Campbell Scientific CR10 CR10 Measurement and Control - Page 178

SECTION 13. CRlO MEASUREMENTS CR1 O vx/J HI OR LO v5 t vlg P 'tF I _ ttl-n\\ \-w I SHITLD FTGURE 13.3-8. Measuring Input Settling Error with the CR10 CR1 O HI OR LO FIGURE 13.3-9. lncorrect Lead Wire Extension on Model 107 Temperature Sensor 13.4 THERMOCOUPLE MEASUREMENTS A thermocouple consists of two wires, each of a different metal or alloy, which are joined together at each end. lf 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 thermocouple voltage and adding the corresponding temperature difference to the reference temperatu re. 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 (lnstruction 13 or 14). The CR10 calculates the voltage that a thermocouple of the type specified would output at the reterence 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. 13-12