OIML R51 vs R76: Which Standard Applies to Your Scale?

OIML R 76 and OIML R 51 are the two international recommendations that define how weighing instruments are classified, tested and given maximum permissible errors. Which one applies depends on a single question: does an operator intervene in each weighing, or does the instrument weigh automatically? This article explains the split, compares the accuracy classes and shows how motion at sea is handled. This article is general information, not legal advice: check the requirements for your application with your national fisheries control authority and metrology authority.

What OIML recommendations are, and what they are not

OIML recommendations are model technical requirements that national and regional legislators adopt; they only become binding through law. OIML R 76-1:2006 specifies requirements for non-automatic weighing instruments "that are subject to official metrological control" (clause 1). OIML R 51-1:2006 does the same for automatic catchweighing instruments "subject to national metrological control" (clause 1.1). In the EU the equivalent legal instruments are the NAWI Directive 2014/31/EU and Annex VIII (MI-006) of the MID, Directive 2014/32/EU, explained in NAWI and MID directives explained.

Automatic or non-automatic: the deciding test

A non-automatic weighing instrument is one "requiring the intervention of an operator during weighing" (Article 2(2), Directive 2014/31/EU). An automatic weighing instrument determines mass "without the intervention of an operator" following a predetermined programme, and an automatic catchweigher weighs "pre-assembled discrete loads" or single loads of loose material (Annex VIII definitions, Directive 2014/32/EU).

R 51 splits catchweighers into category X, only for checkweighers checking prepackages under OIML R 87, and category Y for all others, such as weigh-price labellers and instruments weighing single loads of loose material (clause 2.1).

Accuracy classes compared

Both recommendations use the same framework: a verification scale interval, which is the mass step on which accuracy class and permissible errors are based, and the number of such intervals that fit into the maximum capacity. A 30 kg scale with a verification scale interval of 10 g, for example, has 3 000 intervals. The table combines Table 1 of R 51-1, Annex I Table 1 of the NAWI Directive and clause 2.2.2 of R 51-1.

One difference matters for fish graders: for category Y scales "used for grading", R 51 allows a minimum capacity of 5 verification scale intervals (clause 2.2.2), because individual fish can be light relative to the instrument's range.

Maximum permissible errors

For a non-automatic class III instrument, the error on initial verification may not exceed half a verification scale interval for loads up to 500 intervals, one interval for loads up to 2 000 intervals and one and a half intervals for loads up to 10 000 intervals; in service, twice those values apply (NAWI Annex I, points 4.1-4.2). For a category Y(a) catchweigher in automatic operation, R 51 allows one, one and a half and two intervals over the same load bands at initial verification, and one and a half, two and a half and three and a half intervals at in-service inspection (Table 5). Worked example for a 30 kg instrument with a verification scale interval of 10 g (3 000 intervals):

How motion at sea fits into R 76 and R 51

Neither recommendation contains a chapter on ships, but both regulate the effects that vessel motion creates: tilt, disturbance and vibration. The physics are explained in why ordinary scales fail at sea and how motion compensation works.

• Tilt, R 76: for class II-IIII instruments liable to be tilted, the limit is set by a level indicator, by a manufacturer-defined value where an automatic tilt sensor is fitted, or otherwise 50/1000. A tilt sensor may compensate but must blank the display and inhibit printing and data transmission when the limit is exceeded (clause 3.9.1.1).
• Tilt, R 51: instruments not fixed in position must meet requirements at 5 % tilt; instruments mounted in vehicles at 10 %, or a lower value with an automatic tilt-limiting device (clause 2.9.3).
• Stable equilibrium, R 76: printed or stored values may not deviate by more than one verification scale interval from the final value, and during continuous or temporary disturbance the instrument must not print, store, zero or tare (clause 4.4.2).
• Vibration, R 76: where vibration is a normal feature of the environment, the instrument must operate correctly despite it or be protected against it (clause 3.9.5).

For ships specifically, WELMEC Guide 2:2023 section 2.3.1 describes how notified bodies examine NAWIs installed in ships: gravity (acceleration) compensation to at least ±3 m/s² unless the display blanks at a lower value, tested dynamically at no more than 0.3 Hz; tilt tests up to 25 % (15°); dynamic tilt at 0.03-0.3 Hz; a tare test in dynamic mode; and treatment of a compensating tilt sensor as an essential part subject to temperature, humidity and EMC tests. In-service control and periodic verification are left to national rules.

Graders and catchweighers at sea and ashore

A grader that weighs each fish on a belt and routes it to a class without operator action is an automatic catchweigher; see also weight grading of fish. Testing follows R 51 and, in Europe, WELMEC Guide 2.6 on testing automatic catchweighing instruments. Belt speed, product characteristics and rate of operation are part of the rated operating conditions a manufacturer must specify (MID Annex VIII, Chapter I, point 1.4), and means must limit the effect of tilt, loading and rate of operation so that errors stay within limits (point 3.1). The MID's M1-M3 mechanical classes do not apply to automatic weighing instruments; for instruments under special mechanical strain, such as those in vehicles, the manufacturer defines the mechanical conditions (point 1.3).

Choosing the right standard: a short checklist

1. Decide whether an operator intervenes in each weighing. If yes, R 76 and the NAWI Directive; if no, R 51 (or another automatic recommendation) and MID MI-006.
2. Pick the verification scale interval and maximum capacity so that your lightest regulated load is above the minimum capacity and the maximum permissible error at your typical load is small compared with the tolerance you have to meet.
3. For shipboard use, ask whether the type examination covered motion, tilt and gravity compensation as described in WELMEC Guide 2.
4. Confirm with the national authority which approval and verification it requires; see EU weighing rules for catches.

How WPL approaches this

WPL builds operator-driven, motion-compensated bench and platform scales, such as the M2 and M6, for weighing fresh and frozen fish at sea. For which documentation applies to a given model and use, contact WPL and your competent authority. More background is on the legal metrology hub.