OIML R111-1:2004 is the internationally accepted standard for classifying mass reference weights. It defines a total of 9 accuracy classes: E1, E2, F1, F2, M1, M1-2, M2, M2-3 and M3.
Typical Uses of the Classes
- E1: national metrology institutes, the highest accuracy
- E2: accredited calibration laboratories, reference transfer
- F1: precision analytical balance calibration (0.1 mg resolution)
- F2: semi-precision balances (1 mg–10 mg resolution)
- M1: industrial weighing, trade
- M2–M3: heavy industry, coarse weighing
The Rule for Choosing the Right Class
General rule: the maximum permissible error (MPE) of the reference weight must be at least 3 times smaller than the balance’s resolution.
Example: for an analytical balance with 0.1 mg resolution, the weight used at the 100 g point must have an MPE ≤ 0.033 mg. Under OIML R111, the MPE of class F1 at 100 g is ±0.5 mg — which is insufficient. Instead of F1, E2 (±0.16 mg) or E1 (±0.05 mg) should be chosen.
Tests in Balance Calibration
- Repeatability: at least 10 repeated measurements with the same load; the standard deviation is calculated
- Linearity: 5–10 different points, measured in both ascending and descending order
- Eccentric load: measurement with ~1/3 of full capacity at the 4 corners and centre of the pan
- Sensitivity check: a small load is added to and removed from the pan to examine the instrument’s response
Critical warning
Handling weights with bare hands can leave 1–3 mg of contamination on the mass. The use of tweezers or gloves is mandatory for the E1/E2 classes.
Uncertainty Budget
The uncertainty budget in balance calibration includes the uncertainty of the reference weight, balance repeatability, resolution, air buoyancy effect and eccentricity components. The air density correction is especially critical in precision weighing (E1/E2).