Verifying Metrological Characteristics in Balances & Scales

Mass standards and weights are used to verify the metrological characteristics of balances, scales and mass comparators. Specified mass, measurement uncertainty and traceability feature in mass standards, while weights have suitable accuracy class. Section 3.7 of EN 45501 'Non-automatic weighing instruments' standard dictates that the requirements of OIML R 111-1 recommendation must be fulfilled by the mass standards. Section 5.2 specifies the essential requirements. Expanded uncertainty:

For each weight, the expanded uncertainty, U, for k = 2, of the conventional mass, no more than one-third of the maximum permissible error in Table 1 [15].

The readability of the tested object needs to be taken into account first when selecting weights for testing. The dimensions, shape, material, surface smoothness and maximum permissible error all define the weight, due to legal requirements. The maximum permissible error of weight, if the accuracy class is known, is less than the value given in table 1, OIML R111-1. In the case of very accurate measurements, the possibility of using such a weight is complicated when its conventional mass is not known.

Calibration is a determination of a weights real mass and uncertainty. Currently, when the weight is subjected to calibration, it becomes a mass standard. Therefore, two measuring instruments can be used simultaneously if they feature the same design.

Set of weights from 1 mg to 200 g

Figure 1. Set of weights from 1 mg to 200 g. Image Credit: Radwag Balances & Scales

Weight and mass standard basic differences.

Figure 2. Weight and mass standard basic differences. Image Credit: Radwag Balances & Scales

Summary

To define, carry out, maintain or reproduce a unit of mass, measuring equipment called mass standards is used. These can be of any shape and any material, which ensures the stability of mass over time. An identification and calibration certificate must be available for mass standards providing details on traceability and estimated measurement uncertainty. Measurement uncertainty estimated during the calibration process forms the basis of the elementary classification for mass standards. Mass standards cannot be used as weights within the meaning of legal metrology.

The units of measure are weights, determined by the following documents: OIML R111-1 and ASTM E617. Previously, weighing instruments of I and II accuracy classes were also adjusted using weights. Metrological requirements for weights, in terms of compulsory verification worldwide, have been specified by the International Legal Metrology Organisation. OIML R111 (2004) refers to weights ranging from 1 mg to 50 kg. The requirements are specified in terms of accuracy classes, material, shape, identification and protection.

A hierarchical proportion 1:3 is used to determine the accuracy classes E1 up to M3, where E1 is the highest class and M3 is the lowest. Supervision over measuring equipment has to be carried out as periodic tests and/or calibration with the use of measuring equipment, mass standards and weights (section 7.1.5.2. of ISO 9001). The results of such tests have to be documented.

Bibliography

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[5] Guidelines on the Calibration of Non-Automatic Weighing Instruments EURAMET/cg-18/v.02, January 2009.

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[14] OIML D 28, “Conventional value of the result of weighing in air” Edition 2004 (E).

[15] OIML R 111-1, “Weights of classes E1, E2, F1, F2, M1, M1–2, M2, M2–3 and M3, Part 1: Metrological and technical requirements”.

[16] Pan Sheau-shi, Lu H., Chang C., “Instruments used for measuring the magnetic properties of one kilogram mass standard in Center for Measurement Standards (CMS)”, TC3 IMEKO’2005 Proceedings, pp. 1-7.

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Last updated: Nov 29, 2019 at 9:53 AM

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