Invar composition knowledge check (thermocouple materials): Invar is an iron–nickel alloy used in instrumentation. Approximately what percentage of nickel does invar contain?

Introduction / Context:
Invar is renowned for its extremely low coefficient of thermal expansion, which makes it valuable for precision instruments, supports, and certain sensor applications. Familiarity with its composition is part of core materials knowledge in instrumentation and control.

Given Data / Assumptions:

• Invar is an Fe–Ni alloy engineered for dimensional stability.
• Applications include precision rods, springs, and instrument parts.
• We seek the approximate nickel content.

Concept / Approach:
The classic invar alloy composition is about 36% nickel and 64% iron. This balance produces very low thermal expansion near room temperature, minimizing thermally induced dimensional changes—critical for accurate and stable measurements.

Step-by-Step Solution:

Verification / Alternative check:
Materials handbooks list “Invar 36” explicitly, reinforcing the 36% nickel identification.

Why Other Options Are Wrong:

• 12, 54, 68, 82: These do not correspond to the standard invar composition and would not deliver the signature low expansion.

Common Pitfalls:
Confusing invar with other Fe–Ni–Co low-expansion alloys (e.g., Kovar) that have different compositions and properties.

Final Answer:
36