CALIBRATION OF PLATFORM SCALE Flashcards
are large industrial platforms used for weighing pallets, containers and
other heavy goods
Platform scales
dedicated regulations for weighing instruments and their calibration
(EURAMET Calibration Guide, NIST Handbook 44, OIML)
The weighing instrument should be switched on at least 30 minutes before the
calibration. The temperature of the weights should be stabilized to the same
temperature where the calibration is to be done.
The weighing instrument should be at a horizontal level, especially for small and
accurate weighing instruments. Perform a few pre-tests by placing weights close to the
maximum of the range on the instrument and to ensure it works normally.
In case the weighing instrument fails in calibration and it is adjusted, you should make
an “as found” calibration before adjustment and an “as left” calibration after adjustment.
Preparations before calibration
the reference load is placed in a few different specified locations
on the load receptor. First, the load is placed in the center of the load receptor (the
load’s center of gravity) and the result is observed. Next, the load is placed in four
different sectors of the load receptor
Eccentricity test
is performed by replacing the same load on the same place on
load receptor (to avoid any eccentricity error) multiple times. Test should be done in
identical and constant conditions and with identical handling.
The load used should be close to the maximum load of the instrument. Often a
repeatability test is done with one load only, but it can be done also with several
different load values separately.
The load does not necessarily need to be a calibrated load, as the aim is to find out the
repeatability. If possible, the load used should be a single load (not several small loads).
Repeatability test
purpose is to test the accuracy (calibrate) of the weighing
instrument throughout its whole range in several steps, with increasing and decreasing
weight.
The most common practice is the following: start with zeroing the instrument without any
load. Set the loads of the first test point, wait for stabilization, and record the indication.
Continue increasing the loads through all the increasing test points. Once the maximum
load is recorded, start decreasing the loads through the decreasing test points.
Weighing test
means that the instrument does not measure equally accurate throughout the range. Even the zero and
full span are correct, there may be errors in the middle of the range,
Linearity issues
is the difference in the indication when a test point is approached with
increasing or decreasing weight.
Hysteresis
is a test that is not always required to be done. This test is anyhow
required within some industries, like the pharmaceutical industr
Minimum weight test
There are also some other tests specified in the standards, although these are typically
not done during a normal calibration, but can be done as a type of approval test or in
the initial verification
. Other test
Example of other tests
Tare test
Discrimination test
Variation of indication over time
Test of magnetic interaction
is no more a part of
modern science or commerce than is the Pony Express. Without this now-quaint
machine, though, none of the scales in modern use would have arisen.
center beam balance
These are built so that the plate is
at rest under the influence of gravity alone, and the machine balances this off by
internally determining the force required to keep the plate precisely still.
Analytical balance
Most today are
digital, but “old-school” analog models persist.
Bathroom scale
This is used to weigh multiple objects known to have a uniform
weight (for example, precision ball bearings) and display a total piece count based on
the result.
Counting scale
