Campbell Manufacturing CR10 SECTION 13. CR10 MEASUREMENTS, 13.4THERMOCOUPLE MEASUREMENTS, FIGURE 13.3-8.Measuring Input Settling Error with the CR10

SECTION 13. CR10 MEASUREMENTS

FIGURE 13.3-8. Measuring Input Settling Error with the CR10FIGURE 13.3-9. Incorrect Lead Wire Extension on Model 107 Temperature Sensor

13.4THERMOCOUPLE MEASUREMENTS

A thermocouple consists of two wires, each of a different metal or alloy, which are joined together at each end. If the two junctions are at different temperatures, a voltage proportional to the difference in temperatures is induced in the wires. When a thermocouple is used for temperature measurement, the wires are soldered or welded together at the measuring junction. The second junction, which becomes the reference junction, is formed where the other ends of the wires are connected to the measuring device. (With the connectors at the same temperature, the chemical dissimilarity between the thermocouple wire and the connector does not induce any voltage.) When the temperature of the reference junction is known, the temperature of the measuring junction can be determined by measuring the

13-12

thermocouple voltage and adding the corresponding temperature difference to the reference temperature.

The CR10 determines thermocouple temperatures using the following sequence. First, the temperature of the reference junction is measured. The reference junction temperature in °C is stored in an input location which is accessed by the thermocouple measurement instruction (Instruction 13 or 14). The CR10 calculates the voltage that a thermocouple of the type specified would output at the reference junction temperature if its reference junction were at 0°C, and adds this voltage to the measured thermocouple voltage. The temperature of the measuring junction is then calculated from a polynomial approximation of the National Bureau of Standards (NBS) TC calibrations.