Laboratory manual for the evaluation of non-automatic weighing devices

Part 2, section 15—Zero setting mechanisms

Table of contents


Reference

Sections 30 to 33, 40, 41, 42, 43 and 44 of the Specifications Relating to Non Automatic Weighing Devices (1998)

15.0 Zero setting mechanisms—Generalities

There are several requirements that establish the proper operation of zero setting mechanisms to ensure accuracy and to prevent the perpetration of fraud or inadvertent measurement errors. These requirements depend upon the type of zero setting mechanisms and applications.

Definitions

Manual zero-setting mechanism (MZSM):

mechanism for setting the indication to zero by the operator (balance ball or potentiometer).

Semi automatic zero-setting mechanism (SAZSM):

mechanism for setting the indication to zero automatically following a manual command (Push-button Zero).

Automatic zero tracking (AZT):

mechanism for maintaining the zero indication within certain limits, automatically.

Initial zero setting mechanism (IZSM):

mechanism for setting the indication to zero automatically at the time the device is switched on and before it is ready to use.

Automatic zero-setting mechanism (AZSM):

mechanism for automatically returning the device to a zero condition from a negative indication.

15.1 Manual zero-setting mechanism (MZSM)

15.1.1 If the balance at zero‑load is achieved by the addition of supplementary material, the material must be enclosed in a cavity covered with a screw cap (or a cover with screws, etc.) so that it cannot be readily removed or altered and so that it cannot shift position in such a way that the balance condition of the device is affected during weighing operation.

15.1.2 If a device is provided with a balance ball and a captive screw or nut arrangement, the maximum effect must not exceed 4 verification intervals per revolution, and means must be provided to ensure sufficient friction to prevent a zero change during weighing operation.

15.1.3 A device intended for direct sales to the public may not be fitted with a MZSM unless operated only with a detachable tool.

15.1.4 The total range of zero setting (positive and negative portion) may not exceed 4% of the device capacity unless the gross load that can be weighed is not increased beyond the device capacity limit. Perform the test described in LG-15.03.

15.2 Semi automatic zero-setting mechanism (SAZSM)

15.2.1 SAZSM must operate only when the scale provide a stable indication
(See section 18.0, Motion Detection, test LG-18.01).

15.2.2 SAZSM (Push Button Zero) must set the indication to only zero, within ± ¼ e. The SAZSM of a Class I or II device equipped with an auxiliary reading means must set the indication to zero to ± ½ d. Perform the test described in LG-15.01.

15.2.3 The activation of SAZSM must not clear stored tare weight values. Enter a platter or keyboard tare and attempt to clear it by pressing the zero button when the platter is empty (zero or negative indication) and loaded (positive weight indication).

15.2.4 The total zero-setting range (negative + positive) must not exceed 4% of the device capacity. The range may exceed 4% of Max provided that the gross load that can be weighed is not increased beyond the capacity limits of the device. Perform the test described in LG-15.03.

15.3 Automatic zero tracking (AZT)

15.3.1 The device AZT must not re-zero (in one shot) a weight value in excess of 0.6 e. Perform the test described in LG-15.02.

15.3.2 The total zero-setting range (negative + positive) must not exceed 4% of the device capacity. The range may exceed 4% of Max provided that the gross load that can be weighed is not increased beyond the capacity limits of the device. Perform the test described in LG-15.03.

15.4 Initial zero setting mechanism (IZSM)

15.4.1 The IZSM must set the device to zero within ± ¼ e. The IZSM of a Class I or II device equipped with an auxiliary reading means must set the indication to zero ± ½ d. Perform the test described in LG-15.01.

15.4.2 Determine if the value that can be "zeroed" by the device IZSM exceeds 20%. Perform the test described in LG-15.04. Record this value.

Clarification This information is needed to establish which procedure will be followed for certain performance tests. If the IZSM range does not exceed 20%, the performance tests are done once with the maximum dead load. If the range exceeds 20%, certain performance tests are performed twice: one with the minimum dead load, the second with the maximum dead load. Individual procedures described in Part III indicate to which value IZSM must be set.

