Thermocouple wiring effects: What is true about the connections to a thermocouple (lead junctions) in practical measurement systems?

Introduction:
Thermocouple accuracy depends not only on the sensing junction but also on the reference (cold) junction formed where the thermocouple wires meet copper leads or terminal blocks. These additional junctions can create unintended thermoelectric voltages if not handled correctly.

Given Data / Assumptions:

• Use of dissimilar metals inherently generates Seebeck voltages at junctions.
• Lead connections and terminal blocks are at ambient (reference) temperature.
• Signal levels are in millivolts.

Concept / Approach:
The extra junctions act like additional thermocouples. Unless their temperature is known or compensated, they introduce error. Cold-junction compensation (CJC) senses the terminal temperature and corrects the measured EMF. Therefore, the wiring can produce an unwanted thermocouple effect that must be compensated.

Step-by-Step Solution:

Verification / Alternative check:
Instrument manuals specify internal CJC or require external reference junctions in ice-bath or controlled blocks, confirming the necessity of compensation.

Why Other Options Are Wrong:

• Extra desirable effect: It is an error source, not an advantage.
• High voltages present: Thermocouple outputs are small millivolt signals; “high voltage” does not apply in normal operation.
• Combined statement: Still incorrect for the same reasons.

Common Pitfalls:
Ignoring lead materials; running thermocouple wires directly to measurement devices without correct extension wire types; omitting CJC results in several degrees error.

Final Answer:
can produce an unwanted thermocouple effect, which must be compensated for