Weighbridge calibration and verification in Kenya: what the law requires.

Calibration and verification are two different things

Calibration is the technical job: the installer loads the bridge with certified test weights across its range, checks each corner and the linearity of the reading, and adjusts the indicator until the bridge reads true. It is done at installation, after any load cell or indicator is replaced, and whenever the bridge drifts.

Verification is the legal step. A Weights and Measures inspector examines the calibrated bridge, tests it against the tolerances for its accuracy class and, if it passes, stamps it. Only a stamped bridge may be used for trade. Calibration gets the bridge ready; verification makes its tickets legally valid.

What the Weights and Measures Act says

The Act and its Rules set five requirements that matter to a weighbridge owner.

• Annual verification. Section 27(1) allows an inspector, not more often than once in every year, to call on anyone with weighing instruments in use for trade to produce them to be examined, verified, stamped or restamped. The time and place are published in the Gazette and a local newspaper at least fourteen days ahead (section 27(2)).
• Verified on site. Rule 113 of the subsidiary rules states that weighbridges and dormant platform machines shall be verified and stamped in situ, in addition to any preliminary test. The inspector comes to the bridge; the bridge does not go to the inspector.
• A current stamp when sold. Section 25(1) makes it an offence to sell or expose for sale a weighing instrument intended for trade unless it bears a valid stamp of verification dated not earlier than one year before the sale.
• No false or unjust instruments. Section 21 makes it an offence to use, or possess for use in trade, a weighing instrument that is false or unjust, and the instrument is liable to forfeiture.
• Annual inspection of premises. Rule 228 places a duty on inspectors to visit every trader's premises in their district at least once a year to inspect all instruments in use for trade, with surprise visits in between.

Cited from the Weights and Measures Act, Cap. 513, Revised Edition 2012. Read the Act itself for the full wording; the sections are listed under Sources below.

The accuracy class a weighbridge is tested to

Weighbridges are non-automatic weighing instruments and are tested to the international recommendation OIML R 76-1. Vehicle weighbridges are built and verified to Class III (medium accuracy). The class fixes two things: how many verification scale intervals the bridge may have, and how far its reading may be from the true weight.

The verification scale interval, e, is the step the bridge is verified to, for example 20 kg on a 60-tonne bridge. Table 3 of R 76-1 requires a Class III instrument to have between 500 and 10,000 intervals across its capacity.

Clause 3.5.2 sets the permitted error in service at twice the initial-verification figure. So a bridge that passed at ±1 e when new may read up to ±2 e at a later inspection and still pass.

Worked example: a 60,000 kg bridge with e = 20 kg has 3,000 intervals, which is within the Class III range. On initial verification it may be out by no more than 10 kg up to 10 tonnes, 20 kg up to 40 tonnes, and 30 kg from 40 to 60 tonnes. In service those limits double to 20, 40 and 60 kg.

What a calibration visit involves

• Certified test weights loaded across the range of the bridge, from zero to close to capacity, so linearity is checked and not just one point.
• Corner tests: the same test load placed over each load cell in turn. A corner that reads differently points to a load cell, cable or junction box fault, not a deck fault.
• Repeatability: the same load weighed several times, with the bridge returning to zero between them.
• Adjustment of the indicator's span and, on digital bridges, the per-cell trim, until the readings sit inside the class tolerances.
• A calibration record for the file, and the bridge prepared for the inspector's verification and stamp.

Signs a bridge needs recalibration before the next inspection

In most cases the cause is a load cell, a cable or a junction box rather than the deck, and the fix is a same-day replacement and recalibration. Leaving it until the annual inspection risks a failed verification and a bridge that cannot legally issue tickets until it is fixed and restamped.

• Different readings for the same truck depending on where it stops on the deck.
• Weights that drift through the day, often with temperature.
• A persistent disagreement with a buyer's or supplier's bridge on the same loads.
• Zero that will not settle, or that moves after rain or lightning.

Where the software fits

Verification proves the bridge is accurate. It does not stop an operator typing a different figure on the ticket. Weighbridge software closes that gap by reading the weight straight from the indicator, locking it only when the reading is stable, recording who weighed what and when, and flagging any weight that was entered by hand. On a bridge with position sensors it also refuses a reading until the vehicle is fully on the deck.

KiloSahihi is commissioned as part of every AEA installation, and the same engineers handle calibration checks under a service contract so the bridge stays accurate between inspections.