To measure the velocity of air by using vane anemometer.

Apparatus:-

Vane anemometer, stop watch.

Theory:-

Importance of air velocity:-

Air velocity is the most frequently measured dynamic physical property of mine air. This is because air velocity is of vital importance in the calculation of airflow quantities, fan power requirements, and fan efficiencies (theses are major factors in quantity control). There are also minimum and maximum air-velocity requirements in headings and active working places for air quality control involving gases and dusts.

Construction of vane anemometer:-

Vane anemometer is a small windmill-like instrument consisting of a number of radial blades or impellers, a gearing mechanism, a clutch system and a pointer. In operation it can be hand held but is preferably mounted on an extension rod and is held with the blades oriented normal to the direction of air motion.

Working:-

The blades rotate at a speed proportional to the air velocity. The gearing mechanism translates the rotation directly into linear airflow in feet, and the pointer integrates the flow on a marked dial. The clutch system engages the gears at the start and is disengaged at the completion of measurement. Disengaging stops the recording and locks the pointer at the current value of the integrated air velocity. Most anemometers are provided with a switch for setting the readings to zero before the start of measurement. Digital anemometers provide direct readout of the linear feet of air travel.

Some factors for its working:-

• It works for intermediate and high-velocity ranges, although specially designed anemometers are available for low or very high air velocity ranges(2000 to 10,000 fpm or 10.16 to 50.8 m/s).
• It works also during traversing. But speed of traversing should be less than 40 ft/minute.
• If distance of vane anemometer from the body is less than 4 ft then error will be negligible.

  Procedure:-

  • First of all press the switch lever to take the pointer at zero.
  • Then press the clutch and at the same time start the stop watch to measure the time.
  • After some time stop the watch and also stop the rotation of impellers by pressing the clutch.
  • Note the time from stop watch and distance from the meter.
  • Calculate the velocity of air.

  Supposed Calculation and observation:-

initial reading =0 ft

final reading =168 ft
time  =48.7 sec

air velocity = 3.45 ft/sec

Velocity of air=3.45 ft/sec=206.98 ft/minute