The software you are using with the thermocouple may have a calibration , zeroing tab while you setup. check that.

@ Saghir - Whether we need to have a constant temperature bath of high temperature for performing this calibration expt ?

I,m not sure but you check your software. for validation may put your thermocouple in some over where you know the temperature by built in thermocouples.

There is an ASTM procedure, or an ASME PTC procedure to calibrate thermocouples.

Take a look to ASME PTC 19.3.

According the mentioned temperature, maybe you have to use a thermocouple calibration furnace instead a constant temperature bath.

Dear Dr. Pintea, the thermocouples are calibrated by the prodicers if you pay for it. Otherwise you can refer to the tables of conversion, but this does not mean that the thermocouples are calibrated.

to do the calibration it is necessary to compare the measured value of your sensor with a reference point or the value reads by a calibrated higher accurancy sensor

of course.

The International Practical Temperature Scale has several defined reference points.  You can use them to calibrate your temperature, assuming that you can obtain several grams each of the pure materials.  Heat the material until it is melted and then let it cool slowly measuring the thermocouple signal.  There will be a constant signal at the melting point of the substance.

IPTS-68 REFERENCE TEMPERATURES

EQUILIBRIUM POINT                 K             °C

Freezing Point of Zinc           692.73        419.58

Freezing Point of Silver       1235.08       961.93

Freezing Point of Gold        1337.58      1064.43

The IPTS-68 is an old rule. The new one is ITS-90.

I'm surprised at how much difference there is, 0.25°C at the melting point of gold.  Not that it makes all that much difference for thermocouple measurements where the accuracy at 1000°C is typically quoted as ±0.5°.

                                K                  °C

FP Zn                   692.677        419.527

FP Al                    933.473        660.323

FP Ag                 1234.93          961.78

FP Au                 1337.33        1064.18

FP Cu                 1357.77        1084.62