Left panel: effect of diffusion, metallicity changes and helium abundance changes on the core regions for models S_A,C1, S_A,C2, S_A,L1, S_A,U1 on the target function of t_u. Since the quantity is integrated, the sensitivity is greatly improved. Right panel: the Y(x) profile of these models is illustrated, thus showing the link between t_u and chemical composition and thus, its diagnostic potential.