⚖️ Balance Calibration and Verification

Metrology PN-EN 45501 / OIML R 76

Verification of non-automatic weighing instruments with determination of indication errors, repeatability, eccentricity and minimum sample weight. Foundation of measurement traceability in every laboratory.

Overview

Calibration (verification) of non-automatic weighing instruments is a fundamental metrological procedure ensuring measurement traceability of weighing results in the laboratory. Standard PN-EN 45501 defines the maximum permissible errors for balances in individual accuracy classes (I — special, II — high, III — medium, IIII — ordinary), and recommendation OIML R 76 defines construction and metrological requirements for non-automatic weighing instruments.

Calibration includes determination of indication errors at selected points of the weighing range using mass standards of appropriate class (E1, E2, F1, F2 according to OIML R 111), repeatability assessment, eccentricity (off-center loading) test, and determination of minimum sample weight with specified uncertainty. The procedure is performed under ambient conditions close to the normal operating conditions of the balance.

Every accredited testing and calibration laboratory is obliged to periodically calibrate balances — this is a requirement of the ISO/IEC 17025 standard. The calibration certificate contains measurement results with expanded uncertainty, allowing the user to assess the suitability of the balance for intended measurements.

In Poland, balance calibration services are provided by laboratories accredited by PCA (Polish Center for Accreditation), including RADWAG Laboratory (AP 069), Matbor, TOPS and others. Calibration is performed both at the laboratory premises and at the customer's site (in situ).

Method principle

Balance calibration consists of comparing the balance indications with the conventional values of reference standard masses under specified environmental conditions. The indication error E = I − m_ref (where I — balance indication, m_ref — conventional mass of the standard) is determined at at least 5 points of the weighing range. Repeatability is assessed as the standard deviation of a series of at least 10 weighings of the same standard. Eccentricity is tested by placing the standard at different positions on the pan. Minimum sample weight is determined based on repeatability and required uncertainty (usually U ≤ 1% of the weight).

Applications

Calibration of analytical balances (d = 0.01–0.1 mg) in chemical laboratories
Calibration of precision balances (d = 1–10 mg) in pharmaceutical production
Legalization of commercial balances according to directive MID 2014/31/EU
Determination of minimum sample weight for pharmacy preparations
Confirmation of measurement traceability according to ISO/IEC 17025
Control of balances in GLP/GMP laboratories

Key parameters

Parameter	Value
Balance accuracy classes	I (special), II (high), III (medium), IIII (ordinary)
Mass standard classes	E1, E2, F1, F2, M1 according to OIML R 111
Calibration points	Min. 5 points in the weighing range (including near Min and Max)
Repeatability	≥ 10 measurements at ~50% Max and ~100% Max
Environmental conditions	18–25°C, stability ±1°C/h, humidity 40–60%
Expanded uncertainty	k = 2, confidence level ~95%

Standard

Standard number: PN-EN 45501:2015-03 / OIML R 76-1:2006
Title (PL): Zagadnienia metrologiczne wag nieautomatycznych / Wagi nieautomatyczne
Title (EN): Metrological aspects of non-automatic weighing instruments / Non-automatic weighing instruments

Step-by-step procedure

1. Acclimatization

Acclimatize mass standards and balance in the calibration room for min. 12 h. Record environmental conditions (T, RH, pressure).

⏱ Time: ≥ 12 h • 🌡 Temperature: 18–25°C

2. Balance inspection

Check pan cleanliness, leveling (spirit level), absence of mechanical damage. Turn on the balance min. 30 min before measurements.

⏱ Time: 30 min

3. Internal adjustment

Perform internal adjustment (calibration) of the balance according to manufacturer's instructions. For balances without internal adjustment — external adjustment with standard.

4. Repeatability test

Perform min. 10 consecutive weighings of the same standard (~50% Max). Calculate standard deviation. Repeat at ~100% Max.

⏱ Time: 15–20 min

5. Eccentricity test

Place the standard (~1/3 Max) successively in the center and 4 corners of the pan. Record indications for each position.

⏱ Time: 10 min

6. Determination of indication errors

Load the balance with standards at min. 5 points of the range (e.g. 10%, 25%, 50%, 75%, 100% Max). For each point record the indication and calculate error E.

⏱ Time: 20–30 min

7. Measurement in near-zero range

Examine indication stability at small loads — determine minimum sample weight based on repeatability and required uncertainty.

8. Uncertainty calculation

Determine uncertainty budget considering: repeatability, resolution, mass standard error, environmental conditions (buoyancy correction), eccentricity.

9. Conformity assessment

Compare results with user requirements (MPE, minimum sample weight). Apply decision rule according to ILAC-G8:09/2019.

10. Documentation

Prepare calibration certificate containing: results, uncertainties, environmental conditions, standard identification, next calibration date.

Required equipment and apparatus

Equipment	Example	Indicative price
Analytical balance class I	Mettler Toledo XPR205, RADWAG XA 82/220.5Y, Sartorius Cubis II	15 000–80 000 PLN
Mass standards class E2/F1	Set 1 mg – 500 g, RADWAG, Mettler Toledo, Häfner	3 000–25 000 PLN (set)
Thermohygrometer	Testo 176 H1, Vaisala HMP110, Rotronic HC2A	1 500–5 000 PLN
Anti-vibration table	RADWAG SAL/STONE, Mettler Toledo	5 000–15 000 PLN
Barometer	Vaisala PTB330, Druck DPI 142	3 000–8 000 PLN

Reagents, media and consumables

Reagent	CAS	Details
Mass standards class E2	—	Set of steel weights, calibration certificate, traceability to the national standard at GUM (Central Office of Measures)
Mass standards class F1	—	For calibration of precision balances class II, stainless steel, with certificate
Antistatic agent	—	Air ionizer to eliminate electrostatic charges on the pan and weights

Health and safety (OHS)

Handle mass standards only with tweezers or gloves — sweat from hands changes the mass
Anti-vibration table — do not lean on it, do not cause vibrations during measurements
Avoid drafts and temperature changes in the room (closed windows and doors)

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