Weighbridge Calibration in Nigeria: Requirements, Process, and How Often You Need It

Your weighbridge might look like it's working perfectly. The display shows a number. Trucks drive on and off. But without proper calibration, that number could be wrong — costing you millions in product losses or exposing you to regulatory penalties.

Calibration is not paperwork. It's the process that guarantees your scale is accurate. This guide explains what it involves, what the law requires in Nigeria, and how to ensure you're compliant.

What Is Weighbridge Calibration?

Calibration is the process of testing and adjusting a weighing instrument against known reference standards to ensure it measures accurately.

For weighbridges, this means:

1. Applying certified test weights of known mass to the scale
2. Comparing what the scale reads against what it should read
3. Adjusting the electronics until the readings are within acceptable tolerances
4. Issuing a calibration certificate confirming the scale meets required accuracy standards

A calibration certificate is your proof — to regulators, customers, and suppliers — that your measurements are reliable.

Why Calibration Drifts

A newly installed weighbridge doesn't stay accurate forever. Several factors cause readings to drift over time:

Temperature changes — Load cells use strain gauges that are sensitive to temperature. As ambient temperature changes seasonally, output can shift slightly.

Mechanical wear — The steel deck, load cell mounting hardware, and junction box connections experience wear under constant loading and vibration.

Overloading — If trucks consistently exceed the scale's rated capacity, load cells can be permanently deformed, causing permanent inaccuracy.

Electrical interference — Lightning strikes, generator surges, and nearby heavy electrical equipment can damage load cell electronics or junction box components.

Settling — Newly installed weighbridges often experience some foundation settling in the first few months, particularly with pit-mounted designs.

Environmental damage — Dust ingress, water ingress (especially during Nigeria's rainy season), and corrosion degrade electronic components.

The Calibration Process Step by Step

Pre-Calibration Inspection

Before applying test weights, a qualified technician should inspect:

• Platform condition — looking for damage, debris under the deck, and structural issues
• Load cell connections — checking cable integrity and terminal security
• Junction box — inspecting for moisture, corrosion, and correct wiring
• Weight indicator — testing display, printer, and software functions

Any mechanical issues must be resolved before calibration begins. Calibrating a damaged scale only masks the underlying problem.

Zero Test

With the platform completely empty and clear, the indicator should read exactly 0.00 kg (or 0.000 tonnes). If it doesn't, the zero is adjusted electronically.

A zero that won't hold (continuously drifting) indicates a mechanical problem, typically:

• Debris or material stuck under the deck
• A damaged load cell
• Loose platform components rubbing against fixed structure

Span Calibration

Certified test weights are placed on the platform in a prescribed sequence. Standard calibration sequences for a 60-tonne weighbridge might include:

At each point, the indicator reading is compared to the known weight. If the error exceeds the allowed tolerance, the span calibration factor in the indicator is adjusted.

Linearity Check

This tests whether the scale is accurate across its entire range, not just at the calibration point. Non-linearity can occur when:

• One or more load cells have different sensitivities
• The junction box trimming resistors are incorrectly set
• Load cells have been damaged or overloaded

Repeatability Test

The same test weight is applied and removed three or more times, and the readings compared. A well-functioning scale should give readings that vary by no more than 1 division (typically 10kg or 20kg depending on scale capacity).

Poor repeatability often indicates:

• Loose or damaged load cell mounting hardware
• Platform contact with fixed structure at one or more points
• Unstable foundation

Eccentricity (Corner Load) Test

The same test weight is placed at each corner and in the centre of the platform. All readings should be within tolerance. Large eccentricity errors indicate uneven load distribution, typically due to:

• Unequal load cell sensitivities
• Platform structure issues
• Incorrect load cell positioning

Certificate Issue

If all tests pass within the required tolerances, the calibration certificate is issued. It should contain:

• Scale owner and location
• Scale description and serial number
• Maximum capacity and scale divisions
• Date of calibration
• Test weights used (with their own calibration certificates)
• Test results at each test point
• Name and credentials of the calibrating technician
• Certificate validity period
• Technician/company stamp or seal

Acceptable Tolerances in Nigeria

Nigeria's Weights and Measures Act and associated regulations adopt the International Organisation of Legal Metrology (OIML) standards. For commercial weighbridges:

OIML Class III tolerance (most common for trade):

A "division" is the smallest increment the scale displays. For a 60-tonne weighbridge with 20kg divisions, 1 division = 20kg.

