Specifications

Section 4. Sensor Support
4-25
When both junctions of a thermocouple are at the same temperature there is no
voltage produced (law of intermediate metals). A consequence of this is that a
thermocouple cannot have an offset error; any deviation from a standard
(assuming the wires are each homogeneous and no secondary junctions exist)
is due to a deviation in slope. In light of this, the fixed temperature limits of
error (e.g., ±1.0 °C for type T as opposed to the slope error of 0.75% of the
temperature) in the table above are probably greater than one would experience
when considering temperatures in the environmental range (i.e., the reference
junction, at 0 °C, is relatively close to the temperature being measured, so the
absolute error — the product of the temperature difference and the slope error
— should be closer to the percentage error than the fixed error). Likewise,
because thermocouple calibration error is a slope error, accuracy can be
increased when the reference junction temperature is close to the measurement
temperature. For the same reason differential temperature measurements, over
a small temperature gradient, can be extremely accurate.
To quantitatively evaluate thermocouple error when the reference junction is
not fixed at 0
o
C, limits of error for the Seebeck coefficient (slope of
thermocouple voltage vs. temperature curve) are needed for the various
thermocouples. Lacking this information, a reasonable approach is to apply the
percentage errors, with perhaps 0.25% added on, to the difference in
temperature being measured by the thermocouple.
4.4.1.3 Accuracy of Thermocouple Voltage Measurement
The -25 to 50 °C accuracy of a CR1000 differential voltage measurement,
without input reversal, is specified as ± (0.12% of the measured voltage plus an
offset error of 3 times the basic resolution of the range being used to make the
measurement plus 2 μV). The offset error reduces to 1.5 times the basic
resolution plus 1 μV if the differential measurement is made utilizing the
option to reverse the differential input (RevDiff = True).
For optimum resolution, the ±2.5 mV range is used for all but high temperature
measurements (TABLE 4.4-2). Using the 0.67 μV b
asic resolution of the
±2.5 mV range in the offset equations above, the offset portion of the accuracy
specification is 4 μV without input reversal or 2 μV with input reversal. This
offset portion of the accuracy specification dominates the voltage measurement
error for temperatures in the environmental range. At the full scale the other
part of the accuracy term is 0.12% of 2.5 mV = 2 μV. For example, assume
that a type T thermocouple is used to measure a temperature of 45 °C and that
the reference temperature is 25 °C. The voltage output by the thermocouple is
830.7 µV. At 45 degrees a type T thermocouple outputs 42.4 µV per
o
C. The
percent of reading error in the voltage measurement is 0.0012 * 830.7 µV =
1 µV or 0.023
o
C (1 / 42.4). The basic resolution on the ±2.5 mV range is
0.67 µV or 0.016
o
C. The 2 μV offset is an error of 0.047
o
C. Thus, the
possible error due to the voltage measurement is 0.07
o
C when reversing
differential inputs, or 0.118
o
C when not reversing differential inputs.
Error in the temperature due to inaccuracy in the measurement of the
thermocouple voltage is worst at temperature extremes, particularly when the
temperature and thermocouple type require using the 250 mV range. For
example, assume type K (chromel-alumel) thermocouples are used to measure
temperatures around 1300
o
C. The TC output is on the order of 52 mV,
requiring the ±250 mV input range. At 1300
o
C, a K thermocouple outputs
34.9 µV per
o
C. The percent of reading error in the voltage measurement is