January/February 2014

ISA Certified Control Systems Technician (CCST) program

CCST

Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.

CCST question

Cold junction compensation is required for which type of temperature measurement device?

A. Resistance temperature detector
B. Thermocouple
C. Infrared pyrometer
D. Bi-metal thermometer

CCST Answer

The correct answer is B, thermocouple. A thermocouple is comprised of two wires of dissimilar metals, connected at one end, which is placed in the process to be measured. This is called the measuring junction.

Thermocouples generate millivolt signals, which can be correlated to the measured temperature. As with all voltage signals, the millivolt signal developed by the thermocouple must be referenced to a known voltage. For example, in current loops, we often use earth ground as a voltage reference (0V).

Any connection a thermocouple makes (including the connections of the thermocouple wires to a terminal block or PLC input card) will form an additional measuring junction. Therefore, a reference junction, called a cold junction, is used to provide a junction with a known, stable output. This allows the thermocouple to generate a millivolt signal along the wires that can be correlated directly to the temperature reading of the measured process.

The other three temperature measurement devices listed in the problem statement do not require a cold junction reference.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.