15.4.3 The IZSM range of indicators tested and approved separately must not exceed 20%.

15.5 Automatic zero setting mechanism (AZSM)

15.5.1 The AZSM must set the device to zero within ± ¼ e. The AZSM of a Class I or II device equipped with an auxiliary reading means must set the indication to zero ± ½ d. Perform the test described in LG-15.01.

15.5.2 AZSM must only operate when the weight display is below zero.

15.5.3 AZSM must only operate when the device is in Gross mode.

15.5.4 There must be a means to enable/disable the AZSM and it must be sealable

15.5.5 The indication must be stable for at least 5 seconds before this function activates. Perform the test described in LG-15.05

Summary of zero setting mechanisms ranges and accuracy requirements
Zero setting mechanism (ZSM) type
Limit type MZSM AZT SAZSM AZSM IZSM
Accuracy see 15.1.2 None ± ¼ e
± ½e for Class I, II Footnote 1
± ¼ e
± ½e for Class I, II Footnote 1
± ¼ e
± ½ e for Class I, II Footnote 1
Total Zero Setting Range 4% 4% Footnote 2 4% Footnote 2 4% Footnote 2 20% Footnote 3
Footnotes
Footnote 1

The ZSM of a Class I or II device equipped with an auxiliary reading means must set the indication to zero to ± 2 d.

Return to first footnote 1 referrer

Footnote 2

The total zero-setting mechanism range (negative + positive) must not exceed 4% of the device capacity. The range may exceed 4% of Max provided that the gross load that can be weighed is not increased beyond the capacity limits of the device.

Return to first footnote 2 referrer

Footnote 3

The IZSM range of indicators tested and approved separately must not exceed 20%. For complete devices if the IZSM range exceeds 20% certain performance tests are performed twice: one with the minimum dead load, the second with the maximum dead load. Individual procedures described in Part III indicate to which value IZSM must be set.

Return to footnote 3 referrer

LG‑15.01 Center-of-zero indication and setting zero within ± ¼ e

Application

Applicable to complete electronic scales and separate electronic indicators having SAZSM or IZSM or equipped with a "center-of-zero" indicator.

Purpose

This test is to verify that the SAZSM and IZSM automatically set the device to zero within ± ¼ e, and to verify that the device range of center-of-zero indication is equal to or less than ± ¼ e. Note that the range of center-of-zero indication of Class I or II devices equipped with auxiliary reading means is ± ½ d. This must be taken into consideration when performing the following tests and interpreting the results. Take into account that the ZU of weight classifiers is adjacent to the graduation.

Procedure
Setting zero to within ± ¼ e.
  1. Switch the AZSM and AZT off or set its value to zero effect; zero the DUT.
  2. Place a load of at least e (comprised of 10 × 0.1e weights) on the platter and RE-ZERO the device.
  3. Successively remove small denomination weights in 0.1e steps until the low end of the interval is reached (the indication begins to alternate between 0 and −1 e). Record the weights removed as being the negative portion of the interval.
  4. Successively add small denomination weights in 0.1e steps until the high end of the interval is reached (the indication begins to alternate between 0 and +1 e). Record the value of the weights added and subtract the value of the weights recorded in the preceding step. The difference is the positive portion of the interval.
  5. Determine if the zero position as set by the SAZSM does not deviate from the true zero reference point by more than 3 e. Generally, the zero position, as automatically set by the SAZSM, should coincide with the zero reference point; in such a case, the negative range would equal the positive range. See illustration.
  6. Repeat the test by zeroing the device using the IZSM.
Width of the centre-of-zero indication (Annunciator)
  1. Switch the AZSM and AZT off or set its value to ZERO effect.
  2. Set the device to zero.
  3. Place a load of at least e (comprised of 10 × 0.1 e weights) on the platter; zero the DUT.
  4. Successively remove small denomination weights in 0.1 e steps until the visual confirmation of zero goes off. Record the value of the weights removed as being the negative portion of the center of zero indication.
  5. Successively add small denomination weights in 0.1 e steps until the center-of-zero indicator goes off (positive limit). Record the difference between the value of the weights remaining on the platter and the value of the weight recorded in the preceding step as being the positive portion of the center-of-zero indication.
Interpretation of results

The DUT is deemed to comply with the requirements if the zero setting mechanisms automatically set the device to zero within ± ¼ e from the true zero reference point, and if the centre-of-zero annunciator indicate a zero balance condition within ± ¼ e of the true zero reference point.

