This invention relates to a cap for use with a fluid container.  In particular, this invention relates to a cap that encloses a vial container of various volumes and shapes.  The vial cap allows for penetration by pipette or sampling tube while avoiding the problems associated with the conventional vial cap discussed below. 
A problem common to conventional vial caps is the propensity for the pipette or sampling tube to cause a back-pressure during filling or a vacuum condition during aspiration of the vial container.  Back-pressure and vacuum conditions can cause errors in the precise transfer of fluid to or from the vial container.  For example, a vacuum condition created in the vial container during a pipetting operation may cause the amount of fluid removed from the container to be less than the desired amount.  Thus, problems transferring the precise amount of fluid from the vial container to the pipette or vice versa may occur when the pipette engages the vial cap and begins to add or withdraw containment fluid from the vial container. 
Another problem with conventional vial caps is lack of symmetry and flange portions that overhang the vial container.  Robotic manipulating arms are designed to grasp vial containers of a particular diameter or width.  When conventional vial caps are utilized with vial containers, they oftentimes have flanges or other extremities that extend beyond the outer perimeter of the vial container and may cause the robotic manipulating arm to fail to grasp the vial container properly.  These conventional vial caps may cause a transport or pipetting operation to completely fail. 
Another problem of the conventional vial cap is that it may be damaged during a pipetting operation because the vial cap often has a shallow slope leading to a center flap portion of the vial cap.  The shallow slope may cause the pipette to impinge on an area of the vial cap that is not penetrable rather than reaching the center flap portion. 
Another problem may occur when the pipette penetrates the center flap portion.  The center flap portion may tear-away and fall into the containment fluid causing pipetting and/or contamination problems. 
What is needed is a vial cap that works well with robotic manipulating arms, maintains its operability during pipetting operations, keeps its center flap portion from falling into the containment fluid, and can facilitate the transfer of containment fluid without creating back-pressure or vacuum conditions that interfere with the proper pipetting of the containment fluid.