I want to know the reasons for drift in the readings of K type thermocouples?

Question

The Thermoucoples are in SS sheath with MgO insulation but no thermo wells are provided.

Answer 1

Uncompensated cold junction temperatures....

Using the improper interconnecting / extension wire type and creating addition temperature sensitive junctions outside of the monitored process. You need to use thermocouple wire all the way back to the meter from the TC and special junction blocks. OR Transistion to copper wire via an instrument amp and convert the TC signal to a standard like 4-20 or 0-10V close to the monitored temp as practical

Contamination and or corrosion ocurring at the junction. Exposure to sulfur bearing oil combustion products will cause a large shift. Corrosion eats away at the junction and can change the composition of the junction over time. Corrosion as well as deposits also provide a leakage path across the junction which allows some of the seebeck voltage produced by the TC to short across the TC.

High temperature operation can also cause oxidation at the junction which will result in a shift. Try changing to a N type TC which is less succeptabel to this type of oxidation.

Along the same idea as leakage due to corrosion would be moisture condensing within the sheath especially if calibration is done at lower temps with corrosion / moisture present. THe moisture dries out at higher operating temps causing a shift. With moisture issues the amount of drift may change with the moisture content.

Answer 2

drift in a t/c is generally attributed to the following. Try checking one by one.
1. Poor soldering of the two metals. specially responsible at higher temperatures.
2. in-accurate cold junction compensation. Check instrument.
3. unbalance is lead wires. Check for proper extension wires from t/c to the instrument.
4. corrossion/oxidation of the metal if ur temp. is high & chemistry of the process material. Use thermowell if possible.