ADM-870 05/23/0815
6.0 VELOCITY MEASUREMENT
Air velocity measurements obtained with the AirData Multimeter are automatically corrected for the density effect
of barometric pressure. The TemProbe must be used to obtain readings corrected for the density effect of
temperature. If the TemProbe has not been connected to the meter, STD 70
/ F will be flashed during the
calculation time, and all data will be processed using the standard temperature of 70
/ F.
An optional temperature value associated with the previous velocity measurement (associated temperature) may
be recalled, without leaving the velocity mode, by pressing the ASSOC key after the velocity reading has been
displayed (if the TemProbe was connected during the reading). The associated differential pressure value may
also be displayed by pressing the ASSOC key again.
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 resultant 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.
2. Determine the "actual" volumetric air flow through the given AMD air movement device. Pitot tube
duct traverse is likely the most reliable means of determining the actual air flow. Direct air flow
measurements can also be used in areas where duct air velocity measurements are not practical,
by using the FlowHood with custom designed tops.
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