Care of dry shippers

For optimal transportation of your crystals it is imperative that you look after your dry shipping dewars. Here are some tips on how to make sure that you don't lose your crystals or introduce ice onto your pucks and pins.

Handling and loading

• Handle with care. Sudden impact can damage the dewar: please ship your dewars in the appropriate shipping container.
• Be careful when placing heavy items (puck holders for example) in the dewar. Do not drop them in, since this could damage the neck of the dewar.
• Do not pack items that are too tall for the dewar, nor incorrectly install its lid to make the items fit. Forcing the lid down can damage the neck

Charging with liquid nitrogen

• Charge the dewar as recommended in the manufacturer's instructions (photocopy these and tape to the side of the dewar if necessary), ensuring all residual liquid nitrogen is poured out of the dewar prior to shipment. 
• Minimise liquid nitrogen spillage on the vacuum release valve near the top of the dewar.
• Check that the 'charged' dewar weight is in line with manufacturer's recommendations (e.g. in the case of the Taylor Wharton CX-100 dry shipper dewar, a charged dewar will weigh 3.6kg more than the 'dry' weight, and thus provide enough absorbed liquid nitrogen to keep the dewar at liquid nitrogen temperatures for 21 days, provided the vacuum has not been compromised).
• Check the dewar every time you use it (or at least every 3 months).

Drying

• Ensure the dewar is dried between uses. Residual moisture can cause damage to the foam material if frozen in situ.
• How do I speed up drying? What is the most effective procedure?
  • Purge with dry nitrogen or air.
  • Leave in a dehumidified room.
  • Be careful: it may still take several days to fully dry.
• How do I know if it is dry?
  • Weigh the dewar at time of purchase (record this on the dewar).
  • When dry, the dewar should be within 0.5kg of the original dry weight.
  • Weigh during drying until it no longer decreases.
• If you don't have time to dry the dewar properly between visits to synchrotrons, we strongly advise you to purchase more shipping dewars.

A well cared for dewar that is properly charged with liquid nitrogen before use should keep crystals safe for at least a week without refilling being required.

Dry shippers rely on two things to keep your samples cold during transit; an absorbent that immobilises a certain mass of liquid nitrogen and a vacuum dewar that insulates the contents and reduces the loss of this liquid nitrogen. Once the liquid nitrogen is all lost, the dewar warms up. We suggest you assess your dewars regularly as outlined below to ensure their integrity and fitness to ship your valuable samples.

The quantity of nitrogen held is dependent on the condition of the absorbent material (degraded absorbent can sometimes be seen as a fine white powder in your dewar). As the absorbent degrades, the amount of nitrogen stored decreases. The easiest way to damage the absorbent is to cool the dry shipper before the absorbent material has fully dried. Dry shippers should be warmed upside down and left until completely dry before cooling. You can speed this process up by directing a flow of dry air into the dry shipper

You can easily assess the absorbent material condition. Weigh your dry shipper when it is dry and warm. Now cool the shipper as you would prior to shipping, pour off any residual liquid and weigh it again.

Cold weight – warm weight = mass of stored nitrogen

This should be around 3kg depending on dry shipper model. On some models this material can be replaced. Do this test periodically and compare values over time.

The loss rate of nitrogen relates to the insulating performance of the dewar (mainly the vacuum level, but also the neck plug condition).

The combined effect of these can be measured by simply weighing the dry shipper when cold over a period and calculating the rate of mass loss. Once at a stable temperature (leave the dry shipper for an hour or so after charging), the mass loss is very linear and two measurements 24h or so apart should give a good estimate.

For a new dry shipper, this mass loss should be around 150-200g a day depending on model.

Diamond colour classifications based on weight loss are as follows:

• Green:               under 300g per day
• Yellow:              300-400g per day
• Red:                   over 400g per day

Total lifetime can be easily calculated from the previous two experiments.

Mass of stored nitrogen/ mass loss per day = maximum storage lifetime of dry shipper

We would recommend relying on no more than half of this value as a sensible safety margin for your samples. At Diamond, dry shippers are topped up in storage every five days.