When thermocouple is placed in the high temperature region for considerable long time we cannot be sure of the reading of the temperature shown by it.This is because the emf produced by the TC can change over time even though it is at same temperature.This change in emf is called thermocouple drift.
Causes This is change is due to change in chemical and metallurgical properties of metal. of thermoelements at its surface or at volume or both.It result in change in seebeck coefficient in the some or entire regions of the thermoelements.
At surface following may occur
- Oxidation of surface material mainly if the thermocouple wires are not protected by sheath or where open wire configuration is used.
- Corrosion or depletion because of the gas or chemical around the thermoelements.
- Chemical reaction with the insulator or sheath.
Inside of the thermocouple ie in its bulk changes may occur becuase
- The metal of the thermoelement may change is metallic phase like.it may undergo allotropic change,diffusionof foriegn mateial.
- Metal can undergo grain growth
- Continous stress may cause the internal lattice to undergo deformation and develope voids etc
- Ordering transformation which is changee in order or symmetry to disorder of the lattice when heated.
Things to be noted about thermal drift is
It is not 100 % avoidable.
Drift is not caused if seebeck constant changes near the hot end of the thermoelement.
Drift s mainly caused if the thermoelements in between the thermocouple wire undergo change in seebeck constant.
This is because the temperature profile of is some thing like this
As it is clear that at hot and cold end region of Tc the temperature in uniform.So any
No effect of change in seebeck coefficient.But at the middle region the temperature is not uniform so if seebeck cooefficient changes drift may occur.