An algorithm for in process optimum heat input control of arc welding is discussed, in which an analytical model of heat conduction is used to evaluate the temperature field during welding and the convex programming method is used in the optimisation of the heat input. The thermal response calculated using an analytical model corresponds well with that measured during welding and it is possible to identify the heat efficiency using the model. Furthermore, an in process optimising algorithm which includes the real time identification of heat efficiency is examined to estimate the optimum heat input for the required temperature field in gas tungsten arc welding under conditions where there is a change in the arc length. It is ascertained that the optimising algorithm is sufficient to determine the optimum heat input in real time during welding.