Note: For Class I or II devices with auxiliary reading means, the range is ± 0.5 d.

LG-15.02 AZT – Maximum incremental load that can be zeroed

Purpose

To determine that the largest load that can be deposited at one time on the device, while the DUT is at zero load, is equal to or less than 0.6 e.

Procedure

Check for the addition of small loads:

  1. Ensure that the AZT is on and the AZSM is disabled; Zero the device.
  2. Place a known test load "A" equal to or greater than e (e.g. 10 e), plus a load "B" equal to 0.7 e on the device load receiving element.
  3. Remove known test load "A", wait at least 10 seconds to see if the device will automatically re-zero test load "B".
  4. If the device did not zero test load "B", repeat with smaller test loads (0.6 e, 0.5 e, 0.4 e, etc) to determine the maximum positive AZT range. Record the maximum value that the AZT will re-zero.

Check for the removal of small loads:

  1. Disable the AZSM (if applicable)
  2. Place a small known test load "A" equal to or greater than e (e.g. 10 e), plus a load "B" equal to 0.7 e on the load receiving element.
  3. Re-zero the device.
  4. Remove all loads. A negative value ("A" + "B") should now be displayed.
  5. Place the test load "A" back on the device and wait 10 seconds. The device should not reset to zero.
  6. If the device resets to zero, repeat with smaller test loads "B" (0.6 e, 0.5 e, 0.4 e, etc) to determine the maximum negative AZT range. Record the value.
Interpretation of results

The device is deemed to comply with the requirement:

LG-15.03 Re-zero range (maximum total range of SAZSM/AZSM/AZT/MZSM)

Application

Applicable to complete electronic scales, electronic indicators, and mechanical scales equipped with balance ball type of zero setting.

Purpose

This test is used to determine whether the total range of the zero setting mechanism, other than the initial zero setting mechanism (IZSM), exceeds 4%; and if the total range exceeds 4%, to ensure that the gross load that can be weighed is not increased beyond the capacity limit of the device.

The procedure depends upon the type of zero setting mechanism. On mechanical devices, the procedure is relatively simple.

Procedure for mechanical devices (beam scales, dial scales, etc.)
  1. Place a load (8-10 % of Max) on the load receiving element.
  2. Set the balance ball to the lowest setting (low end of the range) and note the indication.
  3. Set the balance ball to the highest setting (high end of the range) and record the indication.
Interpretation of results

The difference between the indications at the low and high ends must not exceed 4% of Max.

Procedure for electronic digital devices

On electronic devices, the SAZSM, AZT and AZSM setting ranges must be determined. As illustrated below, in some scale designs, there is only one common parameter that limits the range over which the SAZSM, AZT and the AZSM can function.

When so designed, it is relatively easy to determine the total range because one only has to find the SAZSM range using weights and then to confirm that the AZSM and AZT have also been disabled once the limits of the range have been reached. For this type of design, use the "Weight method".

In other scale designs, there are two distinct range limiting parameters; one for the SAZSM and one for the AZT. It is then necessary to find the limits of the SAZSM range and then to find the limits of the AZT range. The total range is then a sum of the two.

