Understanding Accuracy Classes in Legal Metrology – EU Directive 2014/31

Legal metrology is essential for maintaining fairness, transparency, and accuracy in trade and commerce. It encompasses the regulatory requirements for measuring instruments and measurements, ensuring that consumers and businesses alike can trust the accuracy of the devices used in transactions. One of the key aspects of legal metrology is the concept of accuracy classes, which define the precision and permissible errors of measuring instruments.

What are accuracy classes?

Accuracy classes are a way to categorize measuring instruments based on their precision and the maximum permissible errors (MPEs) they are allowed to have under specific conditions. These classes ensure that instruments used in trade, such as weighing scales, fuel dispensers, or electricity meters, meet consistent standards of accuracy.

Accuracy classes contribute to the categorisation of scales in many ways:

• They are used to determine the suitability of the balance for certain applications.
• They define the tolerance limits that are applied when testing the balance.

The different accuracy classes of non-automatic weighing instruments

In the EU Directive 2014/31, so-called accuracy classes are defined. The following mass and weight units are permitted:

• SI units: Kilogram, microgram, milligram, gram and tonne.
• British units: Troy ounce (when weighing precious metals).
• Other units: Carat (when weighing precious stones).

For a scale to be assigned to a specific accuracy class, it must meet certain criteria. Classification is based on two aspects:

1. The number of verification scale intervals(n): This refers to the ratio of the capacity of the scale to its smallest representable unit, i.e. resolution.
2. The verification scale interval (e): This is the smallest weight interval that can be used in weight-based price determination with a particular scale in a trading context.

What is the verification scale interval (e)?

For weighing instruments, the verification scale interval (e) refers to the smallest legally approved and technically possible scale interval. In other words, it determines the smallest weight that the scale can reliably measure.

The verification scale interval (e) is given in the same unit of measurement as the weight value (for example, grams, kilograms, i.e. the SI units mentioned above). A scale with a verification scale interval (e) of 1 gram would, for example, round weights to the nearest whole gram. A scale with a verifitication value of 0.01 grams, on the other hand, could measure weights to the nearest hundredth of a gram.

Accuracy classes: A decisive categorisation for non-automatic weighing instruments

The following accuracy classes – related to the verification scale interval (e) – have been defined by the legislaton depending on the requirements:

1. Class I: Special These scales are particularly precise and are suitable for accurate measurements in laboratories or special applications. The verification scale interval (e), i.e. the smallest unit of weight that the balance can measure, is at least 0.001 grams.
2. Class II: High. These scales have a slightly lower accuracy than class I, but are still precise.
3. Class III: Medium. These scales are used in commercial environments such as shops or supermarkets. They are less precise than class II, but still sufficiently accurate for commercial use.
4. Class IIII: Ordinary. These scales are used for coarse measurements where high accuracy is not required. The verification scale interval (e) is at least 5 grams.

The following table shows the accuracy classes with the number of scale intervals (n) and the verification scale interval (e).

The accuracy classes illustrated with an example

Accuracy class I

Analytical balances are a special type of balance usually used in laboratories or medical facilities. It is used to measure the mass or weight of substances or objects with high accuracy. Analytical balances are designed to take very precise measurements and can often be accurate to several decimal places.

If we want to calculate the number of scale intervals (n), then the weighing range of 120 grams here corresponds to the capacity and the readability remains at 0.0001 grams.

With the formula n = ((Max)/(e)) = 120/0.0001 we get the result of 1 200 000. Thus this scale belongs to accuracy class 1.

Accuracy class II

A precision balance is a measuring instrument used to measure the weight or mass of an object or substance with high accuracy. Precision scales are often used in laboratories and industrial environments where accurate measurements are critical.

Using the formula n = ((Max)/(e)) = 500/0.001, we get the result of 500.00. Thus, this scale belongs to accuracy class 2.

Accuracy Class III

Accuracy Class III scales are commonly used for commercial weighing of products such as precious metals, semi-precious stones, animals, laundry, mail, and vehicles. These scales are designed to handle a wide variety of applications, including general commercial uses that require moderate precision.

For scales in this class, the readability (e) typically falls between 0.1 g and 2 g, and the scale’s capacity can range accordingly.

If we want to calculate the number of scale intervals (n), for example, for a scale with a maximum capacity of 3000 grams and a readability of 0.1 grams, we can use the formula:

With the formula n = ((Max)/(e)) = 3000/0.1, we get the result of 30,000. However, due to legal restrictions, the number of scale intervals is limited to 10,000. Therefore, this scale belongs to Accuracy Class III.

For higher capacities and heavier loads (e.g., livestock, vehicle scales), if the readability (e) is 1 gram, we would calculate the number of intervals as follows:

Using the formula n = ((Max)/(e)) = 5000/1, we get the result of 5,000. This still meets the specifications for Accuracy Class III.

Accuracy Class IV

Accuracy Class IV scales are commonly used for weight checks in road traffic, such as vehicle scales. These scales are typically used for weighing vehicles, including trucks and large machinery, where precise measurements are important, but the highest accuracy is not required.

For scales in this class, the readability (e) typically ranges from 5 g to 0.1 g, with the minimum value for the number of scale intervals (n) being 100 and the maximum being 1,000.

Example with a capacity of 10,000 g (10 kg) and readability of 20 g:

Using the formula n = ((Max)/(e)) = 10,000 / 20, we get the result of 500. Therefore, this scale belongs to Accuracy Class IV.

Example with a capacity of 10,000 g (10 kg) and readability of 50 g:

Using the formula n = ((Max)/(e)) = 10,000 / 50, we get the result of 200. This scale also belongs to Accuracy Class IV, as the value for n falls within the allowed range of 100 to 1,000.

Example with a capacity of 20,000 g (20 kg) and readability of 100 g:

Using the formula n = ((Max)/(e)) = 20,000 / 100, we get the result of 200. This scale would also belong to Accuracy Class IV.