What two instruments would be the instruments of choice where the exhaust opening is large and air velocities are low as in spray booths or chemical hoods?

What two instruments would be the instruments of choice where the exhaust opening is large and air velocities are low as in spray booths or chemical hoods?

• A. Swinging Vane Anemometer or Velometer
• B. Hot-Wire Anemometer
• C. Pitot Tube
• D. Rotating Vane Anemometer

Answer: (A)

Measuring airflow in large, low-velocity exhaust openings calls for an instrument that can give an accurate average velocity across a wide cross-section and withstand the typical spray booth or hood environment. A swinging vane anemometer, also known as a velometer, fits this need. It uses a lightweight vane mounted on a torsion spring; as air moves, the vane deflects in proportion to the velocity. This design is well suited for low speeds and can be traversed or used to sample across a large opening to build up an average velocity, which leads to a reliable calculation of volumetric flow. Hot-wire instruments are extremely sensitive at low speeds but are fragile and easily fouled by dust, overspray, or chemical vapors common in spray booths and hoods, making them impractical for routine field measurements in these settings. Pitot tubes provide accurate point measurements at higher velocities and would require many samples across the opening to obtain a reliable average, which is less efficient for large openings. Rotating vane devices work for moderate, not very low, velocities and can be less accurate at the smallest end of the range or in very turbulent, dirty air.

Measuring airflow in large, low-velocity exhaust openings calls for an instrument that can give an accurate average velocity across a wide cross-section and withstand the typical spray booth or hood environment. A swinging vane anemometer, also known as a velometer, fits this need. It uses a lightweight vane mounted on a torsion spring; as air moves, the vane deflects in proportion to the velocity. This design is well suited for low speeds and can be traversed or used to sample across a large opening to build up an average velocity, which leads to a reliable calculation of volumetric flow.

Hot-wire instruments are extremely sensitive at low speeds but are fragile and easily fouled by dust, overspray, or chemical vapors common in spray booths and hoods, making them impractical for routine field measurements in these settings. Pitot tubes provide accurate point measurements at higher velocities and would require many samples across the opening to obtain a reliable average, which is less efficient for large openings. Rotating vane devices work for moderate, not very low, velocities and can be less accurate at the smallest end of the range or in very turbulent, dirty air.