5. THERMOCOUPLE CALIBRATION
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The object of calibrating any thermocouple or wire is to deter-
mine temperature-EMF output (voltage produced at a given tem-
perature) as compared to the calibration table or curve.
Comparison method
This method is just what it implies: the comparison of the EMF
of an unknown thermocouple with a working standard (usually
another thermocouple) at the same temperature. Accuracy is
frst limited by the accuracy of the standard. A secondary effect
limiting accuracy is the ability of the observer to bring the un-
known thermocouple junction to the same temperature as the
standard’s measuring element.
Fixed point method
This method entails measuring unknown thermocouples at a
known temperature as defned by the International Temperature
Scale.
No thermocouple can be more accurate than the wire from
which it is made. Certain limits of error have been established by
manufacturers and Engineering Societies to defne acceptable
wires for use in thermocouples.
The accuracy with which wire conforms to the tables, is deter-
mined by checking the wire at predetermined points against
NBS Certifed Platinum. Checking against platinum insures that
individual wires can be paired and remain within standard limits.
For instance: measurement at 150 °C with a type K thermocou-
ple insures that the result will be 150 °C ±2.5 °C for standard
grade material and 150 °C ±1.5°C with premium grade mate-
rial.
Measurement at 550 °C with the same type K thermocouple in-
sures that the result will be at 550 °C ±0.75% for standard gra-
de material and at 550 °C ±0.4% for premium grade material.
Fixed points available for selecting thermocouples
The fxed points for which values have been assigned or deter-
mined accurately and at which it has been found convenient
to calibrate thermocouples are given. In selecting the points at
which to calibrate a thermocouple, one has a choice between a
boiling point and a freezing point.
THERMOCOUPLE CALIBRATION NO LIMITS OF ERROR
LIMITS OF ERROR: STANDARD & PREMIUM GRADE