NABL Scope for Calibration of Weighing Balance – Complete Technical Guide for Accreditation-Seeking Laboratories

Calibration of weighing balances and weighing scales is one of the most critical disciplines under ISO/IEC 17025 accreditation. Laboratories applying for the NABL scope for calibration of weighing balance must demonstrate technical competence, availability of suitable master instruments, appropriate environmental controls, and validated calibration methods.

This comprehensive guide prepared by V-CARE IMPEX, India’s leading manufacturer of E1, E2, F1, F2 and M1 Class Standard Weights, provides a complete technical understanding of the requirements for both high-precision balances and high-capacity industrial scales.

This blog is structured to serve as a single technical reference for new and existing NABL laboratories.

1. Understanding the NABL Scope for Calibration of Weighing Balance

The NABL scope defines:

• The capacity (maximum load) of instruments you intend to calibrate

• The readability (minimum resolution) you can handle

• The uncertainty capability your lab can achieve

• The calibration methods used (direct loading, substitution, distributed loading etc.)

• The range and class of standard weights available in your lab

NABL assessors evaluate:

• Availability of correct master weights

• Environment measuring instruments

• Documented calibration procedures (SOPs)

• Uncertainty Budget for each calibration type

• Traceability of all standards

• Implementation of ISO/IEC 17025:2017 requirements

2. Standard Weights Required for Different Types of Balances

Different balances require different classes of weights as per OIML R111, based on their readability and accuracy class.

The weight class must always be better than the device under calibration to ensure reliable uncertainty.

2.1 Micro Balance (0.1–1 µg Readability)

Weight Class Required: E1

Microbalances have extremely low uncertainties and require the strictest class of weights.

Typical E1 Fractional Set Includes:

• 1 mg

• 2 mg

• 5 mg

• 10 mg

• 20 mg

• 50 mg

• 100 mg

Why E1?

• Ultra-low tolerance

• Non-magnetic

• Very polished surfaces

• Suitable for air buoyancy correction

Environmental Requirements:

• Temperature stability ±1°C

• Dust-free, vibration-free environment

• Cleanroom recommended

2.2 Semi-Micro Balance (0.01 mg Readability)

Weight Class Required: E2 (ideal) or F1 (permissible)

Typical Set Required:

• 1 mg to 200 g

Semi-micro balances require tight mass tolerances; E2 is recommended for national-level accuracy.

2.3 Analytical Balance (0.1 mg Readability)

Weight Class Required: F1 (standard) or E2 (if required)

Common Weight Range:

• 1 mg to 200 g or up to 500 g depending on balance capacity

Applications:

• Pharmaceuticals

• Chemical analysis

• R&D and QC labs

2.4 Precision Balance (1 mg – 10 mg Readability)

Weight Class Required: F1 or F2

Typical Range:

• 1 g to 1 kg / 2 kg

Used for routine laboratory weighing, production QA, and chemical testing.

2.5 Bench Scale (0.1 g – 1 g Readability)

Weight Class Required: F2 / M1

Typical Range:

• 1 kg to 20 kg

These weights are commonly cylindrical SS weights.

3. Standard Weights Required for High-Capacity Platform Scales (3T, 5T, 10T)

A common myth in India is that NABL requires purchasing full capacity weights (e.g., 3000 kg, 5000 kg, 10000 kg).NABL does not require this.

Reasons:

• Most platforms cannot support stacking such a large quantity of weights

• Labs usually have limited storage and no lifting equipment

• Cost of large weights is extremely high

• Safety risks increase significantly

Therefore, NABL permits alternative scientific calibration methods, which are widely used and acceptable across the industry.

3.1 Substitution Method (Most Used in India)

This is the primary NABL-accepted method for high-capacity scales.

Process:

1. Apply your available standard weights (for example, 20 kg × 20 pcs = 400 kg)

2. Take the reading

3. Replace this load with customer’s material (bags, containers, drums etc.)

4. Repeat this process until the entire capacity is covered

Why NABL accepts this method:

• Traceability is maintained

• Incremental loads are scientifically valid

• Full capacity can be simulated in cycles

3.2 Cyclic Build-Up Method

A controlled method where:

• 200–400 kg of weights are loaded

• Reading is noted

• Unloaded

• Cycle is repeated until total theoretical load is achieved

This demonstrates:

• Linearity

• Hysteresis

• Repeatability

3.3 Distributed Loading (5-Point Method)

Used when platform area is small.

Load limited weights at:

• Center

• 4 corners

This checks eccentricity and corner error.

3.4 Using Client’s Material (Substitution Support)

Widely used for:

• Warehouses

• Packaging factories

• Chemical processing units

• Logistics

Customer’s material acts as additional load after verifying 10–20% with standard weights.

