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Sterilisation of thermometer

  1. Dec 27, 2006 #1
    1. The problem statement, all variables and given/known data
    A clinical thermometer cannot be sterilised by placing it in boiling water. Why not?

    2. Relevant equations

    3. The attempt at a solution

    I assume the liquid is mercury. In the book they said a clinical thermometer has a kink at the end (so when it is out of the mouth it dosen't drop the down as fast - I made this up). Mecury boils at 630K so it should still be a liquid at the temperture water boils. I don't see why it can't be sterilised by placing it in boiling water as the sterilisation is meant to kill off microbes. I don't see any side effects. So I can't answer the question.
  2. jcsd
  3. Dec 27, 2006 #2


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    What is the typical range of a [mercury] clinical thermometer? Bear in mind that a clinical thermometer is designed to measure temperatures around 37oC.
  4. Dec 27, 2006 #3
    Does it make a difference what liquid is inside the thermometer? (Actually, since alcohol boils at a lower temperature than water, would it be even more fun seeing an alcohol clinical thermometer dropped in boiling water?)
    Last edited: Dec 28, 2006
  5. Dec 27, 2006 #4
    Mercury freezes at -39 and boils at 357 centigrade so 100 centigrade of boiling water shouldn't do any harm.

    Do you think the questioneer was thinking about alcohol thermometer (but didn't specify it)?
  6. Dec 28, 2006 #5
    Well, no, it wouldn't harm the mercury... I mean the mercury wouldn't evaporate or anything. But the heat would certainly effect whatever's inside the thermometer.
    What do you think is the maximum temperature that a clinical thermometer can measure? Where is the liquid at that temperature? What will happen if you exceed that temperature?
  7. Dec 28, 2006 #6


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    I bracketed the mercury since the OP specified a mercury thermometer (not in the question, but in the response). However, whether the thermometer be ethanol or mercury this does not affect the reason why you cannot boil a glass clinical thermometer. As I have said previously and alluded to by mbrmbrg, consider the range of a typical clinical thermometer. Bear in mind, that the maximum core temperature we can survive is around 44oC (if memory serves).
    Last edited: Dec 28, 2006
  8. Dec 28, 2006 #7
    'coz,the glass capillary would break
  9. Dec 28, 2006 #8
    A normal glass cup doesn't break at 100 degree celcius. You'd expect a thermometer to be stronger than that. Although if taking Hootenanny's advice, of "consider the range of a typical clinical thermometer. Bear in mind, that the maximum core temperature we can survive is around 44oC" it may be the case that the glass breaks. Would the kink in a clinical thermometer have anything to do with it?
  10. Dec 28, 2006 #9
    Ah, but this is not a normal empty glass cup.
    Have you ever frozen a can of soda (or any sealed container filled to the top with liquid)?

    I don't think so.
  11. Dec 28, 2006 #10
    When I said 100 degree celcius for the glass cup, I was thinking of filling it with 100 degree celcius of water. Glass absorbs heat more quickly than water so the temperture of the glass could goto 100 degrees celcius and my glasses don't break when I do that.

    I don't think I have frozen a can of soda before.
  12. Dec 28, 2006 #11
    OK, let's look at this from the top.

    How does a thermometer work? You shove it in someone's mouth, and something happens to the liquid inside the thermometer that allows you to read the temperature. What is that something that happens to the liquid?
  13. Dec 28, 2006 #12
    It expands. What is your point?
  14. Dec 28, 2006 #13
    OK, so the hotter it gets, the more the liquid expands.

    Say the maximum temperature that this thermometer can measure is To.
    Where is the liquid at To?
    What will the liquid try to do at temperatures greater than To?
    More specifically, where will the liquid try to go at temperatures greater than To?
    Is there anything trying to stop it from doing that/going there?
    What will happen to the thing that's in the liquid's way?
    Last edited: Dec 28, 2006
  15. Dec 28, 2006 #14
    So you are saying a clinical thermometer only goes up to say 50 celcius and the glass stops there. Putting it with an 100 celcius oject will mean it will reach 100 celcius if waited long enough and so it will push against the glass. The force might be so much that it might break it? Is that your point?
  16. Dec 29, 2006 #15
    Mercury is dense. Even the slightest expansion against glass should break the (extremely) thin glass capillary. Remember that Mercury resides in the capillary which is very thin, not the outer tube.
    BTW, even an Alcohol thermometer would break if its heated to a greater than the limit of capillary to hold the alcohol.

    IMO, you should try this out pratically. o:)
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