SM0486
p. When the expression "tare" is used, we refer to a round container in which
the calibrated masses (weights) are placed before the weighing. Taring devices are
constructed with loops or hooks on the top so that they can be lifted off or
suspended above the pan so that masses can be added to or taken from the container.
The taring device permits setting the balance readout to zero even when an empty
container (tare) on the pan. The readout then becomes a direct indication of the
net weight of the material placed in the container. By use of a taring device, you
eliminate the necessity for making two weighings and subtracting the tare weight
from the gross weight to determine the net weight. After you zero the balance by
removing the appropriate tare discs stored in tie drawer beneath the pan shown in
Figure 5B, the containers placed on the pan for zero are tared and become a "part
of the balance."
q. You use the front panel knobs (9.1g, 1g, and 10g) shown in Figure 5A, to
add calibrated weights to, or remove them from the weight measurement arm. One of
the knobs which is not shown in Figure 5A is the arrestment knob, shown in Figures
5B, and 6.
This control arrests (secures) or releases the balance pan.
You use
the digitizer control shown in the illustration to move the optical scale line,
which is just below the reference gap into the gap, and to center this scale line
in the reference gap.
Additional information on balance construction is included
in the explanation on balance operating theory.
r. Balance operating theory. The analytical balance illustrated in Figures 5
and 6 is typical of the substitution type balances. Although many new variations
of the analytical balance exist, an age-old principle is still applied. A pivoted
lever called the balance beam has the unknown mass suspended from one end and the
standard suspended from the other.
The beam is brought into a near equilibrium
state by bringing the center of gravity directly underneath the pivot, shown in
Figure 6, and by making the pivot area as small as possible (a few thousandths of
an inch). Under these conditions, any small weight difference between the unknown
and standard masses results in a large beam deflection (rotation), and the beam
angle deflection becomes a sensitive indication of equality (or inequality). The
mass of the standard is changed by manipulating the controls in Figure 5A, marked
"10G" (grams), "1G," and "0.1G," and the digitizer drum until an equality is
indicated.
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