The following information was kindly provided by Terese Bergfors, Uppsala Universitet, Sweden.
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).
- 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, it 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 22 days (16 days for the CXR-100; see below for details), and only when brand new, and provided the vacuum has not been compromised.
- How do I check the dewar holds its charge? Do a NER (normal evaporation rate) test:
- Record the tare weight (= the empty dewar plus its lid).
- Properly charge the dewar as per manufacturer’s instructions, i.e. fill with liquid nitrogen and leave the dewar for 2-3 hours.
- Excessive accumulation of condensation or ice indicates damage.
- Pour off the excess LN2. Record the drained weight = charged weight. Should be ≈3.6 kg more than the dry weight. Per the specs of a new CXR100, the absorbed amount of LN2 is approx. 3.7 litres of LN2. Once you see that number start going down, it is time to change the socks (usually every 3 years).
- After 24 hours, weigh it.
- Repeat again after 24 hours=48 hours total.
- The maximum acceptable NER (normal evaporation rate) is 0.14 kg per 24 hours.
- Check the dewar every time you use it (or at least every 3 months).
- 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.
- It is okay to use a boot drier (e-mail correspondence with Taylor-Wharton 2015.07/tex)
- 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.
What is the difference between CX100 and CXR-100?
- CXR100 vs. CX100
They are the same price in Sweden, so is one better than the other? The only difference I see is that one has a sock and one does not. Does the sock make the CXR100 a better dewar? (Better=holds temperature longer). No – Both units have socks inside of them. The CXR model allows you to replace the socks over time. The CX model the absorbent sock is permanently built into the unit, you can’t replace it. The costs are the same but the performance is different. There is a tradeoff – if you want the ability to change out the socks you go with the CXR model but the performance of the unit is different compared to the CX model. The Static Holding Time for the CX100 is 22 days for the CXR100 is 16 days. The CXR has a removable sleeve in the holding cavity that keeps the socks in place. That removable sleeve is made of a hardened plastic. This sleeve introduces more direct heat into the cavity of the unit thus having a shorter Static Holding Time.
- What is Static Holding Time? Static Holding Time is a value we use to judge the performance of a unit. The value is made up of the amount of LN2 in liters divided by the NER performance. This will give you the number of days or length of time a unit can hold product until it goes empty. Remember the static holding time is nominal. Actual rate may be affected by the nature of the contents, atmospheric conditions, container history and manufacturing tolerances.