Correctly charging a dry shipper with liquid nitrogen before use is a critical step that directly determines how long the vessel will maintain cryogenic temperatures during transport. An improperly charged dry shipper may appear to be performing correctly but will have a significantly shorter hold time, potentially putting samples at risk during longer journeys.
The charging process works because the inner wall of a dry shipper contains a highly absorbent porous matrix, typically made from engineered aluminium, which soaks up and retains liquid nitrogen. The goal of charging is to saturate this matrix as fully as possible before your samples go in.
Here is the correct step-by-step process for charging a dry shipper:
Begin by ensuring your dry shipper is at room temperature and completely dry internally. Any residual moisture can freeze and block the absorbent matrix, reducing its nitrogen uptake capacity.
Place the dry shipper in a stable, upright position on a non-slip surface. Put on your cryogenic gloves and face shield before handling any liquid nitrogen.
Slowly pour liquid nitrogen into the dry shipper through the neck opening. You will hear vigorous boiling as the nitrogen contacts the warm matrix. Continue adding nitrogen as the boiling subsides. The initial charge will evaporate quickly as it warms the matrix and vessel walls.
Allow the vessel to continue absorbing nitrogen. Add more liquid nitrogen periodically as the level drops. A fully saturated matrix will absorb significantly more nitrogen than the first pour suggests. For a 3-litre dry shipper, the initial charging process typically takes 20 to 40 minutes and may require several top-ups.
Once the boiling has slowed significantly and the nitrogen level stabilises, the matrix is approaching saturation. At this point, allow the vessel to sit for a further 10 to 15 minutes to ensure full saturation.
Pour out any excess free liquid nitrogen. Tilt the vessel carefully to drain it. Shake gently to check for any free liquid. A properly charged dry shipper should have no free liquid nitrogen remaining, only absorbed nitrogen in the matrix.
Allow the vessel to equilibrate for 5 to 10 minutes after draining before loading samples. The internal temperature should be well below minus 150 degrees Celsius at this point.
Load your samples into the precooled internal configuration, close the lid, and proceed with transport.
A common mistake is under-charging, typically caused by not allowing enough time for the matrix to fully saturate, or by using nitrogen that has been contaminated with moisture. Another common error is failing to drain all free liquid before loading samples, which risks liquid nitrogen contact with samples during transport.
For the full range of dry shippers suitable for clinical, veterinary, and research transport, including units from 2 litres to 10 litres, visit the CryoStork dry shipper collection at Cryolab.
