An apparatus forming a measuring device for ascertaining and monitoring a chemical or physical process variable in a high temperature process in a container, wherein the measuring device is formed from at least a sensor element located in the process and at least a measurement transmitter located outside the process. At least a first sealing element is provided for sealing against penetration of process medium into the sensor element. The sensor element has a boundary location between a first sensor element region, which faces the process and which is embodied with a high thermal resistance, and a second sensor element region, which faces away from the process and which has a low thermal resistance, and that the temperature sensitive element is arranged in the sensor element at the boundary location.