Field Inspection Manual — Non-Automatic Weighing Devices

Part 3, Section 29: STP-29: Calibration of Counterpoise and Other Trade Weights

Reference

Sections 71- 90 of the Weights and Measures Regulations, Divisions II — Weights.

General

Many weighing devices, both Automatic and Non-Automatic, use weights as part of their operation. These weights may either be counterpoise weights, designed to increase the capacity of the device, or they may be trade weights used to compare the load being weighed to a known load. These weights are not considered standards, but rather Trade Weights.

The Design, Composition and Construction as well as the Performance (Limits of Error) and Use of these weights must be in accordance with the applicable sections of the W&M Regulations. Trade weights are normally tested at the same time the device is being tested.

Examination

Examine the trade or counterpoise weights to ensure that they are in good condition. Ensure:

  • the weights are free from rust, cracks or any other significant flaws.
  • there is lead in each adjusting hole. (Note: Maximum two adjusting holes)
  • the lead is stamped with suitable inspection marks.
  • the value of the weight is indicated,

and in the case of counterpoise weights;

  • the ratio or weight represented is marked.
  • the ratio is suitable for the device the weights are associated with.

Note: Where the size of each adjusting hole does not allow for the inspection marks to be made in numbers or letters of not less than 3 mm, a smaller inspection stamp may be used. In such situations, the inspection marks shall be located on the container where the counterpoise or the trade weight is kept when not in use, while the inspection stamps are on the weight.

Appropriate inspection stamps for ASP inspectors include the organization number or a recognized symbol described in their Quality Documentation and authorized in Appendix 4 of the relevant requirements (Accreditation or Registration program). Measurement Canada inspectors use the symbol of the flag (bar & maple leaf) or of the crown.

If calibration of these weights is required, then it shall be done as per the following procedure.

Procedure

Substitution Method — Direct Reading — Automatic Indicating Scale

Using the attached table, choose a balance that has n interval size equal to or smaller than the emax shown for the corresponding nominal value of the trade weight to be verified or calibrated.

Note: emax is the maximum interval size that a balance can have in order to calibrate trade weights and is calculated so that one scale interval represents at most 13 of the applicable tolerance for the trade weight to be verified or calibrated.

Note: In order to reduce any eccentricity error, always place the local standard and trade weight in approximately the same location on the load receiving element (LRE).

  1. Place on the LRE, the local standard of the nominal value of the weight to be calibrated and note the indicated value as Dv1. Repeat this step 5 times. Dv1 must not change by more than 1 emax. If it does then the balance should not be used to verify/calibrate this value of trade weight.
  2. Remove the local standard and zero the device. Place the local standard back on the LRE along with a standard equivalent to the "Over Marked Weight" Acceptance or In-Service Limit of Error (i.e. at the same time) and note the indicated value as Dv2. The scale indication must have changed by at least 3 scale intervals. If not, the load discrimination ability of the scale is not adequate for calibration of the trade weight.

    Note: Dv2 is the target displayed value for "Over Marked Weight" from the nominal value of that trade weight for both Acceptance and In- Service Limit of Error.

  3. Determine the target displayed value equivalent to the Limit of Error for "Under Marked Weight" from nominal value (Dv3) for the In-Service Limit of Error. Dv3 = Dv1 − (Dv2 − Dv1) ÷ 2 rounded up to the nearest scale interval (based on emax).
  4. Place on the LRE the trade weight to be calibrated or verified.
    • For Acceptance Limit of Error, the displayed value must fall between Dv1 and Dv2.
    • For In-Service Limit of Error, the displayed value must fall between Dv3 and Dv2.
  5. If the displayed value is outside the Limit of Error, adjust the trade weight so that it falls between Dv1 and no more than 1 emax "Over Marked Weight" for either Acceptance or In-service LOE.
  6. Once 5 trade weights have been calibrated/verified as above, and again when all the weights of the same nominal value have been calibrated or verified, place the Local Standard on the platform to verify that Dv1 had not changed. If it has changed by more than 1 emax, you must re-establish the values of Dv1, Dv2 and Dv3 and recheck the 5 previous weights.
  7. Repeat for each nominal value of trade weight to be calibrated/verified.
Table of Values of emax for the Verification of Trade Weights (METRIC)
Trade Weight Nominal Value emax
gram(s) milligram(s)
1 1
2 2
5 2
10 5
20 5
50 10
100 10
200 20
500 20
kilogram(s) gram(s)
1 0.05
2 0.1
5 0.2
10 0.5
20 1
Table of Values of emax for the Verification of Trade Weights (AVOIRDUPOIS)
Trade Weight Nominal Value emax
pound(s) ounce(s) pound(s)
  116 0.000 005
  18 0.000 005
0.01   0.000 005
  ¼ 0.000 01
0.02   0.000 01
  ½ 0.000 01
0.05   0.000 02
  1 0.000 02
0.1   0.000 02
  2 0.000 02
0.2   0.000 05
  4 0.000 05
0.5 8 0.000 05
1 16 0.000 1
2   0.000 2
5   0.000 2
10   0.000 5
20   0.001
50   0.002

Note: emax is calculated so that one scale division represents at most 13 of the applicable tolerance for the trade weight to be verified or calibrated.

Revision

Original

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