If available, the steady data of the first normal-stress difference is also useful. Data can be acquired using a cone-plate rheometer or using a technique based on the hole pressure. It is also possible to estimate the first normal-stress difference on the basis of capillary extrudate swell, by using an empirical formula suggested by Tanner [11].

If such data cannot be acquired, you can instead extract information about the steady first normal-stress difference using the empirical Cox-Merz rule [4] (p. 201), assuming that the storage modulus can reveal a good indication about the first normal-stress difference. See Empirical Rules and Principles for details.

Even though the knowledge of the first normal-stress difference is not important for all processes, the automatic fitting procedure can benefit from it. While extrapolating from measured quantities can provide additional data, data should not be extrapolated over more than one decade.