During troubleshooting of an exhaust gas temperature (EGT) indicating system, a technician needs to replace a section of damaged thermocouple lead wire. Which characteristic of the replacement wire is most critical to maintain accurate readings?
- AThe wire must be made of the same dissimilar metal alloys as the original thermocouple leads
- BThe wire must have the same color coding as the aircraft's DC power wiring
- CThe wire must be shielded with a braided copper jacket for EMI protection
- DThe wire must be the same gauge as standard copper hookup wire used elsewhere in the aircraft
Show answer & explanationAnswer & explanation
Correct answer: A. The wire must be made of the same dissimilar metal alloys as the original thermocouple leads
A thermocouple circuit generates a small voltage based on the junction of two dissimilar metals (such as chromel-alumel or iron-constantan). Using the correct matching alloy lead wire is essential because substituting different metals introduces additional junctions with their own thermoelectric voltages, causing inaccurate temperature readings.
Why the other options are wrong
- B. Color coding conventions for power wiring are unrelated to accurate temperature sensing.
- C. Shielding may be beneficial for noise but does not address the fundamental accuracy requirement of matched metals.
- D. Standard copper wire is not used for thermocouple circuits because it would create erroneous junction voltages.
Thermocouple Lead Wire
Lead wire connecting a thermocouple probe to its indicator must be made of the same dissimilar metal alloys as the thermocouple itself to avoid introducing false junctions that alter the generated voltage.
- Thermocouples generate voltage from junction of two dissimilar metals
- Common EGT thermocouple types: chromel-alumel or iron-constantan
- Mismatched lead wire alloys create parasitic junction voltages
- No external power source is needed; thermocouples are self-generating
Memory trick: Match the metals like matching puzzle pieces — mismatched alloys create a false extra junction that lies to the gauge.