Discharge-proof cryocontainers are vacuum-insulated, double wall containers which have an internal chamber for holding the material samples to be cooled. This chamber is surrounded by the cooling medium, generally liquid nitrogen. The liquid cooling medium is stored in a porous mass which is, therefore, saturated, for example, with liquid nitrogen. The liquid nitrogen can, therefore, not run out when the container tips over or is upside down. The double wall container is closed with a stopper which forms a gas escape gap with the neck of the container.
The cooling medium evaporated as a result of incoming heat escapes through this gas escape gap, for example, gaseous cold nitrogen. The material sample to be cooled can be shipped in these containers.
When these containers are upside down, for example, as a result of a mishap, they do not properly cool any longer. This applies, in particular, for large containers. Large containers in which, for example, animal carcasses such as goats must be kept and shipped at very low temperature are, to be sure, also discharge-proof but cool very poorly when upside down. The reason is that in such containers, determined by the construction, the gas escape gap has a much larger flow cross section than in a small container. When such a large container is upside down, surrounding air enters through the large gas escape gap into the container and causes the liquid nitrogen to evaporate which, in the form of gaseous cold nitrogen, flows downward through the gas escape gap. The relationships can be directly described by free convection since the specific gravity of the cold nitrogen is about four times higher than air.
Penetration of the warm surrounding air into the inside of the container causes a rapid evaporation of the liquid nitrogen so that the container can maintain its cooling function for a short time only. For large containers, hazardous oxygen enrichment inside the container may then, moreover, occur. In addition to the air, water arrives inside the container which contaminates the stored material.
An improvement can be attained by constructing the locking stopper as a locking cover which grasps the outside of the container neck. When the container is upside down, the locking cover then assumes the function of a siphon trap. Penetration of the surrounding air into the container is then no longer possible. But when the container lies on its side, such a siphon does not prevent the penetration of air into the container.