Technical Measurements Flashcards

Air moves through a 2-square-foot duct at 500 feet per minute. What is the airflow?

Answer: 1,000 CFM

CFM equals velocity times area: 500 fpm × 2 sq ft = 1,000 CFM.

Using the sensible heat formula, what load is carried by 1,000 CFM with a 20°F temperature difference?

Answer: 21,600 BTU/h

Sensible heat = 1.08 × CFM × ΔT = 1.08 × 1,000 × 20 = 21,600 BTU/h.

Filter pressure drop across an air handler is typically measured in which unit?

Answer: Inches of water column

Low air-side pressure differentials are measured in inches of water column with a manometer or magnehelic gauge.

A sling psychrometer uses wet-bulb and dry-bulb readings to determine what?

Answer: Relative humidity

The difference between wet-bulb and dry-bulb temperatures is used to find relative humidity on a psychrometric chart.

A chilled water coil has 10 GPM flowing with a 10°F temperature rise. What is the approximate heat transfer?

Answer: 50,000 BTU/h

Water-side heat transfer = 500 × GPM × ΔT = 500 × 10 × 10 = 50,000 BTU/h.

Which instrument directly measures air velocity at a diffuser or duct traverse point?

Answer: Anemometer

Anemometers (vane or hot-wire) measure air speed, typically in feet per minute.

A pump gauge shows 10 psi of pressure. Approximately how many feet of water head does this represent?

Answer: 23.1 feet

One psi equals about 2.31 feet of water head, so 10 psi ≈ 23.1 feet.