Once your roll is wound, you might be interested to know how its temperature might change if placed into a hot or cold store, or how humidity might vary throughout the roll or how solvent vapours will diffuse out of the roll. The RDC does those calculations for you.
The calculations for all three cases are the same. They all require the obvious basics: Roll diameter, Core diameter and Roll width. They need a From value (e.g 30 for 30deg or 30%RH) and a To value (e.g. 5 for 5deg or 5%RH). And they need an estimate, Time, in hours of the timescale for diffusion. If you select too short a Time then you will get a curve that hardly changes across the width. If it’s too large then your first data point will reach equilibrium. Long timespans take a long time to calculate, so always use smaller Times when you are first experimenting.
Finally, the calculations need a Diffusivity. Note that the model assumes linear, isotropic properties – so the diffusion coefficient is the same in the axial and radial directions, and diffusion obeys Fick’s Laws.
Not many of us know values of diffusivities. A typical value for a polyester web and question of thermal equilibration is 0.001cm²/s (0.00015in²/s). A typical value for a paper roll and hygroscopic equilibration is 0.0001 (0.000015in²/s). But in real life you will have to do some measurements on a small roll (which equilibrates quickly, making it easy to do the experiments) by which you calibrate the diffusivity value that you can use on larger rolls (where experimentation is much more difficult).
To experimentally determine the effective thermal diffusivity you can bury a thermocouple in a wound roll. Change the Diffusivity value in the RDC until the temperature predicted matches the temperature measured at that location. To be most practical, the thermocouple should be as far from all edges as practical and the roll chosen to be of a size that could cool noticeably in a few hours or a few days.
To experimentally determine the effective hygroscopic diffusivity you merely need to weigh a small (drying/wetting) roll and change Diffusivity value in the RDC until calculations match measurements. To be most practical, you should trial something like a delta of 5% moisture (such as 10% > 5%) in about the range you will be most interested in (because hygroscopic diffusivity is not truly a constant like it is with thermal problems). Also, roll size should be small enough to noticeably change weight in a few days. This would be something about the size of a loaf of bread which could be obtained as a butt roll from your slitter rewinders.
On pressing Calculate you see two things.