A bob for a viscometer in a portable rheology unit is provided. The bob may include a bob portion having a cylindrical portion and a first end portion, the cylindrical portion defining a first end of the bob. The bob may also include a plastic portion interfacing with an interior surface of the first end portion, wherein the plastic portion includes a conical area extending from a conical area first end to a conical area second end, and the conical second end is connected to an interior surface of the cylindrical portion. The bob may also include a sleeve portion connected to the plastic portion and a bob second end portion connected to the sleeve portion.