Method and apparatus for monitoring fill level of a medium in a container

1. A method for monitoring a predetermined fill level of a medium in a container with at least one measuring probe and an electronics unit, comprising the steps of:

operating the measuring probe alternately in a conductive and in a capacitive operating modes,

supplying the measuring probe with an exciter signal, which is composed of two different, time alternatingly sequential, periodic signal components;

generating in a first time interval a first periodic signal component for the conductive operating mode and in a second time interval a second periodic signal component for the capacitive operating mode, wherein a rectangular signal is used for said exciter signal in said first time interval and a triangular signal or sine signal is used in said second time interval, and wherein a response signal in both time intervals is a rectangular signal;

ascertaining from said response signal obtained from the measuring probe, which depends on said first and second signal component, whether the predetermined fill level has been reached according to the capacitive or conductive operating mode; and

generating a report upon an exceeding or subceeding of this fill level;

wherein in the case of non electrically conductive media said response signal obtained during application of the capacitive operating mode is evaluated;

in the case of electrically conductive media said response signal obtained during application of the conductive operating mode is evaluated; and

for media with conductivity within a transitional region, said response signal of both operating modes are evaluated and measured values won from said response signal are provided with corresponding weighting factors as a function of a conductivity of the medium.

2. The method as claimed in claim 1 , wherein:

said rectangular signal and said triangular signal are so sized that they have the same arithmetic average value.

3. The method as claimed in claim 1 , wherein:

at least one medium specific property is ascertained from said response signal.

4. An apparatus for monitoring a predetermined fill level of a medium in a container, comprising:

at least one measuring probe and an electronics unit, wherein:

said electronics unit is so embodied that measuring probe is operated alternately in a conductive and a capacitive operating mode;

said measuring probe is supplied with an exciter signal, which is composed of two different, time alternatingly sequential, periodic signal components, wherein in a first time interval a first periodic signal component is generated for the conductive operating mode and in a second time interval a second periodic signal component is generated for the capacitive operating mode, wherein a rectangular signal is used for said exciter signal in said first time interval and a triangular signal or sine signal is used in said second time interval, and wherein a response signal in both time intervals is a rectangular signal; and

from said response signal obtained from the measuring probe, which depends on the actual signal component, it is ascertained, whether the predetermined fill level has been reached according to the capacitive or conductive operating mode; and

a report is generated upon an exceeding or subceeding of this fill level;

wherein in the case of non electrically conductive media said response signal obtained during application of the capacitive operating mode is evaluated;

in the case of electrically conductive media said response signal obtained during application of the conductive operating mode is evaluated; and

for media with conductivity within a transitional region, said response signal of both operating modes are evaluated and measured values won from said response signal are provided with corresponding weighting factors as a function of a conductivity of the medium.

5. The apparatus as claimed in claim 4 , further comprising:

a module for generating said rectangular signal and a module for generating said triangular signal or sinusoidal signal.

6. The apparatus as claimed in claim 4 , wherein:

said measuring probe is composed of a sensor electrode and a guard electrode.

7. The apparatus as claimed in claim 6 , further comprising:

at least one measuring resistor, via which in the conductive operating mode a ratio of an electrical current through said sensor electrode and an electrical current through said guard electrode is determined.

8. The apparatus as claimed in claim 4 , further comprising:

at least one voltage divider for generating the first periodic signal component in the conductive operating mode.

9. The apparatus as claimed in claim 7 , further comprising:

at least one difference amplifier, with which in the capacitive operating mode via said measuring resistor a difference of the voltages on said sensor electrode and said guard electrode is determined.

10. The apparatus as claimed in claim 6 , further comprising:

an ESD protection circuit, comprising at least one diode circuit and one bleeder resistor, wherein:

said at least one diode circuit and said at least one bleeder resistor are connected with said guard electrode and via said guard electrode with a ground connection.

11. The apparatus as claimed in claim 4 , wherein:

said measuring probe is so embodied that after installation in the container in a region facing the medium it is essentially flush with a wall of the container or at least partially protrudes inwardly into the container.