Insulation..Which way round ie. foil facing inwards or outwards?

Discussion in 'Builders' Talk' started by Lombear, Oct 11, 2007.

  1. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    And then there's the all-stainless-steel vaccum flask, which makes a mockery of the whole friggin lot.

    Er, NO shiny surfaces(not 'mirror' shiny, anyway).


    Off you go, carry on.


    Mr. HandyAndy - really
     
  2. Mr GrimNasty

    Mr GrimNasty Active Member

    And then there's the all-stainless-steel vaccum
    flask, which makes a mockery of the whole friggin
    lot.

    Why? Polished s/s is a good mirror. Two walls of s/s separated with a vacuum presumably or some 'space age' insulator. Not a mockery - just true to the design idea.
     
  3. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    The stainless steel is DULL.

    The only place a shiny surface in a s/s flask would be of ANY use, is in the contents part. And that will NEVER be shiny.



    Mr. HandyAndy - really
     
  4. Mr GrimNasty

    Mr GrimNasty Active Member

    The stainless steel is DULL.

    The only place a shiny surface in a s/s flask would
    be of ANY use, is in the contents part. And that will
    NEVER be shiny.


    I'm not familiar with the whole design Handy, can't really comment.
     
  5. Stoday

    Stoday New Member

    A load of nonsense - heat radiation is just 'light'
    outside of the visible spectrum. If you could see
    infrared you could see the tea losing heat. The
    physics of light/heat reflection has nothing (in this
    sense) to do with temperature.

    I see. So the Stephan-Boltzmann Law, which relates radiation to the forth power of the temperature is a load of nonsense eh?

    Two examples:

    You don't put a cosy on a silver coffee or tea pot. The inside's murky if not black, but the polished outside keeps the heat loss down and the contents hot. A cosy on a silver pot increases the heat loss. Touch the pot & it's hot, so the heat is not being reflected back into the pot.

    The silver foil used to keep accident victims warm is not improved by wrapping additional blankets on the outside. For maximum efficiency, the silver side has to be outside.
     
  6. Mr GrimNasty

    Mr GrimNasty Active Member

    I see. So the Stephan-Boltzmann Law, which relates
    radiation to the forth power of the temperature is a
    load of nonsense eh?

    Obviously, I was being simplistic - and I tried to indicate this with the caveats in brackets.

    We are getting far from the point that you and Handy told Charlie he was wrong about the construction of a thermos flask - which as far as he went - he wasn't.

    If you care to look back through my post you will see I mentioned black body radiation where your (Stephan-Boltzmann constant) applies. This is radiation (heat loss through <u>emission</u>).

    We are/were talking about heat <u>reflection</u>.

    .....cosy ... silver coffee or tea pot.....

    That's all a bit confused/wrong. A single-skinned metal tea pot is a very poor way to keep tea hot. A cosy would <u>slow down heat loss</u> by simple insulation. This means if you whip the cosy off, the outside of the pot will be very hot.

    The silver foil used to keep accident....

    As far as I am aware they are silver both sides, having a silver side against the body is what provides the usefulness, this reflects 90% of body heat back. True as they are so efficient, additional layers are pretty pointless.
     
  7. Stoday

    Stoday New Member

    The inside of the silvered surfaces (that you cannot
    see) would look like the back of a mirror (without
    addition protection if that makes you happy - and I
    suppose the back of the deposit may not be oxidised
    like a mirror in air, but it will still be rough/matt
    and a poor reflector).

    The silver surface is every bit as polished as the side you see through the glass. Telescope mirrors are silvered or aluminised on the reflecting surface (Ag for infra red; Al for UV).

    As for the coffee pot, posh hotels use silver plated coffee pots because they hold the heat better than pot. They are not polished inside.
     
  8. Mr GrimNasty

    Mr GrimNasty Active Member

    The silver surface is every bit as polished as the
    side you see through the glass. Telescope mirrors are
    silvered or aluminised on the reflecting surface (Ag
    for infra red; Al for UV).

    It's a different (and more expensive) mirroring process for optics;- you don't want the light passing through glass to the reflective <u>back</u> surface - you need a thin-film metal deposit on <u>top</u> of a surface so there is less light loss/distortion etc.). Some optic mirrors are just highly polished metal surfaces - not a film at all.

    As for the coffee pot, posh hotels use silver plated
    coffee pots because they hold the heat better than
    pot. They are not polished inside.

