Abstract: A cover device is adapted for use with a casing which has an open top end, and a confining wall which confines an interior that is divided into a plurality of cell compartments by partition plates. The cover device includes a lower cover body with a top wall and a surrounding wall which confines a cavity that is divided into a plurality of chambers by means of dividers. Each of the dividers has a bottom edge that is adapted to be mounted sealingly on a top edge of a respective one of the partition plates to prevent fluid communication among the cell compartments below the top wall. The top wall has a vent area that is formed with a plurality of vent holes for communicating an upper side of the top wall with the chambers, respectively, and a plurality of upwardly projecting annular lips around the vent holes, respectively. Each of the annular lips is formed with a notch unit, and has a pressure cap sleeved movably thereon for covering the respective vent hole.