A weighing instrument can be accurate and still not be lawful for use in a commercial transaction. This is the point that often causes confusion.

Calibration checks measurement performance against a reference standard. Legal verification confirms that an instrument meets the requirements for its intended regulated use and has been passed as fit for trade. These processes support each other, but they are not interchangeable.

For businesses using industrial or commercial weighing equipment to determine quantity, price or another value in a transaction, understanding the difference is important. It affects the equipment you choose, the records you keep, the work carried out after a repair and whether the instrument can continue to be used legally.

This guide explains legal verification of weighing scales in the UK, with a particular focus on Non-Automatic Weighing Instruments (NAWI).

What is legal verification?

Legal verification is a formal regulatory process used to confirm that a weighing instrument is fit for its intended legal-for-trade application.

In Great Britain, the framework is underpinned by the Weights and Measures Act 1985 and the Non-Automatic Weighing Instruments Regulations 2016.

Under section 11 of the Weights and Measures Act, equipment used for trade must be passed as fit for use for trade and bear the appropriate stamp or mark. This may be carried out by a Weights and Measures Inspector or, within the relevant scope, an Approved Verifier.

Verification considers more than whether the displayed reading appears correct. The verifier must consider the instrument’s approval, markings, configuration, accuracy class, maximum permissible errors, installation and intended use. The instrument must also correspond with the relevant conformity documentation and approval requirements.

For NAWI, verification testing follows the technical principles of EN 45501 and OIML R76. Depending on the instrument and application, checks may include:

• Visual inspection of the instrument and its metrological markings.
• Confirmation of the declaration of conformity and relevant approval information.
• Loading and unloading performance.
• Gross, net and tare weighing.
• Zero-setting and tare-setting accuracy.
• Repeatability.
• Eccentric loading.
• Discrimination and response to small changes in load.

The test equipment must also be suitable. UKWF guidance states that test weights should meet the relevant OIML tolerances, such as M1 or better for Class III instruments and F2 or better for Class II instruments. Annual calibration of these working weights is recommended to help confirm that they remain within calibration.

When does a weighing instrument require verification?

A NAWI generally requires the appropriate conformity assessment and verification route when it is used for one of the regulated applications covered by the legislation.

Examples in industrial and commercial environments can include:

• Manufacturing processes where weight determines the quantity or price of materials.
• Food production and the sale of goods by weight.
• Logistics, freight and vehicle weighing.
• Weighbridges and axle weighing systems.
• Waste and recycling transactions where material is bought or sold by weight.
• Precious metals and other high-value materials.
• Bulk materials, aggregates and chemicals.
• Industrial weighing where results feed into a commercial transaction.
• Laboratory or precision weighing where the result is used to determine a trade value.

Whether a specific instrument requires legal verification depends on how it is used, not simply on its size or model. A scale used for internal process monitoring may have different requirements from an instrument used to determine the charge for goods or services.

A trade-approved scale therefore involves two connected elements:

1. The instrument must be an approved type and have followed the appropriate conformity assessment route.
2. The individual instrument must have been verified and remain in qualified condition for its intended use.

Regulated NAWI placed on the GB market must meet the applicable requirements. The manufacturer draws up a declaration of conformity and applies the relevant UKCA or CE and M markings, depending on the applicable arrangements. OPSS is responsible for market surveillance of NAWI and compliance with the relevant regulations.

Calibration and legal verification are different

The difference between calibration and verification can be summarised simply:

> Calibration determines how accurately an instrument measures against a traceable standard. Legal verification confirms whether the instrument is compliant and fit for use in a regulated trade application.

Calibration is a technical measurement process. A technician compares the instrument’s readings with known, traceable standards and records the results, including error and, where applicable, measurement uncertainty. The instrument may be adjusted as part of an agreed service, but calibration itself does not confer legal acceptance for trade use.

Legal verification is a formal regulatory act. It considers whether the instrument is suitable for the approved application, complies with its approval and falls within the permitted legal tolerances. The result is recorded through the prescribed certification, stamp or mark.

