ADM-880C 07/20/09
31
6.0 VELOCITY MEASUREMENT
Air velocity measurements obtained with the 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.
Pressing ALT / VIEW while a pitot tube velocity reading is being displayed will display the temperature, absolute pressure,
differential pressure (velocity pressure), time and date associated with that pitot tube velocity reading. Pressing ALT / VIEW
while a AirFoil probe or VelGrid velocity reading is being displayed will display the temperature, absolute pressure, time and
date associated with that velocity reading.
The AirData Multimeter default air density correction for flow and velocity readings is to local air density with reference to
barometric pressure. Comparison with "hot wire" anemometer readings may require the correction of the "hot wire" readings
to local density conditions. See Section 6.6 VELOCITY: LOCAL VERSUS STANDARD DENSITY.
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 diffusers with uneven throw 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 CleanRoom 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.
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.
Summary of Contents for AIRDATA ADM-880C
Page 42: ...ADM 880C 07 20 09 37 FIGURE 6 3 VELGRID ASSEMBLY...
Page 50: ...ADM 880C 07 20 09 45 FIGURE 10 1 FRAME STORAGE FIGURE 10 2 FLOWHOOD IN CASE...
Page 51: ...ADM 880C 07 20 09 46 FIGURE 10 3 FLOWHOOD ASSEMBLY...
Page 53: ...ADM 880C 07 20 09 48 FIGURE 10 7 1X5 FRAME ASSEMBLY FIGURE 10 8 3X3 FRAME ASSEMBLY...