An apparatus for determining and/or monitoring a physical and/or chemical, measured variable of a medium, including a housing, especially an elongated housing, and a sensor arranged in the housing. The sensor serves for registering the measured variable, wherein the housing has at least one opening, through which the sensor comes in contact with the medium. The housing has a first housing section, in which the sensor and a sensor electronics are arranged. The sensor electronics is arranged on a first carrier element. The sensor electronics serves for changing the measured variable into an analog electrical signal. An insert is applied in the first housing section, in which the sensor electronics is arranged. The insert is tubular.