A Seal a Buyer Can Read: A Tamper-Evident Sealing Design for Market Scales, and the Attack It Cannot Stop

Official seals on market scales are usually designed to be read by inspectors: small lead stamps and fine wire that a buyer cannot interpret and rarely looks at. This paper designs a sealing arrangement whose state can be read by an untrained buyer in 16 seconds, for a total cost of 0.94 US dollars per scale at a quantity of one hundred under stated price and labour assumptions, or 56.40 dollars to seal a market of sixty scales. The arrangement has three parts: a large frangible printed label, carrying a serial number in characters a buyer can read aloud, bridging the case seam and the adjustment cover; a doubled loop of 0.8 mm galvanised wire through the cover and a case lug, closed with a crimped ferrule, whose computed breaking force is 503 N, above an assumed strong hand pull of 300 N, so that opening the case requires cutting; and a matching serial card held by the market office, with an ultraviolet ink mark as a secondary feature. Entry through the sealed surfaces must cut the wire, fragment the label — whose computed tear force across its 40 mm width is 12.8 N — or remove the ferrule, each leaving a change visible in the four-step inspection set out here. The attack the design cannot stop is stated plainly: substitution of the whole instrument. A replica seal set costs 0.50 dollars in materials under stated assumptions, and a substitute second-hand scale with a copied serial fitted to it is costed at 19.30 dollars in total; against a buyer who never compares the serial with the registry, the chance of detection by the seal alone is zero at every number of visits. The seal is therefore specified as one half of a system whose other half is registry comparison, and the paper quantifies how detection grows with the frequency of that comparison. AI disclosure: Dave, an AI research system, assisted in the preparation of this work.