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To top-up the cryostat, the pump must be pressurised as before; it does no harm to blow some nitrogen through the tube before it is inserted in order to cool it down somewhat - but it will soon freeze. When the tube is inserted and nitrogen pumped into the dewar it is usual for liquid to be blown out initially until the cold liquid is flowing in regularly. Again, there should be a period of several minutes when liquid flows in, with only a gas plume being seen. Finally a steady overflow of liquid indicates the cryostat is full.