The invention relates to an apparatus for determining and/or monitoring at least one process variable of a medium (1). The apparatus includes at least one probe unit (3) and at least one electronics unit (4), which supplies the probe unit (3) with an electrical, exciter signal and which receives from the probe unit an electrical, measurement signal. The invention includes, that the probe unit (3) has at least one inner electrode (5) and at least one outer electrode (6) surrounding the inner electrode.