ADM-860C 04/14/0857
APPENDIX A - NIST VELOCITY TESTING
The AirData Multimeter is primarily an electronic micromanometer which measures pressures very accurately.
The velocity pressure generated by the various probes is used to calculate and display air velocity and air flow.
Confirmation of the meter pressure measurement accuracy is fairly simple using NIST traceable transfer standard
gages.
The testing of the meter with various velocity probes, however, is not as straight forward. Velocity accuracy
testing requires the use of a fairly large wind tunnel to reduce boundary layer and wall effects. The wind tunnel
must provide a very flat velocity profile and a range from 50 to 9000 fpm.
Such a facility does exist at NIST and can provide directly traceable testing and certification.
National Institute of Standards and Technology
Rt. #I-270 & Quince Orchard Road, Bldg. 230
Fluid Mechanics Bldg.
Gaithersburg, MD 20899
Phone: (301) 975-5947
RECOMMENDED TEST PROCEDURES
The AirData Multimeter automatically corrects air velocity and flow readings for the density effects of barometric
pressure and temperature. Readings will be corrected to local density true air speed unless standard density,
mass flow readout is specifically selected. No such selection is required, since the NIST velocity correction is
normally for local air density. The AirData Multimeter does not correct air velocity readings for relative humidity.
The AirData Multimeter TemProbe and extension cord must be plugged into the meter and placed in the wind
tunnel airstream to correct the readings for the density effect of variations in temperature. Record the AirData
Multimeter temperature and absolute (barometric) pressure reading at the beginning and end of each probe test
range. These readings must fall within the specified tolerances to allow velocity testing.
The hose lengths connecting the probes to the AirData Multimeter should not exceed 15 feet of 3/16" ID flexible
tubing.
The air velocity test points should be preselected to provide data over the full range of each probe, with additional
test points at the velocities of most interest for a given application. Suggested test points might include:
Pitot tube:
50, 75, 100, 150, 200, 300, 500, 1000, 1500, 2000, 2500, 3000, 4000, 5000, 6000, 8000 fpm
AirFoil probe:
50, 75, 100, 150, 200, 300, 500, 750, 1000, 1500, 2000, 2500, 3000, 4000, 5000 fpm
VelGrid:
50, 75, 100, 150, 200, 250, 300, 500, 1000, 1500, 2000, 2500 fpm
NIST LASER DOPPLER VELOCIMETER WIND TUNNEL TESTING
The AirData Multimeter probes must be positioned at the wind tunnel centerline.
Select the automatic storage mode on the AirData Multimeter. When the air velocity has stabilized, begin the
AirData Multimeter velocity sampling interval. The AirData Multimeter must be taking readings throughout the
LDV sampling period. The simultaneous sampling period must be long enough for the AirData Multimeter to
obtain the average of at least 10 readings at velocities less than 200 fpm. Record the LDV and AirData
Multimeter averages for each test measured velocity.
The tests described above are extensive and relatively expensive. The expense of obtaining such traceability
is justifiable when a transfer standard meter and probes are designated as a "Sheltered Set". This "Sheltered
Set" should only be used to verify the accuracy of instruments which are used in the field.
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