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Measuring temperature in a metal bar

  1. Feb 24, 2014 #1
    Good day !

    I am trying to read a temperature on a heated cylindrical metal (steel) bar (about 15 mm in diameter) using a pyrometer. However I cannot measure the temperature on this bar. The bar has a rather dim shine on it and I think it affects the reading. Also, maybe because it has a small diameter and the pyrometer can't fix the bar for reading.
    How can I measure the temperature of this bar ? Thinking of maybe using electricity in my favor, but not yet willing to give up the pyrometer. Any advices ?

    PS : For reference : I am trying to measure the displacement at one end of the metal bar using an interferometric method (Michaelson), fixing the bar at one end and at the other one a reflective mirror.

    Also, do you see anything else to study here besides the displacement ?

    Thanks in advance !
  2. jcsd
  3. Feb 24, 2014 #2


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    Put a non-reflective coating on the portion of the bar that is to be measured. For example, coat it with soot.
  4. Feb 24, 2014 #3

    Simon Bridge

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    Welcome to PF;
    I suppose you could always use spectrographic measurements - but I agree with UltrafastPED, coat the bar.
    It may work just to run the bar through an emery cloth to rough up the surface.

    If the bar is really too narrow to be picked up, you may want to try attaching something to the bar to make it wider - fold a sheet of tin foil over it maybe and hope it ends up close to the temperature of the bar. You are probably going to be more interested in the change in temperature anyway.
  5. Feb 25, 2014 #4
    Thank you for the ideas. Coating it with soot did not work. I tried some heat resisting paint. It did the job for the temperature I work for (about 300-400 degrees Celsius). The pyrometer won't let me do the trick. I tried gluing a thin sheet of a similar material to the bar using solder. I don't yet know if it did the trick. The day after tomorrow I will test it. I am interested in the changes of temperature from a theoretical point of view. Experimentally, I can have a small error.
    But in theory, is the heat spread uniformly along the bar ? Is the thin sheet of metal taking the necessary heat ? It is cooling faster than the bar, taking into consideration that it has a larger surface in contact with air ? Things like that bother me...
    The change of temperature cools the bar and I electronically count the fringes to see the distance. I also do some stress analysis.
    I was thinking of sending some waves in the bar to check the changes, but I don't have an oscilloscope. Any ideas here on how to measure without an expensive tool like that ?

    Thank you !
  6. Feb 25, 2014 #5

    Simon Bridge

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    In general - no. It won't even have a uniform temperature. The details depend on how the heating happens.

    Some heat will be in the thin metal yes, but you were interested in temperature.
    In order to measure temperature, some heat must be transferred to the thermometer somehow - you cannot avoid losses.

    Yes - it will do the duty of a heat sink.
    You need to make the bit of metal as small as possible, yet still big enough to give your instrument a reading.

    Note: having air around it makes the surface of the bar cooler than if you didn't.

    Heat experiments are very tricky because everything you do upsets the system.

    You need to work out what tolerances are important.
    i.e. if you only care about the change in length vs change in temperature, then the fact the metal cools the bar quicker may not matter so much.

    How to measure what?

    You could just trust your multivibrator.

    Note: audio frequency oscilloscopes can be obtained cheaply as a computer program that uses the computer's mic input.
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