Campbell Scientific CR10X Measurement and Control System - Manual (Page 206)

SECTION 13. CR10X MEASUREMENTS
13-14
FIGURE 13.4-1. Thermistor Polynomial Error
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 cannot 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. In other words, 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
°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 CR10X voltage
measurement is specified as 0.2% (0.1% 0 to
40
°C) of the full scale range being used to
make the measurement. The actual accuracy
may be better than this as it involves a slope
error (the error is proportional to the
measurement being made though limited by the
resolution). The error in the temperature due to
inaccuracy in the measurement of the
thermocouple voltage is worst at temperature
extremes, where a relatively large scale is
necessary to read the thermocouple output. For
example, assume type K (chromel-alumel)
thermocouples are used to measure
temperatures at 600
°C. The TC output is on
the order of 24.9 mV, requiring the +25 mV
input range. The accuracy specification of 0.1%
FSR is 25 uV which is a temperature error of
about 0.60
°C. In the environmental
temperature range with voltage measured on an
appropriate scale, error in temperature due to
the voltage measurements is a few hundredths
of a degree.