The Measurement of temperature, which is an important process variable is becoming increasingly important in many fields of activity. For practical day-to-day thermometry a more readily realizable scale of temperature is the International Temperature Scale ITS-90 covering the temperature range – 270.15° C to 1084.63° C. It is defined by temperature values assigned to a number of fixed temperature phenomena, i.e. the triple points, boiling points and freezing points of certain materials. Resistance thermometers and thermocouples constructed in a specified manner are used to interpolate between fixed points. ITS-90 relates closely enough to the fundamental thermodynamic temperature scale for all practical purposes (See Table I) At the National Physical Laboratory, the temperature sensors are calibrated at fixed points and other values for different Temperature are obtained by interpolation .

At the Institute constant temperature furnaces or baths are used for calibration. The method adopted is by comparing the mV of the under test with the Standard Thermocouple in the cases of thermocouples or comparison of resistance of the under test Platinum Resistance Thermometer with the Standard PRT. In both the cases the deviation can be worked out Table II gives in detail the various baths/furnaces used Table III lists the primary standards for temperature measurement at IDEMI laboratory.