Campbell Scientific CR9000X Measurement and Control System - Manual (Page 102)

Section 3. CR9000X Measurement Details
junctions is required for a thermocouple to output 2.285 mV, the voltage at
which 0.07% of the reading is equal to 1 A/D count (1.6 mV).
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). An A/D count on the
±50 mV range is worth 1.6 µV or 0.038 o C. Thus, the possible error due to the
voltage measurement is 0.166 o C on a single-ended or non-reversing
differential, or 0.052 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 = 36.4 µV or 1.04 o C (36.4/34.9). An A/D count on the 200
mV range is worth 6.3 µV or 0.18 o C. Thus, the possible error due to the
voltage measurement is 1.77 o C on a single-ended or non-reversing
differential, or 1.22 o C with a reversing differential measurement.
TABLE 3.1.4-2. Voltage Range for maximum
Thermocouple resolution
Thermocouple
Type and
temperature
range o C
Temperature
range for ±50
mV range
Temperature
range for ±200
mV range
T -270 to 400 -270 to 400 not used
E -270 to 1000 -270 to 660 >660
K -270 to 1372 -270 to 1230 >1230
J -210 to 1200 -210 to 870 > 870
B 0 to 1820 0 to 1820 not used
R -50 to 1768 -50 to 1768 not used
S -50 to 1768 -50 to 1768 not used
N -270 to 1300 -270 to 1300 not used
When the thermocouple measurement junction is in electrical contact with the
object being measured (or has the possibility of making contact) a differential
measurement should be made. If the voltage potential exceeds the common
mode range of the CR9050 module (e.g., the +12 V terminal of an automotive
battery) it is possible to use the 9055 ±50 V Analog Input Module to make the
Thermocouple measurement. The resolution and noise level are much worse
than with the CR9050 Module. The ±500 mV range offers the best resolution,
1 A/D count is 16 µV, about 0.4
o C for most thermocouples.
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