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.