A temperature sensor assembly for use with a process vessel wall includes a base structure, a first temperature sensor, a second temperature sensor, and a processor. The base structure forms a contact area with an external surface of the process vessel wall. The first temperature sensor extends through the base structure to measure a temperature of the external surface of the process vessel wall. The second temperature sensor is at a second surface spaced from the first surface to measure a temperature of the second surface of the base structure. The processor is connected to the first and second temperature sensors, and adapted to determine an internal process vessel wall temperature value as a function of the measured temperature of the external surface of the process vessel wall, the measured temperature of the second surface of the base structure, base structure parameters, and process vessel wall parameters.