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Safety Note If you store LN in a thermos with a cap that seals the thermos, you've made a time bomb, and thermos shrapnel does not improve the flavor of

__ Safety Note __ If you store LN in a thermos with a cap that seals the thermos, you've made a time bomb, and thermos shrapnel does not improve the flavor of cocktails.

jeffwass 12y ago
Also remember - any dewar itself, improperly handled, can be a bomb! One professor I worked for hammered this concept into us when we started work in his lab. Yet my PhD adviser did not. Sometime after I graduated, one of my fellow grad students blew up a dewar!! (Luckily no one got hurt). He obtained an old dewar from another lab, and went through the usual cool-down procedure - evacuate the Outer Vacuum Chamber, fill the LN2 shroud, then pre-cool the main bath with LN2. Stupidly he did not notice there were no pressure relief safety valves!!! (Not only should you verify these exist, but you should also verify they work by slightly pressurizing the dewar.) He put in a bunch of LN2, capped it, and left for the weekend. When he came back, the dewar had exploded, pretty much along the weld seams, strewing the metal body and super-insulation all over the lab. The first professor I mentioned above made sure we understood all valves and release valves. And also noted that if opened for too long while full with LHe, it's possible ice could form in the dewar neck, below the safety relief valves. Always kept me on my toes while working with LHe (well, that and the fact that LHe burns way worse the LN2 if it makes accidental skin contact).
- tptacek 12y ago
  The dewar I bought from Amazon isn't pressurized. It is literally a big thermos. I assume it loses more LN to the atmosphere than a pressurized dewar; on the other hand, the explosion risk isn't big.
  - jeffwass 12y ago
    Hmm, does the cap screw on tight like a normal thermos? Maybe it's designed to always have a loose cap for venting. The dewars I was talking about above hold several hundred liters, are about 5-8 feet tall, and primarily for liquid helium (where evaporation is way more of a factor. The pressure relief valves were built in to the storage dewar's we would get from LN2 and LHe suppliers, but I don't think the cryostats in our lab had them built in, you needed to add them separately by connecting onto the dewar's main flange. The one my colleague exploded didn't have the extra valve hardware attached, and he just capped it and walked away.
    - tptacek 12y ago
      The "cap" is a long styrofoam tube with a plastic top that comes on and off with no resistance (if you upended the dewar, that cap would slide right out).
    - mechanical_fish 12y ago
      The non-pressurized LN dewars I've seen had a "plug" that was just a cylinder of foam that slid into the neck of the dewar, with a light plastic cap on top. It was lightweight enough to bob upward at the slightest pressure, releasing gas. It probably had a design with channels such that the lip never sealed completely anyway. They could easily preserve LN for days. Dewars are impressive.
      - tptacek 12y ago
        Static hold time: 100 days. Evaporation rate: 0.20 liters/ per day. 2.25" Necktube inside diameter. 15" Outside diameter. 25.50" Overall height. Empty weight: 23 lbs. Full weight: 60 lbs. :)
        mechanical_fish 12y ago
        Wow, I forgot just how many days that was, I see. Though I now remember that one of my labs had a storage dewar full of precious biological samples that was a bigger version of this design, and we only refilled it a couple times a month. $500, eh? Hmmmmmmmmmmmmm.
        tptacek 12y ago
        I felt bad about the purchase before we actually used it, but now I'm feeling like it made at least as much sense as the circulator did as a purchase. Malort ice cream!

__ Safety Note __ If you store LN in a thermos with a cap that seals the thermos, you've made a time bomb, and thermos shrapnel does not improve the flavor of

__ Safety Note __ If you store LN in a thermos with a cap that seals the thermos, you've made a time bomb, and thermos shrapnel does not improve the flavor of cocktails.

jeffwass 12y ago

Also remember - any dewar itself, improperly handled, can be a bomb! One professor I worked for hammered this concept into us when we started work in his lab. Yet my PhD adviser did not. Sometime after I graduated, one of my fellow grad students blew up a dewar!! (Luckily no one got hurt). He obtained an old dewar from another lab, and went through the usual cool-down procedure - evacuate the Outer Vacuum Chamber, fill the LN2 shroud, then pre-cool the main bath with LN2. Stupidly he did not notice there were no pressure relief safety valves!!! (Not only should you verify these exist, but you should also verify they work by slightly pressurizing the dewar.) He put in a bunch of LN2, capped it, and left for the weekend. When he came back, the dewar had exploded, pretty much along the weld seams, strewing the metal body and super-insulation all over the lab. The first professor I mentioned above made sure we understood all valves and release valves. And also noted that if opened for too long while full with LHe, it's possible ice could form in the dewar neck, below the safety relief valves. Always kept me on my toes while working with LHe (well, that and the fact that LHe burns way worse the LN2 if it makes accidental skin contact).

tptacek 12y ago
The dewar I bought from Amazon isn't pressurized. It is literally a big thermos. I assume it loses more LN to the atmosphere than a pressurized dewar; on the other hand, the explosion risk isn't big.
- jeffwass 12y ago
  Hmm, does the cap screw on tight like a normal thermos? Maybe it's designed to always have a loose cap for venting. The dewars I was talking about above hold several hundred liters, are about 5-8 feet tall, and primarily for liquid helium (where evaporation is way more of a factor. The pressure relief valves were built in to the storage dewar's we would get from LN2 and LHe suppliers, but I don't think the cryostats in our lab had them built in, you needed to add them separately by connecting onto the dewar's main flange. The one my colleague exploded didn't have the extra valve hardware attached, and he just capped it and walked away.
  - tptacek 12y ago
    The "cap" is a long styrofoam tube with a plastic top that comes on and off with no resistance (if you upended the dewar, that cap would slide right out).
  - mechanical_fish 12y ago
    The non-pressurized LN dewars I've seen had a "plug" that was just a cylinder of foam that slid into the neck of the dewar, with a light plastic cap on top. It was lightweight enough to bob upward at the slightest pressure, releasing gas. It probably had a design with channels such that the lip never sealed completely anyway. They could easily preserve LN for days. Dewars are impressive.
    - tptacek 12y ago
      Static hold time: 100 days. Evaporation rate: 0.20 liters/ per day. 2.25" Necktube inside diameter. 15" Outside diameter. 25.50" Overall height. Empty weight: 23 lbs. Full weight: 60 lbs. :)
      - mechanical_fish 12y ago
        Wow, I forgot just how many days that was, I see. Though I now remember that one of my labs had a storage dewar full of precious biological samples that was a bigger version of this design, and we only refilled it a couple times a month. $500, eh? Hmmmmmmmmmmmmm.
        tptacek 12y ago
        I felt bad about the purchase before we actually used it, but now I'm feeling like it made at least as much sense as the circulator did as a purchase. Malort ice cream!