Procedure "Weight method"
Determination of the negative portion of SAZSM/AZSM/AZT
  1. Place on the LRE a load approximately equal to 6% of Max. This load should be made up of several smaller weights allowing for the gradual withdrawal of the load.
  2. Activate the Initial Zero Setting Mechanism by switching the DUT "OFF" and "ON" (Note: on some devices, it may be necessary to unplug and reconnect the power cord in order to activate the IZSM). The DUT should then have zeroed the 6% load and display a zero registration.
  3. By successively removing loads and activating the SAZSM (i.e. zero button), determine to the nearest d the maximum load that can be removed from the LRE while still allowing the DUT to perform a semi-automatic zeroing. Record this value.
  4. Once the DUT no longer performs a semi-automatic zeroing (The scale is just below the low range of SAZSM), remove a load equal to 0.5e; wait 1 minute. Remove an additional 0.5e and wait 1 minute. If the DUT's AZSM or AZT has not re-zeroed the 1e (0.5e + 0.5e) (and thus the DUT is displaying a −1e registration), this will have confirmed that the SAZSM, AZT and AZSM share the same range limit in the negative direction. Record this value.
  5. If the AZSM, AZT and SAZSM do not share the same range continue removing loads until the AZSM no longer functions, or all weight has been removed. Record the value of all the weights removed.
  6. Once the DUT no longer performs automatic zeroing (The scale is just below the low range of AZSM), remove a load equal to 0.5e; wait 10 seconds. Remove an additional 0.5e and wait 10 seconds. If the DUT's AZT has not re-zeroed the 1e (0.5e + 0.5e) (and thus the DUT is displaying a −1e registration), this will have confirmed that the AZT and AZSM share the same range limit in the negative direction.
  7. Record the total amount of weights removed as being the negative portion of the Total Range of SAZSM/AZSM/AZT.

    Note: If the DUT's AZT has re-zeroed the 0.5 e load that was removed, and if the DUT appears to continue automatically zeroing additional 0.5 e loads upon removal, then the "Water Method" will be used to determine the AZT range.

  8. Restore all weights removed (thus restoring the 6% of Max on the LRE) and re-zero using the SAZSM. (The DUT should now be indicating a zero registration).
Determination of the positive portion of SAZSM/AZT
  1. By successively adding loads and activating the SAZSM (i.e. zero button), determine to the nearest e the maximum amount of load that can be added to the LRE while still allowing the DUT to perform a semi-automatic zeroing.
  2. Once the DUT no longer performs a semi-automatic zeroing, add an additional load equal to 0.5 e; wait few seconds. Add an additional load of 0.5 e. If the DUT's AZSM has not re-zeroed the 1e (0.5 e + 0.5 e) (and thus the DUT is displaying a +1 e registration) this will have confirmed that the SAZSM and AZSM both share the same range limit in the positive direction. Record the total amount of weights added as being the positive portion of the Total Range of SAZSM/AZT.

    Note: If the DUT's AZSM has re-zeroed the 0.5 e load that was added, and if it the DUT appears to continue automatically zeroing additional 0.5 e loads upon removal, then the "Water Method" will be used to determine the AZSM range.

  3. The sum of the positive and negative portion is the total range of SAZSM/AZSM. If the sum exceed 4% of Max, proceed with the test LG-14.01.
Interpretation of results

If the total SAZSM/AZSM/AZT range(s) exceed 4% of the device capacity, the device is deemed to have complied with the requirement if the gross load that can be weighed is not increased beyond the capacity limit of the device.

Example 15 kg × 0.005 kg computing scale

Step 1 0.600 kg (4%) placed on the scale; scale is zeroed. The scale indicates zero.

Step 2 An additional 5 kg is placed on the scale. The scale is zeroed.

Step 3 Load the scale until it indicates an overload condition. The maximum value that may be indicated is 10.045 kg (15 kg − 5 kg + 9d).

LG-15.04 IZSM range (maximum range of initial zero setting mechanism)

Application

This test is applicable to electronic indicators and to complete electronic devices.

Purpose

The purpose of this test is to determine whether or not the total range of the initial zero setting mechanism exceeds 20%. The initial zero setting mechanism is the mechanism that sets the scale to zero upon power up.

Clarification—Electronic indicators tested and approved separately

The load receiving element to which an electronic indicator tested and approved separately will be interfaced will not have been tested up to 200% of Max. Consequently, the maximum Initial Zero Setting Mechanism range of electronic indicators must be limited to 20% of Max.

