Another good way to test the dry shipping dewar is to its dry/empty weight.
If you don't know your "as new" mass, you can weigh it, then
purge with dry air/nitrogen for 24 hours and check the weight again,
if it is unchanged, then you can use that as your dry/empty weight.
Next you can charge the shipping dewar according to the manufacturer's
specs. Pour out the excess LN2. Then weigh the unit. This will tell
you the capacity of LN2 retained in the absorbent material. (You can
compare this to the manufactures specs). Then you can leave the unit
for 24 hours and then weigh it again. From the difference of the mass,
you can calculate the nominal evaporation rate. E.g. how much LN2
is lost each day. Then divide the mass of the retained LN2
by this evaporation rate to find the number of days of holding
time.
For a CX100 dewar, the absorbent material should hold 4.4 L of LN2
which is 3.6 kg and should have a holding time of ~21 days when new.
For the MVE SC4/2 it holds 3.8 L (3.1 kg) with an expected holding
time of 14 days when new. (note 1 l LN2 = 0.808 kg)
A good check you can do each time before sending out your samples
is to fill the shipper the day before (you can leave excess liquid
in the shipper). After you have fully charged it and it is cold (an
hour or so) record the weight. Then take another measurement 12-24 hrs
later and calculate your evaporation rate to confirm it is close to
what you expect.
There are some nice guides at
https://smb.slac.stanford.edu/facilities/hardware/cryotools/shipping-dewar-testing.html
https://www.diamond.ac.uk/Instruments/Mx/Common/Common-Manual/Shipping-Samples/Care-of-dry-shippers.html
I know that the LS-CAT group at the APS has the correct vacuum operator
to pump out the CX100 dewars. If that is the same one you have, they
might be able to tell you the source/supplier for it. I had looked for
the operator for the MVE SC4/2 type dewars, but did not find confirmation
on the correct thread pitch. When I contacted tech support at the
vendor we purchased the dewar from, they contacted the manufacture
and were told that they don't consider it user serviceable and would not
relay the diameter and thread pitches.
Here is a general video on the evacuation process --
https://www.youtube.com/watch?v=hnPZrGdcTSs
Here is a DIY solution ---
http://www.nuclearphysicslab.com/npl/npl-home/spectroscopy/software_and_hardware/refurbishment-of-ln2-dewars/
Regards,
Mitch
Quoting Artem Evdokimov <[email protected]>:
Cheapest possible system to check dry shipper would be a wooden stick
lowered into the shipper then taken out and exposed to moist air.
Condensation would indicate a cold zone...
Second cheapest option is a red LED on a pair of long thin wires. If it
changes to green as it dips into the dewar, again- it is cold in there. The
change will be partial depending on the actual temperature. Please note
this only works with inorganic LEDs.
If you crave precision and actual numbers, then you should invest a dollar
or two in a type K thermocouple. It is good down to -200 C which is lower
than LN2 temperature. These are on ebay and Amazon for a few bucks. A few
more bucks will get you a thermometer body with the right connector, or you
can use a multimeter and recall that the voltage change should be about 41
microcolts per degree C. You can calibrate your system in LN2 and ice or
boiling water to get the correct thermal coefficient and offset. Just don't
forget to use the same connection and instrument all the time since the
reference junction will change if you change the setup.
As to re-evacuating the dewar - this varies depending on how your
particular device is constructed. Some have a 'nipple' style valve, others
have a needle style valve and some are permanently welded shut... it's not
obvious that your mechanics shop will have the requisite vacuum connector
to service your shipper. We used to send ours to manufacturer for
reconditioning.
Best of luck,
Artem
On Fri, Oct 18, 2024, 20:23 Pedro Matias <[email protected]> wrote:
Dear All,
I'm looking for a low temperature measuring system to check our dry
dewars, preferably one that is not very expensive. Any recommendations ?
Also, I was told that it is possible to regenerate the vacuum in those
vessels. Does anybody have any experience with this? If so, how can it
be done?
Thanks in advance,
Pedro Matias
--
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Macromolecular Crystallography Unit
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