Theory Of Operation - TSI Alnor Balometer 6461 Manual Del Usuario

The Balometer is an adaptation of the Velometer
system which allows this instrument to be used for
measurement of air volume, by reading average
velocity as the air moves through a cross-section
of known constant dimension.
The Balometer consists of six major subassemblies
which function as a system to provide accurate air
volume measurements:
1. Air Collection System—All the air issuing
from or entering a diffuser is collected through
a cloth "hood" which is chosen to fit a specific
diffuser at one end, and translates this variable
size to a standard opening which is slightly
larger than one foot square. The diffuser end
of this hood is foam-edged to allow for a
complete seal around the opening.
2. Instrument Base—The base, in addition to
housing the range selector, manifold, and
Velometer, provides the standard cross-section
through which the air is passed. The bottom
flared portion acts both as a secure footing for
the instrument, and provides the exit for the
captured air. This is also the entry point for
exhaust or return air measurements.
3. Manifold—This is a grid which contains
calibrated holes in a regular pattern spaced
within the standard base opening. It is actually
two grids back-to-back, with the pattern of
holes in a separate network on each side. Air
can therefore enter the manifold from either
side and be discharged at the other. The
amount of air which flows through the
manifold is directly proportional to the
average velocity of the air moving past the
sensing holes. Therefore this velocity, which
is moving through a known cross-sectional
area, is proportional to the volume of air flow
through the instrument.
4. Range Selector—By means of a single control
the range selector provides for the
measurement of air flow in either direction,
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THEORY OF OPERATION

and provides calibrated orifices for three
ranges, which effectively triples the scale
length of the meter. Accuracy of the
measurement in either direction is assured by
separate calibrations of each range, in each
direction.
5. Velometer—TSI's standard air measuring
instrument is provided with a special scale
graduated either in cubic feet per minute,
cubic meters per hour, or liters per second.
6. Low flow Adapter—This screen assembly
makes use of the standard area and velocity
principles mentioned above. The adapter
reduces the effective area through which the
air flows, causing a corresponding increase in
velocity past the manifold. The screen blocks
50% of the standard opening, increasing the
velocity to twice the original. This means, for
example, that with the adapter a volume of
250 CFM will create the same velocity as 500
CFM without the adapter. A fourth scale has
been provided on the Velometer to be used
with the adapter, on the most sensitive range
setting.
Note that the adapter provides higher
sensitivity at low flow rates, but can more
easily upset the system being measured at flow
rates above 125 CFM (200 cmh, 60 l/s)
because of additional resistance. It is
therefore suggested that measurements above
this value be taken without the adapter.
Note: All capture hoods are susceptible to "odd"
air flow patterns. "Odd" could refer to any
air flow pattern different from the pattern
where it was calibrated. For example,
substantial negative effects can be observed
when using a large hood on a small
diffuser. This creates large recirculation
regions on the sides of the fabric hood and
causes an "odd" air flow pattern as it
passes over the manifold. It is
recommended to match the hood closely to
the size of the diffuser.
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