Procedure for electronic indicators

Note: The following explanation of the test procedure is based upon the use of a load receiving element connected to the electronic indicator because it is easier to explain the procedure this way. However, the actual test can be performed using a load cell simulator or a rheostat provided that the basic principles are maintained. The procedure consists of calibrating the electronic indicator so that it only uses a small portion of the total capacity of the LRE.

  1. Connect the indicator to a LRE and calibrate the indicator so that it is at a zero balance condition when the LRE is loaded at 50% of its maximum capacity and that it displays the number of intervals "n" (as requested by the applicant) when the LRE is fully loaded (Cap).
  2. By trial and error, find the negative (-ve) portion of the electronic indicator's IZSM range by removing a load(s) from the LRE and by trying to zero it using the electronic indicator's IZSM. The IZSM is triggered by unplugging and replugging the power cord (Note: on some devices, it is sufficient to switch it off and on).
  3. Similarly, by trial and error, find the positive (+ve) portion of the electronic indicator's IZSM range by adding a load(s) to the LRE and by trying to zero it using the electronic indicator's IZSM.
Procedure for complete electronic scales

Note: Whenever possible, perform the procedure described above for electronic indicators tested separately; or:

  1. Remove the platter in order to reach the lowest point of the IZSM range.
  2. By trial and error, find the positive (+ve) portion of the electronic indicator's IZSM range by adding a load(s) to the LRE and by trying to zero it using the electronic indicator's IZSM. The IZSM is triggered by unplugging and replugging the power cord (Note: on some devices, it is sufficient to switch it off and on).
Interpretation of results

An electronic indicator tested and approved separately is deemed to comply with the requirements when the total range of the Initial Zero Setting Mechanism (absolute value of negative (-ve) portion of the range plus the positive (+ve) portion of the range) does not exceed 20% (or can be set to a maximum of 20% and sealed) of the DUT's maximum capacity (Max);

The IZSM range of a complete electronic device may exceed 20% of Max if the device performs within tolerances when the IZSM is set at the minimum and maximum points of its range.

When the IZSM range is limited to 20%, performance tests are conducted once: at the maximum IZSM setting. When the IZSM range exceeds 20%, certain performance tests are conducted twice: at the minimum and at the maximum setting of the range. See description of the performance tests in Part 3.

LG-15.05 AZSM – Maximum load that can be re-zeroed

Application

Applicable to complete electronic scales and electronic indicators

Purpose

This test is used to determine whether the range of the automatic zero setting mechanism exceeds 4% and does not activate for at least 5 seconds after the indication is stable.

Procedure

Both the range and time limit of the AZSM must be determined. The AZSM may only re-zero negative quantities.

  1. Place a load (5% of Max) made up of small weights on the load receiving element.
  2. Activate the IZSM
  3. Remove all weight from the platter (device should now indicate −5%, give an error, or blank)
  4. Allow device to stabilize and wait at least 5 seconds (or the time indicated by manufacturer for activation of the AZSM). If the device re-zeroes the load then record the value of the weights removed and the time required for activation of the AZSM. If not, continue with the test
  5. Successively add small loads, waiting at least 5 seconds (or the time indicated by manufacturer for activation of the AZSM) to see if the AZSM activates. Continue adding weights until the AZSM range of the device is determined. Record this value and the time required for activation of the AZSM
Interpretation of results

The device should not re-zero a negative load greater than 4% of the device capacity and may only re-zero after the indication has been stable for at least 5 seconds. If a load greater than 4% was re-zeroed complete LG-14.01.

Revision

Rev. 1 (March 2010)
  • AZSM renamed to AZT
  • added new definitions for AZT and AZSM
  • added requirements and test for new AZSM function (LG-15.05)
  • removed Method 3 from LG-15.03 and altered procedure to reflect new AZSM/AZT
  • table outlining all zeroing functions added
Rev. 2 (May 2013)
  • Removed method 2 from LG-15.03
Date modified: