When a retailer weighs out his product in front of the customer, can the customer trust the scales? It is usually up to local government to certify the accuracy of such scales, though in recent years, certification of measuring devices within much of the world has been privatised with various countries’ national laboratories overseeing the certification process.

With the advent of digital weighing devices, the increased sensitivity of devices has become an issue as has the threat of multiplicity of differing local regulations. To promote international harmony, the International Organization of Legal Metrology (OIML) has introduced various recommendations which member states can adopt into their legal structures. The EU was in the forefront of adopting these recommendations and when the United Kingdom left the EU, these rules remained in place. The three recommendations that are of interest to non-automatic weighing devices (i.e. weighing devices that you see at food and vegetable markets, at the deli counter, in the doctor’s surgery, at the Post Office etc) are:

• R 52 – Hexagonal Weights – metrological and technical requirements
• R 76 – Non-automatic weighing instruments
• R 111 – Weights of classes E1, E2, F1, F3, M1, M1-2, M2, M2-3 and M3

Recommendation R 52 – Hexagonal Weights

Recommendation R 52 was the first of these recommendations to be published. It defined the physical sizes of cast-iron weights that are sometimes seen in markets. The recommended sizes are 100 g, 200 g, 500 g, 1 kg, 2 kg, 5 kg, 10 kg, 20 kg and 50 kg. Such weights are made of cast iron and have a hollow in their base. This hollow is partially filled with lead to bring it up to the required weight. Once this is done, the lead plug is stamped by an auditing organisation to certify its weight. The allowable tolerance is ±0.05% apart from the 100 g weight where a tolerance of ±0.1% is allowed.

Typical hexagonal 2 kg weight with a credit card to show its relative size.

The same 2 kg weight showing the assayer’s mark on the lead plug.

In view of recommendation R 111, there have been moves to deprecate this standard but a counter argument that these weights are easy to manufacture and are often still used in third world markets has prevailed.

Recommendation R 76 – Non-automatic weighing instruments

Recommendation R 76 covers almost all weighing devices that display the weight in question and covers anything between jeweller’s balances and industrial weighbridges.

The OIML originally categorised weighing devices in one of four classes defined by the allowable tolerance of the device, but in recent years have expanded Class III, when it is used for weights above 4 tonnes, has been split into two classes – Class III and Class III L.

All classes must meet the following criteria:

• For a weight to be valid on the device, it must be equal to or exceed the minimum number of divisions with each division usually being equal to one increment.
• The maximum range of the device must be equal to or exceed the value given in the column “Maximum number of divisions.”
• The device must be certified by an accredited certification agency, both when it is new and at regular intervals as specified in the manufacturer’s documentation.
• Other criteria also apply, which are discussed later.

Since the recommendation covers such a wide range of devices the recommendation uses the symbol “e” to represent the minimum increment on the scale. To put this into perspective, consider two different weighing devices that I own. One is a jeweller’s scale which has a capacity of 200 g in 0.01 g increments. The value of “e” for this device is 0.01 g and the maximum value is equivalent to 20 000 times the basic increment. My bathroom scales on the other hand have a different value for “e.” In the range 50 kg to 100 kg, the display is rounded to the nearest 0.2 kg, so on this device, “e” has a value of 0.2 kg thus at 100 kg, the display is 500 times the basic increment.

The classes are defined as follows:

Class	Minimum number of divisions at minimum weight	Minimum number of divisions at maximum weight	Maximum tolerance at maximum weight	Applications
I	50 000	Not specified	Not specified	High precision laboratory work
II	100 (Note 1) 3 000	100 000	±0.001%	Laboratory work and transactions of high value goods (such as jewellery)
III	100 (Note 2) 500	10 000	±0.01%	Most commercial and medical transactions
III L	2 000 (Note 3)	10 000	±0.01%	Weighbridges etc (Weights above 4 tonnes)
IIII	100	1 200	±0.08%	Portable devices used in medical clinics etc

Note 1: e <= 50 mg

Note 2: e <= 2 g

Note 3: e >= 2 kg

Outline definitions of the various classes of weighing devices

The recommendation runs to almost 100 pages as it covers a variety of weighing devices, both analogue and digital including yardarm balances, point of sale weighting devices, medical scales etc. The points that the recommendation covers include:

• Default temperature ranges of operation which varies by class. If the device is designed to operate in extreme heat or extreme cold, the ranges should be catalogued and certification tests planned accordingly.
• If a device has multiple ranges, then each range should be tested separately. For example, my bathroom scale has 0.1 kg increments for weights below 50 kg, 0.2 kg increments for weights between 50 and 100 kg and 0.5 kg increments for weights above 100 kg – this being typical of multiple-range devices.
• The default range of permitted power fluctuations is defined. Operational use must not be affected if the electrical power fluctuates within the specified limits. If the power fails or is outside limit, the device should either turn itself off or display an error message.
• Specification of printouts and displays.
• If the device has a dial and pointer the recommendation defines the spacing of graduation marks.
• The acceptable accuracy of weights used during certification and must be “e/3”.

The above list is a fraction of the criteria that are listed in the recommendation. The interested reader is referred to the recommendation itself.

Recommendation R 111 – Weights of varying classes

Recommendation R 111 covers three principal categories of weights – those used as references for the calibration of other weights, those use for the calibration of weighing devices and those used for everyday purposes in conjunction with a weighing device. The recommendation is limited to weights that are greater than or equal to 1 mg and less than or equal to 5 tonnes.

The standard defines nine classes of weights known as E1, E2, F1, F2, M1, M2, M3, M1-2 and M2-3 (in descending order of accuracy). Three classes of weights are defined for calibration purposes of weighing devices of Classes I, II and III respectively and the remainder are intended either for everyday use for scales of Classes I, II, III, III L and IIII or for calibration purposes of Class IIII devices.

Weights of Class E1 are designed to be calibrated against the national prototype kilogram and to serve as working copies for calibration of other weights. A certificate of traceability against the national prototype accompanies such Class E1 weights. For Class E1 weights of 100 g and above, the permitted tolerance of ±0.5 parts per million. At the other end of the scale, the tolerance weights of class M3 having a nominal weight of 100 g or more is 0.05%. Below 100 g, the tolerance level for all classes of weight is relaxed due to the difficulty of manufacturing of such weights.

Most of the recommendation is devoted to detailed rules concerning the physical construction of the weights, the environment in which they are to be used and the way in which tests are to be carried out. Of note is that the calibration of the weights assume that they will be used in air and that the buoyancy effects need to be considered. The recommendation states that the weight should be accurate when used in air with a density of 1.2 kg/m³ against a target object having a density of 8000 kg/m³. (The density of water is approximately 1000 kg/m³).

Other parts of the recommendation deal with the way in which the calibration is to be recorded, the requirement that the apparatus reaches thermal stability before performing the test and ensuring that magnetic fields are either absent, are negligible or that there is compensation for them.

The interested reader is referred to the recommendation for further details.

Set of weights, range 10 mg – 100 g used for laboratory work.

References:

• R 52 Hexagonal Weights – https://www.oiml.org/en/files/pdf_r/r052-e04.pdf
• R 76 – Non-automatic weighing machines – https://www.oiml.org/en/files/pdf_r/r076-1-e06.pdf
• R 76 explained (plus US Customary extensions): https://adamequipment.com/content/post/what-are-scale-balance-accuracy-classes-divisions/
• R 111 Weights of Various Classes – https://www.oiml.org/en/publications/recommendations/en/files/pdf_r/r111-p-e04.pdf