CR10X Reference Manual
13-14
temperature, the chemical dissimilarity between the thermocouple wire and the
connector does not induce any voltage.) When the temperature of the reference
junction is known, the temperature of the measuring junction can be determined
by measuring the thermocouple voltage and adding the corresponding temperature
difference to the reference temperature.
The CR10X determines thermocouple temperatures using the following sequence.
First, the temperature of the reference junction is measured. The reference junc-
tion temperature in
o
C is stored in an input location which is accessed by the
thermocouple measurement instruction (Instruction 13 or 14). The CR10X calcu-
lates the voltage that a thermocouple of the type specified would output at the
reference junction temperature if its reference junction were at 0
o
C, and adds this
voltage to the measured thermocouple voltage. The temperature of the measuring
junction is then calculated from a polynomial approximation of the National
Bureau of Standards (NBS) TC calibrations.
13.4.1 Error Analysis
The error in the measurement of a thermocouple temperature is the sum of the
errors in the reference junction temperature, the thermocouple output (deviation
from standards published in NBS Monograph 125), the thermocouple voltage
measurement and the polynomial error (difference between NBS standard and
CR10X polynomial approximations). The discussion of errors which follows is
limited to these errors in calibration and measurement and does not include errors
in installation or matching the sensor to the environment being measured.
Reference Junction Temperature
The junction which is created when a thermocouple is wired to the wiring panel is
referred to as the reference junction. The temperature of the reference junction
must be known in order to calculate the absolute temperature of the measuring
junction. The 10TCRT Thermocouple Reference Temperature is used to measure
the temperature of the reference junction (terminal strips).
The 10TCRT uses a precision thermistor to measure the relative temperature of
the terminal strips. The accuracy of this measurement is a combination of the
thermistor’s interchangeability specification, the precision of the bridge resistors,
and the polynomial error. In a ‘worst case’ example, all errors add to yield a
±0.4
o
C error over the range of -20°C to +48°C. It is emphasised that this is the
worst case. Campbell Scientific’s experience shows that the overall accuracy is
typically better than ±0.2°C. The major error component in the -20°C to +60°C.
range is the ±0.32°C thermistor specification. When a CR10X is outside this tem-
perature range, the polynomial error becomes much worse (see Figure 13-13), and
may necessitate the use of an external reference junction to improve the accuracy.
If the terminal that the thermocouple is wired into is at a different temperature
than the 10TCRT thermistor, this difference in temperature becomes an error in
the thermocouple temperature measurement. With the CR10X in an enclosure (see
Section 14) this error will generally be less than 0.3
o
C.