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A What temperature do thermocouples show? Static or Total?

  1. Aug 12, 2016 #1
    What temperature does thermocouples show? Static or Total?
    And also in formulas for example Q(dot) = m(dot) * C_p * (T2 - T1) which temperature should be used precisely?
     
    Last edited: Aug 12, 2016
  2. jcsd
  3. Aug 12, 2016 #2
    What is the difference between static and dynamic temperature?
     
  4. Aug 12, 2016 #3
    I'm sorry .I should have actually said "Total" and I corrected it. Total temperature is (if I'm not mistaken) Static + Dynamic temperature, and total temperature itself is defined like this:
    upload_2016-8-12_21-47-52.png
    where gamma is the ratio of specific heats and M_a is Mach number.
     
  5. Aug 12, 2016 #4

    tech99

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    I think they show the difference in temp between the hot and the cold junction.
     
  6. Aug 12, 2016 #5

    jbriggs444

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    How do you contrive to wire up a thermocouple where the hot junction is moving at mach 1 relative to the cold junction?
     
  7. Aug 12, 2016 #6

    boneh3ad

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    It will measure whatever temperature it is subjected to at its junction. Exactly what temperature that is in the sense of a compressible flow depends on how you have it situated. If you can find a way to place a thermocouple into a flow without stagnating the fluid, then congratulations on that.

    Generally, though, placing an object in the flow bring a its velocity to zero and you measure something higher than static temperature. Unfortunately (for the sake of convenience), as the Mach number increases, the temperature you measure is increasingly different from the total temperature, and is instead going to be the so-called recovery temperature.
     
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