MIL-PRF-32001
fan unit operation to stabilize for 10 minutes.
Verify air flow is produced in a downward direction within 5 minutes of reaching the test base
temperature. Measure the air velocity and calculate the total air flow produced by the fan unit as
follows. The shroud inner area shall be divided into 5 equal areas as shown in Figure 4. The air
velocity transducer probe shall be inserted through the holes in the side of the shroud to the
middle of each area and a measurement shall be recorded. Since each velocity measured is for an
equal area, the measurements shall be averaged and multiplied by the total area to obtain the air
flow rate. Insert the probe tip to a depth equivalent to that listed in Table III for point one. Note
that this depth is based on the fan unit outer radius or half the diameter (D/2). Record the air
velocity at the first point. Repeat for each of the five points listed in Table III. Remove the probe
from the shroud and insert it into the hole which is 90 degrees from the first hole. Repeat the five
measurements of air velocity at the locations listed in Table III. Add all ten measurements and
divide by 10 to obtain the average air velocity in feet per minute. Multiply this number by the area
of the fan shroud to obtain the air flow rate in cubic feet per minute (cfm). Note that the diameter
in Table III is in inches while the diameter in the area equation is in feet.
Repeat the entire procedure for each of the base temperatures listed in Table I. Air flow rates
produced by the fan unit shall be equal or above the minimum values listed in Table I for the
corresponding base temperatures.
FIGURE 4. Air velocity measurement points
TABLE III. Air velocity transducer probe insertion lengths
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