An official bulk-weighing scale is calibrated
using precision test weights, which together must
weigh at least 1/10 of the scale's capacity.
Larger weights make the tests of the scale go
faster. These weights are connected to the weigh
hopper by cables, but the cables are normally
slack, so their weight is supported entirely by
the floor. During a scale test, hydraulic
cylinders attached to the cables lift the test
weights off the floor, so their weight is
supported by the load cells. The difference
between the scale reading with the weights lifted
and the scale reading with the weights lowered
should be equal to the mass of the test weights.
If it is not, the scale's error over the weight
range just tested is found by subtracting the mass
of the test weights from this difference.
Ideally, after the test weights have been
lifted and the weight has been recorded, more
precision test weights would be added and the
scale's accuracy would be tested at a higher
weight. This process would be repeated until the
scale's capacity is reached. However, no elevator
has enough test weights to allow a test to be
performed this way. Instead, the scale is tested
by a "Build-Up Test": Starting with the weigh
hopper empty, the scale is read, the test weights
are lifted, and the scale is read again. Then the
weights are set down, grain is added to the scale
until the scale reads almost the same as it did
with the weights lifted, but slightly less, and
the weights are lifted again. This cycle is
repeated, each time adding a mass of grain
slightly less than the mass of the test weights,
until the scale has been tested to capacity. The
amount of grain added each time must be less than
the mass of the test weights, so the weight ranges
over which the scale is tested will overlap.
For each lift of the test weights, the scale's
error is calculated by subtracting the weight of
the test weights from the increase in the scale
reading when the weights are lifted. The absolute
value of this error must be less than a certain
tolerance, or the scale must be adjusted. If the
scale cannot be adjusted to bring the error for
every lift within tolerance, the scale is removed
from service until it can be repaired. For a new
scale, an "Acceptance Tolerance" equal to 0.05% of
the total test weight is used. For scales which
have been in service for at least three months, a
"Maintenance Tolerance" of 0.1% of the total test
weight is used.
The "Accumulated Error" is the algebraic sum of
the errors determined for the individual lifts of
the test weights. It must be within an Acceptance
Tolerance of 0.05% of the scale capacity or a
Maintenance Tolerance of 0.1% of the scale
capacity, plus half of a scale division. Because
the weight ranges covered by the individual lifts
overlap, it is possible for a scale to be (barely)
within tolerance on each lift and still be out of
tolerance on the Accumulated Error.
A demonstration of a build-up test follows. The
scale has a capacity of 40,000 pounds and has
8,000 pounds of test weights. Clicking on the
button below will either lift the weights or lower
the weights and add grain. At the same time, it
will record the scale reading in the table and
calculate the error. The scale has a digital
weight display with five-pound increments.
Occasionally the display will flash between two
values, in which case the recorded weight will be
the average of the two values. This particular
scale is within the maintenance tolerance.
Three points about this scale test need
(1) The scale indicates a positive weight (the
tare weight) when it is empty. A bulk-weighing
scale does not have to be adjusted to read zero
when it is empty. The tare weight is subtracted
from the gross weight to give the weight of grain
in the hopper.
(2) The scale indication at the end of the test is
slightly over its rated capacity. A bulk-weighing
scale may indicate weights up to 5% over capacity.
Beyond that, the display is required to go blank.
(3) The type of test performed, known as a
"tolerance test", is limited in its accuracy to
one scale division. When a scale is first tested
after being placed in service or modified, a more
accurate test known as an "error test" is
performed. In this procedure, small weights called
"error weights," equal to 1/5 of a scale division,
are placed on the scale until the scale indication
flashes between two values. After the test weights
are lifted, error weights are added or removed
until the scale again flashes between two values.
The error is calculated to within 1/5 of a scale
division by including the number of error weights
added or removed.