Campbell Scientific CR7 Measurement and Control System - Manual (Page 135)

SECTION 13. CR7 MEASUREMENTS
13-11
FIGURE 13.3-9. Incorrect Leadwire Extension
on Model 107 Temperature Sensor
13.4 THERMOCOUPLE
MEASUREMENTS
A thermocouple consists of two wires, each of a
different metal or alloy, which are joined
together at each end. If the two junctions are at
different temperatures, a voltage proportional to
the difference in temperatures is induced in the
wires. When a thermocouple is used for
temperature measurement, the wires are
soldered or welded together at the measuring
junction. The second junction, which becomes
the reference junction, is formed where the
other ends of the wires are connected to the
measuring device. (With the connectors at the
same 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 CR7 determines thermocouple
temperatures using the following sequence.
First the temperature of the reference junction is
measured. If the reference junction is the CR7
I/O Module, the temperature is measured with
the PRT in the 723-T Analog Input Card
(Instruction 17). The reference junction
temperature in oC is stored in an input location
which is accessed by the thermocouple
measurement instruction (Instruction 13 or 14).
The CR7 calculates the voltage that a
thermocouple of the type specified would output
at the reference junction temperature if its
reference junction were at 0oC, and adds this
voltage to the measured thermocouple voltage.
The temperature of the measuring junction is
then calculated from a polynomial
approximation of the NBS TC calibrations. If
the CR7 has been instructed to calculate the
temperature difference between the reference
and measuring junctions it will subtract the
reference temperature before storing the
temperature value.
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
linearization error (difference between NBS
standard and CR7 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
WITH 723-T
The PRT in the CR7 is mounted in the center of
the 723-T terminal strip. This resistance
temperature device (RTD) is accurate to ±0.1oC
over the CR7 operating range. The I/O Module
was designed to minimize thermal gradients. It
is encased in an aluminum box which is
thermally isolated from the CR7 enclosure.
Heavy copper grounding bars underlying the
terminal strips on the I/O cards and large brass
bars running the length of the I/O Module
provide thermal conduction for rapid
equilibration of thermal gradients. Sources of
heat within the CR7 enclosure exist due to
power dissipation by the electronic components
or charging batteries. In a situation where the
CR7 is at an ambient temperature of
approximately 20oC and no external
temperature gradients exist, the temperature
gradient between one end of an Analog Input
card to the other is likely to be 0.05oC, and the
gradient between the cards, from one end of the
I/O Module to the other, is likely to be 0.1 to
0.2oC. The end of the module with the CPU
card will be warmer due to heat dissipated by
the processor.
Given the above conditions, if it is desired to
make a series of thermocouple measurements
with the reference junctions within 0.05oC of the
RTD temperature, the temperature obtained
from the 723-T card can be used for