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Allen,
That's a great pic!! Bonafide Steam-Punk!!
...Still without a research grant, I poked around a bit and got the impression that the bed density refers to the material, and where it falls into the 'Geldart Groupings'

The type of fluid bed one chooses, is based on what group you're working with. Our beloved bean falls into catagory 'D', which is the highest density group. A sprouting bed is a common approach, as well as a shallow bed (bubble?)

On the testing; It seems you have a background in mechanical systems?
Cheers, -Scott