Practical example: A 60-tonne scale with 20kg divisions weighing a 30-tonne load. The reading must be within ±1 division (±20kg) of the true weight. Error rate: 0.067%.

How Often Must You Calibrate?

Legal Requirements

For scales used in trade transactions (buying or selling by weight), Nigerian law requires:

• Initial verification before first use
• Annual re-verification at minimum
• Re-verification after any repair or modification
• Re-verification if the scale has been moved

Practical Recommendations

Legal minimum and best practice are not always the same. We recommend:

Signs Your Scale Needs Immediate Calibration

Don't wait for the scheduled date if you observe:

• Unstable readings — display fluctuating with no load on the scale
• Zero drift — scale won't zero correctly or zero changes during the day
• Inconsistent readings — same truck weighing differently on consecutive days
• Customer disputes — multiple complaints about weight accuracy
• Physical damage — truck drove off the side, impact to the deck or load cells
• Power event — scale was on during a lightning strike or major power surge

Who Can Calibrate Your Weighbridge in Nigeria?

Government Calibration Services

The Federal Ministry of Industry, Trade and Investment through the Standards Organisation of Nigeria (SON) and state Weights and Measures offices are the primary government calibration authorities. They:

• Issue the legal-for-trade verification stamp
• Are required for scales used in commercial transactions
• Can be slow and have limited geographic coverage

Private Calibration Companies

Private weighing equipment companies, including Kira Scales Limited, provide calibration services with:

• Faster scheduling and response times
• Technical depth beyond what government agencies typically offer
• Comprehensive inspection and maintenance services combined with calibration
• Detailed calibration reports and certificates

For many businesses, the practical approach is annual calibration by a qualified private company (with a proper certificate) supplemented by SON verification for legal-for-trade requirements.

Test Weights: The Unspoken Problem in Nigeria

Calibration is only as accurate as the test weights used. Many calibration companies in Nigeria use:

• Uncertified weights with unknown accuracy
• Old weights that haven't themselves been calibrated recently
• Insufficient weights (a 60-tonne scale calibrated with only 5 tonnes of test weights is not properly calibrated)

What to insist on:

1. Test weights must have their own calibration certificates from a traceable source
2. The total test weight must reach at least 80% of scale capacity (preferably 100%)
3. For a 60-tonne scale, the company should arrive with 48+ tonnes of certified test weights

Ask to see the test weight certificates before the calibration begins. If a company can't show them, walk away.

Maintaining Your Calibration Between Services

Calibration is point-in-time. Good maintenance practices preserve accuracy between calibration visits:

Daily:

• Check and record the zero reading at start of operations
• Clear any debris from under the deck
• Check that the platform is free-moving (no contact with fixed structure)

Weekly:

• Clean platform surface and approach areas
• Inspect visible cable runs for damage

Monthly:

• Check junction box for moisture or insect ingress
• Verify printer paper and ink are adequate
• Review zero trend (consistent drift suggests developing issue)

After heavy rain:

• Inspect the pit or drainage channels (pit-mounted scales)
• Check load cell junction box for water ingress
• Perform a zero check before resuming commercial use

Kira Scales Calibration Services

Kira Scales Limited provides certified weighbridge calibration services across Nigeria, including:

• Calibration with certified test weights — properly certified, adequate mass
• Full inspection before calibration — mechanical and electronic check
• Calibration certificate — detailed results at each test point
• On-site troubleshooting — identify and fix issues that affect accuracy
• Maintenance contracts — scheduled visits to keep your scale in optimal condition

We service weighbridges regardless of brand or supplier — you don't need to have purchased from us to use our calibration services.

To book a calibration visit: kirascales.com