Laboratory manual for the evaluation of non-automatic weighing devices

LG-3.02 Load discrimination

Table of contents


Reference

Sections 14, 15 of the Specifications relating to non automatic weighing devices (1998)

Application

Load discrimination tests are performed on complete electronic or mechanical non automatic scales, and weighing elements tested separately using a high precision electronic indicator (Resolution must be at least ten times the DUT resolution). This test is not applicable to electronic indicators tested separately.

The load discrimination test is used to determine whether the device is capable of sensing a small change of load and of changing its registration accordingly.  Frictional forces, binding or system inertia can prevent a mechanical device from sensing the prescribed "Load Discrimination Test Load". Frictional forces, binding, the use of oversized (capacity) load cells, the use of an electronic indicator with an inadequate display sensitivity (µ Volts/Display Digit) or inadequate digital filter algorithms could cause an electronic weighing device to fail to detect the addition of the prescribed "Load Discrimination Test Load".

Settings

Procedure for digital indicating devices

  1. If necessary, put a small load on the platter to bring the scale out of its AZT range. The indication is near zero.
  2. Successively add small weights equal to 0.1d until the ZU is reached; the indication should be alternating between two values; remove one small weight to obtain a solid indication; the indication is at the high point of the scale interval. Record the indication.
  3. Smoothly add a load equal to 1.4d. Record the indication.
  4. Repeat the test near the maximum capacity.

Note: (See IZSM points above)

Note: The test can also be done by removing the 1.4d test weight. To do so, bring the indication right above the ZU and then remove the 1.4d load.

Interpretation of results

The addition or removal of the 1.4d test load must cause a change in the indication of 2d.

Procedure for analogue indicating devices

  1. At the no load condition, press on the platter to generate motion of the indicating element (pointer). Allow the indicator to stabilize.
  2. Smoothly add (or remove) a load equal to 1.4d and allow the indicating element to stabilize. Record the indication.
  3. Repeat the test near maximum capacity.

Interpretation of results

The addition or removal of the 1.4d test load must cause a change in the indication of 1d.

Procedure for non automatic indicating devices—Beam scales with no additional means of indication

  1. Set the device to zero (beam in horizontal position at mid distance between the trig loops or the limiting stops).
  2. Add or remove a load equal to 1d (in-service limit of error) when the test is performed near zero.
  3. Repeat the test near maximum capacity. Add or remove a small load corresponding to the in-service limit of error applicable to the load on the platter, without exceeding 2d.

Interpretation of results

The addition or removal of the test load must cause the weighbeam to change from the center position to the outer limit of the trig loop or limiting stops.

Procedure for non automatic indicating devices—Beam with additional indicating means (over/under indicator having graduations without assigned values)

  1. Set the device to zero.
  2. Add or remove a load equal to 1d (in-service limit of error) when the test is performed near zero.
  3. Repeat the test near maximum capacity. Add or remove a small load corresponding to the in-service limit of error applicable to the load on the platter, without exceeding 2d.

Interpretation of results

The addition or removal of the test load must cause the position of the indicator to change by the distance indicated below:

Revision

Rev. 1 (March 2010)
  • AZSM renamed to AZT
  • Formatting change
Date modified: