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Thermos question

  1. May 30, 2008 #1
    You all know about this invention (vacuum flask) is someone don't its metal bottle with cup inside is vessel of glass which double walls and vacuum between them, outside of walls is painted with mirroring dye, and theres some sprout on the bottom of vessel (i don't know why).

    My question is i've seen them made of pure metal and i want to know can they hold heat as good as ones made of glass? And what this painting and sprout on the bottom are doing?
    Last edited: May 30, 2008
  2. jcsd
  3. May 30, 2008 #2
    the walls are mirrored so that no heat radiates out (from the hot contents inside) or no heat radiates in (into the cold contents). i think that the spout you are referring to is the place where from the air inside was sucked out (to create the vacuum). after the air has been sucked out, the glass has to be sealed right, that what results in that spout.
  4. May 30, 2008 #3
    With regards to your question of glass versus metal, you would be right to assume that the glass type is superior, since glass(or some variant) does not conduct heat nearly as well as a metal.
    But why are there metal thermo's when glass is better? Money.
    It's cheaper to make and they hope consumers don't care about quality.
  5. May 30, 2008 #4


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    Stainless steel is still a pretty good material to make thermos bottles from. Nowadays even lab equipment for low temperature experiments is made from stainless steel. Glass used to be a popular material (there are still plenty of glass cryostats around, I have used a few) but there are a couple of problems.
    One is of course that glass is quite fragil, but another problem is that the vacuum in glass containers tend to go soft (=bad vacuum) after a while. The vacuum in a properly stainless steel container will be ok for several years whereas the pressure in most glass flasks tend go up much quicker and a good insulation vacuum is MUCH more important than any conduction losses (which are pretty small assuming the inner container only touches the outer at the neck).
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