Section 8. Operation
voltage polynomial fit error, the non-ideal nature of the thermocouple (deviation
from standards published in NIST Monograph 175), the thermocouple-voltage
measurement accuracy, and the voltage-to-temperature polynomial fit error
(difference between NIST standard and CR1000 polynomial approximations). The
discussion of errors that follows is limited to these errors in calibration and
measurement and does not include errors in installation or matching the sensor
and thermocouple type to the environment being measured.
Panel-Temperature Error
The panel-temperature thermistor (Betatherm 10K3A1A) is just under the panel in
the center of the two rows of analog input terminals. It 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 estimated in figure
Panel Temperature Error Summary
(p. 329).
In
summary, error is estimated at ± 0.1 °C over 0 to 40 °C, ± 0.3 °C from –25 to 50
°C, and ± 0.8 °C from –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 CR1000 is in a weather-tight enclosure not subject to violent swings in
temperature or uneven solar radiation loading, the temperature difference between
the terminals and the thermistor is likely to be less than 0.2 °C.
With an external driving gradient, the temperature gradients on the input panel
can be much worse. For example, the CR1000 was placed in a controlled
temperature chamber. Thermocouples in terminals at the ends and middle of each
analog terminal strip measured the temperature of an insulated aluminum bar
outside the chamber. The temperature of this bar was also measured by another
datalogger. Differences between the temperature measured by one of the
thermocouples and the actual temperature of the bar are due to the temperature
difference between the terminals the thermocouple is connected to and the
thermistor reference (the figures have been corrected for thermistor errors). Figure
Panel-Temperature Gradients (Low Temperature to High)
(p. 329)
shows the errors
when the chamber was changed from low temperature to high in approximately 15
minutes. Figure
Panel-Temperature Gradients (High Temperature to Low)
(p. 330)
shows the results when going from high temperature to low. During rapid
temperature changes, the panel thermistor will tend to lag behind terminal
temperature because it is mounted deeper in the CR1000.
328
Summary of Contents for CR1000
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