Apparatus designed to determine and/or monitor at least one process variable (P) of a medium (M) comprising a sensor unit with a tuning fork which is able to vibrate mechanically with a first and a second vibrating element , at least a first piezoelectric element and a second piezoelectric element , wherein the first piezoelectric element is arranged at least partially in the first vibrating element and the second piezoelectric element is arranged at least partially in the second vibrating element , wherein the first and the second piezoelectric element are designed to excite the tuning fork to perform mechanical vibrations by means of an excitation signal (A), to receive the mechanical vibrations of the tuning fork and to convert them into a first reception signal (E A ), and wherein the first piezoelectric element is designed to emit an ultrasonic signal (S), and wherein the second piezoelectric element is designed to receive the ultrasonic signal (S) in the form of a second reception signal (Es), a unit designed to determine and/or monitor a conductivity (σ) and/or a capacitance (C) of the medium (M), and an electronics unit , which is designed to determine the at least one process variable (P) using the first (E A ) and/or the second reception signal (Es), and to determine the conductivity (σ) and/or the capacitance (C) of the medium (M) using a third reception signal (E L/C ) received from the unit . a sensor unit with a tuning fork which is able to vibrate mechanically with a first and a second vibrating element , at least a first piezoelectric element and a second piezoelectric element , wherein the first piezoelectric element is arranged at least partially in the first vibrating element and the second piezoelectric element is arranged at least partially in the second vibrating element , wherein the first and the second piezoelectric element are designed to excite the tuning fork to perform mechanical vibrations by means of an excitation signal (A), to receive the mechanical vibrations of the tuning fork and to convert them into a first reception signal (E A ), and wherein the first piezoelectric element is designed to emit an ultrasonic signal (S), and wherein the second piezoelectric element is designed to receive the ultrasonic signal (S) in the form of a second reception signal (Es), a tuning fork which is able to vibrate mechanically with a first and a second vibrating element , at least a first piezoelectric element and a second piezoelectric element , wherein the first piezoelectric element is arranged at least partially in the first vibrating element and the second piezoelectric element is arranged at least partially in the second vibrating element , wherein the first and the second piezoelectric element are designed to excite the tuning fork to perform mechanical vibrations by means of an excitation signal (A), to receive the mechanical vibrations of the tuning fork and to convert them into a first reception signal (E A ), and wherein the first piezoelectric element is designed to emit an ultrasonic signal (S), and wherein the second piezoelectric element is designed to receive the ultrasonic signal (S) in the form of a second reception signal (Es), a unit designed to determine and/or monitor a conductivity (σ) and/or a capacitance (C) of the medium (M), and an electronics unit , which is designed to determine the at least one process variable (P) using the first (E A ) and/or the second reception signal (Es), and to determine the conductivity (σ) and/or the capacitance (C) of the medium (M) using a third reception signal (E L/C ) received from the unit . <EOS>