form the electrodes of a capacitor. If the container wall is not conductive, then a separate electrode must be provided, inside or outside the container. Between the two electrodes—depending on the level of the medium in the container—there is either air or the medium, which because of the different dielectric constants of the two substances is expressed as a change in the measured capacitance. Naturally, the measured capacitance is also dependent on the fill level of the medium in the container at the time, since the variables called “level” and “measured capacitance” both depend functionally on one another. Capacitive sensors can therefore be used both in limit state detection and in continuous level determination.

#From German Patent DE 195 36 199 C2, a capacitive level sensor has been disclosed that is mounted at the height of the level to be monitored. In other words, it is a limit state detector, or limit switch. Limit switches are preferentially mounted as overflow suppressors in containers or as means of securing pumps against running empty. If the capacitive sensor is covered by the particular product to be detected, then it has a higher capacitance than in the uncovered state. By means of a capacitance measuring circuit and a comparator, the measured capacitance is compared with a threshold value; from the outcome, it is clearly detectable when the level to be monitored has been reached.

#The invention will be described in further detail in conjunction with the following drawings. Shown are:

#FIG. 1: a schematic illustration of the device of the invention;

#FIG. 2: a substitute circuit diagram of the measuring/evaluating circuit of the device of the invention;

#FIG. 1 is a schematic illustration of the device 1 of the invention. The capacitive sensor 4 is disposed in the upper region of the container 4, to prevent overfilling, or in other words, in the case illustrated, it functions as a limit switch. The capacitive sensor 4 ascertains the measurement voltage UM, that is, the measurement or apparent current IM. Preferably, the device 1 of the invention employs a sinusoidal measurement voltage UM of constant measuring frequency fM. The measurement voltage UM and measurement current IM vary as a function of the measured capacitance CM, which—as already mentioned several times—is a measure for the level F of the medium 3 in the container 2. Via the connecting line 5, the capacitive sensor 4 is connected to measuring/evaluating circuit 6; it is quite possible for the measuring/evaluating circuit 6 instead to be disposed in the capacitive sensor 4.

#In FIG. 2, a substitute circuit diagram of the measuring/evaluating circuit 6 of the device 1 of the invention is shown. An inductor L is connected in series with the measured capacitance CM. A resistor R is connected parallel to the inductor L. The measured capacitance CM, inductor L, and resistor R form a series resonant oscillating circuit 7. Every real inductor—because it is always accompanied by parasitic capacitances—acts as its own parallel oscillating circuit with its own resonant frequency or its own frequency response. By means of suitable tuning of this resonant circuit, it is therefore possible for unwanted frequency components of the interference spectrum to be largely and purposefully suppressed.

#The resistor R serves to provide purposeful damping of the series resonant oscillating circuit 7. Between the parallel circuit, formed by the inductor L and resistor R, and the measured capacitance CM, there is the insulation capacitor C1 of the capacitive sensor 4, which is represented by the capacitor 8. A further capacitance CA is connected to the internal ground of the sensor 4 at the node point 11 of the parallel circuit comprising the inductor L and the resistor R. This capacitance CA serves to set the operating point of the measuring/evaluating circuit 6. The environment of the sensor 4 to be evaluated, that is, the container 2, the medium 3, and ground, is shown in the substitute circuit diagram in the form of the measured capacitance CM and an external return line to the internal ground of the sensor 4.