Specifications

Section 3. CR3000 Measurement Details
Accuracy of the Thermocouple Voltage Measurement
The -25 to 50 °C accuracy of a CR3000 differential voltage measurement is
specified as ± (0.07% of the measured voltage plus the input offset error of 3
times the basic resolution of the range being used to make the measurement
plus 2 µV). The input offset error reduces to 1.5* the basic resolution +1 µV if
the differential measurement is made utilizing the option to reverse the
differential input.
For optimum resolution, the ±20 mV range is used for all but high temperature
measurements (Table 3.4-2). The input offset error dominates the voltage
measurement error for environmental measurements. A temperature difference
of 45 to 65 °C between the measurement and reference junctions is required
for a thermocouple to output 2.67 mV, the voltage at which 0.07% of the
reading is equal to 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 possible slope error in the
voltage measurement is 0.0007x830.7 µV = 0.58 µV or 0.014
o
C (0.58/42.4).
The basic resolution on the ±20 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 0.10
o
C on a non-reversing differential, or 0.06
o
C with a
reversing differential measurement. The value of using a differential
measurement with reversing input to improve accuracy is readily apparent.
The 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 200 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 ±200 mV input range. At 1300
o
C, a K thermocouple outputs
34.9 µV per
o
C. The possible slope error in the voltage measurement is
0.0007x52 mV = 36 µV or 1.03
o
C (36/34.9). The basic resolution on the 200
mV range is 6.67 µV or 0.19
o
C. Thus, the possible error due to the voltage
measurement is 1.65
o
C on a non-reversing differential, or 1.34
o
C with a
reversing differential measurement.
TABLE 3.4-2. Voltage Range for Maximum Thermocouple Resolution
TC Type and temp.
range
o
C
Temp. range for
±20 mV range
Temp. range for
±50 mV range
Temp. range for
±200 mV range
T -270 to 400 -270 to 395 not used not used
E -270 to 1000 -270 to 300 -270 to 675 >675
K --270 to 1372 -270 to 500 -270 to 1250 >1250
J -210 to 1200 -210 to 385 -210 to 885 >885
B 0 to 1820 0 to 1820 not used not used
R -50 to 1768 -50 to 1690 -50 to 1768 not used
S -50 to 1768 -50 to 1768 not used not used
N -270 to 1300 -270 to 595 -270 to 1300 not used
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