The invention relates to a sealed oxygen reference fluid containing bag adapted for being pierced by an access probe for withdrawal of the oxygen reference fluid and including an access system comprising: a sealing element provided outside the bag and preventing any leakage between the bag and the access probe when the access probe has penetrated the bag, and a longitudinal support element provided inside the bag extending essentially parallel to an edge of the bag and being adapted to support the bag when the bag is penetrated by the access probe.