This automation industry quiz question comes from the ISA Certified Control Systems Technician (CCST) program. Certified Control System Technicians calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. Click this link for more information about the CCST program.
When connecting a “K” type thermocouple to a control system where extension wires are required, it is important to use only properly installed “KX” extension wires because:
a) this prevents the formation of a second temperature measurement junction
b) the manufacturer’s warranty for the thermocouple would be voided if “KX” extension wire is not used
c) “KX” thermocouple extension wire comes with special connectors for making the connection
d) “KX” thermocouple extension wire is cheaper than “JX” thermocouple extension wire and reduces installation cost
e) none of the above
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Proper installation of thermocouple extension wires also requires special terminal blocks to prevent additional junctions from being formed.
The correct answer is A; it prevents the formation of a second temperature measurement junction. A thermocouple measurement junction is formed wherever two dissimilar metals are joined. KX-type thermocouple extension wire is made of the same metals as the K-type thermocouple (chromel and alumel). When extending the thermocouple leads with an extension wire back to the control system input card, KX thermocouple extension wire must be used, and the chromel wire and the alumel wire must be joined to the wire of the same metal in the extension cable. If JX or another type of extension wire is used, another measurement junction is formed. For instance, if JX extension cable is used in the example in this problem, the point where the iron and chromel wires are joined would form another thermocouple. This will negatively affect the intended measurement signal.
Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, 2nd Edition