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4) Comparisons of Accuracy: Pitot vs Anemometer

The following was reported in the CTI paper TP-84-15 by Wheeler, Wilber and Starnes, as a result of testing using anemometer vs pitot tube. Compare average difference in ACFM (two runs each).

• ●  Average ACFM (% difference)......... 0.96%
• ●  Pitot Tube Repeatability was ......... +/- 0.19%
• ●  Anemometer Repeatability was ..... +/- 0.75%

There is no authoritative number you can place on accuracy of a particular instrument. Effect of variations on wind changes during the test, accuracy of calibration, proficiency of the tester, and choice of measurement location affect the overall "absolute" accuracy of the of the flow measurement. For example, the following results using the same operators, measuring the same cell, using the same instrument with same fan setting. Again, from data reported in TP-84-15:

Percent difference between average readings of pitot vs anemometer was 1.35%. Keep in mind these are with will calibrated instruments in the hands of trained technicians.

5) Preparing for Air Flow Test

(1) Decide where you're going to test

(2) Measure the inside diameter of the ring or stack at the measurement plane and the outside diameter of the seal disc.

(3) Calculate the distance to the center of the equal area bands using the net free area calculated from step two.

(4) Prepare your equipment

• ●  Mark off the distances to the measurement points with tape around the pole of your instrument. Lay out from the center or the anemometer to the edge of the fan ring for each equal area band. Calculation method is shown below.
• ●  If you are measuring temperature, attach a thermocouple to the pole with its sensing bulb near the end that airflow and temperature are both measured.

6) Calculate Anemometer/Pitot Locations

A typical airflow test requires measuring velocity at the center of 5 equal area bands in four quadrants, plus one reading directly over the center of the fan. This is 21 readings total and is considered the minimum acceptable number. For fans over 20 ft diameter, 10 equal area bands should be considered.

(1) Let DF equal the inside diameter of the fan ring. Let DSD equal the Seal Disc or Hub diameter.

(2) Calculate the following:

• ●  Gross Area (A_G) = 0.7854(DF)²
• ●  Net Free Area (NFA) = 0.7854(DF² - DSD²)
• ●  Area/Band (A/B) = NFA/(2 x Number of Bands)

(3) Area within center of Band 5 = (A_G - A/B)ft² = A₅
R₅ = (A₅ / p)^1/2ft. Radius to center of band 5

(4) Area within center of Band 4 = (A₅ - 2 x A/B) = A₄
R₄ = (A₄ / p)^1/2ft. Radius to center of band 4

(5) Area within center of Band 3 = (A₄ - 2 x A/B) = A₃
R₃ = (A₃ / p)^1/2ft. Radius to center of band 3

(6) Area within center of Band 2 = (A₃ - 2 x A/B) = A₂
R₂ = (A₂ / p)^1/2ft. Radius to center of band 2

(7) Area within center of Band 1 = (A₂ - 2 x A/B) = A₁
R₁ = (A₁ / p)^1/2ft. Radius to center of band 1

(8) Calculate distances from fan ring to anemometer measurement points.

d₅ = D/2 - R₅
d₄ = D/2 - R₄
d₃ = D/2 - R₃
d₂ = D/2 - R₂
d₁ = D/2 - R₁

Note that one reading is taken over the center of the fan. This will indicate a negative air flow (down). It must be advantage in with one quadrant's velocities.

(9) Sample Problem: Calculate anemometer positions to measure flow a 14 ft. fan in a 14'2" fan ring with a 45" seal disc.

DF = 14'2" = 14.166 ft.
DSD = 45" = 3.75 ft.
A_G = 157.63 ft²
NFA = 146.59 ft²
Area per Band (A/B) = 14.66 ft²

• ●  Area to center of Band 5 = A_G - A/B = A₅ = 142.97 ft²
  Radius to center of Band 5 = (142.97 / p)^1/2= 6.75 ft
• ●  Area to center of Band 4 = A₅ - 2 x A/B = A₄ = 113.65 ft²
  Radius to center of Band 4 = (113.65 / p)^1/2 = 6.01 ft
• ●  Area to center of Band 3 = A₄ - 29.32 = A₃ Radius to center of Band 3 = (84.33 / p)^1/2= 5.18 ft
• ●  Likewise:
  A₂ = 55.01 ft²
  R₂ = 4.18 ft
  A₁ = 25.69 ft²
  R₁ = 2.86 ft
  
  Cheek: A₁ - A/B should equal ASD
  25.69 - 14.66 = 11.03 ft²
  ASD = 11.04 ft² (Checking Okay)

(10) Calculate anemometer positions measured from fan ring: