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Connecting a glass dewar flask to a vacuum system

  1. May 27, 2012 #1
    I'm wondering how a glass dewar flask connects to a vacuum system, i.e. how is it vacuum sealed? Once it is sealed, I see how you can use appropriate flanges to pump out the, say, He gas, but how is a glask flask connected to this vacuum system?

    Thanks.
     
  2. jcsd
  3. May 28, 2012 #2

    ZapperZ

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    My guess is that the glass and metal were brazed together. Someone else who understands the manufacturing process can correct this.

    Zz.
     
  4. May 28, 2012 #3

    f95toli

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    There are probably a few ways of doing this. However, I wouldn't be too surprised if glue could be used.
    Stycast and a few other glues can be used to make strong, vacuum tight bonds between different materials and works surprisingly well as long as one keeps the usual things in mind (expansion coefficients etc)
     
  5. May 28, 2012 #4

    ZapperZ

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    Er, glue? Even for UHV systems?

    I suppose I can expect some sort of an epoxy, but I must admit I don't know of any glue that is UHV compatible.

    Zz.
     
  6. May 28, 2012 #5

    marcusl

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    For Thermos bottles (the type you drink out of), the bottom of the flask ends in a small diameter glass tubing. Large glass lab dewars often have a large-bore (1", e.g.) glass tube, the better to pump down quickly and deeply. It often comes out of the side near the top, for convenience. The transition to the vacuum system is made by slipping over a special brass or stainless collar that has a compression fitting similar to those used to connect 3/8" copper water tubing into the angle stop valve under your bathroom sink. The compression seal is made by a rubber O-ring instead of a brass ferrule, with a little high-vacuum silicone grease to ensure a good seal. A length of flexible piping, which can be heavy-walled rubber or a stainless steel bellows, provides strain relief between the glass dewar and the valve at the input port to the vacuum pump. In the lab, the dewar is always connected. The Thermos bottle, on the other hand, is pumped down and then its little glass tube is heated with a torch, sealed, and broken free. In some Thermos bottles, you can remove the bottom and see this.

    Of course more exotic systems are available to address special requirements.
     
  7. May 29, 2012 #6

    marcusl

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    BTW, glass labware started phasing out a half-century ago, due to its fragility and because highly skilled artisans are needed to produce it. Dewars are now made of fiberglass or metal, which are rugged and can be mass-produced.
     
  8. May 29, 2012 #7

    ZapperZ

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  9. May 30, 2012 #8

    f95toli

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    The OP talked about a glass dewar, so I assume he/she meant the type of glass helium dewar that is still often used for lab demonstrations of e.g. superfluidity and other low-temperature experiments.
    Dewars of this type are not UHV, far from it (you have to pump out the isolation vacuum whenever you warm them up to room temperature).
    Btw, stycast works quite well even for vacuum vessels immersed in liquid helium and the leak rate is quite low; I use it quite frequently.
     
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