Campbell Scientific CR1000 Measurement and Control System - Manual (Page 302)

Section 8. Operation
302
reference junction and 0 °C. This compensation voltage, combined with the
measured thermocouple voltage, can be used to compute the absolute temperature
of the thermocouple junction. To facilitate thermocouple measurements, a
thermistor is integrated into the CR1000 wiring panel for measurement of the
reference junction temperature by means of the PanelTemp() instruction.
TCDiff() and TCSe() thermocouple instructions determine thermocouple
temperatures using the following sequence. First, the temperature (°C) of the
reference junction is determined. Next, a reference-junction compensation voltage
is computed based on the temperature difference between the reference junction
and 0
°C. If the reference junction is the CR1000 analog-input terminals, the
temperature is conveniently measured with the PanelTemp() instruction. The
actual thermocouple voltage is measured and combined with the reference-
junction compensation voltage. It is then used to determine the thermocouple-
junction temperature based on a polynomial approximation of NIST thermocouple
calibrations.
8.1.4.1 Error Analysis
The error in the measurement of a thermocouple temperature is the sum of the
errors in the reference-junction temperature measurement plus the temperature-to-
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.
8.1.4.1.1 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. 303).
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 channels at the ends and middle of each
analog terminal strip measured the temperature of an insulated aluminum bar