Probe unit with - a probe housing, - a probe electrode which is at least partially rod-like in shape - an additional electrode, which coaxially surrounds the probe electrode at least in part and which is isolated from the probe electrode by means of electrical isolation, - a first fastening unit with which the additional electrode is fastened in the probe housing, - a connection module, which can be fitted on a rod-like section of the probe electrode arranged in the probe housing, and which has a printed circuit board with at least a first conductive track for the electrical connection of the probe electrode and at least a second conductive track for the electrical connection of the additional electrode, - and with a second fastening unit designed for fastening the connection module to the probe electrode, characterized in that the printed circuit board is flexible, and in that the connection module has an insulation sleeve and a module housing in addition to the printed circuit board, wherein - the insulation sleeve has an essentially cylindrical meter body for accommodating a section of the rod-like probe electrode and a planar support element which is aligned in an essentially orthogonal manner in relation to the meter body, said element dividing the meter body into a first section and a second section, - in an end section the printed circuit board has a first contact area into which the first conductive track leads, and a second contact area into which the second conductive track leads, wherein the first contact area and the second contact area each has a central opening the diameter of which is at least the same as the outer diameter of the meter body of the insulation sleeve, and wherein at least the central openings of the first contact area and the second contact area can be aligned by folding the printed circuit board such that the first contact area can be placed on the first section of the meter body and the second contact area can be placed on the second section of the meter body, - and the module housing holds together the assembly comprising the support element and the contact areas (71, 74) of the printed circuit board. - a probe housing, - a probe electrode which is at least partially rod-like in shape - an additional electrode, which coaxially surrounds the probe electrode at least in part and which is isolated from the probe electrode by means of electrical isolation, - a first fastening unit with which the additional electrode is fastened in the probe housing, - a connection module, which can be fitted on a rod-like section of the probe electrode arranged in the probe housing, and which has a printed circuit board with at least a first conductive track for the electrical connection of the probe electrode and at least a second conductive track for the electrical connection of the additional electrode, - and with a second fastening unit designed for fastening the connection module to the probe electrode, characterized in that the printed circuit board is flexible, and in that the connection module has an insulation sleeve and a module housing in addition to the printed circuit board, wherein - the insulation sleeve has an essentially cylindrical meter body for accommodating a section of the rod-like probe electrode and a planar support element which is aligned in an essentially orthogonal manner in relation to the meter body, said element dividing the meter body into a first section and a second section, - in an end section the printed circuit board has a first contact area into which the first conductive track leads, and a second contact area into which the second conductive track leads, wherein the first contact area and the second contact area each has a central opening the diameter of which is at least the same as the outer diameter of the meter body of the insulation sleeve, and wherein at least the central openings of the first contact area and the second contact area can be aligned by folding the printed circuit board such that the first contact area can be placed on the first section of the meter body and the second contact area can be placed on the second section of the meter body, - and the module housing holds together the assembly comprising the support element and the contact areas (71, 74) of the printed circuit board.