    They use silver for service because it looks nice - nothing to do with retaining heat.
     
  9. sinewave

    sinewave Screwfix Select

    What was the question again!
     
  10. ­

    ­ New Member

    > What was the question again! :p

    I think it was 'is it better to make tea in a pot or in the cup'
     
  11. Stoday

    Stoday New Member

    The inside of the silvered surfaces (that you cannot
    see) would look like the back of a mirror (without
    addition protection if that makes you happy - and I
    suppose the back of the deposit may not be oxidised
    like a mirror in air, but it will still be rough/matt
    and a poor reflector).

    If you want to see the silver surface on the inside of glass, just look in a reflector lamp. Or a car headlamp.

    Nothing rough/matt or a poor reflector here.

    Grim&Specious, you are wrong on this issue just as you are with the others.
     
  12. steve_r

    steve_r New Member

    Foil backed plasterboard - isn't it normally fitted with foil facing towards outside of the building... isn't this the same principle?
     
  13. Mr GrimNasty

    Mr GrimNasty Active Member

    PB foil is a Moisture barrier primarily.

    Stoday:

    Lamp reflectors - yet another kettle of fish; different purpose, economics of production, materials and manufacturing processes etc. to a vacuum flask. But they are usually metal foil (e.g. chrome) on plastic nowadays - anyway don&#146;t get your point, you can see the reflectors - so what? (That&#146;s rhetorical BTW).

    Anyway, once again, looking inside a vacuum flask: http://bc.barnard.columbia.edu/~schapman/bc3252/images/Dewar.jpg

    And here is a video of a glass mirror being made. The mirroring solution is applied to the concave side of a piece of glass, making the opposite convex side the mirror, and leaving the concave side grey and dull (goes blacker as it oxidises with time).
    http://www.alanmacfarlane.com/glass/birm6.html

    The picture of the in(drink)side of the flask is clearly mirrored - because the silvering is applied to the glass surfaces that are inside the vacuum, and if you could see them from behind, they would be matt as per the concave side above.

    As you&#146;ll note with my remarks on a stainless steel capsule flask, I don&#146;t know how it works, I can only guess - I admit when I don&#146;t know something, and if someone else provides overwhelming evidence that I am wrong, I&#145;m man enough to accept it.

    As I really can&#146;t think of a way to be more convincing about a glass thermos if you still disagree, there&#146;s clearly no point in me arguing further.

    Are you a pseudonym of Handy Andy - you wriggle like him?
     
  14. Swiss Tony

    Swiss Tony New Member

    So why is a flask called a vacuum flask if its all in the mirrors?

    As the foil is a vapour barrier, isn't the answer that it goes on the inside then?

    Bit like in a vacuum flask where the shiney side is inside to keep the moisture in?

    And cars are shiney on the outside to keep the rain out.

    I love physics. Its so easy to follow the logic.
     
  15. Mr GrimNasty

    Mr GrimNasty Active Member

    I think that'll be a detention for not paying attention and for being facetious - now go and read from the beginning to end 100 times.
     
  16. Swiss Tony

    Swiss Tony New Member

    Mr Grim, believe me, having waded through this lot it feels like I have read it 100 times.
     
  17. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Are you a pseudonym of Handy Andy - you wriggle like him? <<



    Steady!! I am actually agreeing with YOU, you know ?


    There is NO shiny surface(mirror-like) in a s/s flask.

    The vacuum part is the important bit, as it is in all 'vacuum' flasks.

    Obviously.

    Otherwise, the common, glass bulb flask would be called a mirror flask.



    Mr. HandyAndy - really
     
  18. Stoday

    Stoday New Member

    The silver on the vacuum side of the glass walls of a vacuum flask won't oxidise because there's no oxygen in a vacuum. In any case silver tarnishes as a result of sulphur making black silver sulphide.

    I spoke of reflector lamps, which necessarily have to be glass, not lamp reflectors, which may be plastic.

    It's fun twisting words around, but you're now looking like Grim&numpty.
     
  19. Mr GrimNasty

    Mr GrimNasty Active Member

    Wrong again, but like I said Stoday - I'm done, you're just showing yourself up as a bad loser now.
     
  20. Mr GrimNasty

    Mr GrimNasty Active Member

    BTW: Look up the definition of chemical oxidisation - you might learn something.
     

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