Hot-wire anemometry: The resistance wire element used to sense air velocity is commonly made from which material?

Introduction / Context:
Hot-wire anemometers measure fluid velocity by relating convective heat loss from a heated, fine wire to flow speed. The choice of wire material is crucial because the sensor operates at elevated temperature, requires mechanical robustness at microscopic diameters, and must have stable electrical properties.

Given Data / Assumptions:

• The sensor is a very fine wire (often 2–10 micrometers in diameter).
• It must sustain high temperatures and repeated thermal cycling.
• Stable resistance–temperature characteristics are desirable.

Concept / Approach:
Tungsten is widely used for hot-wire probes due to its very high melting point, good strength at temperature, and ability to be drawn into extremely fine, robust wires. While platinum or platinum–iridium wires are also used (especially for constant-temperature anemometry and calibration stability), tungsten is the classic choice for many laboratory and industrial probes.

Step-by-Step Solution:
Identify required properties: high melting point, fine drawability, mechanical toughness.Compare materials: copper and aluminium oxidize/soften and lack high-temperature strength; chromium alone is not used for such wires.Conclude tungsten best satisfies hot-wire demands.

Verification / Alternative check:
Instrument catalogs and experimental fluid mechanics texts list tungsten and platinum-family wires as standard, with tungsten being common for constant-current or constant-temperature anemometers in air.

Why Other Options Are Wrong:

• Copper/aluminium: low melting points, oxidation, poor high-temperature strength.
• Chromium: not commonly used as a standalone filament in hot-wire probes.
• Platinum–iridium alloy: used in some designs, but the most common textbook answer is tungsten.

Common Pitfalls:
Assuming general electrical conductivity determines suitability; in reality, high-temperature mechanical stability and survivability at micro-scale are decisive.

Final Answer:
Tungsten