Specifications

Section 3. CR3000 Measurement Details
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 CR3000 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 CR3000 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 CR3000 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
The panel temperature thermistor is just under the panel in the center of the
two rows of analog input terminals.
The thermistor (Betatherm 10K3A1A) has an interchangeability specification
of 0.1 °C for temperatures between 0 and 70 °C. Below freezing and at higher
temperatures this specification is degraded. Combined with possible errors in
the completion resistor measurement, and the Steinhart and Hart equation used
to calculate the temperature from resistance, the accuracy of panel temperature
is ±0.3°C-25 to 50°C and ±.0.8°C-55 to 85°C.
The error in the reference temperature measurement is a combination of the
error in the thermistor temperature and the difference in temperature between
the panel thermistor and the terminals the thermocouple is connected to. The
terminal strip cover should always be used when making thermocouple
measurements. It insulates the terminals from drafts and rapid fluctuations in
temperature as well as conducting heat to reduce temperature gradients. In a
typical installation where the CR3000 is in a weather proof enclosure not
subject to violent swings in temperature or lopsided solar radiation loading, the
temperature difference between the terminals and the thermistor is likely to be
less than 0.2 °C.
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