Measuring device, comprising: a sensor for measuring a physical or chemical measured variable and for generating at least one measured value corresponding to said measured variable; an electronic operating circuit electrically coupled to the sensor, wherein said operating circuit comprises at least a first communication interface, wherein the operating circuit is designed to generate at least a first electronic signal that correlates to the measured value and is transmitted via the first communication interface; and at least a first output unit with at least a second communication interface and at least an electrical output and at least an optical output, wherein the first output unit is designed to receive the first signal via the second communication interface and to convert it into a fourth electrical signal and into a second optical signal, wherein the fourth signal is output via the electrical output and the second signal is output via the optical output; and at least a second output unit comprising at least an optical input and an electrical output; wherein the optical input of the second output unit is coupled to the optical output of the first output unit, wherein the second output unit is designed to receive the second signal via the optical input and to convert it into a fifth electrical signal, wherein the second output unit has a third communication interface to the operating circuit, characterized in that the operating circuit is designed to also send the first signal to the second output unit, wherein the second output unit is designed to compare the first signal sent by the operating circuit with the second signal received from the first output unit and, after comparing the signals, to send a third signal to the operating circuit via the third communication interface, wherein the third signal contains information about the result of the comparison. a sensor for measuring a physical or chemical measured variable and for generating at least one measured value corresponding to said measured variable; an electronic operating circuit electrically coupled to the sensor, wherein said operating circuit comprises at least a first communication interface, wherein the operating circuit is designed to generate at least a first electronic signal that correlates to the measured value and is transmitted via the first communication interface; and at least a first output unit with at least a second communication interface and at least an electrical output and at least an optical output, wherein the first output unit is designed to receive the first signal via the second communication interface and to convert it into a fourth electrical signal and into a second optical signal, wherein the fourth signal is output via the electrical output and the second signal is output via the optical output; and at least a second output unit comprising at least an optical input and an electrical output; wherein the optical input of the second output unit is coupled to the optical output of the first output unit, wherein the second output unit is designed to receive the second signal via the optical input and to convert it into a fifth electrical signal, wherein the second output unit has a third communication interface to the operating circuit, characterized in that the operating circuit is designed to also send the first signal to the second output unit, wherein the second output unit is designed to compare the first signal sent by the operating circuit with the second signal received from the first output unit and, after comparing the signals, to send a third signal to the operating circuit via the third communication interface, wherein the third signal contains information about the result of the comparison.