In our experience, "dry shipper" dewars will "fail" for two reasons:
1) the vacuum gets compromized
2) the "sponge" that soaks up the liquid nitrogen gets contaminated with 
water

Hot and cold cycling can cause the vacuum pump-out valve (that 
sticky-out thingy that always seems to be in the way) to leak. This 
includes spilling liquid nitrogen on the valve while filling the dewar!  
Once the vacuum starts to go, you will notice the outside of the dewar 
is cool or even icy to the touch, and this further weakens the valve 
seal.  Makes one wonder why we bother with a valve, until, of course, 
you actually do need to pump it down.  We have rescued a couple of user 
dewars over the years by pumping them down.  Doesn't always work, but 
sometimes it does. Obtaining the fitting for the port is the tricky 
part.  We got ours from here:
http://www.cryocomp.com/vacuum_valves/product.php?id=50

The contaminated sponge problem is different, and depends on what kind 
of "sponge" you have.  Most modern dewars use a zeolite, which is a 
porous mineral.  The name literally means "boiling stone", which I think 
reflects how hot you have to get it to drive off the water.  Other types 
of "sponge", such as cotton wool can be easier.  However, if you are 
patient, there is nothing quite like a very large volume of reasonably 
dry air blowing over something wet to get it to dry out.  The boil off 
gas from a liquid N2 tank, flowing through a warm dewar for days to 
weeks should eventually dry it out completely.  You can keep weighing it 
to monitor the progress.

And, as always, there is no substitute for testing your shipping dewar 
by prepping it as you would for your precious samples, and then putting 
a thermocouple in it and verifying that it actually does last for 2 
weeks or more without the temperature rising about that of liquid nitrogen.

-James Holton
MAD Scientist



On 7/17/2013 7:33 AM, Mark J van Raaij wrote:
> I heard or read (don't remember where) that repeated warming/cooling cycles are what can make the dewars less efficient and can "break" them.
> So we keep them always with liquid nitrogen, filling them every week if we can. But we are lucky that Madrid has low humidity most of the year. And perhaps our liquid nitrogen supply is also relatively dry (I don't know this), because we haven't had too much problems with icing.
> Given your advice, maybe we should dry our dewars once or twice a year at least.
>
> Mark J van Raaij
> Lab 20B
> Dpto de Estructura de Macromoleculas
> Centro Nacional de Biotecnologia - CSIC
> c/Darwin 3
> E-28049 Madrid, Spain
> tel. (+34) 91 585 4616
> http://www.cnb.csic.es/~mjvanraaij
>
>
>
>
>
> On 17 Jul 2013, at 16:25, Johan Turkenburg wrote:
>
>> We put a supply line of compressed air (not sure how dry this is) in the dewar to wam it up and dry it out. We leave this running overnight, and this seems to result in the dewars being dry enough for later use. We haver never bothered to weight the dewars before and after use etc, so can't comment on how well this really dries the dewars. Icing seems in our case more dependent on manipulation of the samples and the pucks (we think unipucks cause more problems than pucks that hold the pin in a vial, but can't prove that). And maybe on how clean the LN2 is in the sample changer dewars (at home and at the synchrotron).
>>
>> Johan
>>
>>
>> On 17 July 2013 12:34, Phil Evans <[log in to unmask]> wrote:
>>
>> In response I felt I had to post the low-tech version (not mine)
>> Phil
>>
>>
>> <IMG_0694.jpeg>
>>
>> On 16 Jul 2013, at 11:57, Edward Lowe <[log in to unmask]> wrote:
>>
>>> At Elspeth's request, here's a link to a picture of the rack we use for
>>> drying out shipping dewars.
>>>
>>> https://www.dropbox.com/s/8ber2iikra9x73y/2013-02-21%2013.39.38.JPG
>>>
>>> Very simple and both we and the Lea group here have found it to make a
>>> huge difference the the effectiveness and longevity of our dewars.
>>>
>>> Ed.
>>>
>>> -- 
>>> Dr. E.D. Lowe
>>> Department of Biochemistry
>>> University of Oxford
>>> South Parks Road
>>> Oxford, UK
>>> OX1 3QU
>>>
>>> e:[log in to unmask]
>>> t: +44 (0) 1865 613288
>>> f: +44 (0) 1865 613201
>>>
>>>
>>>
>>>
>>> On 12/07/2013 14:27, "Edward Snell" <[log in to unmask]> wrote:
>>>
>>>> Ditto - I was always impressed with the contraption in the Garman lab
>>>> which, if I remember correctly, is made of a thick block of wood and some
>>>> plumbing pipes. It is designed to hold empty open Dewars inverted so they
>>>> could dry.
>>>>
>>>> Edward Snell Ph.D.
>>>> Assistant Prof. Department of Structural Biology, SUNY Buffalo,
>>>> Senior Scientist, Hauptman-Woodward Medical Research Institute
>>>> 700 Ellicott Street, Buffalo, NY 14203-1102
>>>> Phone:     (716) 898 8631         Fax: (716) 898 8660
>>>> Skype:      eddie.snell                 Email: [log in to unmask]
>>>> Telepathy: 42.2 GHz
>>>>
>>>> Heisenberg was probably here!
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of
>>>> Ginell, Stephan L.
>>>> Sent: Friday, July 12, 2013 12:45 AM
>>>> To: [log in to unmask]
>>>> Subject: Re: [ccp4bb] frosted crystals during storage in pucks
>>>>
>>>> My experience with xtals frosting in LN2 either in a dewar, while
>>>> freezing, or in pucks, has been because the LN2 was contaminated with ice
>>>> crystals  The fog you see above your dewar when freezing xtals is frozen
>>>> water vapor...it will fall and collect in the LN2 and also deposit on the
>>>> xtals. Dewars filled with recycled LN2 get contaminated with ice. Dewars
>>>> dried upside down allow the cold gas to flow out and warm moist air to
>>>> flow in and the water to condense inside the dewar (basic physics). To
>>>> dry shipping dewars keep up right while warming.
>>>> Steve
>>>>
>>>> Sent from my iPad
>>>>
>>>> On Jul 11, 2013, at 5:25 PM, "Nathaniel Clark"
>>>> <[log in to unmask]<mailto:[log in to unmask]>> wrote:
>>>>
>>>> At our last synchrotron trip, the beamline staff suggested that the
>>>> problem was due to moisture accumulation in the dry shipper.  They
>>>> recommended storing them inverted (for a few weeks, if I recall), and/or
>>>> putting a supply of dry air in the dewer.  Haven't tried it yet!
>>>> Nat
>>>>
>>>>
>>>> On Thu, Jul 11, 2013 at 5:08 PM, <Rain Field>
>>>> <[log in to unmask]<mailto:[log in to unmask]>> wrote:
>>>> Hi All,
>>>> We found if the crystals are storaged in pucks for 3-4 days in shipping
>>>> dewar (with liquid nitrogen), they are almost frosted.
>>>> Although I can wash them with liquid nitrogen, but it's not convenient to
>>>> do that for each crystals.
>>>> I doubt it's because the humid air in North West America.
>>>> Does anyone has an idea how to avoid this?
>>>> Thank you!
>>
>>
>>
>> -- 
>> Dr. Johan P. Turkenburg                     X-ray facilities manager
>> York Structural Biology Laboratory
>> University of York                               Phone (+) 44 1904 328251
>> York YO10 5DD   UK                          Fax   (+) 44 1904 328266
>>
>> Note new email address [log in to unmask]