Campbell Scientific CR5000 Measurement and Control Module - Manual (Page 66)

Section 3. CR5000 Measurement Details
3-12
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 -25 to 50 °C accuracy of a CR5000 differential voltage measurement is
specified as
±
(0.075% of the measured voltage plus the input offset error of 2
times the basic resolution of the range being used to make the measurement
plus 2
µ
V). The input offset error reduces to the basic resolution if the
differential measurement is made utilizing the option to reverse the differential
input.
For optimum resolution, the
±
20 mV range is used for all but high temperature
measurements (Table 3.4-2). The input offset error dominates the voltage
measurement error for environmental measurements. A temperature difference
of 45 to 65 °C between the measurement and reference junctions is required for
a thermocouple to output 2.67 mV, the voltage at which 0.075% of the reading
is equal to 2
µ
V. 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 0.00075x830.7 µV = 0.62 µV or 0.014 o C (0.62/42.4). The
basic resolution on the ±20 mV range is 0.67 µV or 0.01 o C. The 2
µ
V offset
is an error of 0.047 o C. Thus, the possible error due to the voltage
measurement is 0.081 o C on a non-reversing differential, or 0.024 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,