Campbell CR5000, Руководство оператора

Страница: 62 / 238

Section 3. CR5000 Measurement Details
3-8
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 CR5000 determines thermocouple temperatures using the following
sequence. First the temperature of the reference junction is measured and
stored in °C. If the reference junction is the CR5000 analog input terminals,
the temperature is measured with the built in thermistor (PanelTemp
instruction). The thermocouple measurement instruction measures the
thermocouple voltage (TCDiff or TCSE). The thermocouple instruction
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 thermocouple voltage. The temperature of the
measuring junction is then calculated from a polynomial approximation of the
NIST TC calibrations
3.4.1 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 CR5000 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.
Panel Temperature
A brass bar is just under the CR5000 panel between the two rows of analog
input terminals. This bar helps to reduce temperature gradients between the
terminals. The panel temperature thermistor is in a depression in the center of
this bar.
The thermistor (Betatherm 10K3A1A) has an interchangability specification of
0.1 °C for temperatures between 0 and 70 °C. Below freezing and at higher
temperatures this specification is degraded (Figure 3.4.1). Combined with
possible errors in completion resistors and the measurement, the accuracy of
panel temperature is ±0.25 °C 0 to 40 °C, ±0.5 °C -25 to 50 °C, and ±0.7 °C
-40 to 85 °C.