numerical scheme

ado

Dear friends, i am trying to use numerical methods in order to determine thermal diffusivity and thermal conductivity of unknown material ranged from highly diffused to lower one. The method used is well known and called "parameter estimation" which use the comparison between experimental and numerical temperature-time histories of the material under investigation. I am using a fully implicit numerical scheme. I got very good result in steady state (no variation of temperature with time). This allows to get the thermal conductivity of any material. However, I found something strange. A difference exists between the numerical and the experimental curves in transient state. This difference become more and more important with the "decrease " of thermal diffusivity value. Because of this, it is not possible at the present stage to get a satisfactory results for thermal diffusivity. Personaly, I guess this is due to the numerical scheme i am using which is well known to be unconditionaly stable but not very accurate..But on the other hand, how can this happen only for lower diffuse materials and not for the highers like metal liquids??? waiting for your answers and advise. ado