A measuring instrument (11) for fill level sensors (4) having a measuring electrode (6) and a compensating electrode (9). The measuring instrument (11) has a bridge circuit whose one bridge branch (21) is formed by two series-connected test resistors (23, 24). The other bridge branch (22) is formed by the measuring resistor (17) on the measuring electrode (6) and the compensating resistor (18) on the compensating electrode (9). Between the two bridge branches (21, 22) a diagonal line (26) with a test switch (27) is provided. The measuring instrument (11) is controlled by the control circuit (13) so that the diagonal line (26) is closed outside of preset test time periods, and interrupted at preset test time periods. The end of the diagonal line (26) located between the two test resistors (23, 24) is connected to the base potential (25) of the measuring circuit (12). The resistors (17, 18, 23, 24) are designed so that when the test switch (27) is closed, the imbalance of the bridge is determined by the second bridge branch (22), whereas when the test switch (27) is open, the imbalance of the bridge is determined by the first bridge branch (21), simulating in this way a situation in which the fill level has dropped below the limit value. The control circuit (13) provides an error signal if the signal of measuring circuit (12) is not corresponding. In this way, the measuring instrument (11) automatically tests its functionality.