3.5 Practical Standard Weight Quantities for High-Capacity Calibration

Required NABL Scope	Practical Weight Recommendation	Notes
3T Platform Scale	20 kg × 20–25 pcs	Substitution method
5T Platform Scale	20 kg × 25–30 pcs	Substitution + cyclic
10T Platform Scale	20 kg × 30–40 pcs	Substitution + client load
10T–60T Weighbridge	20 kg × 40–50 pcs	Standard approach

This is realistic, economical and NABL-approved.

4. Environmental Monitoring – Mandatory for NABL Compliance

To perform calibration correctly, environmental conditions must be measured accurately.

Required Instruments:

• Calibrated thermometer

• Hygrometer

• Barometer

• Air flow measurement (for sensitive instruments)

• Data logger (recommended)

Why?

Environmental parameters are required for air buoyancy correction, which affects accurate mass measurement.

5. Understanding Measurement Uncertainty (Simplified Explanation)

Uncertainty in weighing calibration arises from:

• Balance readability

• Standard weight tolerance

• Air buoyancy

• Environmental variations

• Repeatability & hysteresis of the device

• Drift in reference standards

NABL requires laboratories to prepare uncertainty budgets for all types of weighing instruments.

Key formulae follow OIML R76 standards but can be simplified into:

Total Uncertainty = √(Sum of all component uncertainties²)

NABL assessors ensure:

• Weight class supports the uncertainty claim

• Test points are adequate

• Environmental corrections are applied

6. Required Documentation for NABL Scope

Labs must maintain:

A. Technical Documents

• Calibration procedures / SOPs

• Uncertainty calculations

• List of master instruments

• Calibration certificates (NABL traceable)

• Environmental monitoring SOPs

B. Quality System Documents

• Internal audits

• Management reviews

• Corrective actions

• Equipment maintenance logs

• Training records

C. Scope Documentation

• Capacity & readability

• Calibration method chosen

• Uncertainty capability

7. Common Mistakes Labs Make (And How to Avoid Them)

1. Buying wrong class of weights

Example: Using F2 weights for analytical balance → Not acceptable.

2. Thinking full capacity weights are mandatory

This leads to unnecessary cost — avoid.

3. Not maintaining environmental conditions

Temperature fluctuations increase uncertainty.

4. Poor documentation during calibration

NABL audits require traceability and record-keeping.

5. Incorrect handling of weights

Leads to magnetization or surface damage.

8. Calibration Frequency Recommendations

Weight Class	Recalibration Frequency
E1	Every 3 years
E2 to F2	Every 2 years
M1 & M2	Every 1 year
20 kg weights used for field work	1 year (preferred)

Balances should ideally be calibrated:

• Micro / Analytical → Quarterly

• Precision → Semi-annually

• Industrial → Annually

9. Why Choose V-CARE IMPEX for NABL Labs?

V-CARE IMPEX offers:

• Complete range of E1, E2, F1, F2, M1 Standard Weights

• Fractional weight sets for micro & analytical balances

• 20 kg stainless steel weights for industrial calibration

• NABL traceable certificates

• Master balances & accessories

• Guidance on selection of weight class

• Support in framing calibration methods (Substitution, Cyclic, Distributed Loading)

We are trusted suppliers to laboratories across India, Middle East and Asia.

Conclusion

Getting the NABL scope for calibration of weighing balance becomes straightforward once the correct weight classes, calibration methods and documentation structure are understood. You do not need full capacity weights for high-capacity scales — NABL accepts substitution, build-up and distributed loading methods. For high-precision balances, correct E1, E2 and F1 classes are essential. With proper uncertainty budgeting, environmental control and master instruments, any lab can confidently qualify for NABL accreditation.

V-CARE IMPEX is committed to supporting laboratories by supplying world-class standard weights, precision equipment and professional technical guidance.

📌 Disclaimer

The information provided in this blog is based on general industry practices, publicly available NABL guidelines, and professional experience. It is intended solely for educational and informational purposes.This content should NOT be treated as an official, complete or final reference (“not a bible”) for NABL accreditation.

NABL requirements may change from time to time, and actual assessments may vary depending on:

• The assessor’s judgment

• Your laboratory’s scope

• Equipment condition

• Documentation quality

• Uncertainty budget

• Current NABL policy/updates

Readers are strongly advised to:

👉 Verify all technical requirements directly from NABL documents (NABL-100, NABL-102, NABL-141, ISO/IEC 17025:2017) and relevant OIML standards.

👉 Consult a qualified NABL consultant or assessor before making investments or scope decisions.

V-CARE IMPEX is not liable for any decisions, actions, or interpretations made solely based on this blog.