Laboratory manual for the evaluation of non-automatic weighing devices

Part 2, section 10—Indicating zero and negative values

Table of contents


Reference

Sections 30 to 33, and 41 of the Specifications Relating to Non Automatic Weighing Devices (1998)

10.0 Indicating zero and negative values–generalities

The purpose of the following requirements is: to ensure that devices can be set and maintained as close as possible to a true zero value; to eliminate possible confusion between under and over indicated weight values; and to ensure that the smallest value of scale interval for digital devices remains an adequate representation of device accuracy.

Indicating zero

10.1 The device must be provided with means to adequately indicate or record a "zero-balance" condition.

10.2 Automatic indicating devices must provide an out-of-balance indication on both sides of zero.

Clarification 1: Over/Under Indicators: At least one graduation on each side of the zero reference must be provided. Clear indication must be provided to differentiate between the below zero and above zero portions of the chart.

Clarification 2: Digital: The minimum acceptable indication of a zero balance condition must be one of the following:

  1. if a decimal point or comma is used, at least one digit to the left and all digits to the right of the decimal point must be displayed;
  2. if a decimal point or comma is not used, at least one active decade plus any constant (fixed) zeros must be displayed;
  3. all decades to the right of a decimal point must be active, i.e. a fixed zero cannot appear to the right of a decimal point.
Table 1: Examples of displaying zero
Max x d (kg) Minimum zero indication
25 x 0.01 0.00
5 000 x 1 0
50 000 x 10 00 Footnote 1
Footnotes
Footnote 1

The last digit may be fixed.

Return to footnote 1 referrer

In the case of an electronic digital display, the zero balance indication may be a continuous digital weight indication or be indicated by other means provided that the scale either automatically inhibits the scale operation or returns to a digital weight indication when an out-of-zero balance condition exists (See section on sleep mode).

10.3.1 A digital indicating scale must either automatically maintain a "center-of-zero" condition to ± ¼ e or less (AZT) or have a center-of-zero indicator that defines the zero balance condition to ± ¼ e or less. Class I or II devices with auxiliary indicating means must define a zero balance condition to ± ½ d only. Therefore, these devices are not required to have a center-of-zero annunciator or an AZT. Perform test LG-15.01.

10.3.2 If the zeroing range of AZT is less than the width of zero (± 0.4 d ), then a center of zero indication that defines the zero balance condition to ± ¼ d or less is required. Otherwise the scale could possibly display zero and not be truly set to zero within ± ¼ d. In this case, the operator would not be warned that the device needs to be re-zeroed. Perform test LG-15.02.

10.4 If provided, the center-of-zero indicator must be inhibited at all displayed weight values other than zero ± ¼ e (or ½ d for class I or II devices with auxiliary reading means).

Clarification: The centre-of-zero light may only be on at net load zero, or at gross load zero if no tare is entered. (Policy on centre of zero light when can the center of zero light be on.wpd)

10.5 On digital indicating devices, a + or - sign must not appear when the scale is indicating zero in any of the available units.

Displaying negative values

10.6 A display of negative value is required in the net display mode when the gross weight is less than the tare value.

10.7 A negative value must be indicated in a manner that cannot be confused with a positive weight value. A minus sign clearly associated with the weight indication may be used provided that it is located in a manner that does not interfere with the weight.

10.8 Blanking the display is a method that may also be used to indicate an under-zero condition when the device is in the gross mode (no tare); the scale may also display symbols which cannot be interpreted as a quantity value (e.g. eeeeee, underload, S-1, etc.). A row of zeros, flashing zeros or flashing weight values are not acceptable means.

Note 1: For devices such as hopper or tank scales, it is recommended to indicate a negative balance condition and an over-capacity condition differently so that the load condition is evident for the operator.

Note 2: For devices such as hopper or tank scales, if blanking the indication is used, it is also recommended that the indicator has an annunciator to indicate "power on" so the operator does not think that the power has been lost when the display is blank.

Sleep Mode or displaying non metrological information

Definition: The term "Sleep Mode" used in this section means a function of a device that blanks partially or totally indications after a defined period of non use, in order to save the screen or to display information other than weights; it also refers to the shut off condition of a device following a period of non use. Also known as "power save mode" or "economy mode".

A device may go in a sleep mode or may display non metrological information such as advertisement, greetings, time and date, etc. provided that the following conditions are met:

10.9 The scale totally or partially blanks its indications or displays non metrological information only when the device is at gross load zero (no tare) and has reached a zero-balance condition.

10.10 If the device blanks its indications partially only or displays non metrological information, the displayed information cannot be construed as weight indications.

10.11 The scale must be provided with an automatic means to inhibit the weighing operation or return the device to a continuous digital indication when the scale is in an out-of-balance condition. Perform the test described in LG-10.01.

10.12 Printing function must be inhibited when the device is in sleep mode.

Alternative

A scale may go into "sleep mode" with a load on the platter, if the scale is designed to prevent any further weighing operation before the load is removed from the platter, all tares are cancelled and the device is reset to zero. In this case the device must bear a special legend "The scale is set to zero when in the sleep mode" (or similar message) adjacent to the weight display for the customers' view.

LG-10.01 Means of automatically restoring weight indication

Application

This test will be performed on any electronic device with a sleep mode or capable of displaying non metrological messages.

Purpose

To ensure that the device has means of detecting when it is no longer in a zero-balance condition and restoring a weight indication; and also to ensure that the printing function is inhibited when in sleep mode.

Test procedure
  1. Ensure that the "sleep mode" function is activated;
  2. Zero the device and wait until the device goes to the "sleep mode" or display non metrological messages;
  3. In one firm motion (so as to prevent the AZSM from capturing part of the test load) apply a load equal to e. The non-zero/non-metrological registration must have been replaced by a mass registration equal to e;
  4. Wait 5 minutes (or the normal delay for the device to return to the sleep mode) to see if the device will go into a "sleep" mode with a load on the platter;
  5. Zero the device with the load on the platter and wait until the device goes to "sleep mode" or display non metrological messages;
  6. Attempt to print;
  7. In one firm motion, remove the 1 e test load. The device must display a mass of - e, or provide an under‑weight indication.
Interpretation of results

The device complies with the requirements if:

Revision

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