What temperature do thermocouples show? Static or Total?

  • Context: Undergrad 
  • Thread starter Thread starter Omish
  • Start date Start date
  • Tags Tags
    Static Temperature
Click For Summary

Discussion Overview

The discussion centers around the temperature readings of thermocouples, specifically whether they indicate static or total temperature. Participants explore the implications of these definitions in the context of thermodynamic formulas and flow conditions.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question whether thermocouples show static or total temperature, particularly in relation to the formula Q(dot) = m(dot) * C_p * (T2 - T1).
  • One participant defines total temperature as the sum of static and dynamic temperatures, referencing the relationship involving the ratio of specific heats and Mach number.
  • Another participant suggests that thermocouples measure the temperature difference between the hot and cold junctions.
  • A later reply emphasizes that the temperature measured by a thermocouple depends on its placement within a flow, noting that it typically measures a temperature higher than static temperature due to the effects of flow velocity.
  • It is mentioned that as Mach number increases, the measured temperature diverges from total temperature, leading to the concept of recovery temperature.

Areas of Agreement / Disagreement

Participants express differing views on whether thermocouples indicate static or total temperature, with no consensus reached on the precise nature of the temperature readings in various flow conditions.

Contextual Notes

Participants highlight the complexity of measuring temperature in compressible flows and the influence of flow dynamics on thermocouple readings, indicating that assumptions about flow conditions and thermocouple placement are critical to understanding the measurements.

Omish
Messages
25
Reaction score
0
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:
Science news on Phys.org
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?
 
  • Like
Likes   Reactions: Bystander
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.
 

Similar threads

Undergrad Temperature change due to mixing liquids, heating and heat losses
  • · Replies 8 ·
Replies
8
Views
3K
Undergrad Why work is PdV and not (P+dP)dV in an isothermal process?
  • · Replies 6 ·
Replies
6
Views
1K
Undergrad How to understand experiment to measure heat capacity of solid?
  • · Replies 1 ·
Replies
1
Views
2K
Undergrad How to measure average thermal conductivity of a metallic material?
  • · Replies 1 ·
Replies
1
Views
2K
Undergrad Constant length metallic wire as a thermometer, using tension?
  • · Replies 1 ·
Replies
1
Views
1K
Undergrad How Do You Calculate Equilibrium Conditions After Mixing Two Different Gases?
  • · Replies 3 ·
Replies
3
Views
3K
Undergrad Warm-up time for a hollow cylinder
  • · Replies 5 ·
Replies
5
Views
3K
Undergrad Graphical difference between adiabatic and isothermal processes.
  • · Replies 1 ·
Replies
1
Views
4K
Undergrad Why pressure stays the same when doubling both volume and temperature?
  • · Replies 35 ·
2
Replies
35
Views
6K
Undergrad Empirical temperature scales using different thermometers
  • · Replies 7 ·
Replies
7
Views
3K