ADM-870 05/23/0817
6.1 PITOT TUBE
NEGPITOT will be displayed if the pitot tube readings are negative. This will occur if the positive (+) and
negative (-) hoses have been incorrectly connected to the inlet ports of the meter; if the pitot tube has been
improperly positioned in the air stream; or if the pitot tube tip has been placed in an area of flow reversal or eddy.
It is common practice, although not a purely accurate procedure, to consider negative pitot tube readings as zero
in the averages of pitot tube traverse readings. If a memory sequence has been started, a NEGPITOT display
will be immediately replaced by "-0". This negative zero may be entered into memory by pressing the STORE
or the READ key and will be counted as a zero in the velocity average, and will be recalled as "-0". NEGPITOT
(-0) readings will not be entered into memory while the meter is in the SPEED-READ memory mode). A memory
sequence cannot be started with a "NEGPITOT" reading.
Air flow within the duct may be calculated by multiplying the average duct air velocity (fpm) 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 is reading velocity pressure, rather than
velocity. A pitot tube used with the AirData Multimeter reads out directly in velocity when used in the PITOTUBE
mode, and reads velocity pressure when used in the DIF PRES mode.
6.3 AIRFOIL PROBE
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 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
filter 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
static pressure connections are to be connected respectively to the positive (+) and negative (-) connections of
the meter. The length of each 3/16" I.D. 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 AirFoil 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
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