Campbell CR23X, Руководство оператора

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SECTION 13. CR23X MEASUREMENTS
13-14
CR23X
FIGURE 13.3-9. Incorrect Lead Wire 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 CR23X determines thermocouple temperatures
using the following sequence. First, the temperature
of the reference junction is measured. If the
reference junction is the CR23X panel, the
temperature is measured with the built-in thermistor
(Instruction 17). The reference junction temperature
in °C is stored in an input location which is accessed
by the thermocouple measurement instruction
(Instruction 13 or 14). The CR23X calculates the
voltage that a thermocouple of the type specified
would output at the reference junction temperature if
its reference junction were at 0°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 Institute of Standards and Technology
(NIST Monograph 175) 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 NIST Monograph 175), the
thermocouple voltage measurement, and the
polynomial error (difference between NIST
standard and CR23X 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 CR23X PANEL
The CR23X panel temperature is measured
with a BetaTherm 100K6A1 thermistor mounted
under the analog input terminal strip. The
accuracy of this measurement is a combination
of the thermistor’s interchangeability
specification and the precision of the bridge
resistors. In a "worst case" example, all errors
add to yield a ±0.3°C error in the range of -10°C
to +60°C. Figure 13.4-1 shows the thermistor’s
interchangeabiity.
Any difference in temperature between the
thermistor and the actual reference junction (the
channel to which the thermocouple is attached)
will become an error in the measurement. The
terminal strip cover is designed to reduce the
gradient across the terminal strip. In field
applications, with the terminal cover installed and
the CR23X in one of Campbell Scientific’s
enclosures, this error will generally be less than
0.3°C.