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Liquid Nitrogen - Cold burns?

  1. Dec 9, 2008 #1
    Hi,

    I was wondering, what is a cold burn exactly? We were told that liquid nitrogen (or anything that cold for that matter) can cause cold burns when touched... How can something cold burn you?

    Or is it not burning you but is it merely named 'cold burn' even though it is simply freezing, frostbite, etc..?

    Just a simple question I was wondering about...


    Also, is LN really that dangerous? I have poured it into a coffee cup and stuck my finger in a few times, but as long as you take it out quick enough you hardly even feel anything at all. I'm not stupid enough to try and hold it in as long as I can though lol...
     
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  3. Dec 9, 2008 #2

    mgb_phys

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    correct it's nothing to do with a burn, but the symptoms - blisters, dead skin, pain etc are similair

    It depends on how much and how long - cold water isn't dangerous in a shower but it is if you sink in the arctic!
    For a small amount of LN2 for a short time the heat of your finger will boil the liquid in contact with it and form a gas layer which insulates you from the cold liquid.
    If the liquid is held in contact with your skin for long enough it will burn.
    A real danger with LN2 is spilling it on clothing since the 'wet' clothing will hold the cold liquid against your skin, you are also supposed to wear lab coats without pockets.
     
  4. Dec 9, 2008 #3
    Cool, that clears some things up.


    Another quick question. The gas you see coming from any container of boiling liquid nitrogen, is it nitrogen, or is it water? I have heard people say it's nitrogen, while others say it's water... I thought nitrogen gas was invisible?
     
  5. Dec 9, 2008 #4

    mgb_phys

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    The white clouds are water vapour - they are literally clouds!
    Nitrogen is invisible, otherwise it would be difficult to see - 80% of air is nitrogen.

    edit: Russ is correct - it is tiny droplets of water (and possibly ice crystals?)
     
    Last edited: Dec 9, 2008
  6. Dec 9, 2008 #5

    russ_watters

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    Minor technicality: water vapor is invisible too: Clouds are condensed, atomized water particles.
     
  7. Dec 9, 2008 #6

    f95toli

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    Similar, but not identical. I might be wrong but I don't think you can get a blister from LN2; I get LN2 on my hands quite frequently (you'd think I learn, but no:blushing:) and you certainly end up with dead skin and pain (and it hurts a LOT if you get enough of it on you) but I have so far never had a blister (I get my blisters from soldering instead).
    I've also hurt myself on a couple of occasions by getting my fingers into cold He vapours (pulling transfer tubes out of a liquid He dewar without wearing gloves, not something I'd recommend) but the effects are more or less the same as LN2.

    Also, the real danger is in my experience not the LN2/LHe itself but the metal surfaces that it they been in contact with; on a dry day you can't really tell if a metal surface it cold or not and the heat transfer between skin and metal is very efficient. Moreover, since the skin goes numb pretty quickly it doesn't really hurt at first , not until you remove your hand that is...

    Hence, wear gloves
     
  8. Dec 10, 2008 #7
    Extremes in hot and cold temperatures tend to instantaneously saturate our sensory inputs and for this reason, extreme cold can be sensed as a potential extreme heat (and vice versa), which makes us pull away instantly to protect ourselves from potential damage. When we sense temperatures that aren't dangerously extreme, our sensory input allows a determination as to whether something is merely warm or even fairly hot just as we can determine whether something is cool or quite cold. Extremes however, swamp our senses and by the time we detect these extremes, we've automatically flinched to spare ourselves from further damage.

    I've been exposed to frigid temperatures at times where I thought I was being burned when it was actually an extremely frigid temperature that contacted a portion of my skin. It makes you jump regardless whether it's extreme hot or cold. I've also been burned when I actually thought I touched something that was frigid, so I'm speaking from firsthand experience.
     
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