Thermocouple Measurements
Thermocouple measurements are special case voltage measurements.
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 wire connects to terminals of a voltage measurement device, such as the data logger. 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 Induces microvolt level thermal electromotive forces (EMF) across junctions of dissimilar metals in the presence of temperature gradients. This is the principle behind thermocouple temperature measurement. It also causes small, correctable voltage offsets in data logger measurement circuitry..
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 (for example, solder or data logger 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.
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 data logger 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.
See the CRBasic Editor help for detailed instruction information and program examples: 