A device for determining and/or monitoring at least three different process variables (P1, P2) of a medium, comprising a sensor unit with a mechanically resonant unit that is a vibrating fork with one first vibrating element and one second vibrating element, at least one first piezoelectric element and one second piezoelectric element, and a unit that comprises a temperature sensor for determining and/or monitoring a temperature of the medium and with electronics wherein the first piezoelectric element and the second piezoelectric element are configured to cause the mechanically resonant unit to vibrate mechanically using a excitation signal, as well as to receive the mechanical vibrations of the resonant unit and convert these into a first reception signal (E A), wherein the first piezoelectric element is configured to transmit one transmitter signal, which is an ultrasonic signal, wherein the first piezoelectric element or the second piezoelectric element is configured to receive the transmission signal in the form of a second reception signal (Es) and wherein the electronics are configured to determine a density of the medium as a first process variable on the basis of the first reception signal (E A), determine a speed of sound within the medium as a second process variable on the basis of the second reception signal (Es), determine the temperature of the medium as a third process variable on the basis of a third reception signal (E T) that is received by the unit, compensate for an influence of the temperature of the medium on the first reception signal (E A) and/or on the second reception signal (Es), and determine a reference value for the density on the basis of the determined speed of sound and compare this with a value for the density determined from the first reception signal (E A), 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. a mechanically resonant unit that is a vibrating fork with one first vibrating element and one second vibrating element, at least one first piezoelectric element and one second piezoelectric element, and a unit that comprises a temperature sensor for determining and/or monitoring a temperature of the medium wherein the first piezoelectric element and the second piezoelectric element are configured to cause the mechanically resonant unit to vibrate mechanically using a excitation signal, as well as to receive the mechanical vibrations of the resonant unit and convert these into a first reception signal (E A), wherein the first piezoelectric element is configured to transmit one transmitter signal, which is an ultrasonic signal, wherein the first piezoelectric element or the second piezoelectric element is configured to receive the transmission signal in the form of a second reception signal (Es) and wherein the electronics are configured to determine a density of the medium as a first process variable on the basis of the first reception signal (E A), determine a speed of sound within the medium as a second process variable on the basis of the second reception signal (Es), determine the temperature of the medium as a third process variable on the basis of a third reception signal (E T) that is received by the unit, compensate for an influence of the temperature of the medium on the first reception signal (E A) and/or on the second reception signal (Es), and determine a reference value for the density on the basis of the determined speed of sound and compare this with a value for the density determined from the first reception signal (E A), determine a density of the medium as a first process variable on the basis of the first reception signal (E A), determine a speed of sound within the medium as a second process variable on the basis of the second reception signal (Es), determine the temperature of the medium as a third process variable on the basis of a third reception signal (E T) that is received by the unit, compensate for an influence of the temperature of the medium on the first reception signal (E A) and/or on the second reception signal (Es), and determine a reference value for the density on the basis of the determined speed of sound and compare this with a value for the density determined from the first reception signal (E A), 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.