Characteristics of velocity sensors

Adapted from ASHRAE 1997

Measurement Means

Application

Range

Precision

Limitations

ft/min m/s

Smoke puff or airborne solid tracer

Low air velocities in rooms; highly directional

5 to 50

0.025 to 0.25

10 to 20%

Awkward to use but valuable in tracing air movement

Deflecting vane anemometer

Air velocities in rooms, at outlets, etc.; directional

30 to 24,000

0.15 to 120

5%

Needs periodic check calibration

Revolving vane anemometer

Moderate air velocities in ducts and rooms; somewhat directional

100 to 3000

0.5 to 15

2 to 5%

Extremely subject to error with variations in velocities with space or time; easily damaged; needs periodic calibration

Hot-wire anemometer

a. Low air velocities; directional and nondirectional available

1 to 2000

0.005 to 5

2 to 5%

Requires accurate calibration at frequent intervals. Some are relatively costly.

b. High air velocities

Up to 60,000

Up to 300

0.2 to 5%

c. Transient velocity and turbulence

     

Measurement Means

Application

Range

Precision

Limitations

ft/min m/s

Pitot tube

Standard instrument for measuring duct velocities

180 to 10,000 with micromanometer;
600 to 10,000 with draft gages;
10,000 and up with manometer

0.9 to 50 with micromanometer;
3 to 50 with draft gages;
50 and up with manometer

1 to 5%

Accuracy falls off at low end of range

Impact tube and sidewall or other static tap

High velocities, small tubes and where air direction may be variable

180 to 10,000 with micromanometer;
600 to 10,000 with draft gages;
10,000 and up with manometer

0.6 to 50 with micromanometer;
3 to 50 with draft gages;
50 and up with manometer

1 to 5%

Accuracy depends on constancy of static pressure across stream section

Cup anemometer

Meteorological

Up to 12,000

Up to 60

2 to 5%

Poor accuracy at low air velocity (< 500 fpm, [<2.5 m/s])

Laser Doppler velocimeter

Calibration of air velocity instruments

1 to 6000

0.005 to 30

1 to 3%

High cost and complexity limit LDVs to laboratory applications