The storage tank for cryogenic liquids incorporates an ullage vessel that provides for an ullage space. The ullage vessel is in communication through an ullage line to a fill line that provides cryogen to a cryogen space. The junction where the ullage line and fill line meet is of a certain cross-sectional area. Downstream of the junction within the fill line, measured in reference to the direction of cryogen flow during a coruse of filling the cryogen space, is of a greater cross-sectional area than is the case at the junction. This creates a pressure reduction at the junction during a course of filling that causes a net flow of material from the ullage space over the course of filling. Once the cryogen tank is liquid full causing cryogen to be redirected down the ullage line, the smaller cross-sectional area of the ullage line compared to the fill line causes a reduction in flow of cryogen which is detected by the fill pump causing filling to stop providing the ullage space. A means of draining the ullage vessel prior to and during filling of the storage tank is provided.