Campbell Scientific CR3000 CR3000 Micrologger - Page 307

Section 8. Operation Scientific strongly encourages any user of thermocouples to carefully evaluate Error Analysis (p. 307). An introduction to thermocouple measurements is located in Hands-on Exercise: Measuring a Thermocouple (p. 43). The micro-volt resolution and low-noise voltage measurement capability of the CR3000 is well suited for measuring thermocouples. A thermocouple consists of two wires, each of a different metal or alloy, joined at one end to form the measurement junction. At the opposite end, each lead connects to terminals of a voltage measurement device, such as the CR3000. These connections form the reference junction. If the two junctions (measurement and reference) are at different temperatures, a voltage proportional to the difference is induced in the wires. This phenomenon is known as the Seebeck effect. Measurement of the voltage between the positive and negative terminals of the voltage-measurement device provides a direct measure of the temperature difference between the measurement and reference junctions. A third metal (e.g., solder or CR3000 terminals) between the two dissimilar-metal wires form parasitic-thermocouple junctions, the effects of which cancel if the two wires are at the same temperature. Consequently, the two wires at the reference junction are placed in close proximity so they remain at the same temperature. Knowledge of the referencejunction temperature provides the determination of a reference-junction compensation voltage, corresponding to the temperature difference between the 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 CR3000 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 CR3000 analog-input terminals, the temperature is conveniently measured with the PanelTemp() instruction. The actual thermocouple voltage is measured and combined with the referencejunction compensation voltage. It is then used to determine the thermocouplejunction 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-tovoltage 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 CR3000 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 307