Campbell Scientific CR23X Micrologger - Manual (Page 218)

SECTION 13. CR23X MEASUREMENTS
13-18
TABLE 13.4-2. Limits of Error on Datalogger
Thermocouple Output Linearization
(Relative to ITS-90 Standard in NIST
Monograph 175)
Limits of
TC Type Range °C 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
polynomial range. The ranges covered by
these polynomials include the CR23X
environmental operating range, so there is no
problem when the CR23X 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 polynomials 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 CR23X 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 CR23X calculates a temperature
difference of -10.2°C, a -0.2°C error. The
temperature calculated by the CR23X 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)
Limits of
TC Type Range °C 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
ERROR SUMMARY
The magnitude of the errors described in the
previous sections illustrate that the greatest
sources of error in a thermocouple temperature
measurement are likely to be due to the limits of
error on the thermocouple wire and in the
reference temperature determined with the
built-in thermistor. Errors in the thermocouple
and reference temperature polynomials are
extremely small, and error in the voltage
measurement is negligible.
To illustrate the relative magnitude of these
errors in the environmental range, we will take a
worst case situation where all errors are