ADM-860C 04/14/0829
Air flow within a duct may be calculated by multiplying the average duct air velocity (fpm) as measured with the
pitot tube, by the duct area (sq ft). The resultant flow is expressed in cubic feet per minute (cfm).
The standard pitot tube is .3125 inches in diameter and reduces the duct cross-sectional area by only 0.077
square inches in the measurement plane of the duct. This duct area reduction is less than 1% for ducts greater
than three inches in diameter and does not need to be deducted in the duct area calculation.
It is important to note that most publications assume that the pitot tube reading is expressed in velocity pressure,
rather than velocity. The AirData Multimeter used with a pitot tube reads out directly in velocity when used in
the PITOT TUBE mode, and reads velocity pressure when used in the DIFF PRES mode.
6.3 AIRFOIL VELOCITY MEASUREMENT
IMPORTANT: See Section 6.1 VELOCITY CORRECTION FACTORS
The AirFoil probe offers increased versatility in velocity measurements. This accessory amplifies the velocity
pressure signal, giving greatly increased sensitivity at extremely low velocities. It is of particular value in small
diameter ducts since, due to its smaller size and straight configuration, it does not require lateral rotation for
insertion into the duct. The AirFoil probe is also relatively tolerant of rotational misalignment. The measurement
range of the AirFoil probe is 25 to 5,000 fpm.
The AirFoil probe is useful for free point air velocity measurements, such as exhaust hood face velocities, HEPA
filters or laminar hood velocities. A pitot tube senses total pressure at the tip and static pressure several inches
behind the tip, and in many cases is not as suitable for point air velocity measurements.
The AirFoil probe is connected to the meter in a manner similar to the pitot tube. The total pressure and lee side
pressure connections are to be connected respectively to the positive (+) and negative (-) connections of the
meter. The length of each 3/16" ID external tube should be limited to 18 feet.
The air flow should impinge directly onto the total pressure (+) side of the AirFoil probe tip during measurements.
The probe tip should be held perpendicular to the direction of the airstream.
Press the MODE key until AIRFOIL is displayed. Use the retractile cord to connect the TemProbe to the meter.
Insert the AirFoil probe and the TemProbe into 3/8" holes drilled into the side of the duct, being careful to align
the AirFoil probe tip as discussed above. Press the READ key to obtain the air velocity measurement.
NOTE: The AirFoil probe readings will be displayed with a negative sign if the hoses are connected backwards
to the meter or to the probe. The AirFoil probe lee side pressure connector should point downstream with the
air flow.
FIGURE 6.2 AIRFOIL PROBE
The AirFoil probe lee side pressure port is not equivalent to the static pressure port on a pitot tube, and must
not be used to obtain static pressure readings.
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