A temperature sensor assembly for use with a process vessel wall, the assembly comprising: a base structure having a base thermal conductivity (K B) and a base thickness (T B), the base structure including: a first surface adapted to form a contact area with a portion of an external surface of the process vessel wall having a vessel thermal conductivity (Kw), a vessel radius, and a vessel thickness (Tw), and a second surface spaced from the first surface by the base thickness (T B) of the base structure; a first temperature sensor extending through the base structure at the contact area such that, in use, said first temperature sensor is in physical contact with said external surface of said process wall, said first temperature sensor being adapted to measure a temperature of the external surface of the process vessel wall; a second temperature sensor disposed at the second surface of the base structure to measure a temperature of the second surface of the base structure; and a processor connected to the first temperature sensor and the second temperature sensor, and adapted to receive, from the first temperature sensor, the measured temperature of the external surface of the process vessel wall and to receive, from the second temperature sensor, the measured temperature of the second surface of the base structure, and 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, the base thermal conductivity (K B), the base thickness (T B), the vessel thermal conductivity (K W), and the vessel thickness (T W), said temperature sensor assembly being characterised in that said processor is adapted to determine said internal process vessel wall temperature value by using said vessel radius as an additional parameter. a base structure having a base thermal conductivity (K B) and a base thickness (T B), the base structure including: a first surface adapted to form a contact area with a portion of an external surface of the process vessel wall having a vessel thermal conductivity (Kw), a vessel radius, and a vessel thickness (Tw), and a second surface spaced from the first surface by the base thickness (T B) of the base structure; a first surface adapted to form a contact area with a portion of an external surface of the process vessel wall having a vessel thermal conductivity (Kw), a vessel radius, and a vessel thickness (Tw), and a second surface spaced from the first surface by the base thickness (T B) of the base structure; a first temperature sensor extending through the base structure at the contact area such that, in use, said first temperature sensor is in physical contact with said external surface of said process wall, said first temperature sensor being adapted to measure a temperature of the external surface of the process vessel wall; a second temperature sensor disposed at the second surface of the base structure to measure a temperature of the second surface of the base structure; and a processor connected to the first temperature sensor and the second temperature sensor, and adapted to receive, from the first temperature sensor, the measured temperature of the external surface of the process vessel wall and to receive, from the second temperature sensor, the measured temperature of the second surface of the base structure, and 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, the base thermal conductivity (K B), the base thickness (T B), the vessel thermal conductivity (K W), and the vessel thickness (T W), said temperature sensor assembly being characterised in that said processor is adapted to determine said internal process vessel wall temperature value by using said vessel radius as an additional parameter.