Accuracy of the thermocouple voltage measurement – Campbell Scientific CR9000X Measurement and Control System User Manual
Page 101
Section 3. CR9000X Measurement Details
TABLE 3.1.4-1. Limits of Error for Thermocouple Wire (Reference
Junction at 0
o
C)
Limits of Error
Thermocouple Temperature
(Whichever is greater)
Type Range
o
C Standard Special
T
-200
to
0
±
1.0
o
C or 1.50%
0
to
350
±
1.0
o
C or 0.75%
±
0.5
o
C or 0.4%
J
0
to
750
±
2.2
o
C or 0.75%
±
1.1
o
C or 0.4%
E
-200
to
0
0
to
900
±
1.7
o
C or 1.00%
±
1.7
o
C or 0.50%
±
1.0
o
C or 0.4%
K
-200
to
0
0
to
1250
±
2.2
o
C or 2.00%
±
2.2
o
C or 0.75%
±
1.1
o
C or 0.4%
N
-270
to
0
0
to
1300
±
2.2
o
C or 2.00%
±
2.2
o
C or 0.75%
±
1.1
o
C or 0.4%
R or S
0 to 1450
±
1.5
o
C or 0.25%
±
0.6
o
C or 0.1%
B
800
to
1700
±
0.5%
Not Estab.
When both junctions of a thermocouple are at the same temperature there is no
voltage produced (law of intermediate metals). A consequence of this is that a
thermocouple can not have an offset error; any deviation from a standard
(assuming the wires are each homogeneous and no secondary junctions exist)
is due to a deviation in slope. In light of this, the fixed temperature limits of
error (e.g.,
±
1.0
°C for type T as opposed to the slope error of 0.75% of the
temperature) in the table above are probably greater than one would experience
when considering temperatures in the environmental range (i.e., the reference
junction, at 0
°C, is relatively close to the temperature being measured, so the
absolute error - the product of the temperature difference and the slope error -
should be closer to the percentage error than the fixed error). Likewise,
because thermocouple calibration error is a slope error, accuracy can be
increased when the reference junction temperature is close to the measurement
temperature. For the same reason differential temperature measurements, over
a small temperature gradient, can be extremely accurate.
In order to quantitatively evaluate thermocouple error when the reference
junction is not fixed at 0
o
C, one needs limits of error for the Seebeck
coefficient (slope of thermocouple voltage vs. temperature curve) for the
various thermocouples. Lacking this information, a reasonable approach is to
apply the percentage errors, with perhaps 0.25% added on, to the difference in
temperature being measured by the thermocouple.
Accuracy of the Thermocouple Voltage Measurement
The accuracy of a CR9000X voltage measurement is specified as 0.07% the
measured voltage plus 4 A/D counts of the range being used to make the
measurement. The input offset error reduces to 1 A/D count if a differential
measurement is made utilizing the option to reverse the differential input.
For optimum resolution, the
±50 mV range is used for all but high
temperature measurements (Table 3.1.4-2). The input offset error dominates
the voltage measurement error for environmental measurements. A
temperature difference of 40 to 60 °C between the measurement and reference
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