Making a kilogram mean the same thing everywhere

Reliable weighing depends on more than a sensor. Unit definitions, reference weights, calibration and measurement uncertainty must be considered together. “Weight standard” may mean a physical reference weight or a technical document governing an instrument. In weight measurement, these are different concepts; no single document covers every weighing application.

The kilogram is a unit of mass

The kilogram (kg) is the SI unit of mass. The definition introduced in 2019 fixes the numerical value of the Planck constant at 6.62607015 × 10⁻³⁴ J·s. The unit no longer depends on the mass of one metal prototype, although physical reference weights remain essential in everyday calibration.

The difference between mass and weight matters here: mass is expressed in kilograms, while weight, a gravitational force, is expressed in newtons (N). In F = m × g, g is gravitational acceleration. For an instrument that senses force and displays kilograms, the effect of gravitational acceleration deserves attention, particularly after relocation or where high accuracy is required.

Reference weights and ordinary objects

A reference weight provides a measurement reference with an established value and uncertainty. A “1 kg” marking alone is insufficient. OIML R 111 addresses the metrological and technical properties of weights in classes E1, E2, F1, F2, M1, M1–2, M2, M2–3 and M3. It covers error limits, materials, surfaces, density and magnetic properties. Select the class to suit the measurement task; one reference weight class is not suitable for every scale.

Traceability: linking a result to a reference

Metrological traceability links a measurement result to a reference through a documented, unbroken chain of calibrations. Each step contributes uncertainty. A working weight at a factory may be linked to a laboratory’s reference, which is linked to a higher-level standard. Traceability enables comparison, but does not itself prove that uncertainty is small enough for your application.

Calibration, adjustment and verification

Calibration establishes the relationship between reference values and instrument indications, with associated uncertainty, under specified conditions. Adjustment changes the instrument’s indications. Verification provides evidence that specified requirements have been met. Calibration need not include adjustment; after adjustment, recalibration may be needed to establish the instrument’s new behaviour.

In a simplified example, a load with a reference value of 1000.00 g produces an indication of 1002.00 g. The indication error is +2.00 g, and the corresponding correction is −2.00 g at that point. Do not extend this correction across the entire capacity without evidence. Different loads and repeated measurements help assess the scale’s measurement behaviour. Simple checks at home cannot replace documented calibration.

Which document covers which part?

OIML R 76 addresses requirements and tests for non-automatic weighing instruments. Max denotes maximum capacity, Min minimum capacity, d the actual scale interval and e the verification scale interval. d and e need not always be equal. Display increments alone do not establish permissible error; classification, load and applicable requirements must be considered together.

OIML R 60 addresses the metrological and technical requirements for load cells. Sensor conformity alone does not establish conformity of the complete weighing system. ISO/IEC 17025 concerns the competence, impartiality and consistent operation of testing and calibration laboratories; it is not a scale product standard. When choosing a laboratory, check that the relevant measurement is within its accreditation scope and assess the uncertainty it offers.

Conditions that affect measurements on site

Vibration, draughts, temperature changes and off-centre loading can affect readings. Air buoyancy also matters in precision weighing; the relationship between density and weighing helps explain it. Checks assess repeatability, indication errors at different loads and the effect of load position. For weighing systems integrated into production, laboratory-like bench conditions alone may not represent actual operation.

Legal verification and production control

Legal metrology applies legal requirements to measurements and measuring instruments. In commercial weighing, obligations for conformity assessment, verification and marking depend on the country, instrument type and application. A calibration certificate does not automatically replace required legal verification. Following an OIML document alone does not establish compliance with local legislation.

For weighing in quality control, define acceptance limits, the working range and check frequency according to process needs. When recording weight data, retaining instrument identification, dates, measurement conditions and relevant calibration information helps evaluate results. Further applications are covered in the weight measurement FAQ.