United Kingdom Weights and Measures Act 1985
This makes it an offence to sell or have packaged ready for sale, products that weigh less than the value marked on the package. The defence available to a company found to be breaking this law is that they have acted with ‘due care and diligence’ in the preparation of packages.
There are two classes of regulation, Class A and Class B. Class A rules set an average weight for the packages which is not less than the weight printed on the package. Variations around the average are allowed, measured in units of Tolerable Negative Error, specified for various package weights in the Act’s Code of Practical Guidance. Class B rules stipulate that all packages less than the declared weight must be rejected. This is often referred to as the minimum weight rule.
The regulations are easily accommodated by modern software based checkweighers. These machines allow the weighing and recording of results continuously and rapidly up to a maximum of 330packs per minute, recording the weight of each package for audit purposes. The alternative is to weigh and record weights by hand which is expensive. And so most people employ checkweighers.
There are, intrinsic to the mechanisms employed in check weighing, errors arising from basic physical factors. These include settling time variations which can distort results, digitisation errors and sensor linearity. Although small, individually they can add up to have a significant impact on the results so that inadvertently an operator can be working outside the law.
Zones of Indecision
The combined effect of the various fundamental errors described above, are categorised into zones of indecision. Having established what this is for a checkweigher and a particular product, the Tolerable Negative Error and average setting (Class A) or minimum weight setting (Class B) are calculated to be entered into the machine’s software so that irrespective of the errors the rules of the Act are obeyed.
Checkweigher operation should be checked at regular intervals. Standard packages are used representative of the packages being processed. These are weighed statically a number of times to establish the errors and spreads associated with the linearity of the weigher sense head and subsequent digitisation. To establish settling errors, weighing is repeated dynamically at the speed of processing . The results of these measurements indicate the maintenance needed to minimise the errors. This complete, the Zone of Indecision is established and appropriate changes made to average and minimum weight settings so that the law is complied with.
Checking the Checkweigher
It is worth checking the performance of your machine at regular intervals and registering this as a Certificate of Compliance issued by your service provider. Should a subsequent inspection identify a weight error the possession of a Certificate will provide strong evidence of you having undertaken ‘due care and diligence’