ADM-860C 04/14/08
26
6.0 VELOCITY MEASUREMENT
Air velocity measurements obtained with the ADM-860C AirData Multimeter are automatically corrected for the
density effect of barometric pressure on the velocity readings. The TemProbe sensor must also be used to
obtain readings corrected for the changes in density caused by the temperature of the air being measured. If
the TemProbe has not been connected to the meter, STD 70
/
F or STD 21.1
/
C will be displayed during the
calculation period, and all data will be processed using the standard temperature.
Comparison with "hot wire" anemometer readings may require the correction of the "hot wire" readings to local
density conditions. See Section 13.5 HOT WIRE ANEMOMETER VERSUS AIRDATA MULTIMETER
6.1 VELOCITY CORRECTION FACTORS
Prior to the development of capture hoods for measuring air flow directly, face velocity and jet velocity
measurements were used to calculate air flow. Since the primary interest was in determining accurate
volumetric air flow, obtaining accurate velocity measurements was not a priority. Only the repeatability of the
velocity readings was considered to be important.
The manufacturers of the various air movement devices developed what became known as A
k
or "area
correction factors". These A
k
factors actually corrected for the variations in velocity reading for the different
types of instruments being used to measure velocity. It was necessary to develop different A
k
factors for each
type of test instrument used to test velocity, because each type is affected differently by the configuration of a
given air movement device (AMD).
Use of the terms A
k
or area constant diverted attention from the fact that average face velocity readings taken
with different instruments on the same AMD were not the same, nor were readings taken with the same
instrument likely to be the same on two or more AMDs with identical areas, but with different configurations.
We continue to use A
k
factors when calculating the air flow for very large diffusers and other special
applications. The use of an A
k
factor is not appropriate, however, in the measurement of face velocities, work
zone velocities or in calculating air flow from velocity measurements at most air movement devices such as
clean room HEPA filters, chemical exhaust hoods, safety cabinets, laminar flow work stations, coil and filter face
velocities, kitchen exhaust hoods or any air movement device that affects velocity measuring instruments by
its shape or configuration.
Various air measurement instruments will display differing readings when used on various (AMD) air movement
devices, but the resulting calculated velocity or flow will be the same if the correct "k" factor is used for each
particular instrument on that device. This correction factor is not an area correction factor,"A
k
" (and never really
was), but is actually a "Kv" velocity correction factor which must be applied to the velocity readings obtained with
a specific instrument used in a specific manner on a specific AMD.
The area of the AMD is the gross active face area (frame to frame actual face area, plus leakage or bypass
areas). The measured velocity multiplied by the correct "Kv" results in a corrected velocity reading that
represents the true average face velocity relative to the gross active area. The measured velocity multiplied by
the "Kv" multiplied by the active face area results in a calculated volumetric flow in cfm, l/s, etc.
Ideally, the manufacturers of the various air movement devices (AMD) will eventually develop and provide Kv
correction factors and procedures to be used with each of their products and various velocity measurement
instruments.
In the meantime, Kv factors will have to be established through field testing of AMDs in the following manner.
1.
Determine the
gross active area of
the filter, coil, grille, opening or exhaust hood. Be sure to
deduct the area of all obstructions to air passage such as support bands, T-bars, glue line and
repaired areas on HEPA filters. The total intake area of an exhaust hood includes all areas of air
entry, including the space behind and around the sash, under the threshold, and through service
openings. It is accepted practice to assume that the velocity through these additional areas is the
same as that of the sash opening area.
Summary of Contents for ADM-860C
Page 38: ...ADM 860C 04 14 08 33 FIGURE 6 3 VELGRID ASSEMBLY...
Page 47: ...ADM 860C 04 14 08 42 FIGURE 10 1 FRAME STORAGE FIGURE 10 2 FLOWHOOD IN CASE...
Page 48: ...ADM 860C 04 14 08 43 FIGURE 10 3 FLOWHOOD ASSEMBLY...
Page 50: ...ADM 860C 04 14 08 45 FIGURE 10 7 1X5 FRAME ASSEMBLY FIGURE 10 8 3X3 FRAME ASSEMBLY...
