Campbell Scientific CR10X Measurement and Control System - Manual (Page 207)

SECTION 13. CR10X MEASUREMENTS
13-15
THERMOCOUPLE POLYNOMIALS - Voltage
to Temperature Conversion
NIST Monograph 175 gives high order
polynomials for computing the output voltage of a
given thermocouple type over a broad range of
temperatures. In order to speed processing and
accommodate the CR10X's math and storage
capabilities, 4 separate 6th order polynomials are
used to convert from volts to temperature over the
range covered by each thermocouple type. Table
13.4-2 gives error limits for the thermocouple
linearization functions.
TABLE 13.4-2. Limits of Error on Datalogger
Thermocouple Output Linearization
(Relative to ITS-90 Standard in NIST
Monograph 175)
TC Type Range
°C
Limits of Error
°C
T -200 to 87 ± 0.022
87 to -316 ± 0.042
316 to -400 ± 0.060
E -240 to -130 ± 0.40
-130 to -479 ± 0.05
479 to 1000 ± 0.21
K -56 to 413 ± 0.05
413 to 1372 ± 0.40
J -150 to 92 ± 0.02
92 to 412 ± 0.04
B 50 to 1007 ± 0.20
1007 to 1700 ± 0.50
R 0 to 579 ± 0.10
579 to 1450 ± 0.40
S 0 to 972 ± 0.17
972 to 1450 ± 0.40
N -200 to 1000 ± 0.0075
1000 to 1240 ± 0.01
REFERENCE JUNCTION COMPENSATION -
Temperature to Voltage
The polynomials used for reference junction
compensation (converting reference temperature
to equivalent TC output voltage) do not cover the
entire thermocouple range. Substantial errors will
result if the reference junction temperature is
outside of the calibrated range. The ranges
covered by these calibrations include the CR10X
environmental operating range, so there is no
problem when the CR10X is used as the
reference junction. External reference junction
boxes, however, must also be within these
temperature ranges. Temperature difference
measurements made outside of the reference
temperature range should be made by obtaining
the actual temperatures referenced to a junction
within the reference temperature range and
subtracting. Table 13.4-3 gives the reference
temperature ranges covered and the limits of
error in the linearizations within these ranges.
Two sources of error arise when the reference
temperature is out of range. The most significant
error is in the calculated compensation voltage;
however, error is also created in the temperature
difference calculated from the thermocouple
output. For example, suppose the reference
temperature for a measurement on a type T
thermocouple is 300
°C. The compensation
voltage calculated by the CR10X corresponds to
a temperature of 272.6
°C, a -27.4°C error. The
type T thermocouple with the measuring junction
at 290
°C and reference at 300°C would output -
578.7 µV; using the reference temperature of
272.6
°C, the CR10X calculates a temperature
difference of -10.2 °C, a -0.2°C error. The
temperature calculated by the CR10X would be
262.4
°C, 27.6°C low.
TABLE 13.4-3. Reference Temperature
Compensation Range and Linearization
Error (Relative to ITS-90 Standard in NIST
Monograph 175)
TC Type Range
°C
Limits of Error
°C
T -100 to 100 ± 0.025
E -150 to 206 ± 0.040
K -50 to 100 ± 0.030
J -150 to 296 ± 0.045
B 25 to 80 ± 0.100
R -50 to 100 ± 0.020
S -50 to 100 ± 0.080
N -100 to 100 ± 0.001