Shortridge Instruments AIRDATA ADM-880C, Руководство по эксплуатации

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ADM-880C 07/20/09 33
Connect one of the tubing sections from the positive (+) port of the meter to the total pressure connection (in line with the
main shaft) on the pitot tube and connect the negative (-) port to the static pressure connection (perpendicular to the main
shaft).
If the hoses are connected incorrectly, the readings will show as negative air velocity and the meter will display [NEG
PITOT]. All passages and connections must be dry, clean, and free of leaks, sharp bends and other obstructions.
After turning the meter on, press the MODE key until [PITOT TUBE] is displayed. Use the retractile cord to connect the
TemProbe to the meter. Insert the pitot tube and the TemProbe into 3/8 inch holes drilled into the side of the duct, being
careful to align the point of the pitot tube so that it is facing directly into the airstream. If the negative (-) connection of the
pitot tube is exactly parallel to the duct, the point of the pitot tube should be facing directly into the airstream. The shaft of
the pitot tube is marked at one inch intervals to make it easier to control the location of the pitot tube within the duct. Press
the READ key to obtain the air velocity measurement.
The accuracy of pitot tube results depends heavily upon uniformity of air flow and completeness of the duct traverse.
Careful technique is critical to good results. Pitot tubes are available in several different sizes and configurations to simplify
different applications which may be encountered.
When a pitot tube is used in internally insulated ducts, small particles of fiberglass may be dislodged and become caught
in the openings of the tube. This will effect the accuracy of the readings and eventually clog the tube. Remove the
connections to the meter and blow compressed air through the bottom of the inside tube to discharge fiberglass particles
from the tip of the pitot tube.
[NEG PITOT] 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 probe 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 [NEG PITOT] reading will be stored as zero. This zero will be calculated in the
velocity sum and average, and will be recalled as nns NP 0.
A memory sequence cannot be started with a [NEG
PITOT] reading.
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 inch (internal diameter) 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 inch 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.