ADM-870C 07/14/0928
FIGURE 6.1 DWYER SERIES 160 STANDARD PITOT TUBE
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 PROBE 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 air stream.
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.
To Next Page
Loading...
Need help?
General
