A device for determining and/or monitoring at least a first process variable and a second process variable of a medium, comprising a sensor unit with a unit that can mechanically oscillate, at least one first piezoelectric element, a second piezoelectric element, and a unit for determining and/or monitoring a pressure (p), and electronics, wherein the unit that can mechanically oscillate is an oscillating fork with a first oscillating element and a second oscillating element, wherein the first piezoelectric element is arranged at least partially in the first oscillating element, and the second piezoelectric element is arranged at least partially in the second oscillating element, wherein the first piezoelectric element and the second piezoelectric element are configured to cause the unit that can mechanically oscillate to mechanically oscillate by means of an excitation signal, and to receive the mechanical oscillations of the unit that can oscillate and convert these into a first received signal (E A), wherein the first piezoelectric element is configured to send a transmission signal, and wherein the second piezoelectric element is configured to receive the transmission signal in the form of a second received signal (Es), wherein the electronics are configured to determine the first process variable based on the first received signal (E A) and the second process variable based on the second received signal (Es), and to determine the pressure (p) of the medium based on a third received signal (E p) sent to the unit, wherein the electronics are configured to determine a pressure (p) of a gas mixture which is located in the same container as the medium, above the medium, wherein the electronics are configured to work out the CO 2 content in the gas mixture and a resultant CO 2 concentration in the medium based on a change in the pressure (p), and wherein the electronics are configured to compensate for an effect of the CO 2 concentration in the medium on the determination of the first process variable and/or the second process variable. a unit that can mechanically oscillate, at least one first piezoelectric element, a second piezoelectric element, and a unit for determining and/or monitoring a pressure (p), and electronics, wherein the unit that can mechanically oscillate is an oscillating fork with a first oscillating element and a second oscillating element, wherein the first piezoelectric element is arranged at least partially in the first oscillating element, and the second piezoelectric element is arranged at least partially in the second oscillating element, wherein the first piezoelectric element and the second piezoelectric element are configured to cause the unit that can mechanically oscillate to mechanically oscillate by means of an excitation signal, and to receive the mechanical oscillations of the unit that can oscillate and convert these into a first received signal (E A), wherein the first piezoelectric element is configured to send a transmission signal, and wherein the second piezoelectric element is configured to receive the transmission signal in the form of a second received signal (Es), wherein the electronics are configured to determine the first process variable based on the first received signal (E A) and the second process variable based on the second received signal (Es), and to determine the pressure (p) of the medium based on a third received signal (E p) sent to the unit, wherein the electronics are configured to determine a pressure (p) of a gas mixture which is located in the same container as the medium, above the medium, wherein the electronics are configured to work out the CO 2 content in the gas mixture and a resultant CO 2 concentration in the medium based on a change in the pressure (p), and wherein the electronics are configured to compensate for an effect of the CO 2 concentration in the medium on the determination of the first process variable and/or the second process variable.