Instrument calibration is essential for maintaining accuracy in measurement devices, particularly in temperature measurement where factors like material changes can lead to drift. Calibration methods include primary and secondary comparisons against standard instruments, which are certified by bureaus of standards. Specific procedures for calibrating thermocouples involve checking ice and steam points, using insulated baths for accurate temperature control.

1. Calibration
• Instrument calibration is one of the primary processes used to maintain
instrument accuracy.
• Calibration is the process of configuring an instrument to provide a result for
a sample within an acceptable range.
• Eliminating or minimizing factors that cause inaccurate measurements is a
fundamental aspect of instrumentation design.

2. Why Devices need calibration
• Every device used for process-critical measurements should be checked
periodically to verify it continues to deliver the required accuracy. Where
adjustment is possible, a device measuring outside of expected limits
should be brought back to an acceptable performance level, but in the case
of non-adjustable equipment the deviation or measurement performance
should be recorded and a decision made on whether it remains fit for
purpose.
• In the case of temperature measurement equipment, the properties of
bimetals and thermocouple wire change with use and time (especially when
used at elevated temperatures) resulting in measurement drift.
• Additionally, a thermocouple probe may be damaged in service, possibly
mechanically or by corrosion, resulting in rapid deterioration of the wire.
RTD’s and thermistors are also both fragile devices and easily damaged, so
should be checked periodically.

3. Calibration of temperature sensors
The temperature sensors, exclusive of optical pyrometer. Are calibrated by
the following methods:
• By reference to the equilibrium temperature of substance (Primary
calibration)
• By comparison with the standard Thermometer of pyrometer certified
against a standards (Secondary Calibration)
• By comparing with a secondary standard thermometer which has been
calibrated against a standard instrument.

4. • The bureau of standards calibrates and certifies three standard
instruments:
1. Mercury in glass thermometer
2. Platinum resistance thermometer
3. Platinum-platinum rhodium thermocouple
•The certified,mercury thermometer are sufficient accuracy for most
engineering works and are normally employed for checking and
calibrating other thermometers which are to be used for temperature
measurement in industry and laborites works.

5. Calibration of thermocouple
• For calibrating a thermocouple, the ice point and the steam point of the
standard thermocouple are to be checked to make sure that its calibration has
not drifted.
• For checking the ice point, the bulb and the stem of the thermocouple are
surrounded with a mixture of crushed ice and water open to atmosphere.
• The stem point is checking by suspending the thermometer in steam
apparatus.
• The thermometer is surrounded but steam to its full depth of immersion but is
not in contact with boiling water.
• The atmospheric pressure is noted and the observed steam point is corrected
for the barometric deviations

6. Fig : Steam Apparatus

7. • The liquid bath is held in a thermally insulated tank provided with a stirrer and
a heating system.
• The standard thermometer is placed as closely as possible to the
thermometer being calibrated without actually touching it.
• The liquid in the bath may be water for temperature between 0-100o
C,a
mineral or cylinder oil for temperature between 95-300o
C,or a molten salt for
temperature up to 500o
C.
• For a limited range of temperature ,the constant temperature bath method of
comparison with a standard mercury thermocouple is the best available
method even for calibration of thermocouples and resistance thermocouples.
• While calibrating thermocouples, the bath must electrically non-conductive
otherwise the EMF set up by the thermocouple will be a function of surface
temperature of bath rather then the junction temperature.