 These turbines (or rotors) come in a variety of diameters (outside diameter), from 0.70–15.0 mm, and are usually spun on air or nitrogen gas.The rotors are made from a number of different materials such as ceramics e.g.zirconia, silicon nitride or polymers such as poly(methyl methacrylate) (PMMA), polyoxymethylene (POM).