Specifications

Section 4. Sensor Support
4-21
4.4 Thermocouple Measurements
Thermocouples are easy to use with the CR1000. They are also
inexpensive. However, they pose several challenges to the
acquisition of accurate temperature data, particularly when using
external reference junctions. Campbell Scientific strongly
encourages any user of thermocouples to carefully evaluate
Section 4.4.1 Error Analysis of Thermocouple Measurements.
The micro-volt resolution and low-noise voltage measurement capability of the
CR1000 is well suited for measuring thermocouples. A thermocouple consists
of two dissimilar metal wires joined together at one end to form a junction.
Practical thermocouples are constructed from two parallel insulated wires of
dissimilar metals soldered or welded together at the junction. A temperature
difference between the junction and the unconnected wires opposite the
junction induces a temperature dependent voltage between the wires, referred
to as the Seebeck effect. Measurement of the voltage between the unconnected
wires opposite the junction provides a direct measure of the temperature
difference between the junction and the measurement end. Metallic
connections (e.g., solder) between the two dissimilar metal wires and the
measurement device form parasitic thermocouple junctions, the effects of
which cancel if the two wires are at the same temperature. Consequently, the
two wires at the measurement end of the thermocouple, referred to as the
reference junction, are placed in close proximity and thermally connected so
that they are at the same temperature. Knowledge of the reference junction
temperature provides the determination of a reference junction compensation
voltage, corresponding to the temperature difference between the reference
junction and 0°C. This compensation voltage, combined with the measured
thermocouple voltage, can be used to compute the absolute temperature of the
thermocouple junction. To facilitate thermocouple measurements, a thermistor
is integrated into the CR1000 wiring panel for measurement of the reference
junction temperature by means of the PanelTemp() instruction.
TCDiff() and TCSe() thermocouple instructions determine thermocouple
temperatures using the following sequence. First, the temperature (°C) of the
reference junction is determined. A reference junction compensation voltage is
next computed based on the temperature difference between the reference
junction and 0 °C. If the reference junction is the CR1000 analog input
terminals, the temperature is conveniently measured with the PanelTemp()
instruction. The actual thermocouple voltage is measured and combined with
the reference junction compensation voltage. It is then used to determine the
thermocouple junction temperature based on a polynomial approximation of
NIST thermocouple calibrations.
4.4.1 Error Analysis
The error in the measurement of a thermocouple temperature is the sum of the
errors in the reference junction temperature measurement plus the temperature-
to-voltage polynomial fit error, the non-ideality of the thermocouple (deviation
from standards published in NIST Monograph 175), the thermocouple voltage
measurement accuracy, and the voltage-to-temperature polynomial fit error
(difference between NIST standard and CR1000 polynomial approximations).
The discussion of errors that follows is limited to these errors in calibration and
NOTE