This means a weighing instrument can be:

• Accurate but not legally verified.
• Calibrated but not approved for its intended trade application.
• Correctly approved but no longer accurate because of wear, damage or environmental conditions.
• Both calibrated and legally verified, which is generally the appropriate position for regulated use.

Calibration and verification should never be treated as interchangeable terms. Routine calibration supports measurement confidence and quality systems. Verification establishes legal status for the relevant application.

WOMAG provides UKAS-accredited mass calibration up to 2,500 kg, with calibration carried out under ISO/IEC 17025:2017 accreditation. Our wider calibration and maintenance work is supported by test equipment traceable to ISO/IEC 17025:2017 and our ISO 9001:2015 quality management system.

Why can repair or adjustment require reverification?

A repair does not automatically mean that an instrument becomes a completely new product. However, repair, adjustment, alteration, addition or replacement can affect the instrument’s accuracy, function or conformity.

The first question should be:

> Has the work been such that the instrument could be disqualified and therefore requires requalification?

This decision must be made case by case, considering the totality of the work and the instrument’s associated documentation. The assessment should involve a physical inspection and a review of the relevant paperwork. Where requalification is not required, that conclusion should be reached positively and recorded so it remains traceable for audits or future inspections.

Businesses often use the term weighing scale reverification. In the regulatory context, requalification is commonly used for the in-service process of restoring an instrument’s qualified status after it has been disqualified or after work that could affect compliance.

Examples from OPSS guidance include:

• Changing a load cell: this normally requires requalification because the load cell is a critical component and its behaviour can be affected by local installation and environmental conditions. These factors need to be confirmed in situ.
• Replacing a self-contained weighing module like for like: this is not normally subject to disqualification where the replacement is identical and covered by the existing declaration of conformity.
• Software upgrades: requalification is required where the software change is metrologically significant.
• Significant changes: where the work means that a different type approval certificate applies, the instrument may normally be treated as new.

The declaration of conformity is important. If it lists individual component serial numbers, a module replacement may not be covered and reverification may be required. If it lists only test certificate numbers, and those numbers remain unchanged, the replacement may remain within the declaration.

A repair company should therefore be told before work begins that the instrument is used for legal-for-trade weighing. This allows the repair, adjustment, sealing and requalification requirements to be considered together.

Who can verify and reverify weighing instruments?

A Weights and Measures Inspector can test and pass equipment as fit for use for trade. An Approved Verifier may do the same within the conditions and equipment scope of their approval.

The OPSS Approved Verifier Guidance explains that Approved Verifiers operate under an audited quality system and are approved for specific equipment types and approval certificates. They are expected to understand:

• The relevant legislation and instrument approvals.
• Conformity assessment procedures.
• Accuracy and measurement uncertainty.
• Testing equipment and traceability.
• Correct examination and testing procedures.
• Sealing against unauthorised changes.
• The correct use of verification and requalification marks.
• Record keeping and quality assurance.

An Approved Verifier’s scope matters. A provider should not imply that approval to work with one instrument type automatically covers every weighing instrument.

Where an Approved Verifier carries out relevant work, the Local Weights and Measures Authority must be formally notified of the work and whether the equipment was adjusted. This is one reason why legal metrology records and communication between the equipment owner, service provider and relevant authority are important.

What responsibilities does the business have?

The business using the equipment remains responsible for making sure that its weighing instruments are suitable and legally compliant for the way they are used.

Good practice includes:

• Confirming whether the application is regulated before selecting or modifying equipment.
• Using an appropriate approved and verified instrument.
• Keeping the declaration of conformity, verification records and service history.
• Protecting verification marks, seals and metrologically significant settings.
• Arranging calibration and maintenance at suitable intervals.
• Telling the service provider that the equipment is used for trade.
• Stopping use and seeking advice if the instrument is damaged, rejected, outside tolerance or has lost its qualified status.
• Arranging requalification where repair or adjustment may affect accuracy or function.
• Checking that the instrument remains suitable for its environment, capacity, accuracy class and intended application.

