TS – 007 Eccentricity test on weighing instruments

Scope

To detail the test procedures to be used when:

  • Performing an eccentricity test
    • Using Working Standards of mass
    • Using unknown loads
  • Performing a rolling load test

Reference documents

Policy

Trading Standards provides the following instructions on how to complete the eccentricity test on a weighing instrument.

Determining test loads for all capacities of weighing instruments

Test loads for performing an eccentricity test are calculated using the formula in Regulation 60. Depending on the capacity of the instrument and the number of support points the formulae can generate a value for the load that may not be achievable.

Trading Standards permits any calculated test load to be rounded down by the minimum capacity of the instrument under test.

Table 1

Class of instrumentPermitted amount to round down
Class II ≤ 200 e
Class III and Class IIII ≤ 20 e

Eccentricity test

An eccentricity test is performed on a weighing instrument during both verification and inspection tests.

Using Working Standards of mass

When performing an eccentricity test there is no requirement to place the test load in the centre of the load receptor. The test is conducted using the procedure outlined in Regulation 60 of the Weights and Measures Regulations 1999.

Regulation 60 - Eccentricity test - Weights and Measures Regulations 1999(external link)

  • Eccentricity Test where the number of support points (n) is ≤ 4

The test load is calculated by using the formula: L = 1/3rd maximum capacity

A diagram of a weighing pan divided into the four equal parts and four test points marked with a plus sign

  • Eccentricity Test where the number of support points (n) is > 4

The load is calculated by using the formula: L = max / n-1, where n = the number of support points.

A diagram of a weighing pan divided into six equal parts and six test points marked with a plus sign

Eccentricity test using an unknown load

Using a substitution load (L sub)

  • Determine the load (L) to be applied for the eccentricity test using the formulas:
    • L = 1/3rd max where the number of support points is  4
    • L = max / n-1 for instruments where the number of support points is > 4
  • Select a substitution load (normally a forklift and substitution weight)
  • Zero the instrument
  • Place standard weights on the instrument ≤ 20 e greater than the substitution load and allow the indication to stabilise; record this load (L)
  • Apply delta loads (∆L) to the instrument until the indication changes up and stabilises
  • Record the additional delta loads (∆L) and the indication (I)
  • Calculate the error in the weighbridge (E) for the load applied (L) using
    • E = I + 0.5e - ∆L – L
  • Remove all weights , for digital instruments leave 10e on the load receptor if AZT is active
  • Place the forklift with the substitution load onto the load receptor as close as possible to the footprint of where the standard weights where placed , remove the 10e if used
  • Record the indication for the substitution load (I sub)
  • Add delta loads (∆L1) until the indication changes up and stabilizes , leave the delta loads that were applied with the substitution load
  • Calculate the actual load (L sub) of the substitution load using
    • L sub = I sub + 0.5e – E
  • Round the true value of L sub , to a whole scale interval L sub (rounded), by applying or removing additional delta loads ( ∆L2 ), keeping the weights with the vehicle (forklift)
  • Reposition the entire eccentric load ( forklift + substitution load + ∆L1 + ∆L2 + any additional standards weights) to each load support point and record the indications ‘P’
  • Determine if the instrument passes or fails. To pass, each indication must be within the MPE of the applied load L sub (rounded) depending on whether the test is a Verification or Inspection

Rolling load test

A rolling load test must be performed on a verification or inspection test on a weighing instrument that is used to weigh rolling loads.
A load equivalent to between:

  • ≤ 80% of maximum capacity, and
  • ≥ of half of maximum capacity - 10%,

must be applied at the six different positions on the centre line of the load receptor in both directions as indicated in Diagram 1.

Diagram 1

A diagram of a weighing pan marked with six arrows and six test points to illustrate a rolling load test

Test procedure

  • The test load must be given an actual value to perform the test and to determine the readings at the six positions indicated in Diagram 1.
  • The intrinsic weighing error of the IUT needs to be determined at the 1500 e or 2000 e MPE test points, or some other tested point that is ≤ - 10% of half max but ≤ 0.8 max.
  • A suitable rolling substitution load that is -10%, but not greater than the test point selected, can then be weighed and the actual value determined by applying the intrinsic error correction from the weighbridge.
  • The substitution load is then placed in the positions on the load receptor as indicated in diagram 1 and the “P” values of the indications calculated.
  • The MPE is then applied to the indications from the IUT using the actual value of the substitution load as the reference.
  • Determine if the instrument passes or fails. To pass, each indication must be within the MPE of the applied load depending on whether the test is a Verification or Inspection.

Trade Measurement — Technical Policy

Policy number:TS - 007
Effective date: July 2016
Version number: 2
Policy name: Eccentricity test on weighing instruments
Reviewed by: Davis White, Legal Metrology Advisor and Senior Trading Standards Officer
Approved by: Phil Sorrell, Team Leader Compliance and Inspection