A combination weigher comprises actuator units for actuating weighing hoppers, and a body having a side wall attached with the actuator units, and a casing member used to accommodate feeders is formed by the side wall. The venting passage is provided for operatively fluidly connecting the supply passage to an ambient atmosphere for selectively enabling or disabling the hydraulic pump. In selected embodiments, the method enables a pattern author to add new or custom POV types to the \u201csubstitution group,\u201d thereby allowing the pattern author to extend the pattern meta model to include new POV types.