mounting board assembly 14 adjustably mounted in accordance with the present invention the centrifuge 10 includes a stationary framework or stator indicated schematically by reference character 18 the stator 18 includes a drive housing portion 20 which receives a drive cartridge 22 these elements are contained within a vacuum seal assembly housing 24 the drive cartridge 22 contains a suitable source of rotational energy such as an oil turbine or other motive source not shown the drive cartridge 22 is covered by a top cap 26 the rotational output from the motive source is conducted via a shaft 28 to a mounting spud 30 the spud 30 has a spring loaded locking detente 34 thereon an array of teeth 36 is provided on the periphery of the spud 30 for use by an alternate magnetic speed sensing arrangement a rotating element or rotor 38 is removably mounted to the spud 30 the undersurface 40 of the rotor 38 is provided with a speed indicia such as a decal 42 as shown in fig1 a which is a fragmentary bottom view of the rotor 38 the speed decal 42 includes a pattern having alternate radially extending bands of a light reflective and light absorptive material as indicated by reference characters 44 and 46 respectively as seen in connection with fig1 and 2 the top cap 26 extends upwardly from the upper surface of the seal assembly housing 24 the drive shaft 28 is suitably mounted for rotation on bearings not shown the shaft 28 extends through and above the top cap 26 for a distance sufficient to receive the rotor 38 on the spud 30 in a manner which permits rotation of.