[f09]

I have a problem concerning post-processing. My goal is to create a temperature-difference scalar field. Therefore I exported the temperature field from the initial run into an internal xyz table and imported it into the second (current) case. Now I would like to define a user defined field function based on the difference between the initial and the current temperature field. But how can I address the temperature from the xyz table? Or is there a better workflow?
Thanks in advance.