ADM-870 05/23/08
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3.
Calculate the effective average face velocity (fpm) by dividing the actual air flow measured in Step
#2 (cfm) by the gross active face area (sq ft) calculated in Step #1.
4.
Measure the average face velocity at the AMD using the VelGrid, AirFoil probe or other velocity
instrument being tested for a Kv. Document the procedure used to obtain the average face velocity
including all factors such as: the instrument used, the sensing probe positions, spacing of the velocity
sample points and the number of readings taken to obtain the average for each measurement
location. Always record the instrument type and any specific set up conditions such as whether
readings were taken in local or standard air density, and whether or not the correction included
temperature.
5.
Calculate the velocity correction factor "Kv" for this particular AMD by dividing the effective average
velocity obtained in Step #3 above by the measured velocity obtained in Step #4 above. This "Kv"
factor should now be used routinely as a required multiplier to correct velocity readings taken at this
specific AMD design, model and size. The specific procedures developed for measuring air
velocities at a given AMD must always be used to obtain the air velocity measurements.
This "demanding" five step procedure seems to leave little room for the "art" of Testing and Balancing. This is
not altogether true. The measurement of the air velocity in Step #4 is affected by the position and orientation
of the air velocity measuring probe. By selective experimental positioning of the sampling point locations, a
procedure can be developed which will result in a Kv for this particular AMD very near or equal to 1.0.
The face velocity test procedure should be included in the AMD test report. The result is a documented,
repeatable face velocity measurement that can be confirmed by a trained technician using the proper
instrumentation and following the test procedure. This procedure may also be used by laboratory personnel to
retest the air flow at periodic intervals to confirm that the flow still conforms to test report data.
6.2 PITOT TUBE VELOCITY MEASUREMENT
The pitot tube is primarily used to obtain air velocity measurements in ductwork. A pitot tube is stainless steel
with a 90 degree bend at one end and two connectors at a 90 degree angle located near the base. The
measurement range of the AirData Multimeter with the pitot tube is 25 to 29,000 fpm. (calibration accuracy is
certified to 8,000 fpm.) The stainless steel pitot tube included in the AirData Multimeter kit is suitable for use in
temperatures up to 1500
/
F. A "traverse" of the duct is obtained by taking multiple air velocity readings at equal
area locations within the duct cross-section. See the section on AIR BALANCE MANUALS AND TRAINING
PROGRAMS for sources of detailed information on performing duct traverses and other air balance procedures.
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 NEGPITOT. 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 PITOTUBE is displayed. Use the retractile cord to connect
the TemProbe to the meter. Insert the pitot tube and the TemProbe into 3/8" 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. As soon as the display reads CALC, the pitot tube
may be moved to the next traverse position.
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.
Summary of Contents for Airdata ADM-870
Page 25: ...ADM 870 05 23 08 21 6 4 VELGRID ASSEMBLY...
Page 34: ...ADM 870 05 23 08 30 FIGURE 10 2 FLOWHOOD IN CASE FIGURE 10 1 FRAME STORAGE...
Page 35: ...ADM 870 05 23 08 31 FIGURE 10 3 FLOWHOOD ASSEMBLY...
Page 37: ...ADM 870 05 23 08 33 FIGURE 10 7 1X5 FRAME ASSEMBLY FIGURE 10 8 3X3 FRAME ASSEMBLY...