8.1.2.2 Thermocouple Measurements —- Details
Related Topics:
• Thermocouple Measurements — Details
• Thermocouple Measurements — Instructions
Thermocouple measurements are special case voltage measurements.
Note Thermocouples are inexpensive and easy to use. However, they pose
several challenges to the acquisition of accurate temperature data, particularly
when using external reference junctions. Campbell Scientific strongly
encourages you to carefully evaluate the section Error Analysis
(p. 327). An
introduction to thermocouple measurements is located in the section Hands-on
Exercise: Measuring a Thermocouple
(p. 46).
The micro-volt resolution and low-noise voltage measurement capability of the
CR1000 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 CR1000. 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 CR1000
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 reference-
junction 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 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.
8.1.2.2.1 Thermocouple 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-
327
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