An apparatus having at least one sensor for registering at least one chemical and/or physical, process variable, and at least one modularly constructed, measuring transducer circuit, which has at least one sensor unit, which ascertains a measured variable from the process variable registered by the sensor and supplies the sensor with energy as needed, and at least one application specific, signal processing unit for ascertaining a measured value of the measured variable, wherein a conditioned output signal is provided between the sensor unit and the application specific, signal processing unit, wherein the application specific, signal processing unit is exchangeably embodied, and wherein, as a function of a predeterminable accuracy of measurement with which the apparatus ascertains the measured values, a plurality of different types of application specific, signal processing units are provided.