Which instrument measures air velocity by the cooling effect over a heated rod?

Which instrument measures air velocity by the cooling effect over a heated rod?

• A. Measures face velocity at the hood
• B. Measures static pressure with a digital sensor
• C. Determines air velocity by smoke visualization
• D. Measures air velocity by the cooling effect over a heated rod

Answer: (D)

The key idea is measuring air velocity through thermal cooling of a heated element. A hot-surface (or hot-wire) anemometer uses a tiny heated rod; as air flows over it, the moving air carries heat away. The rate of cooling increases with faster velocity. By keeping the element at a constant temperature (or constant power) and watching how much electrical power is needed to maintain that temperature, the instrument converts the cooling rate into a velocity reading. This direct, rapid response method makes it the appropriate choice for measuring air velocity via the cooling effect on a heated rod.

The key idea is measuring air velocity through thermal cooling of a heated element. A hot-surface (or hot-wire) anemometer uses a tiny heated rod; as air flows over it, the moving air carries heat away. The rate of cooling increases with faster velocity. By keeping the element at a constant temperature (or constant power) and watching how much electrical power is needed to maintain that temperature, the instrument converts the cooling rate into a velocity reading. This direct, rapid response method makes it the appropriate choice for measuring air velocity via the cooling effect on a heated rod.