Campbell CR9000, Инструкция по эксплуатации

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Section 3. CR9000 Measurement Details
3-7
3.1.4 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 CR9000 determines thermocouple temperatures using the following
sequence. First the temperature of the reference junction is measured. If the
reference junction is the CR9000 Analog Input Module, the temperature is
measured with the PRT in the 9050 Analog Input Module (ModuleTemp
instruction). The reference junction temperature in
o
C is stored and then
referenced by the thermocouple measurement instruction (TCDiff or TCSE).
The CR9000 calculates 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 NIST TC calibrations
3.1.4.1 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 linearization error (difference
between NIST standard and CR9000 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 9050
The PRT in the CR9000 is mounted on the circuit board near the center of the
9050 terminal strip. This resistance temperature device (RTD) is accurate to
±
0.1
o
C over the CR9000 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 CR9000 fiberglass enclosure. Measurement
modules have aluminum mounting plates extending beyond the edges of the
circuit cards that provide thermal conduction for rapid equilibration of thermal
gradients. Sources of heat within the CR9000 enclosure exist due to power
dissipation by the electronic components or charging batteries. In a situation
where the CR9000 is at an ambient temperature of approximately 20
o
C and no
external temperature gradients exist, the temperature gradient between one end
of an Analog Input card to the other is likely to be less than 0.1
°
C. The
gradient from one end of the I/O Module to the other, is likely to be about