Measuring instrument for fill level sensors

Abstract

A measuring instrument ( ) for fill level sensors ( ) having a measuring electrode ( ) and a compensating electrode ( ). The measuring instrument ( ) has a bridge circuit whose one bridge branch ( ) is formed by two series-connected test resistors ( ). The other bridge branch ( ) is formed by the measuring resistor ( ) on the measuring electrode ( ) and the compensating resistor ( ) on the compensating electrode ( ). Between the two bridge branches ( ) a diagonal line ( ) with a test switch ( ) is provided. The measuring instrument ( ) is controlled by the control circuit ( ) so that the diagonal line ( ) is closed outside of preset test time periods, and interrupted at preset test time periods. The end of the diagonal line ( ) located between the two test resistors ( ) is connected to the base potential ( ) of the measuring circuit ( ). The resistors ( ) are designed so that when the test switch ( ) is closed, the imbalance of the bridge is determined by the second bridge branch ( ), whereas when the test switch ( ) is open, the imbalance of the bridge is determined by the first bridge branch ( ), simulating in this way a situation in which the fill level has dropped below the limit value. The control circuit ( ) provides an error signal if the signal of measuring circuit ( ) is not corresponding. In this way, the measuring instrument ( ) automatically tests its functionality.