Campbell Scientific CR5000 Measurement and Control Module User Manual
Page 68
Section 3. CR5000 Measurement Details
3-14
TABLE 3.4-3. Limits of Error on CR5000 Thermocouple Polynomials
(Relative to NIST Standards)
TC
Type
Range
o
C
Limits of Error
o
C
T
-270
to
400
-270
to
-200
+
18 @ -270
-200
to
-100
±
0.08
-100
to
100
±
0.001
100
to
400
±
0.015
J
-150
to
760
±
0.008
-100
to
300
±
0.002
E
-240
to
1000
-240
to
-130
±
0.4
-130
to
200
±
0.005
200
to
1000
±
0.02
K
-50
to
1372
-50
to
950
±
0.01
950
to
1372
±
0.04
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 linearization range. The ranges covered by these
linearizations include the CR5000 environmental operating range, so there is
no problem when the CR5000 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
one temperature from the other. Table 3.4-3 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
o
C. The compensation voltage calculated by the
CR5000 corresponds to a temperature of 272.6
o
C, a -27.4
o
C error. The type
T thermocouple with the measuring junction at 290
o
C and reference at 300
o
C
would output -578.7
µ
V; using the reference temperature of 272.6
o
C, the
CR5000 calculates a temperature difference of -10.2
o
C, a -0.2
o
C error. The
temperature calculated by the CR5000 would be 262.4
o
C, 27.6
o
C low.