An electrical connector module assembly is disclosed having a connector header usable with multiple module bodies. The selectable torque transfer devices are engaged to yield an EVT with a continuously variable range of speeds and mechanically fixed forward speed ratios. An opaque shield portion provided with a second display section at one end thereof is provided to a transparent cover rotatably attached to the front wall. In addition, various vent pressures can be achieved by varying the vertical position of the reservoir relative to the aspiration line. Air and the sample solution, is transported sequentially into the reaction element in that order.