The Weights and Measures business guidance from OPSS states that equipment used in business must be passed as fit for use for trade by an Inspector or Approved Verifier. It also explains that equipment repaired or adjusted will need to be requalified by an Inspector or Approved Verifier.

What does WOMAG’s NAWI Module D authorisation mean?

WOMAG is authorised by UKMB to verify and reverify a range of Non-Automatic Weighing Instruments within our approved scope. WOMAG holds UKMB Module D.

Under the NAWI Regulations 2016, Module D is a conformity assessment procedure based on quality assurance of the production process. It involves an approved quality system covering production, final product inspection and testing, with ongoing assessment by an approved body.

For customers, this sits behind our ability to carry out relevant verification and reverification work within our authorised scope. It means the work is carried out through controlled procedures and an audited quality system rather than on a self-declared basis.

Our role goes beyond servicing or calibrating scales. We work with the wider legal metrology framework, including instrument approvals, conformity assessment, declarations, sealing, verification requirements and the effect of repairs or adjustments on qualified status.

WOMAG also works to ISO 9001:2015, is UKAS-accredited to ISO/IEC 17025:2017 for calibration up to 2,500 kg, and is a member of the UK Weighing Federation. Our maintenance, repair, hire and calibration procedures are approved by the UKWF Code of Practice.

Why does the choice of weighing provider matter?

Legal-for-trade weighing requires more than a routine service visit and a calibration certificate.

Your provider should be able to identify:

• Whether the application is regulated.
• Which approval and conformity route applies.
• Whether the instrument is within the provider’s verification scope.
• Whether a repair or adjustment requires requalification.
• Which components and software are covered by the declaration of conformity.
• How legally significant settings should be protected and sealed.
• What records and notifications are required.
• How calibration, maintenance and verification should work together.

This is particularly important for industrial weighing systems, vehicle weighing, production equipment, weighing modules and instruments operating in demanding environments. A technically capable repair that does not consider legal status can leave a business with an accurate instrument that cannot lawfully be used for the intended trade application.

Frequently asked questions

What is legal verification of weighing scales in the UK?

Legal verification is the formal process of confirming that a weighing instrument complies with the relevant legal requirements and is fit for use in a regulated trade application. In Great Britain, this is carried out by a Weights and Measures Inspector or an Approved Verifier within the appropriate scope.

Is calibration the same as verification?

No. Calibration determines and documents measurement performance against traceable standards. Legal verification confirms compliance and fitness for a regulated trade use. A calibration certificate does not, by itself, make a weighing instrument legal for trade.

What are NAWI verification requirements?

NAWI verification involves checking the instrument against its approval and conformity documentation, markings, intended use and applicable metrological requirements. Testing can include repeatability, eccentric loading, zero, tare, discrimination and loading performance.

When is weighing scale reverification required?

Reverification, or in-service requalification, may be required after repair, adjustment, alteration, addition or replacement where the work could affect the instrument’s accuracy or function. The decision is case-specific and should be documented.

Does replacing a load cell require reverification?

Changing a load cell normally requires requalification because it is a critical component and its performance depends on the instrument’s installation and local environmental conditions.

Can WOMAG verify any weighing instrument?

WOMAG can verify and reverify a range of NAWI within our authorised scope. The relevant instrument type, approval and application should be confirmed before work is arranged.

Regulatory references

• Weights and Measures Act 1985, section 11
• Non-Automatic Weighing Instruments Regulations 2016
• OPSS weights and measures business guidance
• OPSS Approved Verifier Guidance 2023
• UK Weighing Federation: General Verification
• WOMAG UKAS calibration
• WOMAG service and repairs

Need advice on legal-for-trade weighing?

If you are unsure whether your equipment requires legal verification, whether a repair will require reverification, or whether your existing records and approvals are suitable, WOMAG can help you review the position.

Our specialist team can advise on industrial and commercial weighing applications, verification, requalification, calibration, servicing and maintenance. Contact WOMAG for practical advice on your specific instrument and application, or call 0330 0550 500.