Definition
A temperature-sensing device made by joining two dissimilar metal wires -- Chromel (a nickel-chromium alloy) and Alumel (a nickel-aluminum alloy) -- at one end. When that junction is heated, a small voltage is produced that is proportional to the temperature difference between the heated junction and the reference end. This voltage is read by a calibrated meter in the cockpit and displayed as temperature. Chromel-Alumel thermocouples are commonly used in aircraft engines to measure cylinder head temperature (CHT) on piston engines and exhaust gas temperature (EGT) on both piston and turbine engines, because they tolerate the high temperatures involved.
Plain English
A heat sensor made from two specific metal wires joined together. When the joint gets hot, it produces a tiny electrical signal that the cockpit gauge converts into a temperature reading. It is used in places that get very hot, like engine cylinders and exhaust systems.
Context Anchor
Seen in aircraft engine temperature systems, especially where high temperatures must be measured, such as exhaust or turbine temperature sensing.
Derivation
Chromel and Alumel are trade names. Chromel comes from chromium plus the -el ending; it is mostly nickel with chromium added. Alumel comes from aluminum plus the same -el ending; it is mostly nickel with a small amount of aluminum and other metals. Thermocouple combines thermo- (Greek for heat) with couple (a pair joined together) -- literally a heat-sensing pair. Knowing this tells you the device is simply two specific metal wires paired up to sense heat.
Why Pilots Care
Provides the accurate temperature data needed to keep engine operation within safe limits and avoid overheating damage.
Grounding Statement
Heat at the probe tip creates a tiny signal in the two joined metals, and the instrument reads that signal as temperature.
Intuition Check
Do not think of this as a thermometer filled with liquid or as a powered heater. A Chromel-Alumel thermocouple creates its own tiny electrical signal from heat.
Example Sentence 1
The cylinder head temperature gauge gets its signal from a Chromel-Alumel thermocouple mounted on the hottest cylinder.
Example Sentence 2
The mechanic replaced the faulty Chromel-Alumel thermocouple after the gauge showed inconsistent exhaust temperatures on the ground run-up.