What temperature do thermocouples show? Static or Total?

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SUMMARY

This discussion clarifies that thermocouples measure temperature based on their junctions, specifically addressing the distinction between static and total temperatures. Total temperature is defined as the sum of static and dynamic temperatures, with the formula incorporating the ratio of specific heats (gamma) and Mach number (M_a). The conversation emphasizes that when a thermocouple is placed in a moving flow, it measures a temperature influenced by the flow dynamics, often resulting in a recovery temperature that deviates from the total temperature as Mach number increases.

PREREQUISITES
  • Understanding of thermocouple operation and measurement principles
  • Familiarity with static and dynamic temperature concepts
  • Knowledge of compressible flow dynamics and Mach number
  • Basic grasp of thermodynamic equations, particularly Q(dot) = m(dot) * C_p * (T2 - T1)
NEXT STEPS
  • Research the principles of thermocouple calibration and accuracy
  • Explore the effects of Mach number on temperature measurements in fluid dynamics
  • Learn about recovery temperature and its significance in high-speed flows
  • Investigate methods for minimizing flow stagnation when using thermocouples
USEFUL FOR

Engineers, physicists, and researchers involved in thermal measurement, fluid dynamics, and thermodynamic analysis will benefit from this discussion.

Omish
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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?
 
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What is the difference between static and dynamic temperature?
 
nasu said:
What is the difference between static and dynamic temperature?

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.
 
I think they show the difference in temp between the hot and the cold junction.
 
Omish said:
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?
How do you contrive to wire up a thermocouple where the hot junction is moving at mach 1 relative to the cold junction?
 
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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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